Looking at the universe through very different ‘eyes’

The Small Magellanic Cloud galaxy here seen in infrared light, but it looks different when viewed at other wavelengths. ESA/NASA/JPL-Caltech/STScI

Michael J. I. Brown, Monash University

We are bathed in starlight. During the day we see the Sun, light reflected off the surface of the Earth and blue sunlight scattered by the air. At night we see the stars, as well as sunlight reflected off the Moon and the planets.

But there are more ways of seeing the universe. Beyond visible light there are gamma rays, X-rays, ultraviolet light, infrared light, and radio waves. They provide us with new ways of appreciating the universe.

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X-ray Moon

Have you looked at the Moon during the daytime? You will see part of the Moon bathed in sunlight and the Earth’s blue sky in front of the Moon.

The Moon behind a blue sky. Flickr/Ed Dunens, CC BY

Now put on your X-ray specs, courtesy of the ROSAT satellite, and you will see something intriguing.

The Sun emits X-rays, so you can see the daytime side of the Moon easily enough. But the night time side of the Moon is silhouetted against the X-ray sky. The X-ray sky is behind the Moon!

The Moon seen in X-rays by ROSAT. The night side of the Moon is silhouetted against the X-ray background. DARA, ESA, MPE, NASA, J.H.M.M. Schmitt

Just what is the X-ray sky? Well, X-rays are more energetic than visible light photons, so X-rays often come from the hottest and most violent celestial objects. Much of the X-ray sky is produced by active galactic nuclei, which are powered by matter falling towards black holes.

In X-rays, the Moon is silhouetted against many millions of celestial sources, powered by black holes, scattered across billions of light years of space.

Radio skies

If you’re in the southern sky and away from light pollution (including the Moon), then you can see the Small Magellanic Cloud. This is a companion galaxy to our own Milky Way. With the unaided eye it looks like a diffuse cloud, but what we are actually seeing is the combined light of millions of distant stars.

Visible light images of the Small Magellanic Cloud are dominated by starlight. ESA/Hubble and Digitized Sky Survey/Davide De Martin

Radio waves provide a very different view of the Small Magellanic Cloud. Using the Australian Square Kilometre Array Pathfinder, tuned to 1,420.4MHz, we no longer see stars but instead see atomic hydrogen gas.

Radio waves can trace the hydrogen gas in the Small Magellanic Cloud. ANU and CSIRO

The hydrogen gas is cold enough that the atoms hang onto their electrons (unlike ionised hydrogen). It can also cool further and collapse (under the force of gravity) to produce clouds of molecular hydrogen gas and eventually new stars.

Radio waves thus allow us to see the fuel for star formation, and the Small Magellanic Cloud is indeed producing new stars right now.

Feeling the heat in the microwave

If the universe were infinitely large and infinitely old, then presumably every direction would eventually lead the surface of a star. This would lead to a rather bright night sky. The German astronomer Heinrich Olbers, among others, recognised this “paradox” centuries ago.

A visible light image of the entire night sky is dominated by starlight from the Milky Way. ESO/S. Brunier, CC BY

When we look up at the night sky, we can see the stars, planets and Milky Way. But most of the night sky is black, and this tells us something important.

But lets take a look at the universe in microwave light. The Planck satellite reveals glowing gas and dust in the Milky Way. Beyond that, in every direction, there is light! Where does it come from?

The microwave sky is glowing in every direction. ESA, HFI & LFI consortia

At microwave wavelengths we can observe the afterglow of the Big Bang. This afterglow was produced 380,000 years after the Big Bang, when the universe had a temperature of roughly 2,700℃.

But the afterglow we see now doesn’t look like a 2,700℃ ball of gas. Instead, we see a glow equivalent to -270℃. Why? Because we live in an expanding universe. The light we observe now from the Big Bang’s afterglow has been stretched from visible light into lower-energy microwave light, resulting in the colder observed temperature.

Planetary radio

Jupiter is one of the most rewarding planets to observe with a small telescope – you can see the cloud bands stretching across the giant planet. Even binoculars can reveal the four moons discovered by Galileo centuries ago.

A visible light image of Jupiter, taken by the Cassini spacecraft. NASA/JPL/Space Science Institute

But you get a less familiar view of Jupiter when you switch to radio waves. A radio telescope reveals the dull warm glow of the planet itself. But what really stands out are radio waves coming from above the planet.

Jupiter is a copious emitter of radio waves. CSIRO

Much of the radio emission from Jupiter is produced by synchrotron and cyclotron radiation, which results from speeding electrons spiralling in a magnetic field.

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On Earth we use particle accelerators to produce such radiation. But in Jupiter’s powerful magnetic field it occurs naturally (and copiously).

The synchrotron produced by Jupiter is so powerful that you can detect it on Earth – not just with multimillion-dollar radio telescopes, but with equipment that can be bought for several hundred dollars. You don’t need to be a professional astronomer to expand your view of the universe beyond visible light.

Michael J. I. Brown, Associate professor, Monash University

This article was originally published on The Conversation. (Reblogged by permission). Read the original article.

Black holes are even stranger than you can imagine

Alister Graham, Swinburne University of Technology

Our love of black holes continues to grow as our knowledge of these celestial bodies expands. The latest news is the discovery of a rare “middleweight” black hole, a relative newcomer to the black hole family.

We already knew that some black holes are just a few times the mass of our Sun, while others are more than a billion times as massive. But others with intermediate masses, such as the one 2,200 times the mass of our Sun recently discovered in the star cluster 47 Tucanae, are surprisingly elusive.

So what is it about black holes, these gravitational prisons that trap anything that gets too close to them, that captures the imagination of people of all ages and professions?

‘Dark stars’

As far back as 1783, within the framework of Newtonian dynamics, the concept of “dark stars” with sufficiently high density that not even light can escape their gravitational pull had been advanced by the English philosopher and mathematician John Michell.

Almost immediately after Albert Einstein presented his theory of general relativity in 1915, which supplanted Newton’s description of our Universe and revealed how space and time are intimately linked, fellow German Karl Schwarzschild and Dutchman Johannes Droste independently derived the new equations for a spherical or point mass.

Although at the time the issue was still something of a mathematical curiosity, over the ensuing quarter of a century nuclear physicists realised that sufficiently massive stars would collapse under their own weight to become these previously theorised black holes.

Their existence was eventually confirmed by astronomers using powerful telescopes, and more recently colliding black holes were the source of the gravitational waves detected with the LIGO instrumentation in the United States.

A dense object

The densities of such objects is mind-boggling. If our Sun were to become a black hole, it would need to collapse from its current size of 1.4 million km across to a radius of less than 3km (6km across). Its average density within this “Schwarzschild radius” would be nearly 20 billion tonnes per cubic centimetre.

The increasing strength and pull of gravity as you get closer to a black hole can be dramatic.

On Earth, the strength of the gravitational pull holding you to its surface is roughly the same at your feet as it is at your head, which is a little bit farther away from the planet.

But near some black holes, the difference in gravitational pull from head to toe is so great that you would be pulled apart and stretched out on an atomic level, in a process referred to as spaghettification.

In 1958, the American physicist David Finkelstein was the first to realise the true nature of what has come to be called the “event horizon” of a black hole. He described this boundary around a black hole as the perfect unidirectional membrane.

It’s an intangible surface encapsulating a sphere of no return. Once inside this sphere, the gravitational pull of the black hole is too great to escape – even for light.

In 1963, the New Zealand mathematician Roy Kerr solved the equations for the more realistic rotating black holes. These yielded closed time-like curves that permitted movement backwards through time.

While such strange solutions to the equations of general relativity first appeared in the 1949 work of Austrian-American logician Kurt Gödel, it is commonly thought that they must be a mathematical artefact yet to be explained away.

A video simulation of two black holes merging.

Black and white holes

In 1964, two Americans, the writer Ann Ewing and the theoretical physicist John Wheeler, introduced the term “black hole”. Subsequently, in 1965, the Russian theoretical astrophysicist Igor Novikov introduced the term “white hole” to describe the hypothetical opposite of a black hole.

The argument was that if matter falls into a black hole, then perhaps it is spewed out into our universe from a white hole.

This idea is partly rooted in the mathematical concept known as an Einstein-Rosen bridge. Discovered (mathematically) in 1916 by the Austrian physicist Ludwig Flamm, and re-introduced in 1935 by Einstein and the American-Israeli physicist Nathan Rosen, it was later termed a “wormhole” by Wheeler.

In 1962, Wheeler and the American physicist Robert Fuller explained why such wormholes would be unstable for transporting even a single photon across the same universe.

Fact and fiction

Not surprisingly, the idea of entering a (black hole) portal and re-emerging somewhere else in the universe – in space and/or time – has spawned countless science fiction stories, including Doctor Who, Stargate, Fringe, Farscape and Disney’s Black Hole.

Ongoing productions can simply claim that their characters are travelling to a different or a parallel universe to our own. While it appears to be mathematically feasible, there is of course no physical evidence to support the existences of such universes.

But this is not to say that time travel, at least in a limited sense, is not real. When travelling at great speed, or perhaps falling into a black hole, the passage of time does slow down relative to that experienced by stationary observers.

Clocks flown quickly around the world have demonstrated this, displaying time lags in accordance with Einstein’s theory of special relativity.

The 2014 movie Interstellar played on this effect around a black hole, thereby creating a sense of travelling forward in time for astronaut Cooper (played by Matthew McConaughey).

Despite the strangely endearing name, the phrase “black hole” is perhaps somewhat misleading. It implies a hole in space-time through which matter will fall, as opposed to matter falling onto an incredibly dense object.

What actually exists within a black hole’s event horizon is hotly debated. Attempts to understand this include the “fuzzball” picture from string theory, or descriptions of black holes in quantum gravity theories known as “spin foam networks” or “loop quantum gravity”.

One thing that does seem certain is that black holes will continue to intrigue and fascinate us for some time yet.

Alister Graham, Professor of Astronomy, Swinburne University of Technology

This article was originally published on The Conversation. (Reblogged by permission). Read the original article.

2016: the year in space and astronomy

Alan Duffy, Swinburne University of Technology and Rebecca Allen, Swinburne University of Technology

The achievements of astrophysicists this year were as groundbreaking as they were varied. From reuniting a lander with a mothership on a comet, to seeing the most extreme cosmic events with gravitational waves, 2016 was truly out of this world for science.

Here are some of the highlights of the year that was.

1. Gravitational Waves

The spectacular announcement that ripples in the very fabric of spacetime itself had been found (and from surprisingly massive black holes colliding) sent similarly massive ripples through the scientific community. The discovery was made using the Laser Interferometer Gravitational-Wave Observatory (LIGO) and represents a fundamentally new sense with which to see the universe.

Animation showing how colliding black holes cause a ripple in spacetime that moves outwards into the universe as a gravitational wave.

The gravitational waves cause one arm of the LIGO detector to stretch relative to the other by less than a thousandth of the width of a proton in the centre of the atom. Relatively speaking, that’s like measuring a hair’s-width change in the distance to the nearest star.

This discovery was the end of a century-long quest to prove Einstein’s final prediction that these gravitational waves are real. It also allows us to directly “see” that famously and fundamentally invisible entity: the black hole (as well as definitively proving its existence). The fact that the two black holes collided 1.3 billion years ago and the waves swept through Earth just days after turning the detector on only add to the incredible story of this discovery.

The ‘sound’ of the black holes colliding where the measured signal from LIGO is converted to audio, the rising chirp sound towards the end is the two black holes spiralling together ever more quickly. A surprisingly wimpy sound for the most extreme collision ever detected.

2. SpaceX lands (and crashes) a rocket

The year started so well for SpaceX with the incredible achievement of sending a satellite into orbit, which is no mean feat itself at such low cost, before then landing that launch rocket on a barge in the ocean. A seemingly unstoppable sequence of launches and landings made it appear that a new era of vastly cheaper access to space through rockets that could be refuelled and reused was at hand.

A Falcon 9 first-stage automatically returns to the barge/droneship ‘Of Course I Still Love You’ in the middle of the Atlantic ocean.

Unfortunately, with the explosion of a Falcon 9 on the launchpad, the company was grounded, but apparently hopes for a resumed launch in early January.

SpaceX outlines a vision for travel to Mars with planned Interplanetary Transport System.

Add to that the visionary plans to settle Mars outlined by Elon Musk, albeit not without some audacious challenges, and it’s been a year of highs and lows for SpaceX.

3. Closest star may harbour Earth-like world

Proxima Centauri is our Sun’s nearest neighbour at just over four light years away, and it appears that its solar system may contain an Earth-like world. Until this year, astronomers weren’t even sure that any planets orbited the star, let alone ones that might harbour the best extrasolar candidate for life that spacecraft could visit within our lifetime.

What a trip to the Sun’s closet neighbour would look like.

The planet, creatively named “Proxima b”, was discovered by a team of astronomers at Queen Mary University in London. Using the light of Proxima Centauri, the astronomers were able to detect subtle shifts in the star’s orbit (seen as a “wobble”), which is the telltale sign that another massive object is nearby.

An artist’s impression of Proxima b’s landscape. ESO/M. Kornmesser

While Proxima Centauri is barely 10% the size of our Sun, Proxima b’s orbit is only 11 days long, meaning it is very close to the star and lies just within the so-called habitable zone. However, follow-up with either Hubble or the upcoming James Webb Space telescope is necessary to determine if the exoplanet is as well suited for life as Earth.

4. Breakthrough Listen listening and Starshot star-ted

With a potential Earth twin identified in Proxima b, now the challenge is to reach it within a human lifetime. With the breakthrough initiative starshot, which has been funded by Russian billionaire Yuri Milner and endorsed by none other than Stephen Hawking, lightweight nanosails can be propelled by light beams to reach speeds up to millions of kilometres an hour.

Such speeds would allow a spacecraft to arrive at Proxima b in about 20 years, thus enabling humans to send information to another known planet for the first time.

However, there are many challenges ahead, such as the fact that the technology doesn’t exist yet, and that high-speed collisions with gas and dust between stars may destroy it before it can reach its target.

But humans have proven to be resourceful, and key technology is advancing at an exponential rate. Incredibly the idea of sailing to another world is no longer science fiction, but rather an outrageously ambitious science project.

One of the founders of the Breakthrough initiatives, Yuri Milner, discusses the technology needed for breakthrough starshot.

Perhaps, aliens are already sending out their own information in the form of radio transmissions. In another breakthrough initiative called Listen, also championed by Hawking, astronomers will be searching the habitable zones around the million closest stars to try to detect incoming radio transmissions. Involving Australia’s very own Parkes telescope (as well as the Green Bank Telescope and Lick Observatory at visible wavelengths of light), observations have been running through 2016 and the search for alien signals will continue for the next decade.

5. Philae reunited with Rosetta

In 2014 the Philae lander became the first space probe to land on a comet, and even though its crash landing dictated that its science transmission would be a one-off, its recent rediscovery by Rosetta has allowed it to continue to contribute to analysis of comet 67P.

Philae’s crash location, as well as the orientation of the doomed probe, has allowed astronomers to accurately interpret data taken by Rosetta regarding the composition of the comet.

Where’s Philae? ESA

While Philae has literally been living under (crashed on) a rock for the past two years, Rosetta has been the busy bee, taking numerous images, spectroscopy and other data of the comet.

In fact, data taken from Rosetta’s spectrometer has been analysed and revealed that the amino acid, glycine, is present in the comet’s outgassing, which breaks away from the surface of the comet as it becomes unstable from solar heating. Glycine is one of the fundamental building blocks of life; necessary for proteins and DNA, and its confirmed extraterrestrial confirms that the ingredients for life are unique to Earth, and that we may have comets to thank for providing our microbial ancestors with those crucial ingredients.

Dust and gas emitted from comet 67P reveal an amino acid. ESA

Outlook for Down Under

The future for astrophysics in Australia in 2017 looks particularly bright, with two ARC Centres of Excellence: CAASTRO-3D studying the build of atoms over cosmic time; and OzGRav exploring the universe with gravitational waves; as well as SABRE, the world’s first dark matter detector in the Southern Hemisphere, installed by end of the year.

If you thought 2016 was a great year in space, then you’re in for a treat in 2017.

Alan Duffy, Research Fellow, Swinburne University of Technology and Rebecca Allen, PhD candidate researching galaxy formation and evolution, Swinburne University of Technology

This article was originally published on The Conversation. (Reblogged by permission). Read the original article.

Once upon a time… how the Rosetta mission won our hearts

Tanya Hill, Museum Victoria

Last Friday, September 30, the European Space Agency’s (ESA) Rosetta mission, which explored the Comet 67P/Churyumov-Gerasimenko, reached its final conclusion and was heralded a resounding success.

The mission accomplished great technical feats. It was the first to place a spacecraft into orbit around a comet and Rosetta was in the hot-seat to watch the sun turn this cold icy object into a hive of activity.

In November 2014, Rosetta released Philae, the first probe to land on the surface of a comet. The probe ended up bouncing across the comet’s surfacing before coming to rest in the shadows. But it did spend three successful days gathering scientific data before its primary battery was drained and communication was lost.

Just a month before mission end, Philae was finally found.
Main image and lander inset: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA; context: ESA/Rosetta/NavCam

The mission gathered a wealth of scientific information, as Comet Churyumov-Gerasimenko became the most studied comet in history. The comet’s gravity has been mapped, its various surface terrains have been identified, and its distinctive “rubber ducky” shape is now recognised as two smaller comets that gently melded together as one.

Data from Philae revealed that the comet’s surface is covered with key organic compounds, suggesting that the building blocks of life may be widespread throughout the universe.

But alongside all these great achievements has been the exciting array of science communication that has supported the mission. The goal of ESA was to reach out to as many people as possible and the team looked for new and interesting ways to capture the minds, and also the hearts, of a wide audience.

Once upon a time…

Long, long ago (or in reality back in January 2014), there was a little spacecraft that needed waking up. Launched a decade earlier, the spacecraft had been placed in hibernation for 31 months as it completed the last leg of its journey towards the comet.

The ESA team began a Wake Up, Rosetta! campaign to inform the public about this mission that had begun long ago but had a very exciting year ahead of it.

With wonderful insight, ESA recognised the parallel between Rosetta’s story and the classic fairy tale Sleeping Beauty. This inspired the team to produce a cartoon series, specifically targeted to families and young children that would introduce them to Rosetta.

It was time to wake up Rosetta as well as wake up the public to the fantastic mission that was about to occur. ESA

The end result was a charming cartoon series that has reached a range of audiences and has even won the hearts of the scientists themselves.

Hooking people in

Via the cartoon, complex technical and scientific topics have been tackled in a highly approachable way, one that is widely understood by children as well as appreciated by adults.

The cartoons hooked people in to the process of how a mission unfolds (eg. Preparing for #CometLanding), accurately described the science being undertaken (eg. Living with a comet)
and also brilliantly connected with people on an emotional level, adding to the excitement, anticipation and curiosity inspired by the mission (Are we there yet?).

But what about the bad times?

Producing the series was not without its risks. What would happen if the mission failed? This was put to the test when Philae’s landing did not go precisely as planned. Having brought Philae to life and into the hearts of their audience, would he now be left for dead on the comet?

The team realised they could take advantage of the nature of the cartoon and its strong emotional focus. In the #cometlanding episode, the “mishap” was presented in terms of common feelings: a story of the fear, surprise, commitment and even adding a little humour.

Philae packs his bag for the comet landing: camera, compass, pickaxe, snow boots and importantly a sandwich as he’ll need his own source of energy. ESA

In the end, Philae completes the tasks at hand, is proud of his work and slips gently into a long deep sleep. It’s the stuff of fairy tales but made all the better because it was inspired by real events unfolding millions of kilometres away.

One of many approaches

The ESA team should be applauded for their philosophy of making the Rosetta mission personally relevant to people world-wide and being able to building such strong connections.

The cartoon even spun-off its own merchandising material with stickers given out by scientists at public events and a 3D paper model to be made at home. It was featured on T-shirts, sweatshirts and even became a cuddly soft toy.

However, the cartoon was just one aspect of the Rosetta mission’s broad communication campaign. The Rosetta blog provided news and updates as they occurred, there were plenty of interviews with mission experts, and also a Discovery Channel documentary Landing on a Comet.

In a very bold and innovative move, the ESA team released a high-quality short sci-fi film, using all the glamour of Hollywood to present the scientific, technical and philosophical aspects of the mission.

This beautiful work of fiction introduced the mission and a follow up epilogue, released last week, celebrated the mission end.

Well done to ESA and Rosetta for the amazing scientific work that was accomplished and for inspiring all of us along the way.

A detailed overview of ESA’s communication strategy for the Rosetta mission is presented in the March issue of Communicating Astronomy with Public (CAP Journal).

Tanya Hill, Honorary Fellow of the University of Melbourne and Senior Curator (Astronomy), Museum Victoria

This article was originally published on The Conversation. (Reblogged by permission). Read the original article.

Finding Pluto: the hunt for Planet X

Kevin Orrman-Rossiter, University of Melbourne and Alice Gorman, Flinders University

Our solar system’s shadowy ninth (dwarf) planet was the subject of furious speculation and a frantic search for almost a century before it was finally discovered by Clyde Tombaugh in 1930. And remarkably, Pluto’s reality was deduced using a heady array of reasoning, observation and no small amount of imagination.

The 18th and 19th centuries were thick with astronomical discoveries; not least were the planets Uranus and Neptune. The latter, in particular, was predicted by comparing observed perturbations in the orbit of Uranus to what was expected. This suggested the gravitational influence of another nearby planet.

John Couch Adams and Urbain-Jean-Joseph Le Verrier calculated the orbit of Neptune by comparing these perturbations in Uranus’ orbit to those of the other seven known planets. Neptune was hence discovered in the predicted location in 1846.

Soon after this, French physicist Jacques Babinet proposed the existence of an even more distant planet, which he named Hyperion. Le Verrier wasn’t convinced, stating that there was “absolutely nothing by which one could determine the position of another planet, barring hypotheses in which imagination played too large a part”.

Despite that lack of evidence for perturbations in Neptune’s orbit, many predicted the existence of a ninth planet over the next 80 years. Frenchman Gabriel Dallet called it “Planet X” in 1892 and 1901, and the famed American astronomer William Henry Pickering proposed “Planet O” in 1908.

Comets, the law of vegetable growth and a conspiracy

In addition to the perturbations of known planets there were other hypotheses that foretold unknown bodies beyond Neptune.

In the 19th century, it was understood that many comets had highly elliptical orbits that swung past the outer planets at their farthest points from the sun. It was believed that these planets diverted the comets into their eccentric orbits.

Pluto is not only distant, but it’s small. That makes it very difficult to see from Earth. NASA

In 1879 the French astronomer Camille Flammarion predicted a planet with an orbit 24 times that of Earth’s based on comet measurements. Using the same method, George Forbes, professor of astronomy at Glasgow University, confidently announced in 1880 that “two planets exist beyond the orbit of Neptune, one about 100 times, the other about 300 times the distance of the earth from the sun”.

Depending on how the calculations were done, the results predicted anything from one to four planets.

Other predictions were based on what can be described as numerical curiosities or speculations. One of these was the now-discredited Bode’s law, a sort of Fibonacci sequence for planets. The American mathematician Benjamin Pierce was not a fan, claiming that “fractions which express the law of vegetable growth” were more accurate than Bode’s law.

As well as these earnest astronomers, the trans-Neptunian planet idea attracted cranks and visionaries. An interesting contribution came in 1875 from Count Oskar Reichenbach, who accused Le Verrier and Adams of conspiring to conceal the locations of two trans-Neptunian planets.

The early photographic searches

Theories and calculations were all well and good, but many hoped to actually see the hitherto invisible planet(s). From the late 1800s new powerful telescopes equipped with the latest dry-plate photographic technologies were employed to search for undiscovered planets.

Pluto isn’t easy to spot. This 10 minute exposure shows the apparent magnitude of Pluto compared to some nearby stars. Kevin Heider, CC BY-SA

Amateur astronomers such Isaac Roberts and William Edwards Wilson used the predictions of George Forbes to search the skies, taking many hundreds of photographic plates in the process. They found no lurking trans-Neptunian planets.

The professionals fared no better. Edward Charles Pickering, director of the Harvard Observatory and William’s brother, spent around ten years from 1900 searching using his own data and those of earlier astronomers such as Dallet, all to no avail.

Lowell’s approach

In 1906 a new approach was introduced by the veteran astronomer Percival Lowell. Although best known to us for his (mistaken) observations of canals on Mars, Lowell bought a new rigour to analysing the orbit of Uranus based on observational data from 1750 to 1903.

With these improved calculations, hope for a visual fix on the elusive planet was renewed. With the aid of the brothers Vesto and Earl Slipher, Lowell spend the rest of his life scanning photographic plates with a hand magnifier and finally with a Zeiss blink comparator.

In September 1919 William Pickering kicked off another search for “Planet O” based on deviations in Neptune’s orbit. Milton L Humason, from the Mount Wilson Observatory in California, started a search based on these new predictions as well as Lowell’s and Pickering’s 1909 predictions. This search again failed to find any new planets. Pickering continued to publish articles on hypothetical planets but by 1928 he had become discouraged.

Zeiss Blink comparator at Lowell Observatory used in the discovery of Pluto by Clyde Tombaugh in 1930.  nivium/Wikimedia, CC BY

A planet among 160,000 stars

As part of Lowell’s legacy, the Lowell Observatory built a special astrographic telescope. It was completed in 1929, and under Vesto Slipher’s direction, a young assistant was assigned to take and examine the photographs of the farthest reaches of the solar system. His name was Clyde Tombaugh.

This was grim, unglamorous work. Each plate was exposed for an hour or more, with Tombaugh adjusting the telescope precisely to keep pace with the slowly turning sky. Today a computer would make the comparisons, but in 1929 they were made by eye, manually flicking between two images. Stars would remain motionless while other bodies would seem to jump between views. Some images would have 40,000 stars, others up to 1 million.

Nearly a year had elapsed when, on February 18, 1930, two images fifteen times fainter than Neptune were found among 160,000 stars on the photographic plates. The discovery was confirmed by examining earlier images. On February 20 the planet was observed to be yellowish, rather than bluish like Neptune. The new planet had revealed its true colours at last.

Announcing a discovery

Slipher waited until March 13 to announce the discovery. This was both Lowell’s birthday and the anniversary date of the discovery of Uranus. The announcement set off a worldwide rush to observe and photograph the new planet.

Now that astronomers, amateur and professional alike, knew what they were looking for, it turned out that Pluto had been hiding in plain view. Re-examination of Humanson’s plates showed four images of Pluto from his 1919 survey, and there were many others.

On March 14, an Oxford librarian read the news to his 11-year old granddaughter Venetia Burney, who suggested the name Pluto. It was also suggested independently in a letter by William Henry Pickering.

To complete the circle, some of Clyde Tombaugh’s remains are in a canister attached to the New Horizons spacecraft.

Most people alive today would not remember a universe without Pluto. And from 2015, its patterned surface will enter our visual vocabulary of the planets. Once seen, it can never again be unseen. Planet X, welcome to our world.

Kevin Orrman-Rossiter is Graduate Student, History & Philosophy of Science at University of Melbourne. Alice Gorman is Senior Lecturer in archaeology and space studies at Flinders University.

This article was originally published on The Conversation. (Reblogged by permission). Read the original article.

Beyond Pluto: New Horizons’s mission is not over yet

Jonti Horner, University of Southern Queensland and Jonathan P. Marshall, UNSW Australia

When New Horizons phoned home this morning (Australian time) after its close encounter with Pluto, there was jubilation and excitement.

Now, as Pluto retreats into the distance, the slow trickle of data can begin. Sent to us at a rate of just 1 kilobit a second, it will take months to receive it all, and astronomers around the world are waiting on tenterhooks to get their hands on the data.

Pluto: Once shattered, twice shy

Like our own Earth, Pluto has an oversized satellite, Charon. It was discovered back in 1978 and is more than half the diameter of its parent.

Over the past few years, intense observation of Pluto in preparation for New Horizons’ arrival has revealed four more tiny satellites, Hydra and Nix, and tiny Kerberos and Styx.

Prior to New Horizons, our best view of the Pluto system came from the Hubble Space Telescope. NASA, ESA, and L Frattare (STScI)

But how did this satellite system come to be? And why the striking similarity to our double-planet?

If we look at the great majority of satellites in our solar system we find that they can be split into two groups. First, have those that we think formed around their host planet like miniature planetary systems, mimicking the process of planet formation itself.

These regular satellites most likely accreted from disks of material around the giant planets as those planets gobbled up material from the proto-planetary disk from which they formed. This explains the orbits of those satellites – perfectly aligned with the equator of their hosts and moving on circular orbits.

Then we have the irregular satellites. These are (with a couple of noteworthy exceptions) tiny objects, and move on a wide variety of orbits that are typically great distances from their host planets.

These, too, are easily explained – thought to be captured from the debris moving around the solar system late in its formation, relics of the swarm of minor bodies from which the planets formed.

NASA graphic using New Horizons’ early pictures of Pluto and Charon to compare their sizes to that of the Earth. NASA

By contrast, our moon and Pluto’s Charon are far harder to explain. Their huge size, relative to their host, argues against their forming like the regular satellites. Likewise, their orbits are tilted both to the plane of the equator and to the plane of the host body’s orbit around the sun. It also seems very unlikely they were captured – that just doesn’t fit with our observations.

The answer to this conundrum, in both cases, is violent.

Like our moon, Charon (and by extension Pluto’s other satellites) are thought to have been born in a giant collision, so vast that it tore their host asunder. This model does a remarkable job of explaining the makeup of our own moon, and fits what we know (so far) about Pluto and its satellites.

Pluto and its moons will therefore be the second shattered satellite system we’ve seen up close, and the results from New Horizons will be key to interpreting their formation.

Schematic describing our best theory for the formation of Pluto’s satellite system. Wikimedia/Acom

Studying the similarities and differences between Pluto and Charon will teach us a huge amount about that ancient cataclysmic collision. We already know that Pluto and Charon are different colours, but the differences likely run deeper.

If Pluto was differentiated at the time of impact (in other words, if it had a core, mantle and crust, like the Earth) then Charon should be mostly comprised of material from the crust and mantle (like our moon). So it will be less dense and chemically different to Pluto. The same goes for Pluto’s other moons: Nix, Hydra, Styx and Kerberos.

Pluto, the unknown

The most exciting discoveries from New Horizons will likely be those we can’t predict. Every time we visit somewhere new, the unexpected discoveries are often the most scientifically valuable.

Jupiter and its volcanic moon Io, taken by New Horizons as it tore past the giant planet en-route to Pluto.
NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Goddard Space Flight Center

When we first visited Jupiter, 36 years ago, we found that its moon Io was a volcanic hell-scape. We also found that Europa hosts a salty ocean, buried beneath a thick ice cap. Both of these findings were utterly unexpected.

The Death Star terrorised peaceful planets before Voyager sent back images of Mimas. Flickr/Paul T, CC BY

At Saturn, we found the satellite Mimas looked like the Death Star and another, Iapetus, like a two tone cricket ball, complete with a seam. Uranus had a satellite, Miranda, that looked like it had been shattered and reassembled many times over, while Neptune’s moon Triton turned out to be dotted with cryo-volcanoes that spew ice instead of lava.

The story continues for the solar system’s smaller bodies. The asteroid Ida, visited by Galileo on its way to Jupiter, has a tiny moon, Dactyl. Ceres, the dwarf planet in the asteroid belt, has astonishingly reflective bright spots upon its surface.

Pluto, too, will have many surprises in store. There have already been a few, including the heart visible in the latest images (see top) – possibly the most eye catching feature to date. The best is doubtless still to come.

To infinity, and beyond!

Despite the difficulties posed by being more than four and a half billion kilometres from home, New Horizons is certain to revolutionise our understanding of the Pluto system.

The data it obtains will shed new light on the puzzle of our solar system’s formation and evolution, and provide our first detailed images of one of the system’s most enigmatic objects.

But the story doesn’t end there. Once Pluto recedes into the distance, New Horizons will continue to do exciting research. The craft has a limited amount of fuel remaining, nowhere near enough to turn drastically, but enough to nudge it towards another one or two conveniently placed targets.

New Horizons’ will continue its mission after flying past Pluto, studying objects in the Edgeworth-Kuiper belt. NASA

Since the launch of New Horizons, astronomers have been searching for suitable targets for it to visit as it hurtles outward through the Edgeworth-Kuiper belt, en-route to the stars.

In October 2014, as a result of that search, three potential targets were identified. Follow up observations of those objects narrowed the list of possible destinations to two, known as 2014 MU69 (the favoured target) and 2014 PN70.

The final decision on which target to aim for will be taken after New Horizons has left Pluto far behind, but we can expect to keep hearing about the spacecraft for years to come.

Jonti Horner is Vice Chancellor’s Senior Research Fellow at University of Southern Queensland. Jonathan P. Marshall is Vice Chancellor’s Post-doctoral Research Fellow at UNSW Australia.

This article was originally published on The Conversation. (Reblogged by permission). Read the original article.

Live blog: New Horizons flyby of Pluto

Tanya Hill, Museum Victoria

From 9.30pm AEST (12.30pm BST, 1.30pm ASAT, 7.30am EST), I’ll be blogging live as we follow NASA’s coverage of the New Horizons mission. Refresh this page every few minutes for the latest updates.

NASA live stream:

//www.ustream.tv/embed/10414700?v=3&wmode=direct

10:55am, July 15:

New Horizons phones home! Mission control reports that they have locked on to the signal from New Horizons. The team have 15 minutes to check that all systems are healthy. Currently the check-list is running like clockwork, and it is now confirmed that everything is working as it should be. The spacecraft is where it was expected to be – it will have recorded the data they were after. Congratulations NASA.

11:15pm

To finish up for the night – have you checked out Pluto time? I was surprised to find just how bright the daylight is at Pluto. See here to calculate the time at your location that matches the lighting conditions of local noon at Pluto.

The next Pluto time for my location in Melbourne is 7:28am tomorrow, but here’s how it looked a few days ago from the Three Sisters in Katoomba, NSW. The solar system is full of amazing worlds.

10:55pm:

What happens next? The flyby isn’t all that New Horizons is doing. It will now be moving through the shadows of Pluto and Charon. These occultations will allow New Horizons to probe the atmospheres of the two worlds.

When Pluto is between the spacecraft and the sun, measurements will be made at ultraviolet wavelengths to determine what gases are found in Pluto’s atmosphere. Then when Pluto is between the spacecraft and Earth, the aim is for New Horizons to receive a transmission from the NASA’s Deep Space Network on Earth. By detecting how the signal passes through Pluto’s atmosphere it will provide information on the atmospheric pressure and temperature.

 

10:45pm:

Pluto and Charon are a binary world – no other planet and moon combination have such similar masses to each other. Watch this video captured by New Horizons in January and you can see the two objects orbiting around their common centre of mass. Both objects are wobbling back and forth.

Charon and Pluto are also tidally locked – they both keep the same face pointing towards each other. This is because they each take 6.4 Earth days to spin once on their axis AND it takes 6.4 Earth days for Charon to orbit Pluto.

However, they look very different. It appears that Pluto has a younger surface, while Charon is old and battered. As data comes down, scientists will count the number of craters as a function of their size to work out the ages of different parts of their surfaces. Why is Pluto younger? Possibly due to an internal engine or climate effects due to Pluto having an atmosphere, while Charon doesn’t. More will be known with higher resolution data.

10:40pm:

We will get to see the south pole of Pluto, but it’ll be under “Charon-light”. Pluto’s axis is tilted so that the sun set on Pluto’s south pole 20 years ago and it will not rise again for another 80 years. Shortly after New Horizons’ closest approach to Pluto (perhaps happening right now!), the spacecraft will see Pluto’s night-side.

From the surface of Pluto, Charon appears seven times larger than Earth’s full moon, but five times fainter. But that’s enough for Charon to light Pluto so that this southern region will be seen. However, it won’t be as high resolution as the day-time images.

10:30pm:

Here’s the image again:

The north pole is towards the top of the image, while the darker regions towards the bottom are the equator. There is clearly strong variations in brightness across the dwarf planet. The scientists report that they can also see a history of impacts and surface activity, perhaps tectonic activity that occurred in the past or maybe the present.

The atmosphere also plays a role in shaping the planet – it’s known to snow on Pluto and changes have been detected as the planet varies its distance from the sun. But no plumes or other signs of Pluto’s atmosphere have been found, yet.

10:20pm:

Astronaut and astronomer John Grunsfeld (and a hero of mine!) reveals the first of many rewarding views of Pluto. As shown in the sneak peek below, the resolution is 4km per pixels, which is 1,000 times better than can be done from Earth.

But better is to come. Below is a comparison of what Earth would look like if New Horizons was flying over our planet at the same altitude that it has flown by Pluto. In the satellite image looking down on New York city, you can see Manhattan between the Hudson and East rivers, distinguish ponds in Central Park, count the wharves on the Hudson river and see runways from the airport.

Already Pluto is showing features that make it an interesting world to explore.

NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

10:10pm:

Earlier today the New Horizons team provided the best measurement of Pluto’s diameter. At 2,370km, it confirms that Pluto is bigger than the dwarf planet Eris by a mere 34km. When Eris was discovered in 2003, its brightness suggested that it was bigger than Pluto and while the two are now known to be pretty close in size, Eris is certainly more massive by 27%.

Of course the exciting thing about discovering Eris, is that’s opened up a whole new part of the Solar System – a third zone of icy worlds that contain the building blocks of the solar system in deep freeze.

The three zones of the Solar System: the small terrestrial planets, the gas giants and the icy worlds beyond Neptune.
NASA

9:51pm:

New Horizons makes history – somewhere out there, billions of kilometres from Earth a little spacecraft has flown by a distant world. It’s collecting a treasure trove of data that will come flowing back to us over the next year or so. Congratulations to all the scientists, engineeers and those involved that have made it happen.

The team celebrates together.
NASA

9:45pm:

New Horizons is all alone, firing off commands that have been pre-programmed. The last signal from the spacecraft was received at 1:17pm today (AEST). Right now the spacecraft is focused on Pluto – if it spent time talking to Earth that would take time away from observing Pluto. The spacecraft is due to send its ‘I’m fine and healthy’ message back to Earth at 10:53am tomorrow (AEST).

9:40pm:

Here’s a sneak peek of the latest image, taken 6am this morning (AEST) at a distance of 766,000km. Will be discussed on NASA TV in 20 minutes.

9:30pm:

Then and now. Here’s the discovery image of Charon from 1978. See the slight elongation of Pluto in the left image? That gave Charon away, because none of the background stars were found to change in a similar way between the two images.

1978: Pluto and Charon
US Naval Observatory

And this is what New Horizons is giving us now. We see two very different worlds, one large and red, one small and grey.

2015: Pluto and Charon
NASA-JHUAPL-SWRI

9:10pm:

For most of my childhood, Pluto was closer to the sun than Neptune. Pluto takes 248 years to orbit the sun but for 20 years, between January 1979 and February 1999, Pluto sat inside Neptune’s orbit. Even though their orbits cross paths, the two will never collide. They are in a 3:2 resonance, meaning that for every two orbits of Pluto, Neptune has orbited the sun three times, keeping them apart.

8:50pm:

Not asleep now! For about two-thirds of its flight, New Horizons was powered down and in hibernation. Like a real sleepy-head, the spacecraft would briefly wake up two or three times a year, check that all was ok, then return to deep slumber. The spacecraft woke for good on December 6, 2014.

7:30pm:

The road ahead for New Horizons – note the timings are given in Australian Eastern Standard Time (AEST).

 

7:00pm:

“It feels like you’ve been walking on an escalator for almost a decade, and then you step upon a supersonic transport” says Alan Stern, principal investigator for the New Horizons mission to Pluto.

It’s been a long wait for these scientists and engineers, following a spacecraft that was launched nine-and-a-half years ago. It’s no wonder this has been dubbed the mission of patience.

But now, the fun is about to begin. This evening (Australian time), New Horizons will whizz past Pluto – the last unexplored world in our solar system. It’s a new realm of discovery, seeing a part of the solar system that we’ve never seen before. This is a fantastic story of exploration and one we can all be a part of.

Until then, enjoy some of the latest images to be beamed back from the edge of the solar system.

Tanya Hill is Honorary Fellow of the University of Melbourne and Senior Curator (Astronomy) at Museum Victoria.

This article was originally published on The Conversation. (Reblogged by permission). Read the original article.

New Horizons close encounter with Pluto will reveal its icy secrets

Jonti Horner, University of Southern Queensland and Jonathan P. Marshall, UNSW Australia

At around 10 pm AEST on Tuesday July 14, the New Horizons spacecraft will sweep past the dwarf planet Pluto at a distance of less than 12,500 kilometres. In doing so, it will bring one of humankind’s most remarkable achievements to a thrilling climax.

Despite years of preparation, and the nine and a half years the spacecraft has been in flight, this will be the most fleeting of encounters. New Horizons will zip past Pluto faster than a speeding bullet, spending less than 40 hours within a million kilometres of its icy target.

A NASA computer simulation showing New Horizons’ path past Pluto.

Flung towards Pluto by a fortuitous slingshot

New Horizons is a remarkable spacecraft. It is the fastest spacecraft ever launched, and took advantage of a fortunate alignment of the planets to reach its destination in a timely manner.

New Horizons’ trip to the outer solar system began in 2006, and was boosted by a gravitational slingshot by Jupiter, which was ideally placed to give New Horizons a helping hand.

The path followed by New Horizons to reach Pluto.
http://pluto.jhuapl.edu/Mission/Where-is-New-Horizons/index.php

This flyby cut several years from the probe’s trip to the outer reaches of the solar system. Had it been launched just a few days later, the opportunity would have been lost, and New Horizons would have had to take a much slower route to its destination.

As a result of this game of celestial pinball, New Horizons is now placed to tear past Pluto at a relative speed of some 14 kilometres per second. To make the best of this brief encounter with the solar system’s most famous dwarf planet, it carries a veritable Swiss army knife of scientific instruments.

Seven instruments

To get the best possible views of Pluto, and return the most valuable data, New Horizons has been kitted out with seven separate instruments.

The most well known of these is the Long Range Reconnaissance Imager (LORRI), a telescopic camera that has been returning black and white images of ever-increasing detail over the past months. Complementary to LORRI is Ralph, a visible and infrared camera, adding colour to reveal Pluto’s Mars-esque reddish hue.

A colour image of Pluto taken by LORRI and Ralph on July 3rd, eleven days before closest approach. NASA

Map of Pluto, released on July 7, 2015, based on data from LORRI and Ralph.
http://pluto.jhuapl.edu/Multimedia/Science-Photos/image.php?gallery_id=2&image_id=204

Moving from Pluto’s surface to its atmosphere, we come to Alice. This ultraviolet spectrograph will sample the composition of Pluto’s tenuous atmosphere, and also yield details of the surfaces of Pluto and its satellites, working hand-in-hand with Ralph.

While Alice studies the atmosphere at ultraviolet wavelengths, it will be complemented by the Radio Science Experiment (REX), which will carry out a variety of different experiments through the course of the encounter.

Most excitingly, it will use radio signals from Earth to measure both the temperature and composition of Pluto’s atmosphere at radio wavelengths. By using signals from Earth, REX will be able to sample the most tenuous outer layers of the atmosphere, invisible to Alice and Ralph.

The various instruments being carried by New Horizons. NASA

The next pair of complementary instruments carried by New Horizons are Solar Wind At Pluto (SWAP) and Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI). These will work together to capture and study particles bleeding to space from the outer edges of Pluto’s atmosphere.

By sniffing Pluto’s escaping gas, they will determine its composition with exquisite precision. They will also help us to understand how Pluto’s atmosphere interacts with the Solar wind, which is continually streaming outwards from our beloved Sun.

Last, but by no means least, is the Student Dust Counter (SDC). This instrument, wholly designed and run by students, keeps track of interplanetary debris striking New Horizons as it flies ever outward.

The Zodiacal Light – dust in the Solar system – as seen from ESO’s Very Large Telescope, at Paranal Observatory, Chile. Dust suffuses our Solar system,
and the SDC continually measures it throughout  New Horizons’ flight.  ESO/Y. Beletsky

Unlike the other experiments aboard New Horizons, the SDC has remained awake for the entire duration of the mission. In the process, it has continually monitored dust levels during the voyage. This has provided a unique picture of the dust spread throughout the solar system.

A brief encounter, slowly retold

During the flyby, the spacecraft will gather vast amounts of data. This will range from exquisite images to spectra revealing the makeup of Pluto’s atmosphere and surface. But New Horizons is now so distant that we won’t get to see the data in real time.

Data transmitted by New Horizons faces a lengthy journey back home. Travelling at the speed of light, communication takes almost five hours, one way! And it gets worse.

The great distance begets another problem: low bandwidth. Data returned by New Horizons will trickle back at just one kilobit per second. That’s slower than the speed of the internet during the era of the dial-up modem.

As a result, data obtained during closest approach will take around nine months to be wholly transmitted to Earth.

Not all plain sailing

An excellent illustration of the difficulties involved with missions such as this came earlier this week, on July 4. All of a sudden, as though suffering stage fright with the eyes of the world upon it, New Horizons fell asleep.

This was no planned power nap. Communications with ground control ceased unexpectedly, as the spacecraft went into sleep mode, then switched to its backup computer. There was about an hour and 20 minutes of uncertainty and stress before communications were finally restored.

The cause? The central computer overloaded while simultaneously trying to prepare for new observations and to compress data it had already collected for transmission back home.

The main computer responded by entering safe mode, and switching to the backup, just as it was programmed to do. So while the glitch was unexpected, it wasn’t a catastrophe, although doubtless the staff at mission control had an anxious 80 minutes.

Fortunately, everything is now back online and functioning perfectly, and with any luck, there won’t be any more unplanned naps from our plucky little adventurer.

Jonti Horner is Vice Chancellor’s Senior Research Fellow at University of Southern Queensland. Jonathan P. Marshall is Vice Chancellor’s Post-doctoral Research Fellow at UNSW Australia.

This article was originally published on The Conversation. (Reblogged by permission). Read the original article.

NASA mission brings Pluto into sharp focus – but it’s still not a planet

David Rothery, The Open University

The new pictures that NASA’s New Horizons probe has begun to beam back have revealed Pluto and its largest moon, Charon, in ever greater detail from what is the first ever spacecraft fly-by.

Pluto has an atmosphere and five known moons which have been glimpsed by New Horizons as it closes in, and while we can’t predict what we will find, whatever is revealed is sure to lead to renewed cries that Pluto be re-classified as a planet – a status it lost in 2006.

Two sides of Pluto (larger and browner) and Charon (smaller and greyer) seen as New Horizons approaches. NASA/John Hopkins University APL/SWRI

Pluto was embraced as the solar system’s ninth planet upon discovery by Clyde Tombaugh in 1930. He’d been looking for a planet where faulty data suggested a planet-sized body was perturbing the orbit of Neptune. This, he felt, was it – and the world agreed. Pluto’s mass was at first thought to be roughly the same as the Earth’s, but by 1948 estimates had shrunk it to the size of Mars.

When Pluto’s largest moon Charon was discovered in 1978, Charon’s orbit showed that Pluto’s mass is actually about only 0.2% of the Earth’s (one-sixth that of the Moon), and we now know that its diameter is about 2368km, or two-thirds that of the Moon.

Being so insubstantial, then, should Pluto be classed as a planet? There may seem no obvious reason why not. After all, the Earth is only 0.3% the mass of Jupiter. Planets clearly span a wide range of masses. But the main reasons why delegates to the International Astronomical Union (IAU) voted to demote Pluto from planet status are not based primarily on mass or size.

Pluto is one of many

Since the 1990s, many other roughly Pluto-sized bodies have been discovered beyond Neptune, such as Eris, Huamea and Makemake. There are more than a thousand objects now documented in what is called the Kuiper belt, a region beyond Neptune where it seems no large objects were able to form.

If Pluto had been discovered along with the others rather than 60 years earlier, there can be little doubt that no one would have called it a planet in the first place. There is nothing special about Pluto, other than the accident of having been the first to be discovered.

Eight of the so-called trans-Neptunian objects, including Pluto, and their moons. Lexicon

The crucial part of the definition of planet adopted by the IAU in 2006 is that a planet should have “cleared the neighbourhood of its own orbit”. Neptune, 8,600 times more massive than Pluto, has achieved this because neither Pluto nor anything else that crosses Neptune’s orbit comes close to rivalling Neptune’s mass. On the other hand Pluto clearly does not comply to this definition – it has rivals of comparable mass in addition to being overshadowed by the vastly more massive Neptune.

While it may be that this definition is hard to apply in other solar systems, it works for ours and is a far neater approach than including every Kuiper belt object as a planet – thousands of them, which would be ridiculous. The alternative of defining a size or mass minimum at which an object ceases to be a planet would suffer from our variable and imperfect ability to measure their size or mass remotely.

The Kuiper belt is a busy place. NASA/Johns Hopkins University APL/SRI/Alex Parker

A linguistic fudge

Nevertheless, the IAU shied away from completely stripping the Pluto of its appellation of planet by inventing a new term, dwarf planet. This denotes an object orbiting the sun that has not cleared its orbit, but which has sufficient mass for its own gravity to have pulled it into a near-spherical shape (described as hydrostatic equilibrium). This applies to Pluto, Eris and a few other Kuiper belt objects, and also to the largest asteroid, Ceres.

‘Pluto a planet, Jim? You’ve got to be kidding me.’ NBC Television

I think that was an unnecessary concession to the Pluto-is-a-planet lobby, though it proves that the IAU is not controlled by “a clique of Pluto-haters” as one astronomer has claimed. In fact it’s messy for two reasons. First, shapes cannot be precisely determined for objects that have not been visited by a spacecraft; they have to be assumed on the basis of mechanical models that could easily be wrong. Second, whereas the giant planets (Jupiter, Saturn, Uranus and Neptune) are planets, by the IAU’s own definition the dwarf planets are not planets. As Mr Spock might have said, “That’s illogical, Captain.”

Planetary scientists have a duty to describe the nature of the solar system as clearly as possible, and to lead the public to a clearer understanding of nature – irrespective of how its elements are classified. Appealing to sentiment, seeking celebrity endorsement and posting photos of presidential candidates with “Pluto is a planet” T-shirts is not a good way to advance anyone’s understanding. It’s time to let go of the past, and embrace Pluto as a fascinating world and the most interesting member of the Kuiper belt.

David Rothery is Professor of Planetary Geosciences at The Open University.

This article was originally published on The Conversation. (Reblogged by permission). Read the original article.

Cecilia Payne and the composition of the stars

What are the stars made of? The answer to this fundamental question of astrophysics was discovered in 1925 by Cecilia Payne and explained in her Ph.D. thesis. Payne showed how to decode the complicated spectra of starlight in order to learn the relative amounts of the chemical elements in the stars. In 1960 the distinguished astronomer Otto Struve referred to this work as “the most brilliant Ph.D. thesis ever written in astronomy.”

Cecilia Payne (1900–1979) was born in Wendover, England. After entering Cambridge University she soon knew she wanted to study a science, but was not sure which one. She then chanced to hear the astronomer Arthur Eddington give a public lecture on his recent expedition to observe the 1919 solar eclipse, an observation that proved Einstein’s Theory of General Relativity. She later recalled her exhilaration: “The result was a complete transformation of my world picture. When I returned to my room I found that I could write down the lecture word for word.” She realized that physics was for her.

Later, when the Cambridge Observatory held an open night for the public, she went and asked the staff so many questions that they fetched “The Professor.” She seized the opportunity and told Professor Eddington that she wanted to be an astronomer. He suggested a number of books for her to read, but she had already read them. Eddington then invited her to use the Observatory’s library, with access to all the latest astronomical journals. This simple gesture opened the world of astronomical research to her.

England, though, was not in Payne’s professional future. She realized early during her Cambridge years that a woman had little chance of advancing beyond a teaching role, and no chance at all of getting an advanced degree. In 1923 she left England for the United States, where she lived the rest of her life. She met Harlow Shapley, the new director of the Harvard College Observatory, who offered her a graduate fellowship.

Harvard had the world’s largest archive of stellar spectra on photographic plates. Astronomers obtain such spectra by attaching a spectroscope to a telescope. This instrument spreads starlight out into its “rainbow” of colors, spanning all the wavelengths of visible light. The wavelength increases from the violet to the red end of the spectrum, as the energy of the light decreases. A typical stellar spectrum has many narrow dark gaps where the light at particular wavelengths (or energies) is missing. These gaps are called absorption “lines,” and are due to various chemical elements in the star’s atmosphere that absorb the light coming from hotter regions below.

The study of spectra had in fact given rise to the science of astrophysics. In 1859, Gustav Kirchoff and Robert Bunsen in Germany heated various chemical elements and observed the spectra of the light given off by the incandescent gas. They found that each element has its own characteristic set of spectral lines—its uniquely identifying “fingerprint.” In 1863, William Huggins in England observed many of these same lines in the spectra of the stars. The visible universe, it turned out, is made of the same chemical elements as those found on Earth.

In principle, it seemed that one might obtain the composition of the stars by comparing their spectral lines to those of known chemical elements observed in laboratory spectra. Astronomers had identified elements like calcium and iron as responsible for some of the most prominent lines, so they naturally assumed that such heavy elements were among the major constituents of the stars. In fact, Henry Norris Russell at Princeton had concluded that if the Earth’s crust were heated to the temperature of the Sun, its spectrum would look nearly the same.

When Payne arrived at Harvard, a comprehensive study of stellar spectra had long been underway. Annie Jump Cannon had sorted the spectra of several hundred thousand stars into seven distinct classes. She had devised and ordered the classification scheme, based on differences in the spectral features. Astronomers assumed that the spectral classes represented a sequence of decreasing surface temperatures of the stars, but no one was able to demonstrate this quantitatively.

Cecilia Payne, who studied the new science of quantum physics, knew that the pattern of features in the spectrum of any atom was determined by the configuration of its electrons. She also knew that at high temperatures, one or more electrons are stripped from the atoms, which are then called ions. The Indian physicist M. N. Saha had recently shown how the temperature and pressure in the atmosphere of a star determine the extent to which various atoms are ionized.

Payne began a long project to measure the absorption lines in stellar spectra, and within two years produced a thesis for her doctoral degree, the first awarded for work at Harvard College Observatory. In it, she showed that the wide variation in stellar spectra is due mainly to the different ionization states of the atoms and hence different surface temperatures of the stars, not to different amounts of the elements. She calculated the relative amounts of eighteen elements and showed that the compositions were nearly the same among the different kinds of stars. She discovered, surprisingly, that the Sun and the other stars are composed almost entirely of hydrogen and helium, the two lightest elements. All the heavier elements, like those making up the bulk of the Earth, account for less than two percent of the mass of the stars.

Most of the mass of the visible universe is hydrogen, the lightest element, and not the heavier elements that are more prominent in the spectra of the stars! This was indeed a revolutionary discovery. Shapley sent Payne’s thesis to Professor Russell at Princeton, who informed her that the result was “clearly impossible.” To protect her career, Payne inserted a statement in her thesis that the calculated abundances of hydrogen and helium were “almost certainly not real.”

She then converted her thesis into the book Stellar Atmospheres, which was well-received by astronomers. Within a few years it was clear to everyone that her results were both fundamental and correct. Cecilia Payne had showed for the first time how to “read” the surface temperature of any star from its spectrum. She showed that Cannon’s ordering of the stellar spectral classes was indeed a sequence of decreasing temperatures and she was able to calculate the temperatures. The so-called Hertzsprung-Russell diagram, a plot of luminosity versus spectral class of the stars, could now be properly interpreted, and it became by far the most powerful analytical tool in stellar astrophysics.

Payne also contributed widely to the physical understanding of variable stars. Much of this work was done in association with the Russian astronomer Sergei Gaposchkin, whom she married in 1934.

From the time she finished her Ph.D. through the 1930s, Payne advised students, conducted research, and lectured—all the usual duties of a professor. Yet, because she was a woman, her only title at Harvard was “technical assistant” to Professor Shapley. Despite being indisputably one of the most brilliant and creative astronomers of the twentieth century, Cecilia Payne was never elected to the elite National Academy of Sciences. But times were beginning to change. In 1956, she was finally made a full professor (the first woman so recognized at Harvard) and chair of the Astronomy Department.

Her fellow astronomers certainly came to appreciate her genius. In 1976, the American Astronomical Society awarded her the prestigious Henry Norris Russell Prize. In her acceptance lecture, she said, “The reward of the young scientist is the emotional thrill of being the first person in the history of the world to see something or to understand something.” As much as any astronomer, she had fully experienced that most important of all scientific rewards.

This is an excerpt from COSMIC HORIZONS: ASTRONOMY AT THE CUTTING EDGE, edited by Steven Soter and Neil deGrasse Tyson, a publication of the New Press. © 2000 American Museum of Natural History. To order the book, call 1-800-233-4830, or go to http://www.amnh.org/education/resources/rfl/web/buybook/