Tag Archives: Kuiper belt

New dwarf planet in the outer solar system

The Conversation

Tanya Hill, Museum Victoria

What’s round, orbits the sun and resides in the heavily-populated parts of our solar system such as the asteroid belt or the Kuiper belt? It’s a dwarf planet, and astronomers have just discovered a new one.

Designated 2015 RR245 (based on specific rules), the dwarf planet lies in the solar system’s icy regions beyond the orbit of Neptune.

It was found using the Canada-France-Hawaii Telescope on Maunakea, Hawaii as part of the ongoing Outer Solar System Origins Survey (OSSOS). Since the survey began in 2013, the team has located more than 500 new trans-Neptunian objects but at 700km across, this new discovery is the largest one so far.

2015 RR245 has a 700 year orbit that is highly elliptical. At its most distant the dwarf planet is more than 18 billion km (or 120 astronomical units) from the sun.

The orbit of 2015 RR245 shown in yellow, along with 17 objects (labelled) that are as bright or brighter than the new dwarf planet.
Alex Parker OSSOS team

Slowly it is making a move towards its closest approach to the sun. At present it is nine billion km (or 60 astronomical units) away. It’ll take 80 years to reach its closest point, albeit still a chilly five billion km (or 34 astronomical units) from the sun.

Astronomers will be studying the dwarf planet in detail to improve their estimates of its size. Size is determined by the object’s brightness which is dependent on the object’s albedo or how easily it reflects sunlight.

If it’s nice and shiny, then 2015 RR245 could well be smaller in size. Or perhaps its surface is dark and dull then estimates of its size would need to swell.

When compared to the officially recognised dwarf planets – Pluto, Eris, Haumea and Makemake – this new dwarf planet is about two to three times smaller. According to a statement from the Canada-France-Hawaii Telescope, 2015 RR245 is the 18th largest object in the Kuiper Belt.

Size Matters

When it comes to dwarf planets, a critical measure is that the object must be round. This is what distinguishes a dwarf planet from among the millions of raggedy-shaped asteroids in the asteroid belt and the hundreds of thousands of objects thought to make up the Kuiper belt.

Furthermore, the definition of roundness is grounded in physics. Rather than choose an arbitrary size definition (such as 1,000km in diameter for example), roundness implies that the object must have enough mass so that under its own gravity it can form a spherical shape.

Ceres is the only object in the asteroid belt known to be round and therefore a dwarf planet. Made of rigid, rocky material it has a diameter of about 900km and a mass of around 900 billion billion kg.

New false-colour renderings of the dwarf planet Ceres from the Dawn spacecraft. NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

But trans-Neptunian objects are made of weaker stuff. Their icy interiors are more easily shaped by gravity and so require much less force to obtain a spherical shape. The lower size-limit for an icy object to be a dwarf planet is around 320km with a mass of only 1,000kg.

This certainly stands true when looking at the icy moons that orbit Saturn. The smallest icy object known to be round is Saturn’s moon Mimas, at 400km across. Smaller moons, with diameters around 200km, are not round (see astronomer Mike Brown’s excellent description here).

The numbers game

Officially there are four dwarf planets beyond Neptune but unofficially there are many more. Mike Brown, who discovered the dwarf planet Eris, keeps a list of unofficial dwarf planets.

On that list are ten trans-Neptunian objects which are nearly certainly dwarf planets.

Four confirmed dwarf planets (top row) and four almost certainly dwarf planets (bottom row).
Lexicon

Extend the list to include fainter objects that have diameters of perhaps 600km or more, and another 27 potential dwarf planets (not counting 2015 RR245) are included.

What about fainter still? The uncertainties increase but potentially another 51 dwarf planets could be added to the mix, if they are icy and larger than 500km, as observations currently suggest.

In all, that would be 88 dwarf planets beyond Neptune and 2015 RR245 brings that count to 89. There truly is a lot still to explore within our solar system.

The ConversationTanya 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.

 

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

The Conversation

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.

The ConversationJonti Horner is Vice Chancellor’s Senior Research Fellow at University of Southern QueenslandJonathan 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.


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Live blog: New Horizons flyby of Pluto

The Conversation

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.

The ConversationTanya 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.

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

The Conversation

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.

The ConversationDavid 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.

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