YIR V: ‘Planet Nine’ – Pushing the boundaries of the solar system
In 2016, our understanding of our cosmic home might have taken its most significant leap forward since the discovery of Uranus shattered our then perception of a six-planet system and expanded the solar system for the first time. This year, in January came the announcement of a mathematical and orbital-characteristic inferred “discovery” – more a prediction based on evidence – of a massive, super-Earth, ice giant in a 200 x 1,200 AU orbit that, if confirmed, would represent the ninth planet in our solar system.
‘Planet Nine’ – a hypothetical find beyond the Scattered Disc:
First proposed in 2014 by Chad Trujillo and Scott S. Sheppard, who inferred the existence of a yet-unknown massive body from strikingly similar oddities in the orbits of two Trans-Neptunian Objects (Sedna and 2012 VP113), Planet Nine was thrust firmly into the public and planetary spotlight on 20 January 2016.
On that day, a second team of researchers – Michael Brown and Konstantin Batygin – announced a much more detailed explanation of Planet Nine, including its effect on six observed Trans-Neptune Objects (TNOs) and its proposed mass and orbital characteristics – all of which Brown and Batygin argue would explain the highly improbable configuration of a group of TNOs in the Scattered Disc.
The Scattered Disc is a group of icy objects in the outer reaches of the solar system. By distance, portions of the Scattered Disc overlap with the Kuiper Belt; however, its limits extend farther above and below the ecliptic plane and well beyond those of Kuiper Belt.
In all, the new research and mathematical equations suggest that Planet Nine is 10 times more massive than Earth, has a diameter of two to four times that of Earth’s, and is located in a highly elliptical orbit with a period of 10,000 to 20,000 years and a semi-major axis (radius of the orbit at the orbit’s two most distant points) of 700 AU – 23 times farther from the Sun than Neptune.
Moreover, Planet Nine’s orbit is inferred to hold an aphelion of 1,200 AU and a perihelion of 200 AU with an inclination of 30° to the ecliptic plane – roughly in line with the other known TNOs with large inclinations to those observed elsewhere in the solar system.
At aphelion, Planet Nine would be in the general location of the Orion and Taurus constellations, while perihelion would place the planet in near Serpens, Ophiuchus, and Libra.
This mathematical and other-body observationally inferred discovery of a planet is not new to our exploration of the solar system.
Neptune’s visual discovery on 23 September 1846 was preceded by a mathematical prediction of Neptune’s existence and location based on observed perturbations in Uranus’ orbit.
So precise, in fact, were Urbain Le Verrier’s calculations of Neptune that the planet was discovered within 1° of where Le Verrier predicted it would be on the very first night that observations to find the planet were undertaken.
If it’s there, where did it come from?
Importantly, based on current observations of the extreme outer solar system, the existence of such a massive planet like Planet Nine is possible.
A 2009 survey by NASA’s Wide-field Infrared Survey Explorer (WISE) concluded that a Neptune-sized object could exist at a distance greater than 700 AU; a 2014 study subsequently ruled out the existence of a Jupiter-mass planet to a distance of 26,000 AU – two discoveries that would not rule out Planet Nine’s exitance at its proposed mass, diameter, and distance.
If Planet Nine does indeed exist, there are currently four ideas regarding its creation and how it got to its present-day orbit.
The first, and most likely, explanation is that Planet Nine formed with Jupiter and Saturn and was ejected to its distant, eccentric orbit following close encounters with these two gas giants during the nebular epoch of the early solar system.
Once ejected, Planet Nine’s eccentricity, which was initially much greater, was reduced and its perihelion raised either by a process called dynamical friction (loss of momentum and kinetic energy of moving bodies through gravitational interactions with surrounding matter in space) from the gaseous remnants of the solar nebula that the Sun and solar system formed in, or by gravitational interactions with the other stars that formed in the same birth cluster as the Sun.
This would also serve to explain the mass and size of Planet Nine – as this type of ejection would have greatly halted Planet Nine’s development and left it at roughly the same mass as Uranus and Neptune.
Moreover, for the inferred orbit to be achieved under this model, Planet Nine would have to have been ejected between 3 to 10 million years after the formation of the solar system.
Importantly, this ejection of Planet Nine 3-10 million years after solar system formation would not detract from the Nice model of solar system development – which indicates that a transfer of energy from the outer disk of planetesimals left over from the initial formation of the solar system disturbed the fragile quadruple resonance of Jupiter, Saturn, Neptune, and Uranus.
This destabilization of the quadruple resonance resulted in an increasing eccentricity of the inner ice giant (seventh planet – in this case, Neptune) that subsequently pulled all four gas and ice giants outward from the Sun.
The further breakdown of the secular resonance between the inner ice giant and outer ice giant (Uranus) led to gravitational interactions between the two, which eventually flung Neptune out of its orbit and sent it hurtling through the outer disk.
As Neptune swept through the outer disk, it disturbed the planetesimals – forming the Kuiper Belt while sending some planetesimals even farther out, creating the Scattered Disc seen today.
At the same time, Neptune flung a large number of planetesimals toward the sun. Some of these were grabbed by Jupiter (the gas giant’s trojans seen in its L4 and L5 Lagrange Points) while others continued into the solar system, accounting for the Late Heavy Bombardment between 4.1 and 3.8 billion years ago.
Thus, given the timing of the events believed to have led to the orbital realignment of the gas and ice giants and the Late Heavy Bombardment, Planet Nine would have been ejected 490 million to 697 million years prior – giving enough time for the four remaining gas and ice giants to harmonize and then destabilize.
Thus, Planet Nine would not be responsible for the Late Heavy Bombardment.
The second possibility of Planet Nine’s genesis is that it’s not native to our solar system. Under this explanation, Planet Nine would have formed in the far reaches of its original parent system and in the same birth cluster as the Sun.
As the young stars and systems interacted, the Sun passed close enough to Planet Nine to “grab it” and “steal” it from its parent.
However, given the current orbit Planet Nine is understood to be in, this explanation is only 1-2% probable.
Conversely, Nine could have formed with the rest of the solar system in an extremely distant, circular orbit that was then perturbed to its current eccentricity when the solar system passed close to another star in its birth cluster.
This type of formation would radically reshape our understanding of the early solar system as the Sun would either 1) have to have possessed a massive disk of material far larger and more expansive than previously thought possible or 2) there would have to have been a major outward drift of solid material escaping the accreting disk that formed the eight planets. This outward drifting material would then have had to form a ring in which Planet Nine would then have accreted.
Under this model, with such a distant and eccentric orbit, the odds that Planet Nine would not have been stolen by another star are only 10%, meaning the planet would have to have beaten extraordinary odds to remain with the solar system through its birth and dissolution of the Sun’s birth cluster.
Finally, the fourth possibility is that Planet Nine originated in the inner solar system and was ejected to its current location as Jupiter migrated inward from its formation orbit in the early development era.
This is highly unlikely though not improbable as the chance of such an object being ejected by Jupiter at just the right the velocity to wind up in a stable orbit beyond the Scattered Disc and not be thrown completely out of the solar system is only 2%.
Regardless, Planet Nine will greatly aid our understanding of early solar system development and could help answer the question of why our solar system has no planets with masses between Earth and Neptune (which is 17 times the mass of Earth) – an occurrence that makes our solar system rather unique among observed planetary systems to date.
How are we trying to find it?
While all of this conjecture of how Planet Nine formed and where it came from – as well as the effects it’s having on the extreme TNOs observed – is valid, nothing can be answered completely until the planet is actually found visually or by concrete methods of indirect detection.
To this end, given the extreme distance of Planet Nine, scientists are employing both indirect and direct methods of detection in a concerted effort to narrow the possibilities of where Planet Nine is in its orbit, where its gravitational effects might be seen, and what those gravitational effects could help us explain about the observed solar system.
One of the first instruments employed in the search for Planet Nine was the Cassini spacecraft in orbit of Saturn.
Since Cassini arrived at Saturn in 2004, the craft has taken very specific measurements of Saturn’s orbit of the Sun. An examination of those measurements for unexplained perturbations – deviations – yielded results that, while neither proving nor disproving the existence of Planet Nine, helped scientists eliminate large portions of Planet Nine’s proposed orbit in which the planet could not be.
Simply put, Planet Nine’s gravity, if it were in certain portions of its orbit between 2004 and 2016 (the years bounding the Cassini data investigation), would have noticeable effects on Saturn’s position.
Since all of Saturn’s orbital parameters during this time were exactly as predicted, Planet Nine’s gravity could not have been affecting the massive ringed planet – thus demonstrating that Planet Nine could not be in certain locations of its proposed orbit.
This led to an understanding the Planet Nine could not be located in portions of its orbit bounded by a true anomaly (the angular parameter that defines the position of a body moving along a Keplerian orbit that is the angle between the direction of periapsis and the current position of the body as seen from the point around which it orbits) of -130° to -110° or -65° to 85°.
Further analysis of data from Cassini determined that Planet Nine was likely located at a true anomaly of 107.8° to 128.8° – which would place it at a distance of approximately 630 AU.
Moreover, another way to validate Planet Nine’s existence and further refine where in its orbit it could be is by finding additional extreme TNOs whose orbits could only be explained by a massive perturber (in this case, Planet Nine).
Scheduled for completion in 2023, the Large Synoptic Survey Telescope – which will be capable of mapping the entire sky in a just a few nights – will greatly aid our ability to detect these distant, small, extreme TNOs.
Under Brown and Batygin’s mathematical deductions of Planet Nine, its orbit, and its mass, the pair also find that if Planet Nine does exist, so too will a large population of extreme TNOs with semi-major axes greater than 250 AU and overall orbits with low eccentricities that are aligned with Planet Nine.
Moreover, further analysis released in May indicated that if Planet Nine is indeed in its predicted orbit, it would cause a tenfold increase to the number of extreme TNOs in high inclination and high-perihelion with moderate semi-major axis orbits – something which, if enough extreme TNOs in these kinds of orbits are found, would greatly lend evidence for Planet Nine’s existence.
But orbital studies of Saturn along with the search for more extreme TNOs aren’t the only indirect methods being used.
In fact, one of the other methods relates to an investigation into the mysterious spin-orbit misalignment of the solar system.
As observed, the Sun’s axis of rotation is tilted 6° from the orbital plane of the gas and ice giants.
Why this is remains a mystery; however, if Planet Nine – in its proposed orbit with its proposed mass – is factored in, the spin-orbit misalignment can be explained by the gravitational perturbations from Planet Nine on the Sun and the outer giant planets over billions of years.
While this does not directly or indirectly prove Planet Nine’s existence, it does lend more evidence toward its presence in the far reaches of the solar system – though other explanations for the spin-orbit misalignment are certainly possible.
Given all of the analysis and predictions, Planet Nine could be spotted with existing visual and infrared telescopes.
Indeed, if the planet were located near its perihelion, existing surveys of the sky would contain the photographic proof of Planet Nine’s existence.
However, such searches turned up nothing, and the fact that Saturn’s orbit has had no observed perturbations since 2004 leads to a fairly conclusive indication that Planet Nine is near its aphelion – meaning it would be at its farthest and dimmest (in the visible spectrum).
Unfortunately, this would also place the planet in a section of the sky that is extremely complicated and star-rich – as Planet Nine’s aphelion crosses the plane of Milky Way, where light pollution from the galactic core makes visual detection methods difficult… though not impossible.
Nonetheless, an object of Planet Nine’s size and mass would have a temperature of roughly 47 K (-226° C ; -375° F).
At this temperature, it would be visible in the infrared spectrum and its radiation signature could be detected from Earth-based telescopes and easily detected by the James Webb Space Telescope – which is still just under two years out from launch.
And infrared detection is perhaps the best method of directly observing Planet Nine because of its distance. Near aphelion, Planet Nine is predicted to have an absolute magnitude of 22, making it 600 times fainter than Pluto.
Regardless, visual detection is still possible, and Brown & Batygin and Trujillo & Sheppard are each cooperatively using the Subaru telescope at the Mauna Kea Observatory on Hawaii in what is expected to be up to a five year search for Planet Nine.
Likewise, the Dark Energy Survey (optical and near-infrared) using the Victor M. Blanco Telescope in Chile is also searching for Planet Nine at the portion of its orbit in which data from Cassini indicates the planet may be.
Furthermore, a review of archival visual data from the Catalina Sky Survey (to apparent magnitude 19) and Pan-STARRS (to magnitude 21.5) was undertaken, but the data did not identify the planet.
Data from the WISE mission was also reviewed but returned no detections of the planet.
Other data points, sky surveys, and data are being reviewed as well – and new software to search this already collected information is being developed – to see if Planet Nine could have already been imaged without recognition of what it is.
Missing a planet in the visual record would certainly not be a first.
Uranus was somewhat routinely observed before its official discovery in 1781.
This was mainly due to the fact that, while faint, it is the farthest planet visible to the naked eye and its long orbit produces a very slow motion across the sky.
Neptune, too, was observed before its discovery. Galileo himself observed and plotted Neptune on 28 December 1612 and 27 January 1613, but because the planet had just begun its retrograde trek across the sky on the first day Galileo saw it, he mistook it for a fixed star.
Likewise, James Challis – working from a mathematical prediction of Neptune’s existence from John Couch Adams, a less accurate prediction than Le Verrier’s – observed Neptune on 4 and 13 August 1846 but did not recognize it for what it was.
So too was Pluto observed for its discovery.
The big question: is it a planet or a dwarf planet?
The simple answer is that it is extremely probable – with high certainty – that Planet Nine, if it exists, is indeed a planet under all three definitional requirements set by the International Astronomical Union (IAU).
According to Mike Brown, Planet Nine would contain a mass high enough to clear its immediate orbit over the course of 4.6 billion years of development and would thus dominate its immediate neighborhood.
Thus, it would 1) be a celestial object in orbit of the Sun, 2) have sufficient mass for its self-gravity to overcome rigid body forces so that it assumes hydrostatic equilibrium (a nearly round shape), and 3) has cleared the neighborhood around its orbit.
With its current, TNO-observed inferred parameters, the only way Planet Nine would fall into the grey area between planet and dwarf planet is if it is the smallest of its inferred mass – still ten times that of Earth – and also located in the very farthest possible orbit for such a mass.
Under these circumstances, it is possible – though still not likely – that a debate about whether Planet Nine would then meet the critical third requirement for planethood could occur.
While the third criterion is the most controversial and argued portion of the planet definition (and the one that doomed Pluto while at the same time elevating Ceres, Eris, Haumea, and Makemake), it has since been widely interpreted to mean “gravitationally dominating its area”.
Under this understanding, Planet Nine would certainly qualify as a planet as it is already observed to dominate its area and the known objects around it.
However, should Planet Nine be located, and if its actual mass and/or orbit creates uncertainty toward the third criteria, the IAU would have to make a binding decision regard Planet Nine’s status.
UC Berkeley, NASA looking for citizen scientists to help find Planet 9
Elusive planets and dim failed stars may be lurking around the edges of our solar system, and astronomers from NASA and UC Berkeley want the public’s help to hunt them down.
A previously cataloged brown dwarf named WISE 0855−0714 shows up as a moving orange dot (upper left) in this loop of WISE images spanning five years. By viewing movies like this, anyone can help discover more brown dwarfs or even a 9th planet. (NASA/WISE images)
Through a new website called Backyard Worlds: Planet 9, anyone can now help search for objects far beyond the orbit of our farthest planet, Neptune, by viewing brief “flipbook” movies made from images captured by NASA’s Wide-field Infrared Survey Explorer (WISE) mission. A faint spot seen moving through background stars might be a new and distant planet orbiting the sun or a nearby brown dwarf.
WISE’s infrared images cover the entire sky about six times over. This has allowed astronomers to search the images for faint, glowing objects that change position over time, which means they are relatively close to Earth. Objects that produce their own faint infrared glow would have to be large, Neptune-size planets or brown dwarfs, which are slightly smaller than stars.
UC Berkeley postdoctoral researcher Aaron Meisner, a physicist who specializes in analyzing WISE images, has automated the search using computers, but he jumped at the idea by NASA astronomer Marc Kuchner to ask the public to eyeball the millions of WISE images. NASA and its collaborators, including UC Berkeley, are launching the planet and brown dwarf search Feb. 15.
“Automated searches don’t work well in some regions of the sky, like the plane of the Milky Way galaxy, because there are too many stars, which confuses the search algorithm,” said Meisner, who last month published the results of an automated survey of 5 percent of the WISE data, which revealed no new objects. Online volunteers “using the powerful ability of the human brain to recognize motion” may be luckier, he said.
“Backyard Worlds: Planet 9 has the potential to unlock once-in-a-century discoveries, and it’s exciting to think they could be spotted first by a citizen scientist,” he added.
“There are just over four light-years between Neptune, the farthest known planet in our solar system, and Proxima Centauri, the nearest star, and much of this vast territory is unexplored,” said Kuchner, the lead researcher and an astrophysicist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “Because there’s so little sunlight, even large objects in that region barely shine in visible light. But by looking in the infrared, WISE may have imaged objects we otherwise would have missed.”
People have long theorized about unknown planets far beyond Neptune and the dwarf planet Pluto, but until recently there was no evidence to support the idea. Last year, however, Caltech astronomers Mike Brown and Konstantin Batygin found indirect evidence for the existence of an as-yet-unseen ninth planet in the solar system’s outer reaches. This “Planet 9” would be similar in size to Neptune, but up to a thousand times farther from the sun than Earth, and would orbit the sun perhaps once every 15,000 years. It would be so faint as to have so far evaded discovery.
Video courtesy of the American Museum of Natural History.
At the moment, the existence of Planet 9 is still under debate. Meisner thinks it’s more likely that volunteers will find brown dwarfs in the solar neighborhood. While Planet 9 would look very blue in WISE time-lapse animations, brown dwarfs would look very red and move across the sky more slowly.
WISE images have already turned up hundreds of previously unknown brown dwarfs, including the sun’s third- and fourth-closest known neighbors. He hopes that the Backyard Worlds search will turn up a new nearest neighbor to our sun.
“We’ve pre-processed the WISE data we’re presenting to citizen scientists in such a way that even the faintest moving objects can be detected, giving us an advantage over all previous searches,” Meisner said. Moving objects flagged by participants will be prioritized by the science team for later follow-up observations by professional astronomers. Participants will share credit for their discoveries in any scientific publications that result from the project.
WISE and NEOWISE
The WISE telescope scanned the entire sky between 2010 and 2011, producing the most comprehensive survey at mid-infrared wavelengths currently available. With the completion of its primary mission, WISE was shut down in 2011, then reactivated in 2013 and given a new mission: assisting NASA’s efforts to identify potentially hazardous near-Earth objects, which are asteroids and comets in the vicinity of our planet. The mission was renamed the Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE).
A very blue Neptune-like planet, dubbed Planet 9, may be lurking dozens of times further from the sun than Pluto, as depicted in this artist’s rendering. Citizen scientists who join the Backyard Worlds: Planet 9 project may be the first to spot it. (NASA image)
The new website uses all of the WISE and NEOWISE data to search for unknown objects in and beyond our own solar system, including the putative Planet 9. If Planet 9 exists and is as bright as some predict, it could show up in WISE data.
Meisner said WISE is uniquely suited for discovering extremely cold brown dwarfs, which can be invisible to the biggest ground-based telescopes despite being very close.
“Brown dwarfs form like stars but evolve like planets, and the coldest ones are much like Jupiter,” said team member Jackie Faherty, an astronomer at the American Museum of Natural History in New York. “By using Backyard Worlds: Planet 9, the public can help us discover more of these strange rogue worlds.”
Backyard Worlds: Planet 9 is a collaboration between NASA, UC Berkeley, the American Museum of Natural History in New York, Arizona State University, the Space Telescope Science Institute in Baltimore and Zooniverse, a collaboration of scientists, software developers and educators that collectively develops and manages citizen-science projects on the internet. Zooniverse will spread the word among its many citizen volunteers
NASA’s Jet Propulsion Laboratory in Pasadena, California, manages and operates WISE, part of NASA’s Explorers Program.
Meisner, who specializes in creating high-resolution maps of the universe, is also currently working on the Dark Energy Spectroscopic Instrument, a project at Lawrence Berkeley National laboratory that seeks to learn how mysterious dark energy affects the expansion of the universe.
Quelle: UC Berkeley
ANU leads public search for Planet X
Dr. Tucker said modern computers could not match the passion of millions of people.
The Australian National University (ANU) is launching a search for a new major planet within our solar system, inviting anyone around the world with access to the Internet to help make the historic discovery.
Anyone who helps find the so-called Planet X will work with ANU astronomers to validate the discovery through the International Astronomical Union.
ANU astrophysicist Dr. Brad Tucker is leading the project, which is being launched by Professor Brian Cox during a BBC Stargazing Live broadcast from the ANU Siding Spring Observatory.
"We have the potential to find a new planet in our solar system that no human has ever seen in our two-million-year history," said Dr. Tucker from the ANU Research School of Astronomy and Astrophysics.
Dr. Tucker said astronomers had long discussed the likelihood of a another major planet on the outer edges of the solar system, but nothing had been found yet.
"Planet X is predicted to be a super Earth, about 10 times the mass and up to four times the size of our planet. It's going to be cold and far away, and about 800 times the distance between Earth and the Sun. It's pretty mysterious," he said.
The ANU project will allow the general public to use a website to search hundreds of thousands of images taken by the ANU SkyMapper telescope at Siding Spring.
SkyMapper will take 36 images of each part of the southern sky, which is relatively unexplored, and identify changes occurring within the universe.
Finding Planet X involves volunteers scanning the SkyMapper images online to look for differences, Dr. Tucker said.
"It's actually not that complicated to find Planet X. It really is spot the difference. Then you just click on the image, mark what is different and we'll take care of the rest," Dr. Tucker said.
He said he expected people to also find and identify other mystery objects in space, including asteroids, comets and Kuiper belt objects like Pluto.
"If you find an asteroid or dwarf planet, you can't actually name it after yourself," Dr. Tucker said.
"But you could name it after your wife, brother or sister. We need to follow all of the rules set by the International Astronomical Union."
Dr. Tucker said modern computers could not match the passion of millions of people.
"It will be through all our dedication that we can find Planet X and other things that move in space," he said.
Co-researcher and head of SkyMapper Dr. Chris Wolf said SkyMapper was the only telescope in the world that maps the whole southern sky.
"Whatever is hiding there that you can't see from the north, we will find it," Dr. Wolf said.
From 28 to 30 March at 8 pm London time, BBC Stargazing Live hosted by Professor Cox and comedian Dara O Briain is expected to be viewed by around five million people.
The ABC will broadcast an Australian Stargazing Live program from Siding Spring from 4 to 6 April, hosted by Professor Cox and Julia Zemiro.
SkyMapper is a 1.3-metre telescope that is creating a full record of the southern sky for Australian astronomers.
Four unknown objects being investigated in Planet 9 search
The ANU Planet 9 search team with BBC Stargazing Live hosts Professor Brian Cox and comedian Dara O'Briain at the ANU Siding Spring Observatory.
We've detected minor planets Chiron and Comacina, which demonstrates the approach we're taking could find Planet 9 if it's there.
ANU astronomers are investigating four unknown objects that could be candidates for a new planet in our Solar System, following the launch of their planetary search on the BBC's Stargazing Livebroadcast from the ANU Siding Spring Observatory.
Lead researcher Dr Brad Tucker said about 60,000 people from around the world had classified over four million objects in space as part of the ANU-led citizen search for the so-called Planet 9.
"We've detected minor planets Chiron and Comacina, which demonstrates the approach we're taking could find Planet 9 if it's there," said Dr Tucker from the ANU Research School of Astronomy and Astrophysics.
Dr Tucker said the SkyMapper telescope at Siding Spring used as part of the project was crucial in ruling out areas in the southern sky where Planet 9 could be situated.
"We've managed to rule out a planet about the size of Neptune being in about 90 per cent of the southern sky out to a depth of about 350 times the distance the Earth is from the Sun," he said.
"With the help of tens of thousands of dedicated volunteers sifting through hundreds of thousands of images taken by SkyMapper, we have achieved four years of scientific analysis in under three days. One of those volunteers, Toby Roberts, has made 12,000 classifications."
The team will confirm whether or not the unknown space objects are Planet 9, dwarf planets or asteroids by using telescopes at Siding Spring and around the world.
Dr Tucker said he encouraged people to continue their hunt for Planet 9 through the project website on Zooniverse.org
Professor Chris Lintott from Zooniverse and the University of Oxford said while Planet 9 had not been found, it had been great fun sharing the search with all of the volunteers over the past three nights.
SkyMapper is a 1.3-metre telescope that is creating a full record of the southern sky for Australian astronomers.
People can to participate in the ANU citizen science project to search for Planet 9 at www.planet9search.org
Quelle: The Australian National University, Canberra
Our discovery of a minor planet beyond Neptune shows there might not be a ‘Planet Nine’ after all
Ever since enthusiasm started growing over the possibility that there could be a ninth major planet orbiting the sun beyond Neptune, astronomers have been busy hunting it. One group is investigating four new moving objects found by members of the public to see if they are potential new solar system discoveries. As exciting as this is, researchers are also making discoveries that question the entire prospect of a ninth planet.
One such finding is our discovery of a minor planet in the outer solar system: 2013 SY99. This small, icy world has an orbit so distant that it takes 20,000 years for one long, looping passage. We found SY99 with the Canada-France-Hawaii Telescope as part of the Outer Solar System Origins Survey. SY99’s great distance means it travels very slowly across the sky. Our measurements of its motion show that its orbit is a very stretched ellipse, with the closest approach to the sun at 50 times that between the Earth and the sun (a distance of 50 “astronomical units”).
The new minor planet loops even further out than previously discovered dwarf planets such as Sedna and 2013 VP113. The long axis of its orbital ellipse is 730 astronomical units. Our observations with other telescopes show that SY99 is a small, reddish world, some 250 kilometres in diameter, or about the size of Wales in the UK.
SY99 is one of only seven known small icy worlds that orbit beyond Neptune at remarkable distances. How these “extreme trans-Neptunian objects” were placed on their orbits is uncertain: their distant paths are isolated in space. Their closest approach to the sun is so far beyond Neptune that they are thought to be “detached” from the strong gravitational influence of the giant planets in our solar system. But at their furthest points, they are still too close to be nudged around by the slow tides of the galaxy itself.
It’s been suggested that the extreme trans-Neptunian objects could be clustered in space by the gravitational influence of a “Planet Nine” that orbits much further out than Neptune. This planet’s gravity could lift out and detach their orbits – constantly changing their tilt. But this planet is far from proven.
In fact, its existence is based on the orbits of only six objects, which are very faint and hard to discover even with large telescopes. They are therefore prone to odd biases. It’s a bit like looking down into the deep ocean at a school of fish. The fish swimming near the surface are clearly visible. But the ones even only a meter down are fainter and murky, and take quite a lot of peering to be certain. The great bulk of the school, in the depths, is completely invisible. But the fish at the surface and their behaviour betray the existence of a whole school.
The biases mean SY99’s discovery can’t prove or disprove the existence of a Planet Nine. However, computer models do show that a Planet Nine would be an unfriendly neighbour to tiny worlds like SY99: its gravitational influence would starkly change its orbit – throwing it from the solar system entirely, or poking it into an orbit so highly inclined and distant that we wouldn’t be able to see it. SY99 would have to be one of an utterly vast throng of small worlds, continuously being sucked in and cast out by the planet.
The alternative explanation
But it turns out that there are other explanations. Our study based on computer modelling, accepted for publication in the Astronomical Journal, hint at the influence of an idea from everyday physics called diffusion. This is a very common type of behaviour in the natural world. Diffusion typically explains the random movement of a substance from a region of higher concentration to one of lower concentration – such as the way perfume drifts across a room.
We showed that a related form of diffusion can cause the orbits of minor planets to change from an ellipse that is initially only 730 astronomical units on its long axis to one that is as big as 2,000 astronomical units or bigger – and change it back again. In this process, the size of each orbit would vary by a random amount. When SY99 comes to its closest approach every 20,000 years, Neptune will often be in a different part of its orbit on the opposite side of the solar system. But at encounters where both SY99 and Neptune are close, Neptune’s gravity will subtly nudge SY99, minutely changing its velocity. As SY99 travels out away from the sun, the shape of its next orbit will be different.
The long axis of SY99’s ellipse will alter, becoming either larger or smaller, in what physicists call a “random walk”. The orbit change takes place on truly astronomical time scales. It diffuses over the space of tens of millions of years. The long axis of SY99’s ellipse would change by hundreds of astronomical units over the 4.5 billion-year history of the solar system.
Several other extreme trans-Neptunian objects with smaller orbits also show diffusion, on a smaller scale. Where one goes, more can follow. It’s entirely plausible that the gradual effects of diffusion act on the tens of millions of tiny worlds orbiting in the near fringe of the Oort cloud (a shell of icy objects at the edge of the solar system). This gentle influence would slowly lead some of them to randomly shift their orbits closer to us, where we see them as extreme trans-Neptunian objects.
However, diffusion won’t explain the distant orbit of Sedna, which ha