Artist's impression of a ninth planet: Telescopes are sure to try to track down the object... if it really exists
American astronomers say they have strong evidence that there is a ninth planet in our Solar System orbiting far beyond even the dwarf world Pluto.
The team, from the California Institute of Technology (Caltech), has no direct observations to confirm its presence just yet.
Rather, the scientists make the claim based on the way other far-flung objects are seen to move.
But if proven, the putative planet would have 10 times the mass of Earth.
The Caltech astronomers have a vague idea where it ought to be on the sky, and their work is sure to fire a campaign to try to track it down.
"There are many telescopes on the Earth that actually have a chance of being able to find it," said Dr Mike Brown.
"And I'm really hoping that as we announce this, people start a worldwide search to go find this ninth planet."
The group's calculations suggest the object orbits 20 times farther from the Sun on average than does the eighth - and currently outermost - planet, Neptune, which moves about 4.5 billion km from our star.
But unlike the near-circular paths traced by the main planets, this novel object would be in a highly elliptical trajectory, taking between 10,000 and 20,000 years to complete one full lap around the Sun.
The Caltech group has analysed the movements of objects in a band of far-off icy material known as the Kuiper Belt. It is in this band that Pluto resides.
The scientists say they see distinct alignments among some members of the Kuiper Belt - and in particular two of its larger members known as Sedna and 2012 VP113. These alignments, they argue, are best explained by the existence of a hitherto unidentified large planet.
"The most distant objects all swing out in one direction in a very strange way that shouldn't happen, and we realised the only way we could get them to swing in one direction is if there is a massive planet, also very distant in the Solar System, keeping them in place while they all go around the Sun," explained Dr Brown.
"I went from trying very hard to be sceptical that what we were talking about was true, to suddenly thinking, 'this might actually be true'."
Models and observations
The idea that there might be a so-called Planet X moving in the distant reaches of the Solar System has been debated for more than a hundred years. It has fallen in and out of vogue.
What makes this claim a little more interesting is Dr Brown himself.
He specialises in finding far-flung objects, and it was his discovery of 2,236km-wide Eris in the Kuiper Belt in 2005 that led famously to the demotion of Pluto from full planet status a year later (Dr Brown's Twitter handle is @PlutoKiller).
At that stage, Pluto was thought to be slightly smaller than Eris, but is now known to be just a little bit bigger.
Others who model the outer Solar System have been saying for some years that the range of sizes seen in the objects so far identified in the Kuiper Belt suggest another planet perhaps the size of Earth or Mars could be a possibility. But there is sure to be strong scepticism until a confirmed telescopic observation is made.
Dr Brown and Konstantin Batygin (@kbatygin) report their work in The Astronomical Journal.
The 'ninth planet' - where to look?
The six most distant known objects in the Solar system with orbits exclusively beyond Neptune (magenta) all mysteriously line up in a single direction. Why? Brown and Batygin argue that this is because a massive planet (orange) is anti-aligned with these objects. Can telescopes now find this planet? Is it possible the evidence is already in observational data but no-one has yet recognised it? The hunt is on.
Caltech Researchers Find Evidence of a Real Ninth Planet
Caltech researchers have found evidence of a giant planet tracing a bizarre, highly elongated orbit in the outer solar system. The object, which the researchers have nicknamed Planet Nine, has a mass about 10 times that of Earth and orbits about 20 times farther from the sun on average than does Neptune (which orbits the sun at an average distance of 2.8 billion miles). In fact, it would take this new planet between 10,000 and 20,000 years to make just one full orbit around the sun.
The researchers, Konstantin Batygin and Mike Brown, discovered the planet's existence through mathematical modeling and computer simulations but have not yet observed the object directly.
"This would be a real ninth planet," says Brown, the Richard and Barbara Rosenberg Professor of Planetary Astronomy. "There have only been two true planets discovered since ancient times, and this would be a third. It's a pretty substantial chunk of our solar system that's still out there to be found, which is pretty exciting."
Brown notes that the putative ninth planet—at 5,000 times the mass of Pluto—is sufficiently large that there should be no debate about whether it is a true planet. Unlike the class of smaller objects now known as dwarf planets, Planet Nine gravitationally dominates its neighborhood of the solar system. In fact, it dominates a region larger than any of the other known planets—a fact that Brown says makes it "the most planet-y of the planets in the whole solar system."
Batygin and Brown describe their work in the current issue of the Astronomical Journal and show how Planet Nine helps explain a number of mysterious features of the field of icy objects and debris beyond Neptune known as the Kuiper Belt.
"Although we were initially quite skeptical that this planet could exist, as we continued to investigate its orbit and what it would mean for the outer solar system, we become increasingly convinced that it is out there," says Batygin, an assistant professor of planetary science. "For the first time in over 150 years, there is solid evidence that the solar system's planetary census is incomplete."
The road to the theoretical discovery was not straightforward. In 2014, a former postdoc of Brown's, Chad Trujillo, and his colleague Scott Sheppard published a paper noting that 13 of the most distant objects in the Kuiper Belt are similar with respect to an obscure orbital feature. To explain that similarity, they suggested the possible presence of a small planet. Brown thought the planet solution was unlikely, but his interest was piqued.
He took the problem down the hall to Batygin, and the two started what became a year-and-a-half-long collaboration to investigate the distant objects. As an observer and a theorist, respectively, the researchers approached the work from very different perspectives—Brown as someone who looks at the sky and tries to anchor everything in the context of what can be seen, and Batygin as someone who puts himself within the context of dynamics, considering how things might work from a physics standpoint. Those differences allowed the researchers to challenge each other's ideas and to consider new possibilities. "I would bring in some of these observational aspects; he would come back with arguments from theory, and we would push each other. I don't think the discovery would have happened without that back and forth," says Brown. " It was perhaps the most fun year of working on a problem in the solar system that I've ever had."
Fairly quickly Batygin and Brown realized that the six most distant objects from Trujillo and Sheppard's original collection all follow elliptical orbits that point in the same direction in physical space. That is particularly surprising because the outermost points of their orbits move around the solar system, and they travel at different rates.
"It's almost like having six hands on a clock all moving at different rates, and when you happen to look up, they're all in exactly the same place," says Brown. The odds of having that happen are something like 1 in 100, he says. But on top of that, the orbits of the six objects are also all tilted in the same way—pointing about 30 degrees downward in the same direction relative to the plane of the eight known planets. The probability of that happening is about 0.007 percent. "Basically it shouldn't happen randomly," Brown says. "So we thought something else must be shaping these orbits."
The first possibility they investigated was that perhaps there are enough distant Kuiper Belt objects—some of which have not yet been discovered—to exert the gravity needed to keep that subpopulation clustered together. The researchers quickly ruled this out when it turned out that such a scenario would require the Kuiper Belt to have about 100 times the mass it has today.
That left them with the idea of a planet. Their first instinct was to run simulations involving a planet in a distant orbit that encircled the orbits of the six Kuiper Belt objects, acting like a giant lasso to wrangle them into their alignment. Batygin says that almost works but does not provide the observed eccentricities precisely. "Close, but no cigar," he says.
Then, effectively by accident, Batygin and Brown noticed that if they ran their simulations with a massive planet in an anti-aligned orbit—an orbit in which the planet's closest approach to the sun, or perihelion, is 180 degrees across from the perihelion of all the other objects and known planets—the distant Kuiper Belt objects in the simulation assumed the alignment that is actually observed.
"Your natural response is 'This orbital geometry can't be right. This can't be stable over the long term because, after all, this would cause the planet and these objects to meet and eventually collide,'" says Batygin. But through a mechanism known as mean-motion resonance, the anti-aligned orbit of the ninth planet actually prevents the Kuiper Belt objects from colliding with it and keeps them aligned. As orbiting objects approach each other they exchange energy. So, for example, for every four orbits Planet Nine makes, a distant Kuiper Belt object might complete nine orbits. They never collide. Instead, like a parent maintaining the arc of a child on a swing with periodic pushes, Planet Nine nudges the orbits of distant Kuiper Belt objects such that their configuration with relation to the planet is preserved.
"Still, I was very skeptical," says Batygin. "I had never seen anything like this in celestial mechanics."
But little by little, as the researchers investigated additional features and consequences of the model, they became persuaded. "A good theory should not only explain things that you set out to explain. It should hopefully explain things that you didn't set out to explain and make predictions that are testable," says Batygin.
And indeed Planet Nine's existence helps explain more than just the alignment of the distant Kuiper Belt objects. It also provides an explanation for the mysterious orbits that two of them trace. The first of those objects, dubbed Sedna, was discovered by Brown in 2003. Unlike standard-variety Kuiper Belt objects, which get gravitationally "kicked out" by Neptune and then return back to it, Sedna never gets very close to Neptune. A second object like Sedna, known as 2012 VP113, was announced by Trujillo and Sheppard in 2014. Batygin and Brown found that the presence of Planet Nine in its proposed orbit naturally produces Sedna-like objects by taking a standard Kuiper Belt object and slowly pulling it away into an orbit less connected to Neptune.
But the real kicker for the researchers was the fact that their simulations also predicted that there would be objects in the Kuiper Belt on orbits inclined perpendicularly to the plane of the planets. Batygin kept finding evidence for these in his simulations and took them to Brown. "Suddenly I realized there are objects like that," recalls Brown. In the last three years, observers have identified four objects tracing orbits roughly along one perpendicular line from Neptune and one object along another. "We plotted up the positions of those objects and their orbits, and they matched the simulations exactly," says Brown. "When we found that, my jaw sort of hit the floor."
"When the simulation aligned the distant Kuiper Belt objects and created objects like Sedna, we thought this is kind of awesome—you kill two birds with one stone," says Batygin. "But with the existence of the planet also explaining these perpendicular orbits, not only do you kill two birds, you also take down a bird that you didn't realize was sitting in a nearby tree."
Where did Planet Nine come from and how did it end up in the outer solar system? Scientists have long believed that the early solar system began with four planetary cores that went on to grab all of the gas around them, forming the four gas planets—Jupiter, Saturn, Uranus, and Neptune. Over time, collisions and ejections shaped them and moved them out to their present locations. "But there is no reason that there could not have been five cores, rather than four," says Brown. Planet Nine could represent that fifth core, and if it got too close to Jupiter or Saturn, it could have been ejected into its distant, eccentric orbit.
Batygin and Brown continue to refine their simulations and learn more about the planet's orbit and its influence on the distant solar system. Meanwhile, Brown and other colleagues have begun searching the skies for Planet Nine. Only the planet's rough orbit is known, not the precise location of the planet on that elliptical path. If the planet happens to be close to its perihelion, Brown says, astronomers should be able to spot it in images captured by previous surveys. If it is in the most distant part of its orbit, the world's largest telescopes—such as the twin 10-meter telescopes at the W. M. Keck Observatory and the Subaru Telescope, all on Mauna Kea in Hawaii—will be needed to see it. If, however, Planet Nine is now located anywhere in between, many telescopes have a shot at finding it.
"I would love to find it," says Brown. "But I'd also be perfectly happy if someone else found it. That is why we're publishing this paper. We hope that other people are going to get inspired and start searching."
In terms of understanding more about the solar system's context in the rest of the universe, Batygin says that in a couple of ways, this ninth planet that seems like such an oddball to us would actually make our solar system more similar to the other planetary systems that astronomers are finding around other stars. First, most of the planets around other sunlike stars have no single orbital range—that is, some orbit extremely close to their host stars while others follow exceptionally distant orbits. Second, the most common planets around other stars range between 1 and 10 Earth-masses.
"One of the most startling discoveries about other planetary systems has been that the most common type of planet out there has a mass between that of Earth and that of Neptune," says Batygin. "Until now, we've thought that the solar system was lacking in this most common type of planet. Maybe we're more normal after all."
Brown, well known for the significant role he played in the demotion of Pluto from a planet to a dwarf planet adds, "All those people who are mad that Pluto is no longer a planet can be thrilled to know that there is a real planet out there still to be found," he says. "Now we can go and find this planet and make the solar system have nine planets once again."
The paper is titled "Evidence for a Distant Giant Planet in the Solar System."
Update: 18.45 MEZ
Caltech researchers answer skeptics' questions about Planet 9
Caltech researchers dropped a bomb on the scientific community Wednesday by unveiling new research that suggests there is a ninth planet ranging through our solar system that has so far escaped our notice.
The team came up with the idea as they tried to explain why six objects in an area beyond Neptune, known as the Kuiper Belt, were behaving so strangely. These icy chunks of space rock showed an odd, elliptical orbit that seems to point them in the same direction.
Caltech's Konstantin Batygin said at first they laughed at the notion of a mysterious planet lurking billions of miles from Earth causing this effect.
"We literally looked for every possible explanation you could find where you don't need to invoke this additional massive body in the distant solar system," he explained.
But after exhausting all other theories, they decided to run computer models imagining a gas giant, ten times the mass of Earth, looping through that area of space.
It was that model that proved best at explaining the strange behavior of the Kuiper Belt objects. In essence, the massive gravitational pull of a gas giant in that part of solar system seemed to be the most probable reason for why the six objects were orbiting the way they did.
If it's true, it means a planet roughly the size of Neptune has eluded our detection despite all our increasingly sophisticated observational tools.
It's no surprise then that this big claim has raised some big questions. Here are some of the things skeptics are asking.
DOES IT HAVE TO BE A PLANET CAUSING THESE STRANGE ORBITS?
Plenty of scientists have tried to explain strange findings in the past by imagining a mysterious planet that locks its neighbors in a gravitational vice.
Batygin said his first instinct was to look for a less flashy explanation. He examined whether the odd orbits of the Kuiper Belt objects could have happened randomly. Calculations show just a 0.007 percent chance of that happening, making it very unlikely.
He also looked at whether the solar system as it is currently set up could have somehow led to this configuration.
"We tried... we really failed," he said.
They even used computers to see if the Kuiper Belt itself could create enough gravitational force to be the culprit, but under those models, the belt would need to have 100 times the mass it does today.
In the end, the Planet 9 theory seemed the best option.
Still, Emily Lakdawalla, senior editor with the Planetary Society, noted that just because this theory works the best for now doesn't mean it must be right.
"Any theoretical paper describing one possible explanation for weird features of the solar system often leaves out what could be other possible explanations that we haven't even thought of yet," she said.
IF THIS PLANET IS SO BIG, WHY ARE ONLY SIX OBJECTS INFLUENCED BY IT?
Anything the size of this hypothetical Planet 9 should leave lots of gravitational fingerprints.
However, the initial research here is based on the orbits of just six objects -- each roughly the size of Los Angeles.
That's not a lot of evidence, said Laura Danly, an astronomer and curator with the Griffith Observatory.
"I think we'd want to see more observations of objects that share those strange orbital behaviors to help put more solid foundation on this hypothesis," she said.
The team at Caltech agree, and they said they'd love more data to work with. The problem is Kuiper Belt objects are very distant and hard to spot, so we don't have many known objects to examine.
Still, Caltech's Mike Brown said his team has been able to use the Planet 9 hypothesis to explain the strange behavior of some other distant objects in the same region of space.
"It turns out there are other effects that this predicts that we have started to find too," Brown noted.
Now that this Planet 9 theory is out there, plenty of researchers will be looking for traces of it in other data to either support the idea or bring it down.
"That's the way science works, people should go off and try and prove that we are wrong. I assume that people are starting that today," Brown said.
HOW DID THIS PLANET END UP SO FAR FROM THE SUN?
It's believed our planets formed from a disk of gas and dust swirling around our newly formed sun more than 4.5 billion years ago. Clumps of material stuck together then smashed into other clumps forming the seeds of the planets.
As those seeds orbited the sun, they eventually gathered enough gravity to pull in surrounding objects and eventually cleared all the debris from their path.
The problem with something like Planet 9 is that it lies so far from the sun, that it's likely the disk of dust and gas there was too thin to create a body that size.
Batygin and Brown address this by suggesting perhaps Planet 9 formed closer to our sun but was eventually knocked away by say Jupiter or Saturn, sending it into its current orbit.
This isn't as wild as it may sound since researchers believe our early solar system was a busy place where planets might have bounced off each other like balls on a pool table.
However, if that's the case, it raises another question.
WHY WASN'T THIS GAS GIANT FLUNG OUT OF OUR SOLAR SYSTEM ALTOGETHER?
It would take a huge amount of energy to bounce something the size of Planet 9 out of its original orbit and send it elsewhere. If that were to happen, the planet would likely keep going, perhaps leaving the solar system all together.
Once again, Batygin says the nature of the early solar system might help explain why this didn't happen.
He argues that when our celestial home was forming, it was part of a cluster of thousands of neighboring stars, each affecting the space around it in major ways.
"Once you get far away from the sun, the gravitational pull of neighboring stars can modify your orbit very, very significantly," he said.
Batygin thinks perhaps some of these stars interacted with Planet 9 as it flew from our inner solar system, turning it around and putting it in a very wide but consistent orbit.
GIVEN ITS SIZE, WHY DIDN'T WE SPOT THIS BEFORE?
You think you'd notice something ten times the mass of Earth hanging around in the solar system. So why is this the first time we are hearing about a possible ninth planet?
In fact, in 2009 NASA launched the Widefield Infrared Survey Explorer (WISE) satellite and later tasked it with completing a survey of the sky to find heat from large planets.
Planet 9 didn't show up.
However, that survey was designed to find objects the size of Saturn or larger. Planet 9 would have been too small to be detected.
Planet 9 is also believed to have a very long orbit, taking anywhere from 10,000 to 20,000 years to make one trip around the sun. That means it might have spent recent history quite far from Earth, making it tough to spot.
Still, there are a few telescopes that are up for the job, like the W. M. Keck Observatory and the Subaru Telescope, both in Hawaii.
It's possible the planet also showed up in past photographs of space, but scientists missed it because they weren't looking for it. Brown and Batygin are encouraging researchers to comb through images taken of areas believed to be in Planet 9's orbit to see if the celestial body made a cameo.