THE HUNT FOR PLANET X
Could this hidden planet really exist? All About Space meets the scientists who could be on the trail of an undiscovered world
Astronomers have long wondered if our Solar System could contain a missing, unseen planet. A century ago some were convinced there was a gas giant lurking in the outer reaches, before later being derided for such a suggestion when it proved false. But now momentum is building that there may be another planet that has been hidden from our view and, while we haven’t found it yet, we can see its fingerprints all over the outer Solar System.
This is Planet Nine, which was first seriously proposed by astronomers Mike Brown and Konstantin Batygin from the California Institute of Technology (Caltech) in 2016. Looking at the outer Solar System they noticed something odd – namely that several objects seemed to be grouped together, as if a more distant object was shepherding them into similar orbits with its immense gravity. After eliminating other possible explanations this led them to believe there must be a planet far beyond the orbit of Neptune: a real ninth planet after Pluto’s demotion to dwarf planet.
“This world is the ninth true planet of our
Solar System,” Batygin tells All About Space. “We have never seen it directly, but we know it’s there because of the gravitational signatures that it provides in the Solar System.”
This is not the first time a planet beyond
Neptune has been proposed. In 1906 US astronomer Percival Lowell was convinced there was a large planet hiding out there owing to Uranus appearing to orbit off course. He dubbed it Planet X – with ‘X’ referring to being unknown, rather than ‘ten’, as only eight planets were then known. So convinced was he that he founded the Lowell Observatory in Arizona to hunt for it.
The search for this Planet X was unsuccessful, but the Lowell Observatory did accidentally redefine the entire Solar System. In 1930 a young US astronomer called Clyde Tombaugh used the telescope to discover Pluto. At the time this sparked intense interest that this might be Planet X, but it was later found to be a small world roughly onefifth the size of Earth, not nearly large enough to explain the orbit of Uranus.
The original Planet X was consigned to the bin, and the theory is now often scorned at, which might be why the idea of Planet Nine is so controversial. Batygin is pretty confident things will be different this time, however. “If you go back through the history of astronomy, the number of times that a trans-Neptunian planet [one beyond the orbit of Neptune] has been proposed is quite astonishing,” he says. “The difference is that this
time I think we’re right.”
If they are right, here’s what we know so far. Planet Nine is estimated to be about 5- to 15-times the mass of Earth, although we don’t know its radius. Its eccentric 20,000-year-long orbit around the Sun may take it anywhere from as close as 200times the Earth-Sun distance (200 Astronomical Units, or AU) to the Sun, and up to 1,200 AU away. Such a mass has earned the planet the moniker of super-Earth, a potentially large, rocky planet up to four-times the radius of Earth. But it could also be a mini-Neptune, a gas planet somewhat smaller than Uranus and Neptune.
While we can infer the existence of Planet
Nine in the outer Solar System we can’t yet see it, because if it’s there it’s extremely distant and faint. But astronomers are trying in earnest to find it as we speak. A number of surveys around the world, and more coming online in the future, are scouring the sky to look for any sign of a planet orbiting on its predicted path. And in doing so they are also finding other objects that lend further evidence that Planet Nine exists.
So far about a dozen such objects have been discovered, each clustered on a similar path on the same side of the Sun, including the possible dwarf planet Sedna found in 2003 that takes more than 11,000 years to orbit. The most recent was 2015 TG387, more colloquially known as ‘The Goblin’, which was first spotted in October 2015 and
“We have never seen it directly, but we know it’s there because of the gravitational signatures”
Konstantin Batygin
announced in October 2018 by Scott Sheppard from the Carnegie Institution for Science and his colleagues.
Thought to be a dwarf planet about 300-kilometres (186-miles) across and orbiting the Sun every 40,000 years, The Goblin is yet another piece in the puzzle that Planet Nine exists.
“The Goblin continues the trend seen in the past for this planet that we think is out there,” Sheppard tells All About Space. “If it matches our predictions, then The Goblin is stable to its orbit, and it kind of dances with the planet.”
Objects like The Goblin are given a specific moniker: inner Oort Cloud objects. They are found in a region between the Kuiper Belt and the Oort Cloud, the former extending from 30 to 55 AU and the latter anywhere from 2,000 to 200,000. This region, notes Sheppard, is a ‘no man’s land’ where the objects are too far away to be affected by the eight known planets, but still bound to the Sun and oblivious to outside forces like other stars or the supermassive black hole at the centre of our galaxy.
Only a dozen such objects have been found so far, but the number could be higher. Much, much higher. It’s estimated that thousands of these objects larger than 50 kilometres (30 miles) in size, or even perhaps millions, could be in orbit around the Sun. As their orbits are very elongated, however, they spend most of their time far away, only brightening and becoming visible when they approach the Sun. “We’re just seeing the tip of the iceberg,” says Sheppard.
Not everyone is convinced these objects are proof of another planet, however. There are other explanations that, with a bit of fiddling, can explain how they cluster together, and the theory of Planet Nine is anything but widely accepted just yet. “You can do this with a flyby of another star or a brown dwarf, a massive object early in the Solar System’s history,” says Michele Bannister from Queen’s University Belfast. “Potentially [there are] also some physical processes that involve how the objects interact with the inner Oort Cloud and the tide of the galaxy. [Or] Neptune [could] perturb some of these objects over hundreds of millions of years.”
In mid-2018 a team led by Ann-Marie Madigan from the University of Colorado, Boulder proposed
an additional possibility. They say the natural jostling of the dozen inner Oort Cloud bodies found so far, and space debris in the outer Solar System, could explain their orbits. The collective gravity of the bodies, they say, could provide an answer as to why they clustered together.
The main issue is that these dozen bodies we have discovered so far shouldn’t really be there. “They are small bodies in the outer Solar System that shouldn’t exist,” says Megan Schwamb from the Gemini Observatory in Hawaii. And that has led to the suggestion of Planet Nine, but it doesn’t mean that is the only explanation. “The same evidence for is also the same evidence against,” says Schwamb. “You could have formed these objects detached from Neptune in a stellar cluster. Early on in the Sun’s history many stars coming within 500 to 1,000 AU would have stirred up objects and would have been able to detach them from the outer Solar System and put them in these eccentric orbits.
Other options are a single star passing by, or rogue planets leaving the Solar System.”
Getting to the answer will be no mean feat, but answers may be forthcoming rather soon. At the moment our observations and data for this region of the Solar System come mostly from two telescopes, the Subaru Telescope in Hawaii for the northern sky, and the Blanco Telescope in Chile for the southern sky. Observations were especially helped by the installation of the Dark Energy Camera (DECam) on the Blanco Telescope in 2012. “Up until 2012 it was like looking through a straw,” says Sheppard. “In 2012 the game changed and allowed the floodgates to open for this kind of research.” But even together these two telescopes can see only a small patch of the sky, with about 20 per cent of the sky observed to date, and as the inner Oort Cloud objects are so far away and so faint, they’re extremely hard to see.
The game is going to change again though in 2022, when a new telescope called the Large Synoptic Survey Telescope (LSST) comes online in Chile. Although predominantly designed to find near-Earth asteroids, the LSST will also be able to find more objects in the outer Solar System, covering about 70 per cent of the sky in total. And if there are thousands or millions of objects out there as thought, then there could be a whole swathe of worlds – and possibly more evidence for or against
Planet Nine – awaiting our discovery. “Though we want to find the planet, the main goal right now is finding more of the smaller objects that can lead us to the bigger one,” says Sheppard. “If we can double that number, we can start limiting where the planet can be.”
After that the next big step will be actually seeing Planet Nine. Scientists are already poring through images of the outer Solar System hoping to spot its faint glint but, without knowing its exact location, such efforts are extremely difficult. If it were at its point nearest the Sun right now it would likely already have been spotted owing to its brightness, so if it exists it is more likely much farther out in the Solar System near the outer point of its orbit, known as aphelion, where it also moves at its slowest speed. So far, mostly using the Subaru Telescope, efforts to find the planet have been unsuccessful, but the hunt continues.
Until then, Planet Nine will continue to be hotly contested. While some are convinced it exists, others are not quite ready to throw their hat in the ring just yet, with memories of Planet X and other faux worlds no doubt fresh in the mind.
“I’m agnostic about whether it’s a planet or not,” notes Schwamb, while Sheppard says he is “80 to 85 per cent sure” it exists. But if all the stars – or should we say, inner Oort Cloud objects – align, then perhaps there really is a missing superEarth hiding. “I don’t think we’re going to find it next week, but at the same time
I don’t think it’s going to take 20 years to find this thing,” Batygin says resolutely.