Yyou would think that if you found the first evidence that a larger planet than Earth was hiding invisible in the most distant parts of our solar system, it would be a great time. He would be one of the few people in history to discover such a thing.
But for astronomer Scott Sheppard of the Carnegie Institution for Science in Washington DC, it was a much more peaceful affair. “It wasn̵
He is a master of euphemism. Since he and his collaborator Chad Trujillo of Northern Arizona University first published their suspicions on the invisible planet in 2014, the evidence has continued to grow. However, when asked how convinced he is that the new world, which he calls Planet X (although many other astronomers call it Planet 9), is really out there, Sheppard will only say, “I think it’s more likely than unlikely that it will exist.”
As for the rest of the astronomical community, in most quarters there is a palpable excitement in finding this world. Much of this excitement is centered on the opening of a new giant detection telescope named after Vera C Rubin, the astronomer who, in the 1970s, discovered some of the first evidence of dark matter.
Planned to begin its full investigation into the sky in 2022, Rubin’s observatory could either find the planet directly or provide circumstantial evidence that it is there.
The discovery of the planet would be a triumph, but also a disaster for the existing theory of how the solar system was created.
“It would change everything we thought we knew about the formation of the planet,” says Sheppard, in another characteristic euphemism. In truth, nobody has the slightest idea of how such a large planet can form so far from the sun.
The distant solar system is a place of darkness and mystery. It includes a huge volume of space that starts in the orbit of Neptune, about 30 times farther from the Earth’s sun, or 30 astronomical units (UA), and extends to around 100,000 UA. This is almost a third of the distance from the sun to the next closest star.
It was in the interior regions of this volume that the American astronomer Clyde Tombaugh discovered Pluto in 1930. Although Pluto possessed only two thirds of the diameter of our moon, it was originally classified as a planet.
By the turn of the century, however, telescopes were larger and astronomers were starting to find smaller worlds beyond Neptune. They were all even smaller than Pluto until 2005, when Mike Brown of the California Institute of Technology discovered Eris. It was at least the same size as Pluto and probably larger, so if Pluto was a planet, Eris was too. NASA quickly organized a press conference and announced the discovery of Planet 10.
About a year later, the International Astronomical Union decreed that Pluto and Eris were actually too small to be called planets and renamed them dwarf planets. So the roll-call of the solar system has returned to eight: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. And a cottage industry in which distant objects from the solar system are located has really begun.
The journey to Planet 9 began one night in 2012, when Sheppard and Trujillo were using the telescope of the Cerro Tololo Inter-American Observatory in Chile. They were finding objects more and more distant, but one in particular stood out. Cataloged as 2012 VP113, they nicknamed him Biden in honor of the American vice president of the time (due to the letters VP in the catalog designation). To their amazement, this distant world has never approached the sun by around 80 AU. At most, Biden would have reached 440 AU in deep space, meaning that it followed a highly elliptical orbit. But it wasn’t the most extraordinary thing.
By a strange coincidence, its orbit seemed to be very similar to that of another distant world known as Sedna. This mini-world was discovered in 2003 by Brown, Trujillo and David Rabinowitz of Yale University. It immediately stood out for its highly elliptical orbit, which fluctuates from 76 UA to 937 UA.
“Objects like Sedna and 2012 VP113 cannot form on these eccentric orbits,” says Sheppard. Instead, computer simulations suggest that they are formed much closer and are therefore expelled by gravitational interactions with larger planets. The really strange thing, however, was that the two elongated orbits pointed more or less in the same direction.
And the more Sheppard and Trujillo examined the other objects in their capture, the more they saw that those orbits were also aligned. It was as if something was corrupting those little worlds, like a shepherd dog handling his flock. And the only thing they could think of was able to do it was a much bigger planet.
The curiosity was stimulated, they made some calculations and discovered that the planet they were hinting at their results had to be somewhere between two and 15 times more massive than Earth, on an orbit that is on average somewhere between 250 UA and 1500 UA from the sun. Their results were published in the prestigious magazine Nature in March 2014 and interest in Planet 9 started sweeping the astronomical world.
The next big jump came in 2015 when Sheppard and Trujillo were among the scientists who discovered the 2015 TG387. They nicknamed this the Goblin. It is the third most extreme object behind Sedna and Biden and aligns itself, further reducing the idea that this alignment is a random coincidence.
In 2016, Brown and his collaborator Konstantin Batygin, also from Caltech, published their own data analysis. Agreeing with Sheppard and Trujillo about the size and distance of the planet, they even suggested an area of sky where they thought it could be found.
But not everyone is convinced.
Pedro H Bernardinelli, a graduate student at the University of Pennsylvania, realized that Sheppard’s data was not the only place where far-off worlds could be searched. So he turned to some initial data from a cosmological survey designed to measure how the universe is expanding by looking at distant galaxies. He searched for data for the celestial equivalent of a photo bomb, looking for distant objects in the solar system that just ran into the camera. He found seven.
At first glance, it seemed that these worlds were also aligned as expected, but the more rigorously Bernardinelli analyzed the data, the weaker the alignment felt. “We don’t think we see the signal in our data,” says Bernardinelli, although he admits that he cannot yet definitively rule out the planet and has yet to perform the analysis on the complete survey data. “Our response may change the next time we do it,” he says.
These days, Sheppard can be found regularly using the Japanese Subaru telescope on Mauna Kea, Hawaii, patiently patrolling the sky for further evidence of Planet 9, perhaps even hoping to see the planet itself. The scope of the task is enormous. It’s really like looking for the proverbial needle in a haystack. The planet – if it is also there – is very weak and the sky is very large. But help is coming in the form of Rubin’s observatory.
Rubin is a monster who will devour the sky. While most telescopes would take months or years to survey the entire sky, Rubin will do so in just three nights. So do it again and again and again to see what has changed and then capture the moving objects.
Construction is nearing completion and the telescope will open its giant eye for the first time later this year. Commissioning and modification will therefore take another couple of years.
“This survey will change the science of the solar system as we know it,” says Sheppard. And if Planet 9 is out there, Rubin should see it.
“We can detect a planet of earth mass at around 1000 UA,” says Meg Schwamb, of Queen University of Belfast, who co-chairs the scientific collaboration of the Rubin observatory’s solar system. This easily puts Sheppard’s world within reach. “If others haven’t seen Planet 9 before our survey started, I think all eyes are on Rubin’s observatory,” says Schwamb.
Even if the telescope cannot see the planet directly, it will detect many more distant mini-worlds that can all be used to triangulate the planet’s position more precisely, thus helping to narrow the search area. And if Planet 9 is really out there, the consequences will be enormous.
Astronomers think that the solar system has formed into a disc of matter surrounding the sun. That matter condensed into smaller bodies, which then collided to form larger ones. At the end of this process, the planets were born. But the matter in this disk further expands from the sun, which means that there is not enough raw material to form a large planet in the distant solar system.
To save the standard theory, some suggest that Planet 9 was once destined to become a gas giant like Jupiter or Saturn and was thus forming alongside them. However, a gravitational interaction curbed its growth by throwing it into the dark.
But Durham University’s Jakub Scholtz is skeptical. “It’s possible,” he says, “but it actually requires a lot of coincidences.” This is because a single gravitational interaction cannot do the job. Instead, a series of interactions is needed to position it in an orbit that never takes it back to where it was formed.
Scholtz has a more exotic idea. Together with collaborator James Unwin, of the University of Illinois in Chicago, he suggested that the object that is upsetting these distant worlds is not a long lost planet but a black hole.
In that case, not even Rubin will be able to see it, because black holes do not emit any light – they simply swallow the light and anything else that happens to cross their path. It’s a tempting possibility because Scholtz’s black hole should be part of a population of long-suspected but never proven black holes that formed shortly after the formation of the universe.
But for the moment, most other astronomers seem more than happy with the idea that there is a large planet out there in the dark, just waiting to come to light in the coming years.
And if Planet 9 is really there, then maybe the first time Sheppard sees it through a telescope, he will finally experience something like a moment of eureka.