ANDYou’d think if you found the first evidence that a planet larger than Earth lurked unseen in the far reaches of our solar system, it would be a great time. It would make you one of the few people in all of history who has discovered such a thing.
But for astronomer Scott Sheppard of the Carnegie Institution for Science in Washington DC, it was a much quieter affair. “It wasn’t like there was a eureka moment,” he says. “The evidence slowly accumulated.”
He is a master of the euphemism. Since he and collaborator Chad Trujillo of the University of Northern Arizona first published their suspicions about the invisible planet in 2014, the evidence has only 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 actually out there, Sheppard will only say, “I think it’s more likely to exist. “
As for the rest of the astronomical community, in most sectors there is a palpable emotion to find this world. Much of this excitement centers on the opening of a new giant scanning telescope named for Vera C Rubin, the astronomer who, in the 1970s, discovered some of the first evidence for dark matter.
Scheduled to begin its full survey of the sky in 2022, the Rubin 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 planet formation,” says Sheppard, in another characteristic understatement. In truth, no one has any idea how such a large planet could form so far from the sun.
The distant solar system is a place of darkness and mystery. It encompasses an enormous volume of space that begins in Neptune’s orbit, about 30 times farther from the sun than Earth, or 30 astronomical units (AU), and extends to approximately 100,000 AU. That’s almost a third of the distance from the sun to the next closest star.
It was in the inner regions of this volume that the American astronomer Clyde Tombaugh discovered Pluto in 1930. Although Pluto was only two-thirds the diameter of our moon, it was originally classified as a planet.
By the end of the century, however, telescopes were larger, and astronomers were beginning 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, so was Eris. NASA hastily organized a press conference and announced the discovery of Planet 10.
About a year later, the International Astronomical Union ruled that Pluto and Eris were indeed too small to be called planets and renamed them dwarf planets. So the list of the solar system went back to eight: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. And a craft industry of finding distant objects from the solar system really got going.
The road to Planet 9 began one night in 2012, when Sheppard and Trujillo were using the telescope at the Cerro Tololo Inter-American Observatory in Chile. They found increasingly distant objects, but one in particular stood out. Listed as 2012 VP113, he was nicknamed Biden after the Vice President of the United States at the time (due to the letters VP in the catalog designation). To his surprise, this distant world never got closer to the sun than around 80 AU. At its furthest point, Biden would reach 440 AU in deep space, which means it was following a highly elliptical orbit. But that was not the most remarkable thing about it.
By some strange coincidence, its orbit seemed to be very similar to that of another distant world known as Sedna. This mini world had been discovered in 2003 by Brown, Trujillo, and David Rabinowitz of Yale University. It was immediately noted for its highly elliptical orbit, which ranges from 76AU to 937AU.
“Objects like Sedna and 2012 VP113 cannot form in these eccentric orbits,” says Sheppard. Instead, computer simulations suggest that they form much closer and are then driven off by gravitational interactions with the larger planets. However, the really strange thing was that the two elongated orbits pointed in approximately 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 cornering those little worlds, like a sheepdog maneuvering his flock. And the only thing they could think of was able to do that was a much larger planet.
Curiosity sparked, they did some calculations and discovered that the planet their results hinted at had to be between two and 15 times more massive than Earth, in an orbit that is on average between 250AU and 1500AU of the sun. Their results were published by the prestigious magazine. Nature in March 2014 and interest in Planet 9 began to sweep the astronomical world.
The next big leap occurred in 2015 when Sheppard and Trujillo were among the scientists who discovered the 2015 TG387. They nicknamed this Goblin. It is the third most extreme object behind Sedna and Biden and it also aligns, further reducing the idea that this alignment is a random match.
In 2016, Brown and collaborator Konstantin Batygin, also from Caltech, published their own analysis of the data. According to Sheppard and Trujillo about the size and distance of the planet, they even suggested an area of the sky where they thought it might be found.
But not everyone is convinced.
Pedro H Bernardinelli, a PhD candidate at the University of Pennsylvania, realized that Sheppard’s data was not the only place where one could search for distant worlds. He then drew on some initial data from a cosmological survey that was designed to measure how the universe is expanding by observing distant galaxies. He searched the data for the celestial equivalent of a photopump, searching for distant objects in the solar system that got in the way of the camera. He found seven.
At first glance, it appeared 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 will see the signal in our data,” says Bernardinelli, although he admits that he still cannot definitively rule out the planet and has not yet done the analysis of the full survey data. “Our response could change the next time we do this,” he says.
These days, Sheppard can be found regularly using Japan’s Subaru telescope in Mauna Kea, Hawaii, patiently scouring the sky for more evidence of Planet 9, perhaps even waiting for him to see the planet itself. The scale of the task is enormous. It really is like looking for the proverbial needle in a haystack. The planet, if it is there, is very weak and the sky is very large. But help is on the way in the form of the Rubin Observatory.
Rubin is a monster that 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. Then do it over and over and over to see what has changed to catch moving objects.
Construction is nearing completion, and the telescope will open its giant eye for the first time later this year. Commissioning and adjustments will take a few more years.
“That survey is going to 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 with Earth’s mass at around 1000 AU,” says Meg Schwamb of Queen’s University Belfast, who is co-chairing the Rubin Observatory’s solar system scientific collaboration. That puts the world of Sheppard easily in view. “If others haven’t seen Planet 9 before our survey begins, I think all eyes are on the Rubin Observatory,” says Schwamb.
Even if the telescope can’t see the planet directly, it will detect many more distant little worlds that can be used to triangulate the planet’s position more accurately, helping to narrow the search area. And if Planet 9 really is out there, then the consequences will be enormous.
Astronomers think that the solar system was formed from a disk of matter that surrounds 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 on this disk thins further away from the sun, which means there is not enough raw material to form a large planet in the distant solar system.
To rescue the standard theory, some suggest that Planet 9 was once destined to become a gas giant like Jupiter or Saturn, so it was forming alongside them. However, a gravitational interaction stunted its growth by throwing it into the dark.
But Jakub Scholtz of Durham University is skeptical. “It is possible,” he says, “but it actually requires quite a few matches.” That’s because a single gravitational interaction can’t do the job. Instead, it takes a series of interactions to place it in an orbit that never returns it to where it formed.
Scholtz has a more exotic idea. Together with collaborator James Unwin of the University of Illinois at Chicago, he suggested that the object that corners these distant worlds is not a long-lost planet, but a black hole.
If so, even Rubin won’t be able to see it, because black holes don’t emit any light, they just swallow light and anything else that gets in their way. It’s a tantalizing possibility because Scholtz’s black hole would have to be part of a long-suspected but never-proven population of black holes that formed shortly after the formation of the universe.
But at the moment, most other astronomers seem to be more than happy with the idea that there is a great planet out there in the dark, waiting to appear in the years to come.
And if Planet 9 is really there, then perhaps the first time Sheppard sees it through a telescope, it will finally experience something similar to a eureka moment.