you would think that if you found the first evidence that a planet bigger than Earth was hidden deep in our solar system, it would be a great time. That would make you 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, the matter was much quieter. “There was no eureka moment,” he says. “The evidence has accumulated slowly.”
He is a master of euphemism. Since he and his collaborator Chad Trujillo of the University of Northern Arizona 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 really out there, Sheppard just says, “I think it’s more likely than unlikely to exist “.
As for the rest of the astronomical community, in most sectors there is a palpable emotion in finding this world. Much of this excitement is focused on opening a new giant telescope called Vera C Rubin, the astronomer who, in the 1970s, discovered some of the first evidence of dark matter.
Scheduled to begin its full survey of the sky in 2022, the Rubin observatory could find the planet permanently 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 about 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 fact, 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 a huge 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 about 100,000 AU. This is almost a third of the distance from the sun to the next nearest 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 the moon, it was originally classified as a planet.
By the end of the century, however, telescopes were larger and astronomers began 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 bigger, 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 decided that Pluto and Eris were actually too small to be called planets and renamed them as dwarf planets. So the call from the solar system returned at eight: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. And a home industry to find objects far from the solar system has really started.
The road to Planet 9 began one night in 2012, when Sheppard and Trujillo were using the telescope of the Inter-American Observatory Cerro Tololo in Chile. They were finding objects more and more distant, but one in particular stood out. Cataloged as VP113 2012, he nicknamed him Biden, in honor of the US vice president at the time (because of the letters VP in the catalog designation). To your surprise, this distant world has never come closer to the sun than about 80AU. In the farthest, Biden would reach 440AU in deep space, which means that it followed a highly elliptical orbit. But that was not the most notable thing.
By some strange coincidence, its orbit appeared 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, from Yale University. It stood out immediately because of 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 together and are ejected by gravitational interactions with the larger planets. The strangest thing, however, 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 these orbits were also aligned. It was as if something was cornering those tiny worlds, like a sheepdog maneuvering its flock. And the only thing they could think of that was able to do that was a much bigger planet.
Curiosity aroused, they did some calculations and found that the planet on which their results suggested had to be somewhere between two and 15 times more massive than Earth, in an orbit that averages between 250AU and 1500AU from 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 took place in 2015, when Sheppard and Trujillo were among the scientists who discovered TG387 2015. They nicknamed it Goblin. It is the third most extreme object behind Sedna and Biden and also aligns, 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 analysis of the data. Agreeing with Sheppard and Trujillo on the size and distance of the planet, they even suggested an area of the sky where they thought it could 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 to search for distant worlds. Then he turned to some initial data from cosmological research that was designed to measure the way the universe is expanding, looking at distant galaxies. He searched the data for the celestial equivalent of a photographic bomb, looking for objects distant from the solar system that hindered the camera. He found seven.
At first glance, it seemed that these worldlets were also aligned as expected, but the more rigorously Bernardinelli analyzed the data, the weaker he felt that the alignment became. “We don’t think we see the signal in our data,” says Bernardinelli, although he admits that he still cannot definitively discard the planet and has not yet done the analysis of the complete survey data. “Our response may change the next time we do that,” he says.
Currently, Sheppard can be found regularly using Japan’s Subaru telescope in Mauna Kea, Hawaii, patiently scanning 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’s like looking for the proverbial needle in the haystack. The planet – if it is really there – is very weak and the sky is very big. But aid 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 this in just three nights. Then, do it again and again and again to see what has changed and thus get the moving objects.
Construction is about to be completed, and the telescope is expected to open its giant eye for the first time later this year. Commissioning and adjustments will take a few more years.
“This research will change the science of the solar system as we know it,” says Sheppard. And if Planet 9 is out there, Rubin must see it.
“We can detect a Earth-mass planet at about 1000 AU”, says Meg Schwamb, from Queen’s Belfast University, who co-chairs the scientific collaboration of the Rubin Observatory’s solar system. This puts Sheppard’s world in plain view. “If other people didn’t see Planet 9 before our research started, I think all eyes are on the Rubin observatory,” says Schwamb.
Even if the telescope cannot see the planet directly, it will detect many more distant mini-worlds that can be used to triangulate the planet’s position more accurately, thus helping to narrow the area of research. And if Planet 9 is really out there, the consequences will be enormous.
Astronomers think that the solar system formed in a disk of matter around the sun. This matter condensed into smaller bodies, which then collided to form larger bodies. At the end of this process, the planets were born. But the matter on this disk tapers further away from the sun, meaning that 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 and was thus forming alongside them. However, a gravitational interaction hindered its growth, casting it into the dark.
But Jakub Scholtz, from the University of Durham, is skeptical. “It’s possible,” he says, “but it actually requires a lot of coincidence.” This is because a single gravitational interaction cannot do the job. Instead, it takes a series of interactions to place it in an orbit that will never bring it back 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 surrounding 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 emit no light – they just swallow light and anything else that crosses their path. It is a tempting possibility, because Scholtz’s black hole would have to be part of a population long suspected, but never proven, of black holes that were formed shortly after the formation of the universe.
But for now, most other astronomers seem more than happy with the idea that there is a big planet out there in the dark, just waiting to appear in the years to come.
And if Planet 9 is really there, perhaps the first time Sheppard sees it through a telescope, he will finally experience something similar to an eureka moment.