Growing up, I was taught that there were nine planets in the solar system. That all changed in 2006, when the International Astronomical Union voted to demote Pluto’s status to that of dwarf planet. But now, there is a chance that within the next year or two, the solar system could once again be graced with nine official, full-fledged planets.
The newcomer replacing Pluto presently goes by “Planet Nine.” If and when it finally shows up, it will get the honor of a real name, but so far, the evidence for this hypothetical planet is all indirect. Just as many of the thousands of known planets orbiting other stars have been found by observing their gravitational effects, rather than by actually seeing them, Planet Nine has been identified thanks to the way it has apparently been tugging on a variety of small bodies in the outer solar system and pulling their orbits into lockstep patterns. If it exists, Planet Nine orbits the Sun at an average distance around 550 astronomical units (AU), or 550 times as far as Earth is from the Sun. Astronomers also think it has between five and 10 times Earth’s mass.
Some astronomers think that the body of evidence is already overwhelming. Others are not convinced. But this nearly decade-long debate is reaching a critical point: A new observatory is coming online that should give astronomers the chance to find it. If Planet Nine does exist, the chance to see it may be just around the corner.
Weird orbits
The story of Planet Nine begins with oddities in the orbits of objects in the solar system.
In 2016, Caltech astronomers Michael Brown and Konstantin Batygin reported six small bodies in the outer solar system that had strange, anomalous orbits. These six objects, which orbit far out beyond Neptune, are members of the category of so-called extreme trans-Neptunian objects (ETNOs) who orbit on average 150 times farther from the Sun than Earth. Among the half-dozen is the first known ETNO, the dwarf planet Sedna.
The six curiosities that Brown and Batygin identified have elongated elliptical orbits whose perihelia (their closest points to the Sun) seem to be aligned, although researchers would expect them to be randomly distributed. What’s more, their orbital planes are all tilted by about 18° from the plane of the solar system’s planets, and yet their perihelia all fall almost precisely on that plane. “That grouping, that confinement, requires some kind of gravitational tug,” Batygin says.
The paths of these ETNOs, the pair argued, could best be explained by the influence of a large planet orbiting in a highly elliptical orbit far out beyond Neptune. In the years since that original proposal, additional ETNOs have been found that exhibit this same orbital clustering, bringing the number to 14. Without the influence of another large planet, he says, “if you let them go, they would precess out of alignment,” ending up in the expected random distribution.
The third line of evidence came from a separate population of unusual asteroids. Their orbits are nearly perpendicular to the plane of the solar system. “No one had been able to explain” these odd orbits, Brown says, “and they are, again, a very natural consequence of Planet Nine.” Additionally, there is a population of more than a dozen asteroids within the group known as Centaurs (asteroids that orbit between Jupiter and Neptune) with highly inclined orbits, orbiting the Sun in the opposite direction from the planets and most other asteroids.
“This is another population of bodies that comes out naturally from the Planet Nine hypothesis,” Batygin says. Taken together, these anomalies can’t be “explained with the conventional understanding of how the solar system formed and how it evolved,” Batygin says.
Clustering questions
The initial hypothesis was greeted with skepticism. At least two other groups who have discovered additional ETNOs have concluded that the apparent clustering does not rise to the level of a statistical anomaly. Instead, it may simply be a result of our limited ability to find them.
Because ETNOs are on such extreme orbits, they can only be detected when they happen to be near their perihelia (their closest point to the Sun); otherwise, they are too distant and faint. And the ground-based telescopes that conduct searches for such objects are limited in where they can look on the sky. Samantha Lawler, an astronomer at the University of Regina, Saskatchewan, was co-author of one of these studies, based on a project called the Outer Solar System Origins Survey.
“Our data for these most extreme objects is completely consistent with a random distribution,” she says, and therefore does not require any special explanation such as an extra planet. “I really don’t think there is any clustering,” she adds. Another study using data from the Dark Energy Survey, “which was completely independent of us, also found the same thing.” Lawler says. “So that’s two surveys where we kept very careful track of the biases, and we both don’t see the evidence there.”
But she and her colleagues are still looking, she notes. They are part of an ongoing survey using the Canada-France-Hawaii Telescope on Maunakea, Hawaii, “where we are specifically trying to find the most distant extreme Kuiper Belt objects that we can, in uniform directions around the solar system. So the idea is we will test the clustering.”
Because both of the studies that failed to confirm the clustering involved small numbers of objects — each only found four ETNOs — her team’s study has been criticized for insufficient data. “The critique we have gotten from [Brown and Batygin’s] group is that we didn’t find enough extreme Kuiper Belt objects,” Lawler says. Of the four they found, two were consistent with the suggested clustering, one was perpendicular to the line of that cluster, and the other was 180° away. “So, of the four we discovered, two were completely outside the clustering. So statistically that is consistent with a uniform distribution,” she says.
New lines of evidence
Just last year, Brown and Batygin, along with three co-authors, published research in The Astrophysical Journal Letters citing new lines of evidence in support of the Planet Nine hypothesis. They hope that the question will be answered once and for all within the next year or so.
The new work found that it’s not only objects in the far outer solar system that are perturbed by the gravity of this distant giant planet. According to hundreds of thousands of simulations of asteroid orbits and their evolution, they found that there should be a clearly discernible effect on asteroids orbiting inside Neptune’s orbit as well.
It’s all part, says Batygin, of the way “the giant planets play soccer with these smaller objects.” In this game, Neptune is the goalie. Every once in a while, objects from the Kuiper Belt get perturbed by some random encounter and sent inward, but the influence of Neptune’s gravity — the first giant planet they would encounter on their inward journey — tends to trap them and prevent further progression. Eventually, by lingering too close to Neptune’s orbit, these objects will get ejected from the solar system. This should create a sharp drop in the population of the objects with elongated orbits as you get closer to the Sun than Neptune, because Neptune gets in the way.
But if Planet Nine is farther out and constantly taking shots on goal, perturbing more Kuiper Belt objects and sending them inward all the time, then Neptune just can’t keep up, and some will get through. “As a consequence,” Brown says, “if Planet Nine exists, there should be a pretty smooth population of objects that have a perihelion distance from just outside Neptune’s orbit to all the way in to Uranus and even further.”
And that’s just what they found when they analyzed the orbits of about a dozen ETNOs. “With a pretty nice analysis that’s hard to argue with, we showed that it is definitely a smooth population that goes in all the way to Uranus and beyond, and doesn’t stop at Neptune,” Brown says.
Batygin says that “a Planet Nine-free solar system is incompatible with this data at the five-sigma level,” meaning the odds are 3.5 million to 1 against it. On the other hand, he says, “a solar system simulation that includes Planet Nine is indistinguishable from the real data.”
Something out there?
Brown concedes that none of the lines of evidence alone provides proof of Planet Nine’s existence. But all of these different anomalies require explanation, and the Planet Nine concept ties them all together into a single consistent picture. Without Planet Nine, each would require individual explanation, which nobody has yet proposed in a consistent way, he says. And although some astronomers are skeptical of the apparent clustering that formed the basis for the initial Planet Nine hypothesis, Lawler — who is one of those skeptics — concedes that some of the features of those distant orbits, including some of these newer findings, cannot be easily accounted for.
“I think there’s a lot of the orbital distribution in the Kuiper Belt that we can’t explain currently,” she says. “You can’t explain it with just Neptune. There is other physics going on. Some of it can be explained by galactic tides,” which come from the gravitational effect of the solar system’s oscillating motion above and below the plane of the Milky Way as our Sun orbits the center of the galaxy. “But some of it — there has to be something out there that we haven’t discovered yet,” she says.
What else could it be? Some researchers have suggested the gravitational effects of a star passing nearby long ago. Others imagine an uneven distribution of objects in the Kuiper Belt left over from the massive disk of debris that formed the solar system. Still others think that perhaps a much closer and less massive ninth planet exists, about the size of Mars instead of five to 10 times Earth’s mass.
“There’s a whole bunch of different theories,” she says. It’s true that the Planet Nine hypothesis can tie together several different kinds of anomalous findings into one single, simple explanation, she agrees. “That’s lovely; that would be great. But the original line of evidence for Planet Nine is this clustering, which I don’t think there is evidence for. … I don’t think Planet Nine in that exact configuration is something that we’re going to find,” Lawler says.
Brown says that although the results of their latest research are not as obvious as the similarity of orbits that triggered the proposal of a planet in the first place, they do support the initially hypothesized Planet Nine parameters. “Although the results of this paper aren’t as dramatic as the big clustering of objects in the outer solar system, they’re pretty irrefutable,” he insists. “There’s kind of no way to have the results that we did without there being some sort of giant planet out there.”
Putting it to the test
The analysis of the anomalous orbits enabled Brown and Batygin to calculate the likely orbit of Planet Nine. Since then, they have been searching the part of the sky where it is most likely to be.
In a second, more recent paper last year they, along with co-author Matthew Holman from the Harvard-Smithsonian Center for Astrophysics, conducted an exhaustive search using the database of the Pan-STARRS survey. Combined with results of previous searches, they say they have now ruled out 78 percent of the possible locations the giant planet could be. “We can give pretty good guesses,” Batygin says, “but we can’t predict where it is in its orbit.” That’s different from the way French astronomer Urbain Le Verrier was able to predict Neptune’s precise position in 1846, leading almost immediately to its discovery.
But there’s hope on the horizon. The Vera C. Rubin Observatory, nearing completion on a mountaintop in Chile, is expected to come online later this year. When it does, it will image the entire southern sky every three nights, using the largest digital camera ever built.
Planet Nine should be detectable by that telescope. But it would require an adjustment in the planned observing protocol to extend the telescope’s survey farther north than planned, Brown says, and that has not yet been decided. If the observatory does make the alteration, he says it should detect Planet Nine — unless the world lies at the extreme edge of its predicted distance or at the low end of its possible brightness.
“There are some reasons to think it could be farther away than our initial predictions,” Brown says, “and in that case, Vera Rubin will not be able to track it down either, and I’ll be depressed.” But if it lies almost anywhere along the predicted path and isn’t extraordinarily dark, it should pop into view, likely within Rubin’s first year of operation, Brown says.
Even if the planet itself is not directly seen, Rubin could provide new evidence. “A nice thing about Rubin is that whether you find Planet Nine or not, it will put to rest any questions about whether there is clustering,” Brown says. “There will be many more objects discovered, and it’ll be quite obvious that this clustering is real.” Planet Nine “is really the only explanation. There are no other explanations for the other strange phenomena out there,” he says.
And if Rubin does find the planet, proving its existence and providing an image of it, albeit as just a point of light, what then?
“What I’m looking forward to is not just the discovery, but what we get to do once we discover it,” Brown says. “We’ll study it with every telescope on the planet and off the planet, and learn if it has moons, if it has rings, what kind of atmosphere or surface does it have, what the full orbit really is. And that’ll start to tell us things about how it got there.”
Astronomers have been puzzled by the finding that other star systems commonly contain planets with masses between that of Earth and Neptune, called super-Earths. And yet our solar system has no such planet, making the properties of this whole class of abundant planets relatively unknown to us. Finding Planet Nine, which fits smack dab into that range, would change all that. “It’s our chance, in a sense, to study what many of these planets that are around other stars might be like, a little bit up close and in person,” Brown says.
And the scientific community will want a closer look. “There will be quite a big incentive to actually go there. It’ll take a while, it’s way out there,” some 400 to 800 AU away, he says. (One AU equals the average Earth-Sun distance of 93 million miles [150 million km].) “But as soon as we nail down its position, I think the excitement to go send at least a quick probe out there to take some close-up pictures is going to be too hard to resist.”
If Planet Nine is finally discovered, as one of those who first postulated its existence, does Brown have any suggestions for a possible formal name for this new member of the solar system?
“No,” he declares. “As an observational astronomer who has to stay up all night and worry about clouds, I’m a very superstitious person. I think most astronomers are. And one of my strongly held superstitions is that if you do propose a name, you’ll never find it. So we are just waiting and seeing.”
