In 2006, astronomy lovers mourned the demotion of
what had been the Solar System’s ninth planet: Pluto. A decade later, researchers hypothesized that
there still might be a ninth planet after all, lurking hundreds of times farther from the Sun than Earth, and detectable mainly by the way it warps the orbits of the objects around it.
Now, in a paper posted last week on the arXiv preprint server, a team of astronomers from Taiwan, Japan, and Australia claims it has
found hints of the so-called Planet Nine in archival images of the night sky. Some experts are skeptical that the signal, just a single pair of faint dots, will survive scrutiny and follow-up observations. But if it does, the object lies on an orbit far outside the original Planet Nine prediction—rendering it an entirely different planet.
This mismatch “doesn’t mean it’s not there, but it means it’s not Planet Nine,” says Mike Brown, an astronomer at the California Institute of Technology who, along with his colleague Konstantin Batygin, came up with the Planet Nine proposal nearly a decade ago. “I don’t think this planet would have any of the effects on the Solar System that we think we’re seeing.”
The existing evidence for Planet Nine comes from the most distant objects in the Kuiper belt, a region of space beyond the orbit of Neptune. The orbits of some Kuiper belt objects appear to be unusually clustered and aligned, despite being too far from the influence of Neptune’s gravity. By repeatedly simulating the evolution of the Solar System and comparing it with real observations, Batygin and Brown determined these objects’ unusual orbits could be due to the gravitational tugs of a faraway world roughly five to 10 times more massive than Earth that orbits the Sun once every 10,000 years.
“It is pretty amazing to think that something as big as Neptune could be sitting out there and no one would have ever noticed it,” says Gary Bernstein, an astronomer at the University of Pennsylvania. “But if you put it far enough away, it gets fainter and fainter very fast.”
Most Planet Nine searches have looked for the object’s reflection in visible light: sunlight that must first reach the object and then reflect off its surface back to Earth, getting dimmer the entire time. That’s why Terry Long Phan, an astronomy graduate student at National Tsing Hua University, and his Ph.D. adviser, astronomer Tomotsugu Goto, decided to search for Planet Nine’s intrinsic glow in the far infrared, a signal that travels directly to Earth and could potentially appear stronger.
Phan and Goto used sky surveys from two infrared space telescopes launched 23 years apart: the Infrared Astronomy Satellite (IRAS), a NASA-Netherlands-U.K. satellite launched in 1983; and AKARI, a Japanese satellite launched in 2006. Because of Planet Nine’s long orbit, the researchers hypothesized that the time gap between the two data sets would be enough to see the potential planet move incrementally across the night sky.
From an initial catalog of about 2 million objects within the IRAS and AKARI data sets, the researchers whittled down to pairs of dots of light whose spacing could be explained by a moving planet with a Planet Nine–like mass and distance. Then, they removed known sources such as stars, sources that didn’t move over time, and sources with too much noise, such as those near the bright galactic center. When 13 pairs remained, they checked each by eye. Only one candidate pair survived the scrutiny. The two dots had matching colors and brightnesses—a sign they were the same object.
“I felt very excited,” Phan recalls, not least because
a previous study using the same surveys found no candidates. “It’s motivated us a lot.” The new work has been accepted for publication in the
Publications of the Astronomical Society of Australia.
However, when Brown, who was not involved in the work, calculated the candidate signal’s orbit himself, he found that the planet would have an orbit tilted roughly 120° from the plane of the Solar System: a tilt so extreme that the planet would orbit the Sun in the opposite direction than the other planets. Brown’s models predict that to explain the clustering of distant Kuiper belt objects, Planet Nine needs an orbit tilted only about 15° to 20° from the plane of the Solar System.
If Phan and Goto’s signal really is a far-off planet, its existence would ironically disprove the original Planet Nine, as the two planets could not coexist without making each other’s orbits unstable, Brown adds. “It’s kind of fun that a paper that purports to find a candidate for Planet Nine is really finding something that would basically say that we were wrong the entire time.”
But that’s only if the faint infrared clue holds up after future observations. For one, Bernstein says, it’s difficult to confirm that only two tiny pinpricks of light at such far distances from Earth are anything other than noise—or unrelated impostor objects such as asteroids, stars, or galaxies.
Still others aren’t convinced that a planet, Planet Nine or otherwise, is needed to explain the orbits of objects past Neptune. University of Regina astronomer Samantha Lawler believes the apparent clustering in the Kuiper belt could be explained by biased observations
of what is actually a uniform distribution of objects. “It would be really cool if there was some kind of pattern there,” she says. “But I am not convinced, with current data, that you can’t just go with the simplest explanation.”
Later this year, though, more data will flood in as the Vera C. Rubin Observatory comes online. From its perch in Chile’s Atacama Desert, Rubin will scan the southern sky for 10 years in unprecedented detail—and Brown and Batygin think if Planet Nine is real, Rubin has a good chance of finding it. But whether the telescope spots Goto and Phan’s source or another planet instead, Batygin says he would still cheer on the discovery: “I would be the first person to say, ‘That is not Planet Nine—that is Planet 8.5.’”
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