If Planet 9 is out There, Here’s Where to Look

#Planet9 #space #astronomy #pluto #solarsystem #Pluto #neptune

There are eight known planets in the solar system (ever since Pluto was booted from the club), but for a while, there has been some evidence that there might be one more. A hypothetical Planet 9 lurking on the outer edge of our solar system. So far this world has eluded discovery, but a new study has pinned down where it should be.

Listen to this article

The evidence for Planet 9 comes from its gravitational pull on other bodies. If the planet exists, its gravity will affect the orbits of other planets. So if something seems to be tugging on a planet, just do a bit of math to find the source. This is how Neptune was discovered, when John Couch Adams and Urbain Le Verrier noticed independently that Uranus seemed to be tugged by an unseen planet.

In the case of Planet 9, we don’t have any gravitational effect on a planet. What we do see is an odd clustering of small icy bodies in the outer solar system known as Kuiper belt objects (KBOs). If there were no planet beyond the Kuiper belt, you would expect the orbits of KBOs to be randomly oriented within the orbital plane of the solar system. But instead, we see lots of KBO orbits are clustered in the same orientation. It’s possible that this is just due to random chance, but that isn’t likely.

The possible orbit of Planet Nine. Credit: CalTech/R. Hurt (IPAC)

Back in 2016, the authors looked at the statistical distribution of KBOs and concluded the clustering was caused by an undetected outer planet. Based on their calculations, this world has a mass of 5 Earths and is about 10 times more distant from the Sun than Neptune. The paper even calculated a broad region of the sky where the planet might be. But searches turned up nothing. This led some to conclude the planet doesn’t exist. Orbital oddness doesn’t prove a planet exists. Just ask Planet Vulcan. Others went so far as to argue Planet 9 does exist, but we can’t see it because it’s a primordial black hole.

This new study reexamines the original work in light of some of the criticism it received. One big criticism is that outer solar system bodies are difficult to find, so we look for them where it’s convenient. The clustering effect we see could just be due to biased data. Taking observational bias into effect, the authors find the clustering is still statistically unusual. There’s only a 0.4% chance of it being a fluke. When they recalculated the likely orbit of Planet 9, they were able to better localize where to look.

One interesting aspect of the study is that the newly calculated orbit puts Planet 9 closer to the Sun than originally thought. This is odd, because if it is closer then we should have already found it. The authors argue that observations thus far have ruled out the closest options for Planet 9, which helps narrow down its possible location even further. If the planet exists, it should be detectable by the Vera Rubin Observatory in the near future.

This study isn’t conclusive, and many astronomers still argue that Planet 9 doesn’t exist. But this study makes it clear that we won’t have to argue about it for much longer. Either it will be discovered soon, or observations will rule it out as an explanation for the KBO clustering effect.

Mysterious ‘Planet Nine’ Is Probably 5 to 10 Times the Size of Earth

There could be a planet hiding out on the distant frontiers of our solar system. And astronomers have published new details about what it probably looks like, if it really exists.

Planet 9, according to a new paper published online Feb. 10 in the journal Physics Reports, is probably five to 10 times the mass of Earth. And it probably travels along an elongated orbit that peaks at 400 times Earth’s distance from the sun. That orbit is also likely 15 to 25 degrees off the main orbital plain of our solar system where most planets orbit.

The existence of Planet Nine, as Live Science sister site Space.com previously reported, is an idea that’s become popular among astronomers ever since it was first seriously proposed back in 2014. Researchers suspect the planet’s existence because of patterns of objects in the Kuiper Belt, a ring of debris in the outer solar system. Those objects tend to clump together in ways that suggest that gravity from something big out there is tugging on them.

And the evidence has only gotten stronger. In a separate paper, published Jan. 22 in The Astronomical Journal, some of the same authors of the Physics Report paper calculated the probability of Planet Nine not existing at just 1 in 500. [Amazing Astronomy: Victorian-Era Illustrations of the Heavens]

Strongly suspecting that the dark planet exists isn’t the same thing as knowing it’s real, though. The good news is that this new research suggests that Planet Nine is significantly nearer-by than previously thought. But astronomers still have a lot of space in which to search for it.

The authors of the Physics Reports paper did raise, however, the possibility that there’s no planet out there at all. They added that however strong the current evidence is, that chance should be “taken seriously.”

The likeliest alternative explanation is that humanity’s picture of the Kuiper Belt is incomplete and that the objects only appear to cluster because of some bias in efforts to detect them. It’s also possible, the authors suggested, that the clustering results from the “self-gravity” of the Kuiper Belt acting on its own objects and does not arise from not some hidden planet’s tug.

Still, astronomers have become more convinced by the evidence for Planet Nine in recent years. And now they’re making significant progress toward pinpointing it out in space.