Planet Nine or No Planet Nine?

I saw an very untrue science headline this week that sent me into a tizzy: “Astronomers make groundbreaking discovery of planet hiding in our solar system with 'promising' evidence of life”. To quote Luke Skywalker, “Amazing. Every word of what you just said is wrong.”

The headline is referencing a recent study about the ongoing search for Planet Nine, a possible ninth planet-sized body orbiting the Sun in the distant reaches of the solar system. The study is interesting but also says absolutely none of what that headline says. I was going to try to ignore it, but something about it (maybe the fact that I really do hope we find Planet Nine someday), irritated me enough to generate this blog post.

So what is Planet Nine and what did this study actually see (and why was this headline so wrong)? Let’s head to the outer solar system and find out!

Number 9?

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Back in 2016 a study came out that made a rather remarkable claim. It looked at the orbits of six Kuiper Belt Objects, members of the realm past Neptune where icy bodies like Pluto hang out, and said these orbits look like they’ve been gravitationally interfered with. Essentially these orbits are all highly elliptical and “pointing” in more or less the same direction and have similar tilts. 

The conclusion drawn by the astronomers behind the study was that these six orbits are displaying evidence of the gravitational effects of a large planetary body out in the distant Kuiper Belt. It was predicted this thing would be ten times the mass of the Earth (so unquestionably planet-sized) and orbiting so far from the Sun that it would take somewhere between 10,000-20,000 years to complete a single orbit. These numbers would later be refined to six times Earth’s mass in a roughly 7,400 year orbit.

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(Fun fact: the astronomers behind this study are Konstantin Batygin and Mike Brown. Brown is famous—or notorious, depending on who you ask—for his role in stirring up the discussion about the definition of a planet that led to Pluto being reclassified. He is the one who led the team that discovered Eris, which is about the same size as Pluto and forced the debate to a head. Brown happily leans into his reputation as “the Pluto Killer”).

Dangling the possibility of being able to put “I discovered a planet” on your resume is like catnip to solar system astronomers, and telescopes revved into motion to start the hunt for this potential planet. Of course, this was complicated by the fact it would be really, really faint. And also by the fact that, you know, it might not be there at all. 

Hold Your Horses

Pretty soon after Batygin and Brown made their claim, other studies came out suggesting they may have jumped the gun with their conclusions. Some detractors theory came for the data itself, claiming that the observations had been biased by the location of the telescope, causing Batygin and Brown to see patterns that don’t actually exist.

Other teams went after the conclusions, rather than the data, suggesting that those Kuiper Belt Objects really do have that pattern to their orbits but that a wayward planet is not the only explanation. For instance, one study put forth the idea that the collective gravitational pull of all the objects in the Kuiper Belt on all the other objects could have led to the orbit pattern Batygin and Brown saw.

There’s also the fact that our models of solar system formation don’t easily explain how something so big could be so far from the Sun. That’s not a dealbreaker, because we know planets can be thrown around by gravity and such an object might even be a refugee from another solar system snagged by the Sun’s gravity, but it’s another potential check mark against the likelihood of Planet Nine existing.

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And then there’s the big issue: we haven’t found it. And we’ve been looking. When the study first came out in 2016 it seemed like, if it was there, we’d find it within a couple of years. It always seems like we must just be on the verge of finding it, or that the next observatory that comes online will be all we need to finish the search (in case you’re curious, at the moment the bet is that the soon-to-be-opened Vera Rubin Observatory is the tool that will track down this elusive object). 

Nearly ten years later there has been nary a whisper of Planet Nine in any of the hunts for it. Until…now? Maybe?

The News

What that headline that so irritated me was referring to is a study (which has not yet been peer reviewed) that claims to have found some specks that might be a potential planet candidate.

It looked at two infrared studies that were done 23 years apart, IRAS from 1983 and AKARI from 2006. The goal was to look for specks in IRAS that could represent a potential planet that had corresponding specks in AKARI but in a slightly different position, since in 23 years even an object as far out as Planet Nine would have moved a little.

And they found one! Well, actually they found 13 pairs of specks, but after analysis only one of them looked like it was actually the same object in both infrared surveys. So: mystery solved, right?

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Oh please, you already know it’s not, otherwise this would be a very different blog post. First of all, this is only two datapoints, and if you read my blog post on K2-18b and the five-sigma gold standard you know astronomers never accept something so flimsy. There’s too much chance it’s just noise. So issue #1 is that this Planet Nine candidate might just be noise in the data that happened to appear in the right spots to mimic something else.

Second, even if this detection is solid and this team did find something big out in the vasty darkness of the Kuiper Belt, according to what Mike Brown told Gizmodo it’s not Planet Nine—it would be too far out to fit Brown’s models for Planet Nine’s orbit.

Back to Where We Started

So to go back to the first part of the headline that got me so annoyed in the first place: “Astronomers make groundbreaking discovery of planet hiding in our solar system” is a big no. Astronomers haven’t discovered anything. Astronomers have two specks that might, maybe represent a large object in the outer solar system, but which would require a whole lot of additional observations to confirm (the team behind this discovery did try to look for their mystery object in the WISE dataset, another infrared survey, but didn’t see it).
Even the more reserved headlines spouting this as “the most promising Planet Nine candidate” may be incorrect, if Mike Brown is right and these specks are in the wrong spot to be his Planet Nine. 

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As for the second bit of that headline, “with 'promising' evidence of life”, I think the person who wrote that story just made that up wholesale. Again, we don’t even have a planet here, we have a speck that might, maybe, someday prove to be a planet, a planet that could prove to be a small gas giant orbiting so far from the Sun that it would look more like a particularly bright star than, you know, a sun. That is literally nobody’s idea of a “promising” place to look for life, and we most assuredly have no evidence to here to offer.

And this is why I’m really all riled up over a single headline: space is cool enough to not need exaggerated headlines to be awesome. Let’s look at what we do have here: a potential suggestion of the presence of an unknown massive planet in our solar system.

At this point this detection could very well turn out to be nothing more than noise—but what if it’s not? Then we have a new planet! It might turn out to not be Planet Nine—but isn’t that potentially even cooler? Then we have a completely unexpected new world to geek out over! Our planetary formation models may have trouble accounting for such an object—but that opens up the possibility of things like the Sun rescuing orphaned rogue planets that originated in other solar systems. Imagine if we had a planet in our solar system that formed in a different solar system!

If that’s not cool enough for you on its own, then I don’t know what to tell ya.