Planet 9: Old data could uncover the Solar System's newest world, 38 years late

Michael Rowan-Robinson has never shied away from change.

In 2006, Rowan-Robinson, professor of astrophysics at Imperial College London, was part of the debate over whether or not Pluto should still be considered a planet during the meeting of the International Astronomical Union (IAU).

“My intervention in the debate was to say, ‘For God’s sake, make a decision,’” Rowan-Robinson tells Inverse.

Today, he’s ready to upend the Solar System yet again with another major change: a fabled ninth planet.

Rowan-Robinson recently went digging into 38-year-old data and claims he has found the hypothetical Planet Nine. Although the recent claim is not a clear detection, it helps narrow down the area of the sky in which to look for this distant and mysterious planet.

Rowan-Robinson details his findings in a paper on the preprint server arXiv that has been accepted for publication in the Monthly Notices of the Royal Astronomical Society.

What is Planet Nine?

There’s a controversy in planetary science — some researchers propose that the Solar System has a ninth planet with a distant orbit around the Sun.

The talk of Planet Nine first emerged in January 2015 when a duo of astronomers from the California Institute for Technology (Caltech) suggested that a Neptune-sized planet orbits our Sun in a highly elongated orbit that lies far beyond Pluto.

They based their claim on mathematical modeling and computer simulations and not observation. The duo suggests that a ninth planet is responsible for the gravitational effect on a group of icy objects that lie beyond the orbit of Neptune in the Kuiper Belt. Those small objects have elongated orbits with similar characteristics, as if caused by one massive object.

The planet, so the hypothesis goes, has a mass about 10 times that of Earth and an orbit about 20 times farther from the Sun than Neptune, the eighth and farthest known planet from the Sun. Planet Nine may take between 10,000 and 20,000 Earth years to complete one orbit around the Sun, according to NASA.

“It is a very unusual object,” Rowan-Robinson says. “So it would probably have to be something that was perhaps captured from outside the Solar System.”

WHAT’S NEW — Although he refuses to speculate too much about the origin of Planet Nine, Rowan-Robinson wants to find it.

The astronomer was part of the Infrared Astronomical Satellite (IRAS) team in 1983, the first space telescope to survey the entire night sky at infrared wavelengths.

At the time, there was some talk of a possible tenth planet — Pluto was still a planet back then — but the idea hadn’t really developed. When chatter of a Planet Nine resurfaced again, Rowan-Robinson thought it might be useful to look through the IRAS data again in search of this planet.

“Once this planet stuff started to be talked about, I thought perhaps it was worth looking again at the IRAS data,” Rowan-Robinson says. “So I decided to hunt through these catalogs looking for all sources that were not identified with galaxies or other objects such as stars.”

Rowan-Robinson checked to see if these objects satisfied the pattern that would be expected of a moving object. Objects in our Solar System move in reference to background stars.

“I spent thousands of hours searching through these objects, and gradually I eliminated almost all the possibilities,” Rowan-Robinson says.

Out of around 250,000 objects detected by the satellite, three observations were of interest as a potential candidate for Planet Nine. The same object appeared in June, July, and September 1983, moving across the sky.

“Basically, things that were detected only once went into a reject file because they weren’t confirmed with anything,” Rowan-Robinson says. “Again, there were hundreds of possibilities, almost all of which were eliminated, and finally, I was left with one case which I couldn’t quite eliminate.”

He notes that the object was located in an “awkward place” close to the galactic plane of the Milky Way, so it may be the result of “noise” from filamentary clouds that glow in infrared wavelengths.

WHY IT MATTERS — Although its existence has not been proven yet, direct detection of a ninth planet would be the first discovery of a new planet orbiting the Solar System in two centuries.

But so far, its detection has proven a little tricky.

After the release of the recent paper, Caltech astronomer Mike Brown, who was behind the 2015 Planet Nine model, took to Twitter to suggest that this may be a different planet than the one they had initially proposed.

“The candidate is on an orbit utterly inconsistent with our predictions for Planet Nine, and would not be capable of gravitationally perturbing the distant solar system in the ways that we have suggested. But, of course, that doesn’t mean it isn’t real!” Brown writes on Twitter.

Brown goes on to say that this could be a separate discovery on the hunt for the actual Planet Nine, but still gives it a courteous thumbs up.

“If someone discovers a planet beyond Neptune inconsistent with our predictions WE DID NOT PREDICT IT AND IT IS AN UTTERLY UNRELATED (and quite awesome) DISCOVERY,” he writes.

WHAT’S NEXT — When asked if he plans on any follow-up observations for Planet Nine, Rowan-Robinson proclaims that he is nearing the end of his career with no research students nor booked times at telescopes.

“Obviously, there’s been a certain amount of interest, so I’m hoping that people are hunting through other databases which may have observed this region,” he says.

Rowan-Robinson looks back at the major Pluto moment in 2006 when the IAU set new criteria to define a full-sized planet and how it changed the Solar System as we know it. If astronomers had altered the criteria in Pluto’s favor, the Solar System would have had another 20 planets or so.

“It might have been more fun if we had 20 or 30 planets,” Rowan-Robinson says. “If this thing exists — I certainly wouldn’t put it at more than a 50 percent [chance] — it would be a real planet.”

Abstract: I have carried out a search for Planet 9 in the IRAS data. At the distance range proposed for Planet 9, the signature would be a 60 micron unidentified IRAS point source with an associated nearby source from the IRAS Reject File of sources which received only a single hours-confirmed (HCON) detection. The confirmed source should be detected on the first two HCON passes, but not on the third, while the single HCON should be detected only on the third HCON. I have examined the unidentified sources in three IRAS 60micron catalogues: some can be identified with 2MASS galaxies, Galactic sources or as cirrus. The remaining unidentified sources have been examined with the IRSA Scanpi tool to check for the signature missing HCONs, and for association with IRAS Reject File single HCONs. No matches of interest survive.For a lower mass planet (< 5 earth masses) in the distance range 200-400 AU, we expect a pair or triplet of single HCONs with separations 2-35 arcmin. Several hundred candidate associations are found and have been examined with Scanpi. A single candidate for Planet 9 survives which satisfies the requirements for detected and non-detected HCON passes. A fitted orbit suggest a distance of 225+-15 AU and a mass of 3-5 earth masses. Dynamical simulations are needed to explore whether the candidate is consistent with existing planet ephemerides. If so, a search in an annulus of radius 2.5-4 deg centred on the 1983 position at visible and near infrared wavelengths would be worthwhile.