Artist’s concept of Planet 9. (Image: Caltech/R. Hurt)

Saturn’s Cassini probe is nearing the end of its mission, but that doesn’t mean it’s no longer useful. In fact, astronomers have found a totally new purpose for the plucky little space probe and its vast trove of data: searching for the elusive Planet 9.

Last month, Caltech astronomers Mike Brown—the guy who killed Pluto and is proud of it—and Konstantin Batygin captured the world’s attention when they announced they’d found evidence for a ninth planet, ten times the mass of Earth and lurking somewhere in the far reaches of our Solar System. They didn’t see a giant planet, they inferred its presence by looking at the orbits of several smaller Kuiper Belt Objects. Now, the race is on to find the beast and bring it into the planetary fold.

And help is coming from all corners of the astronomical community, including missions designed to study other large planets. As New Scientist reported this week, scientists are now trying to figure out what sorts of clues Planet 9 might leave behind as it completes its 15,000 Earth-year journey around the Sun. If we had a distinct fingerprint in mind, that could greatly narrow down the fields of view our telescopes have to scour.

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Now, several scientific papers are arguing that we do have a fingerprint. Planet 9 may be a frigid wasteland, but it should still be emitting a tiny amount of energy in millimeter radio wavelengths. It just so happens that Cassini has spent over a decade collecting radio ranging data across the sky as it sails in and out of Saturn’s rings.

Using that data, a team of astronomers has now modeled the motion of all of our Solar System’s heavyweights. And they’ve found they can rule out about half of Planet 9's potential orbits, as the presence of another large planet would have shown up in Cassini’s data. With several more years of radio ranging, astronomers might be able to narrow the search even further. (Cassini is currently slated to plunge suicidally into Saturn’s atmosphere in September, 2017.)

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We’re still a long ways off from finding Planet 9—if it even exists. But efforts like this highlight the creativity of the astronomical community, reminding us that if anyone has a shot at finding a cold, dark world at the ass-end of the Solar System, it’s these brilliant space nerds.

[Astronomy & Astrophysics via New Scientist]