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Evidence for a massive planet in the outer solar system has been found, which may be the elusive Planet Nine that astronomers have long sought.

Planet Nine is a hypothesised world orbiting far beyond Neptune, in our solar system鈥檚 outer reaches. The gravitational clustering of some objects in the outer solar system suggests the presence of such a world, a super-Earth at least five times as massive as our planet, but no concrete evidence for it has yet been found.

at Imperial College London examined data from a now-defunct space telescope called the Infrared Astronomical Satellite (IRAS) to look for Planet Nine. Launched in 1983 and operating for nine months, the telescope surveyed the sky in infrared, discovering objects such as asteroids and comets.

Going back through the telescope鈥檚 data, Rowan-Robinson looked for evidence of a previously overlooked object orbiting at the proposed distance of Planet Nine, and one candidate stood out. IRAS detected evidence for an object three to five times the mass of Earth, orbiting about 225 times further from the sun than Earth does, roughly in Planet Nine鈥檚 expected location. 鈥淚t鈥檚 very tantalising,鈥� he says.

The limitations of the telescope, however, mean there is quite a bit of uncertainty about whether it is really a planet or not. In particular, the candidate is close to our galactic plane 鈥� that is, the thick disc of our galaxy that is full of stars, and some of these might appear deceptively planet-like in a small data set. 鈥淏ut in the data, it does behave quite like a moving object,鈥� says Rowan-Robinson, which would suggest it was a planet rather than a distant star.

at the California Institute of Technology, one of the scientists who proposed the existence of Planet Nine in 2016, says that while the finding was interesting, he couldn鈥檛 be sure the candidate wasn鈥檛 a false positive. 鈥淭his paper was great and I鈥檓 really glad he did this analysis,鈥� says Brown. But the uncertainties in the data resulting from the proximity to the galactic plane meant it was 鈥渉ard to pull out signals from all this dust鈥�.

Even if the candidate did turn out to be a planet, it doesn鈥檛 quite fit the expected parameters of Planet Nine. 鈥淚t鈥檚 a little too small, it鈥檚 a little too close and it鈥檚 quite a bit too inclined to the plane of the solar system,鈥� says Brown. 鈥淚t can鈥檛 cause those gravitational perturbations that we鈥檙e seeing. It鈥檚 not doing what we think is happening.鈥�

Nonetheless, at the University of Regina in Canada says it is worth having a look at the expected location of this candidate planet, to see if it is really there. 鈥淚t鈥檚 a specific prediction for a spot on the sky where there could be something very interesting,鈥� she says. 鈥淪omeone should go observe that spot for sure.鈥�

As to whether this candidate world and Planet Nine could both exist, Rowan-Robinson says that is unlikely. 鈥淚f this object is real and it is not [Planet Nine], then it is a really remarkable coincidence,鈥� he says.

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