spaceSpace and Physics

Where Is Planet Nine?


Jonathan O'Callaghan

Senior Staff Writer

66 Where Is Planet Nine?
Astronomers proposed the existence of the planet in January 2016. Caltech/R. Hurt (IPAC)

We don’t yet know for certain where the hypothesized ninth planet of the Solar System is, if it even exists. But one team of researchers has narrowed down the area that it might be lurking.

Planet Nine was first proposed to exist by astronomers Mike Brown and Konstantin Batygin last month. By modeling the motion of objects in the Kuiper Belt, an area of icy bodies outside Neptune's orbit, they suggested there was a Neptune-sized planet 10 times the mass of Earth orbiting the Sun at a distance of more than 200 AU (astronomical unit, 1 AU is the Earth-Sun distance).


We don’t have any direct evidence for the planet yet, but in an effort to help astronomers locate it, a team of French scientists has refined where it could be. Using radio data from the Cassini spacecraft orbiting Saturn, they looked for the gravitational effects of Planet Nine on the gas giant. Not finding anything, they ruled out certain areas where it could reside.

Their results are published in the journal Astronomy and Astrophysics.

The gravitational effects of Planet Nine are only thought to be detectable for half of its highly elliptical orbit, which can take up to 20,000 years. So if the planet is at the point in its orbit furthest from us, we probably won’t be able to detect its gravitational effect on the outer planets – although we could spot it via other methods, perhaps direct images or its effect on other bodies.

But if the planet is in the nearer half of its orbit, the French scientists say they can exclude 50 percent of the predicted orbital path based on the lack of perturbations on Saturn. "We have cut the work in half," co-author Jacques Laskar from the Paris Observatory told Agence France-Presse (AFP). In fact, they have noted a zone where the planet can most likely be found, seen below.


The red zone is excluded by the study. The pink zone could be studied if Cassini is extended to 2020, and the green zone is the most probable zone for Planet Nine. Fienga et al.

The team said that if the Cassini mission was extended to 2020 – currently it is due to end in 2017 with a dive into Saturn’s atmosphere – then they could further narrow the search field. That is unlikely to happen, but the team notes that the upcoming Juno mission to Jupiter – due to arrive on July 4 this year – could partially help, although the effects of Planet Nine on Jupiter are thought to be less than at Saturn.

Nonetheless, these results will hopefully help astronomers refine where Planet Nine could be. Its hypothesized existence made headlines around the world, so just imagine the excitement if (or perhaps when) a direct detection is made.


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