spaceSpace and Physics

Where Is Planet Nine?


Dr. Alfredo Carpineti

Senior Staff Writer & Space Correspondent

clockApr 25 2016, 21:21 UTC
1213 Where Is Planet Nine?
Planet Nine (in orange) could explain the orbit of other objects in the Kuiper Belt (purple/turquoise). Caltech/R. Hurt (IPAC)

How close are we to finding Planet Nine? A new paper seems to suggest that the solution to the mystery is actually much closer than we previously thought.

Matthew Holman and Matthew Payne from the Harvard-Smithsonian Center for Astrophysics have used observations by the Cassini spacecraft to reduce the potential area in the sky where Planet Nine might be hiding. The region is found in the southern sky, roughly in the direction of the constellation of Cetus. The likely area extends over 20 degrees in all directions; by comparison, the full moon measures only half a degree across.


Planet Nine is a hypothetical planet proposed in January by Mike Brown and Konstantin Batygin to explain why objects beyond the orbit of Neptune have their closest point to the Sun in the same location.

In February, French researchers announced that by using data from the NASA/ESA Cassini spacecraft orbiting Saturn, they were able to narrow down the area where Planet Nine might be hiding. They used the perturbations, or lack thereof, in Saturn's orbit to establish where the planet might be. Saturn would only be perturbed if Planet Nine was at its closest approach, so not seeing any perturbations tells us that Planet Nine is not very close to the Sun right now.

In the latest study, available online, the two researchers used a sophisticated statistical technique called Markov Chain Monte Carlo to reduce the potential hiding place of Planet Nine even further. While the data from Cassini doesn’t show any perturbations that cannot be explained with current models, those studies dismiss the perturbations as noise. Holman and Payne, therefore, decided to look for potential Planet Nine models that would not only fit the values, but also bolster support for the perturbations as a real effect.

“We put Planet Nine at a whole different slew of locations – all different possibilities on the sky, different distances, different masses – and tried to find out whether that constrains things even more,” said Payne to New Scientist.


According to the model, Planet Nine could be located in two narrow strips of the sky. The team then overlapped these regions with Batygin and Brown’s suggested orbit and got an even smaller area.

Astronomers are already looking at the suggested region and will hopefully soon find out whether there are nine planets in the Solar System. 

[H/T: New Scientist]

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