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Was Planet Nine stolen from another star?

Last Updated on Wednesday, 01 June 2016 11:13
Published on Wednesday, 01 June 2016 10:24

Through a computer simulation, astronomers at Lund University in Sweden show that the so-called Planet Nine could be a former exoplanet. If true, this would make it the first object of its kind to be discovered inside our own Solar system. The researchers, who argue that our Sun, in its youth some 4.5 billion years ago, stole Planet Nine from its original star, publish their results in a paper in Monthly Notices of the Royal Astronomical Society.

Planet nine artistic plain smallArtist's impression of Planet Nine as an ice giant eclipsing the central Milky Way, with a star-like Sun in the distance. Neptune's orbit is shown as a small ellipse around the Sun. Credit: ESO/Tomruen/nagualdesign. Click for a full size image"It is almost ironic that while astronomers often find exoplanets hundreds of light years away in other solar systems, there's probably one hiding in our own backyard," says researcher Dr Alexander Mustill, lead author of the new paper.

Stars are born in clusters and often pass by one another. It is during these encounters that a star can "steal" one or more planets in orbit around another star. According to the computer model used in the new work, this is probably what happened when the Sun captured Planet Nine.

"Planet Nine may very well have been 'shoved' by other planets, and when it ended up in an orbit that was too wide around its own star, our Sun may have taken the opportunity to steal and capture it. When the Sun later departed from the stellar cluster in which it was born, Planet Nine was stuck in an orbit around it," says Mustill.

"There is still no image of Planet Nine, not even a point of light. We don't know if it is made up of rock, ice, or gas. All we know is that its mass is probably ten times the mass of Earth."

The existence of Planet Nine has yet to be confirmed, as does the hypothesis that it is the first exoplanet to join our Solar system. But if the theory is correct, Mustill believes that the study of space and our understanding of the Sun and the Earth could take a giant leap forward.

"This is the only exoplanet that we, realistically, would be able to reach using a space probe," he says.


Science contact

Dr Alexander Mustill
Department of Astronomy and Theoretical Physics
Lund University
Phone: +46 46 222 16 12 / +46 72 947 55 85
This email address is being protected from spambots. You need JavaScript enabled to view it.


Video presentation

Alexander Mustill talking through the new result:


Further information

The new work appears in "Is there an exoplanet in the Solar system?", Alexander J. Mustill, Sean N. Raymond and Melvyn B. Davies, Monthly Notices of the Royal Astronomical Society: Letters, in press.


Notes for editors

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