Planet 9 a captured alien, astroboffins suggest

Exoplanet theory for mysterious trans-Neptunian object

Planet 9: Alien or chimera? Image by Caltech / R. Hurt

Astroboffins have boldly suggested that the hypothetical trans-Neptunian Planet 9 is in fact an exoplanet captured by our young Sun from another star.

That's according to astronomers from Sweden's Lund University and the Laboratoire d'Astrophysique de Bordeaux, who questioned the plausibility of a planet some ten times the mass of Earth forming at such a great distance from the solar system's centre.

They then ran a computer simulation demonstrating that as the Sun and its planetary system developed as part of a relatively compact cluster of stars, exoplanet kidnap was certainly possible.

Lund Uni's Alexander Mustill said: "Planet 9 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 Planet 9 from its original star. When the sun later departed from the stellar cluster in which it was born, Planet 9 was stuck in an orbit around the sun."

All well and good, but the existence of Planet 9 has yet to be proved. In January, Konstantin Batygin and Mike Brown of CalTech claimed to have identified the body's orbit from its influence on other Kuiper Belt objects (KBOs).

In March, the case for Planet 9 grew stronger when SETI Institute astronomer Michelle Bannister pinpointed an "as-yet-uncatalogued Kuiper Belt object with an alignment Batygin and Brown say fits with their predictions".

Planet 9's 10,000 to 20,000 year orbit is highly eccentric, at 90 degrees to the ecliptic. This led Batygin and Brown to propose that while the planet formed from the disk of matter surrounding the Sun, it possibly "got knocked out of alignment, possibly by a major object like Jupiter, and sent on a new orbital trajectory".

Alternatively, the body's trajectory might now be explained by the exoplanet theory. Mustill said: "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."

Mustill and his colleagues' findings are published in the Monthly Notices of the Royal Astronomical Society. ®

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