Planet Nine may be a ‘rogue planet’ captured by the Sun

Planet Nine, the hypothetical world believed to exist far beyond Pluto’s orbit, could be a “rogue planet” that was captured by our solar system at some point in the distant past, according to new research presented last week at the 229th meeting of the American Astronomical Society.

James Vesper, an undergraduate student at New Mexico State University (NMSU) and the lead author of the new study, said that computer simulations found it is “very plausible” the object was pulled into our solar system by the sun while it was drifting freely in outer space.

According to Inverse and, Vesper and his mentor, NMSU math and physical science professor Paul Mason, modeled 156 different interactions between our rogue planets of different sizes and our solar system. They found that in nearly 60% of the encounters, the incoming rogue planet would simply be ejected right back out of the solar system.

However, in 40% of the cases, the rogue planet would end up being captured by the solar system in one of two ways: it would either become part of the solar system without incident (which was referred to by Vesper as a “soft capture”) or it would eject at least one of the native planets in its new home to be ejected out into space (which the researchers dubbed a “kick and stay”).

The “kick and stay” scenario occurred in about 10% of the cases, Vesper told, and it would depend upon the characteristics of the rogue planet being captured. However, he also said that the research suggests that the solar system has likely never encountered a rogue planet more massive than Neptune, as that would have caused chaos in the inner solar system.

Orbit consistent with that of a captured rogue world

Originally described by astronomers Chad Trujillo and Scott S. Sheppard in a 2014 letter to the journal Nature, Planet Nine is currently believed to be as much as 10 times as massive as Earth, with a diameter between two and four times that of our home planet and a highly elliptical orbit with an orbital period of approximately 15,000 years.

Trujillo and Sheppard explained that the gravitational influence of an undiscovered object in the outer solar system could explain peculiarities in the orbits of several trans-Neptunian objects like the dwarf planet Sedna. Last January, Caltech astronomers Konstantin Batygin and Mike Brown discovered additional evidence supporting the existence of such a planet, although to date, Planet Nine has never been directly observed.

Batygin and Brown proposed that the object’s orbit takes it up to 1,000 astronomical units (AU) from the sun, said. Since the Earth is 1 AU from the sun, and Neptune is about 30 AU from the sun, this hypothetical world would be on the far outer edge of the solar system – but, as Vesper explained, its orbit would be consistent with that of a captured rogue planet.

Last summer, a paper published in the UK journal Monthly Notices of the Royal Astronomical Society determined that Planet Nine may have originally been an exoplanet that was pulled into our solar system by the sun. According to CNN, the authors behind that study concluded that the exoplanet had been stolen from another star approximately 4.5 billion years ago.


Image credit: Caltech/R. Hurt (IPAC)