Did the Sun Steal Planet Nine?

One of the biggest new mysteries in our Solar System is the purported presence of a large and distant “Planet Nine,” traveling around the Sun in a twenty-thousand-year orbit far beyond Pluto. Although this far-flung world’s existence has yet to actually be confirmed (or even directly detected) some scientists are suggesting it might have originally been an exoplanet around a neighboring star, pilfered by our Sun during its impudent adolescence.

In January 2016 the remorseless “planet killer” Mike Brown — a Caltech professor and astronomer whose discovery of Eris in 2005 prompted the IAU’s reclassification of planets, thereby knocking Pluto from the official list — announced evidence for the existence of a “real” ninth planet orbiting the Sun four times farther than Pluto…and possibly even farther out than the Kuiper Belt is thought to extend. According to Brown and co-researcher Konstantin Batygin their Planet Nine may be almost as massive as Neptune, but they’re still on the hunt for it within the regions where they think it should be.

Formed five billion years ago in a cluster of other stars, our Sun once had hundreds if not thousands of stellar siblings (now long since dispersed through the nearby galaxy.) As the stars developed many likely had planets form around them, just as the Sun did, and with all the young star systems in such relatively close proximity it’s possible that some planets wound up ejected from their host star to be picked up — or possibly even outright stolen — by another.

Brown and Batygin’s Planet Nine could be one of these hypothesized adopted worlds. A team of researchers, led by Alexander Mustill at the Lund Observatory in Sweden, recently investigated the probability of this scenario, described in an April 4, 2016 article on New Scientist.

What the researchers found based on their models — which took into consideration the orbits of known KBOs and trans-Neptunian objects (TNOs) but not the effects of known planets — was that the Sun could very easily capture nearby exoplanets as well as clusters of smaller bodies (like “mini Oort clouds”) given that the objects are far enough from their host star and the relative velocities during the “pick-up” are low.

While the researchers admit that the chances of a heist having actually taken place are quite small — anywhere from 0.1 to 2% — they’re not zero, and so should be considered a reasonable possibility.

“While the existence of Planet 9 remains unproven, we consider capture from one of the Sun’s young brethren a plausible route to explain such an object’s orbit.”
– A. Mustill et al., Is There An Exoplanet In The Solar System? (Source)

It’s previously been suggested that the Sun could have captured worlds from other stars in passing, such as comets and even the approximately 1,800-kilometer-wide KBO Sedna.

There’s also the possibility, note Mustill et al., that a world like Planet Nine could have ended up a member of our Solar System after being forcibly ejected from its own where it formed close to its star but within an orbit that wasn’t stable — especially considering the complexities of multi-body systems.

Of course Planet Nine, if it exists at all and if so, whatever it happens to get named, may also have formed from the same planetary disk as the other planets. But even if that’s the case there will be many questions about its evolution and current location that will then need to be answered.

Sources: New Scientist and arXiv.org

Added 5/30/16: Watch Alexander Mustill discuss his research in a video from Lund University: