Where did Planet Nine come from? Caltech/R. Hurt (IPAC)

Planet Nine may not be from outer space after all. The mysterious giant planet that astronomers think may orbit our sun far beyond Pluto may have been born with the rest of the solar system’s planets instead of stolen from another star or scooped up from interstellar space.

We still haven’t seen Planet Nine directly to confirm its existence, and if it really is lurking at the edge of the solar system we aren’t sure how it got there. From the weird motion of a few distant objects in the Kuiper Belt, researchers have estimated that if there’s a planet out there, it should have a mass around 10 times the mass of Earth.

It probably didn’t form where it is now, since it is nearly impossible for a planet that size to be born so far away from its star, as the amount of material available to build a planet decreases the further out you get.


That leaves two possible options: either the planet was formed closer to the sun and migrated out, or it was stolen from another star.

To find out how likely it is that Planet Nine was captured by the sun early in its lifetime, Richard Parker at the University of Sheffield in the UK and his colleagues turned to a computer simulation of a theoretical cluster of stars.

Dodging planets

Right now, there aren’t many other stars near our own, but most stars – probably including the sun – are born in crowded star-forming regions. Parker and his team simulated a region like this, but adjusted it to make it as easy as possible for any star to acquire a planet.

Even in regions that are just right for stars to grab onto free-floating planets – where there is one planet for every star and everything is moving at similar speeds – the researchers found that less than 6 per cent of the planets get captured.

In simulations of clusters adjusted slightly further to be more like the type our sun’s chemical makeup tells us it was probably born in, only three out of 10,000 stars were able to capture a planet on an orbit similar to what Planet Nine’s orbit seems to be.

“Even if the sun formed in a very hostile, very dynamic, very violent environment, in one of these densely populated clusters, it’s still unlikely that it captured Planet Nine,” says Parker.

Instead, it is possible that Planet Nine was pushed out from the central solar system when the gas giants reshuffled their orbits.

“It might have been formed closer in, perhaps around Uranus and Neptune, and then been kicked out but not totally ejected,” says Matthew Payne at Harvard University. “But in general, you’d expect them to be kicked out totally rather than just hanging around.”

Somewhere out there

But Kat Volk at the University of Arizona says that it’s tough to rule out any formation mechanism at this point, especially since we aren’t even sure that Planet Nine really exists.

“You want something with a high probability to be sure that it’s the answer, but three planets like Planet Nine out of 10,000 still means it’s possible,” she says. “It’s not a likely pathway, but that doesn’t mean it’s not the pathway our solar system took.”

Once astronomers figure out whether Planet Nine is real they can work out how it formed. “If it’s out there and it’s really on this strange orbit, we’re going to have to be creative to figure out how it got out there,” says Volk. “And what it tells us about our solar system depends on what formation mechanism we settle on.”

Journal reference: ArXiv, arxiv.org/abs/1709.00418v1

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