We are discussing the origins of Planet Nine. In part 1 we explored whether it could have been produced during an orbital instability in the early Solar System. In part 2 we showed that Planet Nine could have been captured from another star. Here we will explore another plausible idea that is a little less exotic.
Scenario 3. Planet Nine is a leftover from planet formation (in the Solar System) that was kicked out as Uranus and Neptune grew big
Let’s discuss Planet Nine’s most probable origins story. This is the idea discussed by Batygin and Brown in their paper.
Let’s rewind the clock to when the Solar System’s planets were forming in the protoplanetary disk. Gas giants Jupiter and Saturn formed quickly. Ice giants Uranus and Neptune grew from a population of large ice-rock bodies that were blocked by Jupiter and Saturn. Uranus and Neptune each underwent at least one humongous collision with another large body; we know this because their spin axes are tilted with respect to their orbital planets (they have significant obliquities, especially Uranus).
The growth of the ice giants is not perfectly efficient. About half of the icy building blocks don’t end up in a stable planet but instead get kicked out, usually by Saturn or Jupiter. Planet Nine could be a large leftover that was kicked out. After being kicked onto a wide enough orbit, external gravitational kicks from the Sun’s birth cluster could shift Planet Nine onto its current orbit, like we saw above.
It is a simple and appealing scenario. It’s entirely consistent with our current vision of how the ice giants formed. Sure, there are a few details to be worked out but there are no obvious show stoppers.
This idea does not have any problem with the Oort cloud. Planet Nine must have reached its current orbit early-on, while the Sun was still in its birth cluster. The Nice model instability would then have happened later and produced the Oort cloud. Two separate events at different times and in different environments.
[Paragraph of wild speculation.] If this is how Planet Nine formed, then it fits into a much larger story that connects with exoplanets, planets orbiting other stars. The most abundant class of exoplanets are so-called “super-Earths”, planets somewhat larger than Earth on orbits smaller than Mercury. Super-Earths exist around roughly half of all Sun-like stars. One idea for the origin of super-Earths is that they formed far from their stars and migrated inward. In that model, the Solar System’s lack of super-Earths is due to Jupiter, which formed quickly and became a migration barrier. The building blocks of Uranus and Neptune – including Planet Nine – would have become super-Earths if their migration was not blocked by Jupiter. So, in this (admittedly speculative) story, Planet Nine could represent one of our Solar System’s failed super-Earths! [End of wild speculation]
The punchline: perhaps the simplest origins story for Planet Nine is that it was a building block of the ice giants that was scattered out while the Sun was still in its birth cluster.
A quick summary of the three formation scenarios
- Planet Nine did not form during the Nice model instability.
- Planet Nine could have formed around another star and been captured by the Sun while still in its birth cluster, probably by an encounter with a star hosting a planet on a wide orbit.
- Planet Nine could be a remnant building block of the ice giants scattered outward during the cluster phase.
How can we differentiate between formation scenarios for Planet Nine? Planet Nine’s mass might hold a clue. During the ice giants’ growth it is generally the smaller planets that are scattered out. If Planet Nine formed by this process its mass is likely close to 5 times Earth’s. Building blocks with that mass are the “sweet spot” for forming Uranus and Neptune, so it seems reasonable for Planet Nine to have that mass. However, if Planet Nine was captured from another star its mass could be higher or lower. If Planet Nine is discovered and its mass eventually measured, a mass near 5 Earth masses (say, between 3 and 10 Earths) would strongly suggest a Solar System origin. However, if its mass is much higher – in particular if it is more massive than the ice giants – then it is likely to be a captured extrasolar planet.