The How and Why of Planet Nine

It hasn’t been found yet – let me make that clear.  But with evidence that it should exist, astronomers are looking more closely at the proposed planet nine and how it might have formed, and how it could have ended up in such a distant orbit.

This is an artist’s conception of Planet Nine. Credit: Caltech/R. Hurt (IPAC)

When you start to think about how a planet ten times the mass of Earth could have ended up more than ten times as far from the Sun as Neptune, a few scenarios pop into mind:

  1. It was formed in the inner solar system, where interactions with gas giants or another star pulled it out
  2. It formed in the outer solar system
  3. It was captured from another star or as an ancient interloper

Let’s work backwards.

It was captured from another star

The Sun formed in a cluster of stars (as most stars do), and then the cluster spread out over time.  Early in the Sun’s history, it is plausible that another star could have come close to the Sun and interacted gravitationally.  The problem is that most interactions between objects as massive as stars would give the planet an extreme orbit, sending it flying out of the solar system into interstellar space.  At best, the planet would have ended up with an orbit like a comet, coming very close to the inner solar system, and like a wrecking ball it would create chaos with the inner planets.

We also see a lot of stars with planets that are comparable in distance to theorized planet nine, and so perhaps there is a common mechanism for building larger worlds far from a star.

It formed in the Outer Solar System

Do we need to rethink how planets form in general? When exoplanets were first discovered, astronomers were shocked to see so many massive planets in very close orbits.  And today we know of many solar systems with planets far beyond Neptune.  There is research to suggest planets like Jupiter may have formed in the inner solar system and been booted out, but is it possible to form a gas giant so far from a star?

The problem is two-fold: How does a big planet form when there’s a huge amount of space and subsequently a lower density of dust and gas? And How is there enough material left over to form the gas giants and inner planets?

It formed in the Inner Solar System, and was pulled away

Gas giants have a lot of gravity and angular momentum.  If two of them had a close approach, the huge gravitational interaction would likely send one or both flying out of the solar system.  But if you had one gently tugging on the other every so often, you could get a stable elliptical orbit far from the Sun.

Astronomers Scott Kenyon and Benjamin Bromley use computer simulations to construct scenarios for the formation of Planet Nine in a wide orbit.  And this is just the scenario they are proposing.

“Think of it like pushing a kid on a swing. If you give them a shove at the right time, over and over, they’ll go higher and higher,” explains Kenyon. “Then the challenge becomes not shoving the planet so much that you eject it from the solar system.”

Now What?

With a few different scenarios proposed, and the probability of each calculated, what remains is to find the planet.  Once we know what it looks like, how much gas it has, and get an exact calculation of its orbit, now there is observational evidence to piece together its formation.

All theoretical work aside, finding planet nine, proving its existence, and characterizing its formation is the only real way to unlock the secrets of how our solar system formed.

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