Planet Nine: an intruder among us?

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.  Here we will explore a much more exotic origin.

Scenario 2.  Planet Nine is a planet that formed around another star and was captured by the Sun

The Sun was born in a cluster with many other stars.  It probably looked something like the Trapezium cluster:

Optical (left) and infrared (right) images of the Trapezium embedded cluster of stars. Credit: Hubble Space Telescope/NASA. 

We don’t know how dense the Sun’s birth cluster was.  It probably contained between a few hundred and a few thousand stars.  The cluster lasted about ten million years before expanding and dissolving, sending each star on its own way into the galaxy.  (The Sun is not in a cluster now).

Wide binary stars – systems in which two stars orbit each other on very wide orbits – are thought to form in the following way. Young stars feel the combined gravity of all the other stars as they orbit within their birth clusters. Stars frequently come close to other stars and give each other gravitational kicks. As the cluster expands and dissipates, two stars that happen to be close to each other can find themselves gravitationally bound on a wide orbit.

Stars can also capture planets during the cluster phase if there are a lot of planets floating around among the stars in these clusters.  And there probably are a lot of free-floating planets wandering among the stars.  A dozen or so free-floating (“rogue”) planets have been directly detected.  We don’t know exactly how abundant free-floating planets are (some estimates are very high) but we know that there are a lot of them.

Where do rogue planets come from?  They probably form like most planets do, in disks of gas and dust orbiting young stars.  Then their planetary systems become unstable.  Like the Nice model instability but much much stronger. Here is the cartoon version:


When the orbits of two (or more) Jupiter-like planets cross, there is a series of strong encounters and one or more planets is thrown out into interstellar space.  Check out this awesome animation by Eric Ford and company:

If an instability happens during the cluster phase, the planets may be launched onto star-like orbits within the cluster.  And for every Jupiter-sized planet that is ejected there are several Neptune-sized planets kicked out. We think that the majority of systems of gas giant planets become unstable, creating an abundance of rogue planets.

In its infancy, the Sun’s birth cluster contained only stars (and brown dwarfs) and leftover gas.  Planetary systems were born in disks around most stars.  Many of the stars formed systems of gas giant planets, and most of these became unstable.  By the time the cluster dissipated it must have been teeming with planets.  Like kids at a summer camp, each planet was born to a different star but sent away from home.

Could the Sun have captured Planet Nine as its birth cluster dissipated?  If so, Planet Nine would have formed around a different star in the cluster.  It would be an extrasolar planet lurking within the Solar System.

The answer depends on the orbital dynamics of this complicated system: planets and stars orbiting each other in a cluster as the cluster expands and dissipates.  Calculations suggest that this is a low probability event.  Only 1-10% of stars like the Sun capture a planet, and there is only a few percent chance of that captured planet being on an orbit like Planet Nine’s.  Most of the time, the orbits of captured planets are about ten times wider than Planet Nine’s.

So capture of a rogue Planet Nine during the dissipation of the Sun’s birth cluster is not impossible, but it’s not very likely.  It is at best a roughly 1% event according to current models.  (Remember, of course, that models are imperfect and can change; still, this idea is not looking good at the moment).

But hope remains: there is another way that planets can be captured in clusters, during stellar fly-bys.  Stars in clusters constantly pass near one another.  Of course, stars everywhere in the galaxy pass by one another, but in clusters they pass closer and more often.

If a star passes close enough to a planet-hosting star, it can “steal” the planet.  This only happens when the passage is close, within 2-3 times the size of the planet’s starting orbit.  The captured planet has a much different orbit around its new stellar host: its orbit is much wider and more elliptical.  Here is a cartoon of how this works:

Illustration of how a star can capture a planet from another star.  The key is for the red star to pass closer than 2-3 times the orbital distance of the planet around the yellow star.

Could Planet Nine have been captured during a fly-by with another star?  For this to have happened, another star must have passed relatively close to the Sun.  However, that star could not have passed so close as to disrupt the orbits of the Solar System’s planets. The limit is roughly 100 Astronomical Units; fly-bys closer than that have a good chance of disrupting the Solar System.  This means that the other star’s planet must have been on a much wider orbit than the Solar System’s planets.

Here is one way this could work.  The Sun passed relatively close (say, 200 AU away) to another planet-bearing star.  That star had a planet on a very wide orbit, 100 or more times larger than Earth’s.  Planets on such wide orbits are generally rare; however, in the aftermath of orbital instabilities planets spend time with very wide orbits before being completely ejected.  We can imagine that the planet’s wide orbit was elliptical because it was in the process of being kicked by another, larger planet into interstellar space.

The fly-by was such that the Sun could capture the planet from the other star, but the other star could not capture the Solar System’s planets.  Here is a cartoon view of how this might have happened:

Illustration of how the Solar System could potentially have captured Planet Nine in a fly-by with another planet-hosting star.  While this scenario is speculative it is plausible. For technical details of how this works, see here.

It’s hard to calculate the probability of this scenario playing out, although the pieces fit relatively nicely.  When planetary systems go unstable, the window during which a planet remains on a wide orbit before being ejected is several million years.  The type of star cluster typically lasts 10 million years.  The typical Sun-like star in a cluster akin to the Sun’s birth cluster has an encounter with another star of a few hundred Astronomical Units. Finally, the probability of capturing a planet on a wide orbit is never 100% because it depends on the planet’s actual position.  It can be as high as 30%.  (I am getting numbers from this paper).

I think it is entirely possible that Planet Nine was captured from another star during a fly-by.  For this to have happened requires specific circumstances but nothing too special.  The Sun had to pass close to a star that happened to have a planet on a wide orbit.  That is probably not an exceptional event because about 20% of Sun-like stars have giant planets, most systems of giant planets go unstable, and most instabilities happen relatively early.  To put numbers on it we need more careful models of this process with particular emphasis on Planet Nine.

To sum up, it’s a real possibility that Planet Nine is an extrasolar planet, born around another star then captured by the Sun. 

In part 3 we will discuss what is currently thought to be the most likely origins story for Planet Nine.  We will also think about how to tell these stories apart.

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