PLANETPLANET

It’s official: there might be an extra planet in the Solar System.  It’s called Planet Nine.  Like Hansel, it’s so hot right now.

To find it you must trek past Uranus and Neptune into the dark reaches of the Solar System.  It’s on an orbit hundreds of times larger than Earth’s, that takes 20,000 years to complete a single loop around the Sun.

A quick recap: Planet Nine’s existence has been inferred from the strange distribution of small bodies in the outer reaches of the Solar System (so-called scattered disk objects).  These bodies, whose orbits are detached from the main population of small bodies past Neptune (the Kuiper belt), share a peculiar orbital alignment. This is weird: there is only a tiny (0.007%) statistical chance of those six objects randomly sharing this alignment. This is where Planet Nine comes in.  Konstantin Batygin and Mike Brown at Caltech showed that an extra planet could be pulling the strings, using its gravitation to sculpt the orbits of these objects.  To accomplish this, planet Nine’s orbit must be elliptical and anti-aligned with the orbits of the small bodies.  Planet Nine must also be pretty massive, at least several times more massive as Earth.  Here is what its orbit might look like:

Planet Nine also naturally explains a different class of Kuiper belt objects with orbits that are almost perpendicular to the plane of the planets’ orbits.  Plus, it’s an awesome idea.  I’m a big fan.

Of course, Planet Nine hasn’t been found yet.  It’s really really hard to find a small cold planet that far away from the Sun.  And it might not even exist (there are good reasons to be skeptical (see here, here and here) until it shows up on camera)!

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In a series of three blog posts I’m going to discuss three scenarios for the origin of Planet Nine.  Two scenarios are based on the idea that Planet Nine formed closer to the Sun and was kicked out onto its current orbit. The other scenario invokes the capture of Planet Nine from interstellar space, implying that Planet Nine is of extrasolar origin.  As you will see, one of the scenarios has a fatal flaw but two remain viable. Here we go.

Scenario 1.  Planet Nine was created during an instability in the giant planets’ orbits. 

The orbits of the Solar System’s planets are extremely well-behaved.  Each planet follows a near-circle around the Sun.  To within a few degrees, each planet’s orbit is also located in the same plane.  It’s all very nice.

But there are subtle hints that things were not always so rosy.  One of the biggest advances in Solar System science in recent times is the realization that the Solar System probably went unstable early in its history.  The instability is thought to have been triggered by a gravitational tug of war between the gas giant planets (Jupiter, Saturn, Uranus and Neptune) and a broad disk of planetary leftovers: essentially comets and other pieces of rock and ice that were not incorporated into the planets as they were forming.

The planets’ orbital instability is called the Nice model (because it was developed in Nice, France).  Below is an animation of a computer simulation of the instability.  The animation only shows the outer Solar System – it does not include the rocky planets. The movie shows how, after a delay lasting millions of years, the planets’ orbits go unstable.  When this happens the planets kick the comet-like leftovers all over the place, and end up in a new, wider configuration.  At the end of the movie the planets’ orbits are very close to their current ones.

At the start of the movie there are five giant planets.  In the actual Solar System there are only four (Jupiter, Saturn, Uranus and Neptune).  When the instability happened, one planet was on a very stretched-out, elliptical orbit.  But then that planet disappeared from the movie.  It’s important to realize that that extra planet may have saved the day.  During the instability, the extra planet was kicked around by Saturn and then Jupiter, and  in doing so it prevented those giant gaseous planets from landing in a configuration that would have destroyed the rocky planets (or at least caused a giant collision between two rocky planets).  So, we are glad that extra planet was (probably) there.

But what happened to the extra planet?  It is generally thought to have been launched into interstellar space, gravitationally tossed from the bosom of the Solar System into the cold blackness of the galaxy.  But what if its orbit was somehow changed and it became Planet Nine?

To understand how this could happen we need to look at how gravitational kicks from a planet (say, Jupiter) change another object’s (say, Planet Nine’s) orbit.  This image shows the idea.

Each time Jupiter kicks it, Planet Nine’s orbit gets wider.  But it keeps coming back to the same place – its closest approach to Jupiter – until Jupiter finally kicks it so hard that it never returns.

Planet Nine’s real orbit does not look like any of the yellow curves.  Planet Nine spends most of its time far away from the Sun, and it never crosses the orbits of the other planets.

The story changes when Jupiter is no longer the only thing kicking on Planet Nine.  Imagine Jupiter repeatedly kicked Planet Nine until its orbit was pretty big.  At that point, something farther away gave Planet Nine’s orbit a gravitational kick. That external kick  changes the shape of Planet Nine’s orbit so that it no longer goes close to Jupiter.  In this way, Planet Nine could reach a wide orbit that does not cross Jupiter’s.

Where are the external kicks coming from?  From the Solar System’s surroundings: stars passing by within a few light years, and from the combined gravity of stars and gas in the Sun’s neighborhood.

The Oort cloud – a population of comets surrounding the Solar System – was populated during the Nice model instability.  Remember those planetary leftovers, the green dots in the movie?  They were kicked onto wide orbits by the planets, then external kicks changed the shapes of their orbits and rescued them, just like in the image above.

The Oort cloud is very far away: Oort cloud comets spend most of their time hundreds of thousands of Astronomical Units from the Sun (1 Astronomical Unit is the Earth-Sun distance).  That’s a few light years away from the Sun!

If Planet Nine was captured onto its current orbit during the Nice model instability, that means it has the same orbital history as the Oort cloud comets.  This is important.

The external gravitational kicks that we’ve been discussing can be weak or strong.  At the present they are weak, because the Sun is in a quiet part of the galaxy.  But we know that the Sun formed in a cluster with hundreds to thousands of other stars.  And clusters are busy places.  External gravitational kicks in clusters are strong. The stronger the external kicks, the smaller the final orbit of the comet (or Planet Nine) after it is rescued from getting kicked by Jupiter or the other planets.

Let’s put the pieces together.  If the Nice model instability happened early, while the Sun was still in a cluster, external kicks were strong.  The orbits of Planet Nine and the Oort cloud comets would have been separated from the planets before their orbits got too wide.  Their final orbits would have been a lot like Planet Nine’s current orbit.  Score!

But wait.  If that were the case, then the Oort cloud comets should all have the same kind of orbit as Planet Nine.  And they don’t.  Oort cloud comets’ orbits are hundreds of times wider.

If the Nice model instability happened later, when the Sun was out of its cluster, then external perturbations were weak and the Oort cloud comets would have orbits like their real ones.  Score!

But wait.  Then Planet Nine’s orbit should be in the Oort cloud, not much closer-in.

This scenario cannot match Planet Nine’s orbit and the Oort cloud comets’ orbits at the same time.  It must be wrong (unless we are missing something).  Let’s throw it out and move on to the next one.

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In part 2 we will explore whether Planet Nine is an intruder from another star.  Finally, in part 3 we will discuss the most likely origin for Planet Nine.