Planet or No Planet, the Search is On

Exciting news from the astronomy community grabbed headlines this week — and reignited a conversation about the number of planets in our solar system. Unfortunately for Pluto, however, the debate was not a rehashing of the former planet’s demotion to dwarf planet a decade ago. It was a much older debate, one that goes back to a time before Pluto had even been discovered: the question of whether or not additional planets are hidden in the far corners of our solar system.

This past Wednesday, two scientists from CalTech — Konstantin Batygin and Mike Brown (the self-proclaimed Pluto killer) — published a paper in the Astronomical Journal entitled “Evidence for a Distant Giant Planet in the Solar System.” In it, they present a hypothesis that explains an odd characteristic of the Kuiper Belt, the region of the solar system beyond Neptune.


The orbit of the proposed Planet Nine (in orange), along with the odd orbits of Kuiper Belt objects (purple) that it would help explain. (Credit: Batygin and Brown)

Specifically, some of the objects orbiting beyond Neptune have very similar values for their argument of perihelion — the angle around the object’s orbit between the point where it crosses the ecliptic (the plane given by the orbit of the Earth) moving from south to north and the point where it is closest to the Sun, known as perihelion. This is an unexpected result, because the gravitational effects of the large planets in the solar system should have pulled these Kuiper Belt objects into orbits that have fairly random arguments of perihelia.

In addition, these objects also have poles of rotation that are oriented in very similar directions. Overall, the authors calculate that the probability of this clustering being the result of random chance to be just seven thousandths of a percent (0.007%) — far more likely than your odds of winning the lottery, but not likely enough to accept as the reason for these clustered objects in the Kuiper Belt.

Batygin and Brown put forward a hypothesis that may explain this observation: there may be another planet out there — a fairly big one, ten times more massive than Earth and orbiting hundreds of times farther away from the Sun in a strange, highly elliptical orbit. The paper’s calculations and simulations show that the gravitational pull of this as-yet-undetected planet, which they call Planet Nine, would provide the extra force needed to explain the odd behavior of orbits in the Kuiper Belt.

Sound familiar? Last week’s post was a review of Thomas Levenson’s “The Hunt for Vulcan,”  another story of the search for an unseen planet that could explain unexplained orbital behavior. Neptune was successfully found this way; Vulcan proved to be a dud. However, even though we didn’t get another planet from the search, it did help lead us to general relativity — and that’s not a bad deal.

To be clear: these scientists have not discovered a new planet… yet. All they have done is to show that, if this new planet exists, it would explain odd behavior in the Kuiper Belt. A lot of leg work still must be done to determine whether or not it is actually there. It may be that future observations will find Planet Nine, or it may turn out to be another red herring in the ongoing hunt.

The search for additional planets has a long history of coming up empty, and we should bear the lessons of Vulcan in mind when we consider these findings. As the authors themselves put it, “the suggestive nature of the results should be met with a healthy dose of skepticism.” There are aspects of the Kuiper Belt that remain unexplained, and there are — as is nearly always the case in science — many assumptions behind their analyses.

However, something is going on out there — something that we can’t yet explain — and that’s incredibly exciting. Whether we discover a new planet or discover something new about how the world works, the search is on.


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