January 20 – Scientists Discover New Planet! (Not)

Today’s factismal: If you read about a “new discovery” in the news, odds are it is neither new nor a discovery.

Twitter is all a-twitter today; Mike Brown (the astronomer who found the planet that changed the definition of planet so that Pluto wasn’t a planet anymore) has announced that he believes that there are nine planets in the Solar System. Why has he made this announcement? Does he now agree with the planetologists who thought that Pluto was a planet all along? No such luck. Instead, Brown has found some new data that suggests that there is a large planet hiding 600 times further from the Sun than Earth. This planet would mass about ten times as much as the Earth (making it about half as big as Neptune) and would be responsible for the crazy orbits of some “Kuiper belt” objects. But before you get out your textbooks and start adding new planet names, you need to realize one thing: the “new planet” probably isn’t even there.

Kuiper Belt objects like those that led to the "discovery" of Planet X (Image courtesy NASA)

Kuiper Belt objects like those that led to the “discovery” of Planet X
(Image courtesy NASA)

Say what?
That’s right; the new planet that’s got everyone a-twitter probably doesn’t exist. Right now, our only evidence for this new planet is an oddity in the orbits of a few large chunks of ice and rock out in the remoter regions of the Solar System.The proposed planet hasn’t been seen and has no direct evidence. Based on what is known, claiming that there is another planet in the Solar System is premature at best.

What evidence do they have?
What they do know is that there are six planets (excuse me – “dwarf planets”) out at about 30 AU that have orbits that are clustered together; astronomers estimated that the odds of such a grouping happening by accident is just 0.007%. Put another way, you’d have to have about 300 dwarf planets in the solar system for something like this to happen. (As a point of fact, brown estimates that there are about 400 such objects in our Solar System.) By including a planet that masses about ten times as much as Earth out at a distance of about 600 AU (an astronomical unit is the mean distance from the Earth to the Sun; astronomers use AUs like we use miles), Brown’s team was able to make sense of the odd orbits. The dwarf planets weren’t there by chance; they were being forced!

Another Kuiper Belt Object; the image is made from two snapshots taken hours apart. (Image courtesy NASA)

Another Kuiper Belt Object; the image is made from two snapshots taken 13 hours apart.
(Image courtesy NASA)

Why isn’t that proof enough?
If you are a historian of science, then you’ve heard this story before. The reason that we were looking for “Planet X” and found Pluto was because astronomers saw odd perturbations in the orbits of Neptune and Uranus that could “only” be explained by a large planet out about as far as Pluto is in the Solar System. So astronomers spend decades looking for Planet X and declared victory when we found Pluto. Only once we started getting a better look at Pluto, we realized that it was too small and had too elliptical an orbit to be “Planet X”. And when we looked at the odd perturbations in the orbits of Neptune and Uranus, we found that they weren’t that odd and didn’t need a “Planet X” to explain them. Thus, any claim that the orbits of six incompletely surveyed objects in the outer Solar System indicate a new planet should be treated with skepticism if not outright hostility.

How can they prove Planet X exists?
If Brown’s team is right, then there will only be one way to prove it: they’ll have to directly image the object. One thing that I know for sure; right now, there are lots of astronomers peering at high-resolution images of the outer Solar System trying to locate something big enough and with the right motion to be Brown’s “Planet X”. If someone finds it, then Brown will be the Le Verrier of the new planet; he’ll get credit for predicting its existence but won’t get to name it.

What if it does exist? Can I see it?
If “Planet X” does exist, odds are that you won’t be able to see it. That’s because the planet will be somewhere between half as large as Neptune and 20% larger (how big it is depends on it’s density), but will be much, much farther out than Neptune is. Given that it takes a good set of binoculars and lots of luck to see Neptune in the night sky, you won’t be able to see “Planet X” without a large telescope.

Density Radius (xNeptune)
0.69 (like Saturn) 1.114
1.64 (like Neptune) 0.835
2.06 (like Pluto) 0.774
5.52 (like Earth) 0.557

Yeah, but I want to hunt planets!
Then you are in luck. You can look through the Kepler data to identify new planets around stars very far away. If you find it, then you get to name it! To learn more, go to Planet Hunters:

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