A new planet in the Solar System?

Killing Pluto was fun, but this is head and shoulders above everything else. (M. Brown)

It was only yesterday when I published a post on standard and non-standard existential predictions. Thus, I was very happy today when, opening the newspaper, I read that some astronomers have predicted the existence of a new planet in the Solar System! Michael E. Brown and Konstantin Batygin just published a paper on the current number of the Astronomical Journal, in which they argue in favour of the existence of a ninth planet, nearly the size of Neptune, orbiting the sun every 15,000 years. The news is also reported by the magazine Science. The existence of this “Planet X” has not been confirmed by any discovery yet, therefore for the moment this prediction is nothing but a simple hypothesis.

Whether or not this prediction will be confirmed, it seems to be analogous to Neptune’s prediction. As well as for Neptune, also the existence of this new planet has been postulated in order to “explain away” an anomaly. In this case, the anomaly concerns a clustering of six previously known objects that orbit beyond Neptune. What is anomalous, here, is the fact that these objects are grouped in a cluster and that their orbits are quite similar. As Brown and Batygin say, <<the perihelion positions and orbital planes of [these] objects are tightly confined  and […] such a clustering has only a probability of 0.007% to be due to chance, thus requiring a dynamical origin>>. A ninth planet having the proper characteristics would explain away this “anomaly”.

It must be noted, however, that the anomaly that this ninth planet would “explain away” is a bit different from the anomaly that Neptune explained away. In the Neptune case, the anomaly was—so to say—much stronger than the clusters’ anomaly. There, the anomaly was something that the theory could not account for; here, the anomaly is something that the theory could account for, but with a very low degree of probability. In other words, in the first case the anomaly undermines the theory, whereas in the second case the anomaly is still compatible with the theory but the probability that the disposition of these objects orbiting beyond Neptune is simply due to chance is really low. We can then distinguish between two kinds of anomaly—let’s call them “incompatible” and “compatible” anomalies.

This story also has an ironic shade, since Brown is better known as a planet slayer than a planet discoverer! His 2005 discovery of Eris, a remote icy world nearly the same size as Pluto, revealed that what was previously seen as the outermost planet was just one of many worlds in the Kuiper belt. As a consequence of this discovery, astronomers promptly reclassified Pluto as a dwarf planet. The whole story is recounted by Brown in his 2010 book How I Killed Pluto and Why It Had It Coming. However, as Brown reportedly commented on this, <<Killing Pluto was fun, but this [the discovery of a new planet] is head and shoulders above everything else>>.

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