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What is hot Jupiter pile-up?

The term “hot Jupiter” pile-up refers to the observation that the class of exoplanets known as hot Jupiters occupy a very narrow range in the star-planet distance, and therefore also in the orbital period for revolution around the parent star. We can see this in the mass versus period diagram below (which corresponds to a snapshot of the confirmed exoplanet population on 7 August, 2012 - data from the Extrasolar Planets Encyclopedia).

Exoplanets Period versus Mass

Orbital period versus mass. For comparison, the planets in our solar system are marked as Me, V, E, Ma, J, S, U, N corresponding to Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune respectively (numerical values are from solarsystem.nasa.gov).

The patch of data points in the lower right portion of the diagram represents the enhancement in the population of massive planets with extremely short periods. The masses range from about 50 to 1000 Earth masses (compare with the mass of Jupiter which is 317.828 Earth masses). The pile-up region corresponds to orbital periods between about 2 and 6 Earth days. (The corresponding star-planet distance is in the region of about 1% to 10% of the Earth-Sun distance).

The reason for the clustering in such a narrow band is really not clear and is the subject of intensive research in the field. Could there be an observational bias (selection effect)? The majority of the planets with density measurements were discovered by the transit method, which does indeed favor finding close-in, short-period exoplanets. However, if selection effects were the only explanation, you would expect to see more of a gradual tapering off in the planet-star distance distribution, rather than the sudden cutoff at a period of about 6 days, or approximately 0.1 AU in star-planet distance. One possibility is that a planet at large distances (tens to thousands of AU) migrates so rapidly to small distances that the planet does not spend much of its life at intermediate distances, so the chances of finding a planet at intermediate distances is small. However, this still does not satisfactorily explain what causes the pileup at short distances from the host star, and it is not in fact understood what mechanisms stop a planet migrating too close to the host star. If rapid migration proceeded without a slowing down near the star, there would not be such a pronounced pileup.

Certainly, theories have been advanced for possible reasons for the pile-up. However, they are all speculative and make assumptions and/or propositions of physical processes that are not proven or disproven to be relevant. None of the exoplanations involve a simple physical principle and there is no concensus on a reason for the hot Jupiter pile-up. (Advanced readers: see Chang et al. 2012 for references to various proposed mechanisms).

A more detailed discussion can be found in the book Exoplanets and Alien Solar Systems.

exoplanets and alien solar systems pbk book cover

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File under: Clustering of gas giant close-in planets; the hot Jupiter pile-up problem.

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