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Exoplanets Surface Acceleration Ratio versus Period

Exoplanets Surface Acceleration Ratio versus Period


A diagram showing a quantity called the “surface acceleration ratio” versus orbital period (in Earth days) for a subset of the sample of exoplanets for which an orbital period has been measured, and for which the necessary measurements exist for the surface acceleration ratio to be calculated (subsample from a snapshot on 5 August, 2012). Data are from the Extrasolar Planets Encyclopedia. The surface acceleration ratio is a measure of the ratio of the tendency of the surface of the planet to be tugged away by the host star, due to the star-planet gravity becoming comparable to the planet's self-gravity at its surface. The calculated values of the surface acceleration ratios are actually lower limits because the exoplanet masses are lower limits. 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. The diagram shows that a significant fraction of the exoplanets in the subset show a very strong clustering of the surface acceleration ratio of around 4 and orbital periods of around 2 to 4 Earth days. These are close-in, short-period, hot Jupiters, in which the surface layers of the planets are unstable, and they have no counterparts in our solar system. There appears to be a cutoff (or “wall”, corresponding to the dotted line) at a value of the surface acceleration ratio of around 1, below which only a few exoplanets are found. This likely corresponds to a regime in which most exoplanets cannot exist due to the star-planet gravity overwhelming the planet's self-gravity. A more detailed discussion can be found in the book Exoplanets and Alien Solar Systems.

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Note: The subsample of 216 exoplanets shown in the diagram was taken from a snapshot at time when there were 777 confirmed exoplanets in total (residing in 623 alien solar systems, 105 of which harbored more than one exoplanet).
File under: Surface mass loss from close-in exoplanets; surface acceleration ratio versus orbital period.

© Tahir Yaqoob 2011-2012.