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Exoplanets Semimajor Axis Distribution

Histogram of the exoplanets semimajor axis distribution

A histogram showing the distribution of the orbital size in terms of half of the maximum planet-star distance (more precisely, the semimajor axis of the ellipse), for the confirmed exoplanets. The data are a subset of the sample that had measured semimajor axis values from a snapshot of the confirmed exoplanet sample on 5 August, 2012. Plotted is the percentage of the subsample of exoplanets in each interval of semimajor axis. The exoplanet semimajor axes are shown in astronomical units (AU) (i.e., in units of the semimajor axis of the Earth's orbit around the Sun). Data are from the Extrasolar Planets Encyclopedia. For comparison, the semimajor axes of objects in our solar system are marked as Me, V, E, Ma, J, S, U, N, P corresponding to Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto respectively (numerical values are from

It can be seen that the semimajor axis distribution shows a striking bimodel form, wih a broad peak between about 0.7 to 7 AU, and a narrower peak between about 0.03 and 0.06 AU. The latter peak corresponds to the many hot Jupiters and other close-in planets that have been discovered. Since the star-planet distance is related to the orbital period by Kepler's laws, the distribution of the exoplanet orbital periods also shows a corresponding marked bimodel characteristic. The origin of the form of the distributions is a mystery and raises a number of theoretical issues. There is as yet no concensus on the physics driving the distributions. There are possible selection effects that need to be examined in order to interpret the data. A more detailed discussion can be found in the book Exoplanets and Alien Solar Systems, which includes comprehensive references to the scientific literature.

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Note: The histogram utilizes data from 654 exoplanets, taken from a snapshot when there were 692 confirmed exoplanets in total (residing in 570 alien solar systems, 81 of which harbored more than one exoplanet).
File under: What is the orbital size distribution of exoplanets? What is the distribution of semimajor axis for extrasolar planets? What is the range in orbital size of exoplanets? How far are exoplanets from their host stars?

© Tahir Yaqoob 2011.