Exoplanets Density versus Semimajor Axis

Exoplanets density versus semimajor axis

A diagram showing density versus semimajor axis (i.e., maximum planet-star separation) for the confirmed exoplanets in a subset of the sample that have measurements of radius, mass lower limit, and semimajor axis (sample from a snapshot on 5 August, 2012). Only 29% of the confirmed planets had measurements of all three required parameters at that time. Each exoplanet is marked with a cross. The densities are expressed as ratios relative to the density of the Earth, and the semimajor axes are expressed as ratios relative to the Earth-Sun semimajor axis (i.e. in astronomical units (AU)). Data are from the Extrasolar Planets Encyclopedia, and the densities are calculated values. 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). Amongst other things, the diagram shows that there is a significant clustering of exoplanets that have densities more than 10 times smaller than that of Earth, and star-planet separations more than 10 times smaller than that of Earth. These are the “underdense&rdqup; hot Jupiters representing a kind of planet that does not have a counterpart in our solar system.

Another feature of the density versus semimajor axis diagram is that there appears to be a favored region for the semimajor axis, between about 0.03 and 0.06 AU, and there are planets in this band that stretch over four orders of magnitude in the density. On the other hand, for semimajor axis values greater than 1 AU, the density barely stretches over two orders of magnitude in the semimajor axis, although there is a severe lack of data in this region. It is imperative to consider possible selection effects in order to interpret this diagram (and these cannot always be fully known and evaluated).

Read more about exoplanets, and the density versus semimajor axis relation with Exoplanets and Alien Solar Systems, which includes comprehensive references to the scientific literature, and discussion of selection effects.

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Note: There are 225 exoplanets shown in the plot, taken from a snapshot when there were 777 confirmed exoplanets in total (residing in 623 alien solar systems, 105 of which harbored more than one exoplanet).
File under: How are density and semimajor axis related for exoplanets? How are density and semimajor axis related for extrasolar planets? Is the density correlated with semimajor axis for exoplanets?