Exoplanets Radius versus Mass

Exoplanets radius versus mass relation

A diagram showing radius versus mass lower limit for the confirmed exoplanets in a subset of the sample that have measurements of both radius and mass lower limit (sample snapshot for 5 August 2012). Each exoplanet is marked with a cross. The radii and masses are expressed relative to the corresponding values for Earth (i.e., radii are shown in units of Earth radii, and masses are shown in units of Earth masses). Data are from the Extrasolar Planets Encyclopedia. For comparison, the radii and masses of 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). Note that for clarity, the positions of Jupiter and Saturn correspond to the intersections of the dotted lines that are marked by the letters J and S respectively.

Note that currently, the bulk of confirmed exoplanets do not have radii measurements, and on 5 August 2012, only about 32% of confirmed exoplanets had radii measurements. Therefore, the plot above can only show this rather reduced data set. It can be seen from the radius versus mass diagram that above about 100 Earth masses (or about a third of JUpiter's mass), there is a horizontal band which is quite narrow in planetary size (about 1 to 2 Jupiter radii). This band corresponds to the giant, close-in planets (such as hot jupiters), which have been found to be very common in planetary systems outside our solar system. Below 100 Earth masses or so there are fewer data points, but the trend is a band that shows increasing radius with mass, and quite remarkably, our solar system planets follow this band. A new development since the publication of Exoplanets and Alien Solar Systems (which showed an older version of the diagram with even fewer data points), is the appearance of what looks like another horizontal band above about 100 Earth masses, but with radii in the range of about 2 to 5 Earth radii. These would be very high density “Super-Earths.”

Theoretical studies show that many differnt planet compositions can lead to a similar radius versus mass relation. (Advanced readers: see for example, Seager et al. 2007 and Valencia et al. 2007.) The down side of this is of course that it is not generally possible to uniquely determine the chemical composition of a planet from its mass and radius alone.

One also needs to carefully examine the possible selection effects when interpreting the eoxplanets radius versus mass diagram. Read more about exoplanets, and the radius versus mass 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 241 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). Only the 241 exoplanet shown had both radii and mass measurements or estimates.
File under: How are the radii and masses of exoplanets related to each other. Radius versus mass of exoplanets. Is there a conncetion between exoplanet radius and mass?