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What is the Densest Exoplanet?

In order to calculate a crude, average density we need a mass and radius estimate or measurement for an exoplanet (and a spherical geometry is usually assumed). However, the majority of exoplanets do not have radius (or any kind of size) measurement. For example, on 3 August, 2012, out of 777 confirmed exoplanets, only 252 (just over 32%) had a radius measurements, and only 241 had both mass and radius measurements. Therefore, if we ask “What is the densest exoplanet?” we must remember that we can only answer this question for a fraction of the confirmed exoplanets. Using the same example, on that date, the densest exoplanet in the Extrasolar Planets Encylopedia catalog was Kepler-25 b. This planet has about 12.7 times the mass of Jupiter contained within a size that is less than a quarter that of Jupiter, so we could expect the density to be at least $13 \times 4^{3} = 832 \ $ times that of Jupiter. Using the exact numbers given (12.7 Jupiter masses and 0.23 Jupiter radii), we get a density of 1044 times that of Jupiter, or about 251 times that of Earth. This translates into about $1384.2 \ {\rm g \ cm}^{-3}$, compared to Earth's $5.513 \ {\rm g \ cm}^{-3}$.

Make no mistake: the above high density is absolutely colossal, and the mass of 12.7 is only a minmum mass so the implied density could be higher. No margins of error are given on the radius. Such a high density would require extraordinarily extreme conditions. So extreme that it is questionable whether the object would be a planet at such a high density. What state of matter does it correspond to? Do you think such a high density is reasonable? I think it is more likely that a mistake has been made in one or more of the measurements. In particular, errors in the radius are cubed and therefore greatly amplified in any calculated density (see below).

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For a sphere, density is proportional to mass times radius cubed. If you would like to be able to find out yourself what is the densest exoplanet found so far, here is how you do it. Go to the Extrasolar Planets Encylopedia catalog page. You will then have to download the data table using one of the options given just above the table header. Read the columns of data into an application or program that you are familiar with (for example, EXCEL). Then follow the instructions on the exoplanets density distribution page of this website.

A word of caution: the mass estimates for the majority of exoplanets are lower limits (i.e., minimum values only). This means that most density estimates will be lower limits as well. The radius measurements also have uncertainties and margins of error, which can be as large as a factor of 2 or more in some cases. You should always try to find out what measurement uncertainties are associated with any quoted number (not just for exoplanet masses and radii, but any measurement in general).

See also the exoplanets density distribution.

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