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# What is the Least Massive Exoplanet?

If you would like to be able to find out yourself what is the least massive exoplanet found so far, here is how you do it. Go to the Extrasolar Planets Encylopedia catalog page and click on the button in the header of the table that says “All fields” and then click on the column called Mass. When you click on it, the data in the table should be sorted in order of numerical value in that column (i.e., the mass of the planets). An arrow just to the right of the word Mass tells you whether the numerical order is ascending or descending (the arrow will point upwards for an ascending order). If you click the column header again, the numerical ordering will flip from ascending to descending or vice versa.

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To find the least massive exoplanet you want the mass measurements to be ascending, with the smallest values appearing first. (Note that at the end of the sorted table the rows that appear first will have no entries in the Mass column and these correspond to exoplanets for which the mass has not been measured.) The first non-blank entry in the sorted mass column corresponds to the least massive confirmed exoplanet. The number in the cell in the mass column is the mass in units of Jupiter's mass. You can convert to Earth masses by multiplying the number by 317.828 (which is the mass of Jupiter in Earth masses). If you need the answer in kilograms (kg), then multiply the number in the cell by $1.8981 \times 10^{27} \$ kg (which is Jupiter's mass in kg). From here you can convert to any other units by typing an appropriate statement in a Google search box. See also http://astrophysicsformulas.com for sources for solar system data.

For example, in August 2012, the least massive exoplanet was KOI-55 c. This planet has a radius of about 0.86 Earth radii (no error given), a minimum mass estimate of about 67% of Earth's mass, and an orbital period of about 0.34 days.

A word of caution: the mass estimates for the majority of exoplanets are lower limits (i.e., minimum values only). Regardless, you should always try to find out what measurement uncertainties are associated with any quoted number (not just for exoplanet masses, but any measurement in general).

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