This plot shows the masses and sizes of the smallest exoplanets for which both quantities have been measured. The solar system planets (shown in red) are for comparison.
The three Kepler-138 planets (shown in orange) are among the four smallest exoplanets with both size and mass measurements. Kepler-138b is the first exoplanet smaller than Earth to have both its mass and size measured. This significantly extends the range of planets with measured densities.
By measuring both the mass and size of an exoplanet, scientists can calculate the density and infer the bulk composition to determine if a planet is predominantly made of rock, water or gas. Tiny Kepler-138b's density is consistent with a rocky composition like Earth or Mars, but further observations are needed before astronomers can definitively say that it is a rocky world.
The characteristics of Kepler-138c and Kepler-138d imply that these planets, similar in size to Earth, have a wide variety of compositions.
Planets are primarily composed of three types of constituents: 'rock', which includes metals, is the densest of these components, whereas hydrogen and helium gases are the lightest. All planetary constituents get compressed when they are within massive planets, so for a given composition the density is larger for larger mass planets.
Intermediate density planets can be composed of mixtures of rock plus gas and/or the third major planetary constituent, water and similar materials such as methane (in solid, liquid or gaseous form).
Scientists are working to use these new measurements of small planets from Kepler and NASA's upcoming Transiting Exoplanet Survey Satellite to identify patterns in the relationship between mass and size. These insights will provide context for understanding the history of Earth and other planets in our own solar system, and inform the next generation planet hunters as they search for life beyond the solar system.