Kepler-138b is smallest exoplanet yet to have its density measured

Los Angeles Times : Although the presence and size of distant planets can be determined by the temporary decrease in brightness of their host star as they pass in front of it, determining their mass and density—and thus whether they’re rocky like Earth or gassy like Jupiter—has proven to be more difficult. Scientists have relied on a planet’s gravitational pull on its star, but that works only if the planet is particularly large. Now the mass of the first Mars-sized exoplanet has been determined by measuring its gravitational pull, not on its star but on its fellow planets. Kepler-138b is 0.066 Earth masses, making it somewhat smaller than Mars. In addition, the masses of two other planets in the same solar system—Kepler-138c and Kepler-138d—have also been determined. The findings will help scientists better understand the complex dynamics involved in planetary formation.