An Earth-sized hell circles faraway star

A distant Earth-sized planet’s density suggests it is made of the same stuff as Earth. The planet, named Kepler-78b, is the smallest exoplanet for which researchers know both size and mass.

Kepler-78b, detected this year in data from NASA’s now-crippled Kepler spacecraft, is located 700 light-years from Earth in the constellation Cygnus. The planet’s gravitational tug causes its star to shimmy back and forth slightly, which compresses or expands the wavelengths of light the star emits. Two teams of astronomers used the light’s changing wavelength to calculate the speed at which the stars wobbles; from this speed, they inferred the planet’s mass and, using previously published size measurements, calculated its density.

Reporting October 30 in Nature, one team estimates the planet’s density to be 5.3 grams per cubic centimeter; the other estimates 5.57 grams per cubic centimeter. Earth’s density of 5.515 grams per cubic centimeter falls between these two figures, suggesting Kepler-78b is, like Earth, made of rock with an iron core.

Although similar to Earth in size and composition, Kepler-78b is no life-nurturing paradise. The planet zips around its host star every 8.5 hours in an extremely tight orbit, and its daytime temperatures probably top 2,000° Celsius.