Largest rocky world found

At half Neptune’s size, exoplanet should be gassy — but it’s not

rocky planet

SOLID AS A ROCK  A planet about half the size of Neptune might be made of pure rock, making it larger than other known rocky planets (one illustrated).


When it comes to big balls of rock, exoplanet BD+20594b might have all other known worlds beat. At roughly half the diameter of Neptune, BD+20594b is 100 percent rock, researchers suggest online January 28 at The planet seems to defy recent calculations that indicate a planet this large should be gassy (SN: 8/22/15, p. 32).

BD+20594b sits about 500 light-years away in the constellation Taurus. The planet is about 16 times as massive as Earth but just a little over twice as wide, making its density about 8 grams per cubic centimeter, Néstor Espinoza, an astrophysicist at the Pontifical Catholic University of Chile in Santiago, and colleagues report. Earth’s density, by comparison, is 5.5 grams per cubic centimeter. The new rocky planet was discovered in 2015 with the Kepler space telescope, which looks for the silhouettes of planets passing in front of their stars.

BD+20594b is comparable to Kepler 10c, a rocky “mega Earth” reported in 2014 (SN: 7/12/14, p. 10) to be 2.4 times as wide as Earth with a hefty mass (equal to about 17 Earths). Recent measurements indicate, however, that Kepler 10c isn’t quite as “mega” or as rocky as thought — only 14 times as massive as Earth — which means that the planet is probably encased in shell of gas or water. 

Editor’s note: This story was updated February 17, 2016, to correct the name of the constellation in which BD+20594b resides.

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