LOS ANGELES — NASA scientists have discovered a faraway planet that’s smaller than Mercury — far tinier than they expected they could find when they launched the Kepler space telescope nearly four years ago.

The hot, rocky world orbits a sun-like star that’s about 210 light-years from Earth. Astronomers are excited about it because it’s smaller than any planet in our solar system, said astrophysicist Thomas Barclay of NASA’s Ames Research Center in Mountain View, Calif.

“This is the smallest exoplanet that’s ever been found,” said Barclay, lead author of a report on the discovery published Wednesday in the journal Nature. “We’re breaking new ground here.”