Astronomers at the Keck Observatory at the summit of Mauna Kea have helped to determine the size of three planets smaller than Earth that orbit a star outside the Milky Way galaxy.

The three planets orbit a star called KOI-961, according to W.M. Keck Observatory scientists. Their radii are calculated to be 78, 73 and 57 percent that of Earth, according to a news release.

The sizes of the planets were worked out by Kepler Telescope observations that measured the dimming of the star KOI-961 as each planet passes in front of it, officials said. Scientists were able to determine the sizes of the planets with this information, coupled with crucial observations about the star from Keck and Palomar telescopes, officials said. The Kepler Mission is a space telescope launched by NASA in 2009 that measures light from 150,000 stars. 

The three planets take less than two days to orbit around KOI-961, which is a red dwarf with a diameter one-sixth that of the sun, making it just 70 percent bigger than Jupiter, according to a Keck Observatory news release. Red dwarfs are the most common kind of star in the Milky Way galaxy. 

“These types of systems could be very common in the universe,” said Phil Muirhead, lead author of the new study from the California Institute of Technology in Pasadena. “This is a really exciting time for planet hunters.”