NASA's Kepler mission scientists have discovered a new planetary system that is home to the smallest planet yet found around a star similar to the Sun.
The planets are located in a system called Kepler-37, about 210 light-years from Earth in the constellation Lyra, the U.S. space agency said in a press release.
The smallest planet, Kepler-37b, is slightly larger than Moon, measuring about one-third of the size of Earth. It is smaller than Mercury, which made its detection a challenge. The Moon-size planet and its two companion planets were discovered by scientists with NASA's Kepler mission, which is assigned to find Earth-sized planets in or near the "habitable zone," the region in a planetary system where liquid water might exist on the surface of an orbiting planet.
"While the star in Kepler-37 may be similar to the Sun, the system appears quite unlike the solar system in which we live. Astronomers think Kepler-37b does not have an atmosphere and cannot support life as we know it. The tiny planet almost certainly is rocky in composition," the release said.
Kepler-37c, the closer neighboring planet, is slightly smaller than Venus, measuring almost three-quarters the size of Earth. Kepler-37d, the farther planet, is twice the size of Earth.
The first exoplanets found to orbit a normal star were giants. As technologies have advanced, smaller and smaller planets have been found, and Kepler has shown even Earth-size exoplanets are common. "Even Kepler can only detect such a tiny world around the brightest stars it observes," said Jack Lissauer, a planetary scientist at NASA's Ames Research Center in Moffett Field, California. "The fact we've discovered tiny Kepler-37b suggests such little planets are common, and more planetary wonders await as we continue to gather and analyze additional data," according to him.
Kepler-37's host star belongs to the same class as the Sun, although it is slightly cooler and smaller. All three planets orbit the star at less than the distance Mercury is to the Sun, suggesting they are very hot, inhospitable worlds.
A "year" on these planets is very short. Kepler-37b orbits its host star every 13 days at less than one-third the distance Mercury is to the Sun. The other two planets, Kepler-37c and Kepler-37d, orbit their star every 21 and 40 days.
"We uncovered a planet smaller than any in our solar system orbiting one of the few stars that is both bright and quiet, where signal detection was possible," said Thomas Barclay, Kepler scientist at the Bay Area Environmental Research Institute in Sonoma, California, and lead author of the new study published in the journal Nature.
The research team used data from NASA's Kepler space telescope, which simultaneously and continuously measures the brightness of more than 150,000 stars every 30 minutes.
Kepler is NASA's tenth Discovery Mission and was funded by NASA's Science Mission Directorate at the agency's headquarters in Washington.
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