Kepler-37b, one of three new worlds found around Kepler 37, a star slightly cooler than our sun, is not much larger than Earth's moon, they said.
"This shows us the diversity of exoplanetary systems," Thomas Barclay at the NASA Ames Research Center in Moffett Field, Calif., told NewScientist.com. "We knew there were very large systems -- now we know there are very small ones as well."
The three planets were discovered by NASA's Kepler space telescope, which records decreases in the intensity of starlight when a planet passes in front of the star, as seen from Earth.
The amount of the decrease indicates the size of the planet, scientists said; the greater the decrease, the larger the planet.
Barclay's team made measurements on the star Kepler 37 using a technique called asteroseismology, looking at waves on the star's surface to infer information about its interior, in the same manner geologists use seismic waves to study structure deep inside the Earth.
The technique can provide better estimates of a star's mass, size and density.
"One of our big limitations in studying planets is actually understanding the star," Barclay said, explaining knowing a star's size more accurately allows better estimates of the sizes of its planets. "We've got an accurate measure of just how small they are."