U.S. researchers say they were surprised to find a number of exoplanets in the Milky Way that are only a few times bigger than Earth but covered in gas.

"This indicates these planets formed very quickly after the birth of their star, while there was still a gaseous disk around the star," Yoram Lithwick, an assistant professor of physics and astronomy at Northwestern's Weinberg College of Arts and Sciences, said in a release Wednesday. "By contrast, Earth is thought to have formed much later, after the gas disk disappeared."

Measurements of the planets outside the Earth's solar system broaden scientists' knowledge of exoplanets that are larger than Earth but smaller than Neptune because once their mass and size are known, density can be determined and composition can be inferred.

To measure the mass of the planets found by NASA's Kepler space telescope, Lithwick and graduate student Sam Hadden used a technique involving transit time variation, discovering planets two to three times bigger than Earth have very low density, indicating they are covered in gas.

In contrast, planets slightly smaller than these have much higher density, and are denser than rock, the research indicated. They are similar to or denser than Earth.

Lithwick presented his findings at a scientific session Monday during the annual meeting of the American Astronomical Society in Washington.