During its four-year mission, NASA's Kepler space telescope discovered thousands of "planetary candidates" in our Milky Way galaxy, many of which were found to be covered in gas.
Yoram Lithwick, an assistant professor of physics and astronomy at Northwestern's Weinberg College of Arts and Sciences, measured the masses of approximately 60 exoplanets larger than Earth and smaller than Neptune.
Lithwick said that they were surprised to learn that planets only a few times bigger than Earth are covered by a lot of gas.
He said that this indicates these planets formed very quickly after the birth of their star, while there was still a gaseous disk around the star, asserting that by contrast, Earth is thought to have formed much later, after the gas disk vanished.
To measure the masses of the planets found by Kepler, Lithwick and his graduate student Sam Hadden used a technique called transit time variation (TTV). They discovered that planets two to three times bigger than Earth have very low density, indicating they are covered in a massive amount of gas.
These planets are similar to Neptune, but smaller and fluffier. In contrast, planets that are only slightly smaller than these have much higher density, and are denser than rock. They are similar to Earth or even denser.
The findings have been be published in the Astrophysical Journal.