Astronomers Calculate Mass of 60 Gassy Exoplanets

As scientists convened this week during the 223rd meeting of the American Astronomical Society, Northwestern University's Yoram Lithwick offered a contribution to the ongoing quest to find Earth-like worlds outside of our solar system. The planetary scientist reports to have measured the masses of nearly 60 exoplanets larger than Earth, but smaller than Neptune.

Lithwick and his colleagues made the measurements using data collected from NASA's Kepler space telescope, and in doing so, they substantially added to the body of knowledge surrounding exoplanets in the sub-Neptune size range.

"We were surprised to learn that planets only a few times bigger than Earth are covered by a lot of gas," said Lithwick, an assistant professor of physics and astronomy at Northwestern's Weinberg College of Arts and Sciences.

"This indicates these planets formed very quickly after the birth of their star, while there was still a gaseous disk around the star," he said. "By contrast, Earth is thought to have formed much later, after the gas disk disappeared."

With the assistance of graduate student Sam Hadden, Lithwick measured the masses of the planets using a technique known as transit time variation (TTV), which is typically not used to calculate mass of planets.

With this TTV - which can only be used when two or more planets orbit a star - astronomers can measure pulling forces that lead to slight changes in how planets transit, or pass in front of, their star.

After the TTV calculations revealed the mass of the planter, they researchers were able to extrapolate the density of the planets, finding that planets two to three times bigger than Earth have very low density, which indicates they are covered in a massive amount of gas.

"These planets are similar to Neptune, but smaller and fluffier," the researchers said in a statement. "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."

A separate team of researchers also used the TTV technique to calculate the mass of a strange exoplanet with a mass about the same as Earth's but with a diameter 60 percent larger than Earth. The planet, KOI-314c, shown in this artist's conception above, is the lightest planet to have both its mass and physical size measured.

New developments in how to calculate mass of smaller planets using TTV have made it possible for astronomers to use the technique over the more common radial velocity technique.