New Technique Measures Exoplanet Composition
January 9, 2014

New Technique Unveils Composition Of Gassy Exoplanets

redOrbit Staff & Wire Reports - Your Universe Online

Scientists have successfully measured the masses of approximately 60 exoplanets discovered by the Kepler space telescope – research that will help them determine the composition of these planets and ultimately whether or not they are capable of supporting life.

Their findings, which were presented Monday during the 223rd annual meeting of the American Astronomical Society (AAS) in Washington, DC, covered planets located outside of our solar system that are larger than Earth but smaller than Neptune.

According to the study authors, the measurements will vastly expand astronomers’ knowledge of sub-Neptune exoplanets, because once a planet’s size and mass are known, the density of the planet can be determined and its composition inferred. Their findings will also appear in the upcoming edition of the Astrophysical Journal.

“We were surprised to learn that planets only a few times bigger than Earth are covered by a lot of gas,” explained author and Northwestern University assistant physics and astronomy professor Yoram Lithwick.

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

In order to measure the masses of the planets discovered by Kepler, Lithwick and his colleagues used a technique known as transit time variation (TTV), a sensitive method of detecting exoplanets by observing variations in the timing of a transit that is particularly useful in locating planets approximately the same size as Earth.

They found that planets two to three times larger than ours had extremely low densities, suggesting that they are covered by a tremendous amount of gas. While those planets are comparable to Neptune (only smaller),slightly smaller ones were found to be denser than rock, meaning that they are like a denser version of Earth.

“The TTV technique used in this work requires two planets that orbit the same star, both of which also transit the star,” the university explained in a statement. “The planets pull gravitationally on one another and thereby change the time at which each planet transits its star, relative to when it would have transited if it were orbiting the star alone. Therefore, measuring times of transit reveals the planets' masses.”

One advantage the TTV technique has over the more commonly used radial velocity method (RV) is that it requires no additional observations beyond those already completed by Kepler, Lithwick said. On the negative side, however, the measurement requires interaction between planets which makes it difficult to determine the masses of those worlds.

“As a result, until recently only a handful of sub-Neptune planets have had their masses measured,” the researchers said. To overcome this problem, Lithwick and his colleagues developed a simple formula for turning observed transit times into masses. Lithwick observed that recent mass measurements with RV are confirming the patterns already recorded with TTV, thus corroborating the conclusions reached using the two independent techniques.