20 December 2013 (Daily Galaxy) - To date, scientists have confirmed the existence of more than 900 exoplanets circulating outside our solar system. To determine if any of these far-off worlds are habitable requires knowing an exoplanet’s mass — which can help tell scientists whether the planet is made of gas or rock and other life-supporting materials.

“Knowing the mass is a very important piece of the puzzle,” says Mark Swain, a research scientist at NASA’s Jet Propulsion Laboratory. “If you found the composition of the planet was almost certainly solid, that required a significant amount of water mixed in with a silicate core, and you knew it had habitable zone-type temperatures, you might make a good case for in-depth studies of that world, because it has what seems like the ingredients for a habitable planet.”

But current techniques for estimating exoplanetary mass are limited. Radial velocity is the main method scientists use: tiny wobbles in a star’s orbit as it is tugged around by the planet’s gravitational force, from which scientists can derive the planet-to-star mass ratio. For very large, Neptune-sized planets, or smaller Earth-sized planets orbiting very close to bright stars, radial velocity works relatively well. But the technique is less successful with smaller planets that orbit much farther from their stars, as Earth does.

Now scientists at MIT have developed a new technique for determining the mass of exoplanets, using only their transit signal — dips in light as a planet passes in front of its star. This data has traditionally been used to determine a planet’s size and atmospheric properties, but the MIT team has found a way to interpret it such that it also reveals the planet’s mass.

“With this method, we realized the planetary mass — a key parameter that, if missing, could have prevented us from assessing the habitability of the first potentially habitable Earth-sized planet in the next decade — will actually be accessible, together with its atmospheric properties,” says Julien de Wit, a graduate student in MIT’s Department of Earth, Atmospheric and Planetary Sciences. De Wit is lead author on a paper published today in the journal Science, with co-author Sara Seager, Professor of Physics and Planetary Science.

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