Exoplanet mass revealed in light

A distant planet has been weighed with light that passed through its atmosphere 63 years ago. This new mass-measuring technique should allow researchers to determine which distant planets could support life.

To identify a truly Earthlike exoplanet with the potential for life, astronomers need to confirm three essential attributes: an atmosphere that contains life-supporting molecules such as oxygen and water, plus a size and mass similar to Earth’s. The only attribute easy to measure now is size: Telescopes provide good estimates by measuring planets’ shadows when they cross in front of their stars. Soon, detailed observations of exoplanet atmospheres will become possible. With NASA’s James Webb Space Telescope, set for launch in 2018, astronomers will be able to analyze starlight that passed through a planet’s atmosphere, known as a transmission spectrum. That spectrum can reveal what kinds of molecules the planet’s atmosphere holds.

But an Earth-sized planet with water in its atmosphere could still be made of gas. That’s why astronomers need to know a planet’s mass. Astronomers can use the radial velocity method, which measures a planet’s gravitational tug on its star, but it works mainly for large planets too close to their stars for life.

MIT astronomer Julien de Wit didn’t set out to weigh exoplanets; he was interested in their atmospheres. But he and MIT colleague Sara Seager realized that measurements of a planet’s atmosphere and mass are linked: For planets of the same size, more massive ones will have thinner atmospheres because of their stronger gravitational pull. At the same time, heavier molecules tend to sink toward the surface, also thinning a planet’s atmosphere.

The researchers conclude in the Dec. 20 Science that by determining planet sizes and atmospheric composition, they can calculate planets’ masses. The technique requires astronomers to dissect the same transmission spectrum they would use to characterize a planet’s atmosphere. “It’s almost like you can make this analysis for free,” says Rory Barnes, an astronomer at the University of Washington in Seattle, who was not involved in the study.

To test the technique, dubbed MassSpec, de Wit and Seager calculated the mass of HD 189733b, a sizzling gas giant located 63 light-years away in the constellation Vulpecula. Since the planet is large, its mass had also been estimated using the radial velocity method. The results from the two methods matched almost perfectly.

Because the Webb Telescope will be able to measure transmission spectra of distant planets, de Wit says, researchers should also be able to derive the planets’ masses. That would enable astronomers to evaluate the probability of life on a wide array of worlds, including the appealing ones that are too small and too distant to weigh with the radial velocity method. “This is likely the only way to get the mass of planets that are … potentially habitable,” de Wit says.

Barnes questions MassSpec’s ability to weigh some planets, particularly those with atmospheres obscured by high, thick clouds. But the technique is still invaluable, he says. “It’s a no-brainer that we’d want to do this every chance we get.”