MIT Scientists develop new technique to measure mass of exoplanets

Artistic rendering of a planet's transmission spectrum. 

Credit: CHRISTINE DANILOFF /MIT, JULIEN DE WIT

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.

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.

Spitzer Space Telescope

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.

Julien de Wit

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, the Class of 1941 Professor of Physics and Planetary Science.


Researchers at MIT explain what exactly an exoplanet or extrasolar planet is, why we study them and how you can detect them.

"The mass affects everything on a planetary level, such as any plate tectonics, its internal cooling and convection, how it generates magnetic fields, and whether gas escapes from its atmosphere," de Wit says.

"If you don't get it, there is a large part of the planet's properties that remains undetermined."

Using large telescopes such as the NASA's Spitzer and Hubble Space Telescopes, scientists have been able to analyze the transmission spectra of newly discovered exoplanets.

A transmission spectrum is generated as a planet passes in front of its star, letting some light through its atmosphere.

By analyzing the wavelengths of light that pass through, scientists can determine a planet's atmospheric properties, such as its temperature and the density of atmospheric molecules. From the total amount of light blocked, they can calculate a planet's size.

More information: "Constraining Exoplanet Mass from Transmission Spectroscopy," by J. de Wit et al. Science, 2013. DOI:10.1126/science.1245450