New Technique Measuring Exoplanet Mass
December 19, 2013

Novel Technique Aims To Measure Mass Of Distant Exoplanets

Lee Rannals for - Your Universe Online

A new technique developed by a team at MIT could help scientists determine the mass of exoplanets, using just their transmission spectra. Scientists have discovered more than 900 exoplanets outside our solar system so far, and more are being found every week.

Hunting for exoplanets takes a combination of patience, the right equipment and technique. Space telescopes help keep watch for dimming starlight, indicating that an exoplanet could have passed between the star and Earth. These 'dips' in light are what scientists use to learn much about the exoplanet, including its atmosphere, orbit and size.

A star’s gravity is what causes a planet to orbit it, but the planet isn’t the only object in the dance being influenced by gravity. The star is actually slightly affected by the planets’ gravity, and scientists look for tiny wobbles in a star’s orbit to help derive the planet-to-star mass ratio. This method, called radial velocity, is limited and does not work very well for small planets that orbit much farther from their stars.

The latest technique, developed by Julien de Wit and colleagues, uses only dips in light as a planet passes in front of its star. The team found a way to use dips to interpret the planet’s mass as well.

“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,” explained de Wit, a graduate student in MIT’s Department of Earth, Atmospheric and Planetary Sciences.

Knowing a planet’s mass can provide a glimpse of the planet’s surface and even internal activity, such as any plate tectonics, its internal cooling and convection, how it generates magnetic fields, and whether gas escapes from its atmosphere or not. De Wit, lead author of the paper published in the journal Science, says the mass affects everything on a planetary level, and knowing this bit of information allows one to learn more about the planet’s properties.

Researchers worked out a standard equation describing the effect of a planet’s temperature, gravitational force, and atmospheric density on its atmospheric pressure profile. De Wit realized that if a planet’s mass can be derived from its gravitational force, then the planet’s temperature, pressure profile and density could also help find mass.

The team used an 18th-century mathematical constant called the Euler-Mascheroni constant to reveal the individual effects of each parameter. This constant worked as an encryption key to decode the process by which the properties of an exoplanet’s atmosphere are found in transmission spectrum.

“It really helps you unlock everything and reveal, out of these crazy equations, which atmospheric properties do what, and how,” de Wit said in a statement. “You find this constant in a lot of physical problems, and it’s fun to see it reappearing in planetary science.”

De Wit and colleagues applied the method to the exoplanet 189733b, which is located 63 light-years away. They were able to come up with the same mass measurement as others had done using radial velocity, helping to prove their new technique worked.

“We find good agreement between the mass retrieved for the hot Jupiter HD 189733b from transmission spectroscopy with that from RV measurements. Our method will be able to retrieve the masses of Earth-sized and super-Earth planets using data from future space telescopes that were initially designed for atmospheric characterization,” the authors wrote in the journal.