New Method Will Help Us Find Earth-like Exoplanets by Calculating Their Mass from Atmospheric Properties

December 25, 2013

Artist's impression of planet's transmission spectrum.

Artist's impression of planet's transmission spectrum. Credit: Christine Daniloff/MIT, Julien de Wit

Refinement in techniques we use to detect exoplanets has led to ever increasing number of these planets being confirmed. To this date, over 1000 planets have been discovered orbiting a star outside our solar system. Out of many methods, radial velocity and transit method are two most prominent. Now, scientists have discovered a new method using planet's transmission spectrum that could prove to be very valuable.

Radial velocity is used to measure exoplanet's mass by observing wobbles in star's orbit caused by the planet's gravity as it revolves around the star. This method is, however, less effective the smaller the planet is relative to the star and the farther away it orbits.

Transit method is used to determine the radius of the planet as it passes in front of its star, thus blocking the part of light that reaches our telescopes. This method tells us nothing of planet's mass.

Up until now that is. Scientists at the Massachusetts Institute of Technology (MIT), USA, have found a way to use transit method to calculate planet's mass. "With this method, we realized the planetary mass will actually be accessible, together with its atmospheric properties," said Julien de Wit from MIT. "The mass affects everything on a planetary level. If you don't get it, there is a large part of the planet's properties that remains undetermined."

Mass of the planet can tell us its composition, magnetic fields generation, surface features and even its internal geological activity. If we strive to find Earth-like planets, whether its for colonization purposes or in search of extraterrestrial life, it is crucial to detect small, rocky planets that have been very elusive until now.

This method promises to determine masses of these planets based on three features we are able to detect by using planet's transmission spectrum, analyzing the wavelengths of light that passes through planet's atmosphere: atmospheric temperature, density and pressure profile – the rate at which atmosphere becomes thinner the farther from the surface it is. 

Transmission spectrum of a planet is a result of these three properties, and to determine how each parameter effects it, de Wit made new derivations from the first principles of radiative transfer and found that Euler-Mascheroni constant can act as a key to decode the process at hand.

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

De Wit used this method on a recently discovered exoplanet HD189733b, 63 light-years away. Using this technique, he calculated mass to be identical to the one obtained by using radial velocity method. With new generation of space telescopes planned, such as James Webb Space Telescope, with higher resolution, providing more details, this technique could prove valuable in discovering Earth-like planets in our galaxy.

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