Image: Nature.

Hubble Fellow Mercedes López-Morales is the author of a News and Views piece in this week’s Nature that discusses a breakthrough in exoplanet searches. For years, astronomers have been detecting exoplanets through star-planet binary systems. The star in such systems is much brighter than its orbiting planet, which makes its light and Doppler shift much easier to detect than those of the planet. For this reason, the masses of exoplanets have never been measured directly, but only inferred. In a new study by Snellen et al., the group developed a strategy to observe an exoplanet’s Doppler signal, enabling them to detect the planet’s mass directly.

While exoplanet HD 209458b transited its host star, the team monitored it by using high-resolution spectroscopy, which is sensitive to small changes in a planet’s orbital velocity. They focused their analysis on a small region in the near-infrared part of the spectrum, between 2.29 and 2.35 micrometers, where they predicted that the atmosphere of the planet would produce spectral lines. During a transit, the light from the star passes through the thin atmosphere of the planet, and the chemicals in that atmosphere produce small spectral absorption lines on the stellar spectrum, thus the planet’s atmospheric spectral lines appear shifted by an amount that can be detected through careful analysis. For more information, see López-Morales’ News and Views or the Nature paper.