Atmospheric clouds could hide the true size of exoplanets

Mists and clouds up high in the atmospheres of exoplanets could make them seem bigger than they are, according to a new study published in the Monthly Notices of the Royal Astronomical Society.

In the study, researchers used the European Space Agency (ESA) CoRoT space telescope to analyze the upper atmosphere of two low-mass planets that are frequently seen passing in front of, or transiting, the star they orbit.

The two planets orbit their star in 5 and 12 days. Such quick orbits mean both planets are close to their host sun and should experience spectacular heating. The team learned that the smaller of the two planets would see its atmosphere burn off within 100 million years, if it really is as big as it looks. However, the star is billions of years old, so the planet ought to have lost its atmosphere a long time ago.

Hiding the true size of an exoplanet

The study team concluded the planet is probably about half as big as it seems. An extended, wispy atmosphere would have features that confuse observations, the researchers said.

“The radius is based on what we see when the planet makes its transit,” study author Helmut Lammer, an astronomer from Austrian Academy of Sciences, said in a news release. “This is probably distorted by clouds and haze high in the atmosphere, in a region where atmospheric pressure is otherwise very low.”

Study author Luca Fosseti, from Imperial College London, said his team’s results need to be considered by future efforts to study exoplanets, like the ESA CHaracterising ExOPlanets Satellite (CHEOPS) mission set to launch in next year. The conclusion of the study also means planets observed by NASA’s space-based Kepler observatory may also need to be reassessed.

“Our results show that CHEOPS scientists need to be cautious about their first measurements,” Fossati said. “Since Kepler has also discovered several similar low-density and low-mass planets, it is very likely that the size measured for many of them also differ from the true value, so there could be a bias in the results.”

Since the discovery of the first exoplanet in 1993, astronomers have found more than 3,000 worlds orbiting distant stars. Efforts are now focusing on categorizing exoplanets by mass, size and composition.

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Image credit: NASA-JPL

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