Cloud or haze layers in the upper atmospheres of extrasolar planets may make them appear bigger than they really are, a study led by Dr. Helmut Lammer from the Austrian Academy of Sciences’ Space Research Institute suggests.
A hot Neptune-sized exoplanet behind its parent star. Image credit: NASA / JPL-Caltech.
Since the first confirmed discovery in 1993, astronomers have found more than 3,000 extrasolar planets.
A key goal now is to characterize known planets by mass, size and composition, to better understand the evolution of planetary systems, and the prospects for terrestrial planets that might support life.
In 2014 Dr. Lammer and his colleagues used ESA’s CoRoT (Convection, Rotation and planetary Transits) space telescope to study the upper atmosphere of two low-mass exoplanets that are regularly seen to pass in front of their parent star.
The two planets, CoRoT-24b and CoRoT-24c, orbit the star in 5 and 12 days, appear to be 3.7 and 4.9 times the diameter of the Earth, and have respective masses of 5.7 and 28 times Earth.
CoRoT-24c is similar in mass to Neptune. CoRoT-24b is less than a quarter as massive, but is similar in size, so seems to have a very low density.
With such short orbits, both planets are close to and will experience dramatic heating from their star, CoRoT-24.
Dr. Lammer’s team modeled this and found that CoRoT-24b would see its atmosphere evaporate within 100 million years, if it really is as big as suggested.
But the star is billions of years old, so the planet should have lost its atmosphere long ago.
The solution seems to be that CoRoT-24b is only about half as big as thought.
Dr. Lammer argues that an extended, very thin, atmosphere, surrounds a relatively compact planet, but has high altitude features that confuse observations.
“The radius is based on what we see when the planet makes its transit,” Dr. Lammer said. “This is probably distorted by clouds and haze high in the atmosphere, in a region where atmospheric pressure is otherwise very low.”
“This effect needs to be considered by future exoplanet missions, like ESA’s CHEOPS (CHaracterising ExOPlanet Satellite) mission due to launch in December 2017. Results for some worlds found by NASA’s Kepler space telescope may also need to be re-evaluated,” said co-author Dr. Luca Fossati, also from the Space Research Institute.
“Our results show that CHEOPS scientists need to be cautious about their first measurements.”
“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.”
The results are reported in the journal Monthly Notices of the Royal Astronomical Society.
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H. Lammer et al. Identifying the ‘true’ radius of the hot sub-Neptune CoRoT-24b by mass loss modeling. MNRAS, published online May 16, 2016; doi: 10.1093/mnrasl/slw095
