Hundreds of extrasolar planets – also called exoplanets – have been detected so far, and an intense worldwide research effort is ongoing to unveil the details of these distant worlds. Some of the latest discoveries have been made by EU-funded scientists.
Newly discovered worlds are described as Earth-like planets, depending on the distance between the exoplanet and the host star. This distance is only half of the picture. The other half is determining if an exoplanet has an atmosphere and what its contents are.
In other words, because it is in the habitable zone around its host star, an exoplanet is not necessarily habitable. Recently, EU-funded scientists used data from the Hubble Space Telescope and the Very Large Telescope in the Atacama Desert in Chile in an attempt to detect atmospheres around exoplanets.
The ACE (Atmospheric characterisation of exoplanets) project focused on an Earth-sized world hypothesised in close orbit around α Centauri B, the closest star to us. The exoplanet had been estimated to orbit the neighbouring star in just 3.2 Earth days, making for a very hard and certainly doubtful detection.
The follow-up observations could not repeat the results found with initial 2012 observations from the High Accuracy Radial velocity Planet Searcher (HARPS) at the La Silla telescope. Late in 2015, with new observations, it was concurred that there is no such exoplanet.
This did not rule out the existence of other exoplanets found around other stars, like 55 Cancri e orbiting so close to its host star that temperatures higher than 2 000 degrees Kelvin are expected on its surface. Not habitable, at least not for anything remotely like the kind of life on Earth, 55 Cancri b was found to be surrounded by an extended atmosphere.
The extended atmosphere of this exoplanet is also partially transiting the star, something that had never been seen before. This result of the ACE project suggests that moderately irradiated exoplanets can experience substantial mass loss through thermal winds. Extremely small planets of the size of Mercury, subject to intense irradiation, are expected to undergo periods of atmospheric erosion, but not super-Earths. The most important result of the ACE project is that scientists have begun to probe atmospheric conditions on distant worlds as small as a couple Earth radii and not only on giant exoplanets.
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