The cloud’s shadowIndeed, scientists see the cloud’s shadow when it passes in front of the star. Essentially comprised of hydrogen, it absorbs the ultra violet light emitted by the star and the phenomenon remains invisible on Earth because the ultra violet light is blocked by the atmosphere. It was therefore necessary to use the Hubble spatial telescope for these observations: “When you’re searching in the visible region you only see the shadow of a planet whose size is equivalent to four times that of Earth like Neptune. But if the sensitive eye is pointed toward the Hubble ultra violet light, the planet is transformed into a veritable monster, far bigger than the star”, explains David Ehrenreich. Evaporation is spectacular, but it would not threaten the atmosphere of GJ436b, an exoplanet which was formed several thousands of years ago and which would have only lost 10% of its atmosphere. Massive evaporationBut evaporation might, however, explain the disappearance of atmospheres observed on rocky exoplanets, which rotate very close to their star and are extremely hot, such as the famous “super-Earths” discovered by the Corot and Kepler spatial telescopes. Just like the GJ436b, these might have been hot Neptunes orbiting around more luminous stars which would have circulated in their atmosphere that ended up leaving the rocky centre of the planet bare.
A comet destiny?This type of observation is very promising in the search of habitable planets since “hydrogen from the ocean water that evaporate on slightly hotter terrestrial planets than the Earth could be detected”, as Vincent Bourrier suggests, the second author of these results.
The phenomenon might even explain the disappearance of hydrogen outside our atmosphere, an element that abounded there more than 4 billion years ago.
Finally, such observations might help to envisage the distant future of our planet, when, in 3 or 4 billion years, the Sun will become a red giant and will circulate in our atmosphere making life impossible on Earth. Astrophysicists now hypothesize that our planet would be transformed into a giant comet, thus resembling a «hairy» star just like GJ436b.
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