How do you catch a planet around another star? One of the most important techniques used is looking for transits -that is, the tiny dip in star brightness when a planet passes in front of the star itself. By studying such phenomena, a surprising number of informations can be squeezed out, for example on the remote planet atmosphere composition or temperature. Recently, researchers at the UCSC discovered something really odd about the gas giant HD 209458 b. They compared the transit depth (i.e. the amount of starlight blocked by the planet) in visible light and in the ultraviolet band that is absorbed by hydrogen -the Lyman-alpha line.
They did find that the transit is much deeper in the Lyman-alpha light, just like if the planet was much larger in that frequence (see image above). This means that HD 20958 b has a gigantic hydrogen comet-like cloud around it, that would be quite transparent for our sight but dramatically conspicuous in the ultraviolet.
Read the original story on the Systemic blog.