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Earlier this year, a group of scientists including the new ARC lecturer Dr. Markus Janson reported the detection of a planetary companion to the nearby Sun-like star GJ 504. The planet, named GJ 504 b, is a gas giant more massive than Jupiter in a wide orbit around its parent star. Unlike most exoplanets, which are detected indirectly through the various effects they have on the central star, GJ 504 b was discovered through direct imaging of the planet itself. The ability to resolve the faint planet from its much brighter host star opens up the possibility to investigate the planet in more detail than is otherwise possible, for instance by examining its atmosphere.
Consequently, Dr. Janson has led a follow-up study of GJ 504 b in order to try to determine the composition of its atmosphere. This has now resulted in a clear detection of methane. Like most molecules, methane interacts strongly with light only at some specific wavelengths. As a result, a planet with methane in its atmosphere will become much darker at those particular wavelengths, and can even become completely invisible, if there is a sufficiently large abundance of methane. This is precisely what is observed for GJ 504 b. An image taken in a filter that only lets through light outside of the methane bands shows the planet clearly, but in a simultaneous image with a filter that only lets through light inside of those bands, the planet disappears altogether.
The discovery of methane in the atmosphere of GJ 504 b was made as part of the SEEDS project, which is an international collaboration using the HiCIAO camera at the Subaru telescope, which is run by the National Astronomical Observatory of Japan. The publication describing the results has been accepted for publication in the Astrophysical Journal Letters.