A planet at maximum fluffiness
It’s not often that the word «fluffy» appears in a press release about a new astronomical discovery. It refers to the exoplanet WASP-127b, which orbits a star a good 500 light-years from Earth that is slightly larger than the Sun. An international team of astronomers has now not only detected clouds there, but also measured their height with unprecedented precision.
WASP-127b is a so-called «hot Saturn» – a giant planet with a similar mass to Saturn, but unlike our (cold) Saturn, it orbits very close to its sun. During one orbit around its star, WASP-127b therefore receives 600 times more radiation in about four days than Earth does in a year, heating up to 1100 degrees Celsius. This causes the planet to inflate to a radius 1.3 times larger than Jupiter’s at only one-fifth the mass, making it one of the least dense or «fluffiest» exoplanets ever discovered.
The extended nature of fluffy exoplanets makes them easier to observe, making WASP-127b an ideal candidate for researchers working to characterize its atmosphere. The team observed the planet as it passed by its host star to detect patterns in its spectrogram that become embedded in starlight as it is filtered through the planet’s atmosphere and modified by its chemical constituents. By combining infrared observations from the ESA/NASA Hubble Space Telescope (HST) and visible light measurements with the ESPRESSO spectrograph on the European Southern Observatory’s Very Large Telescope in Chile, the researchers were able to examine different parts of the atmosphere. The results yielded a few surprises.
First, as is common for this type of planet, the researchers detected sodium, but at a much lower altitude than expected. Second, there were strong water vapor signals in the infrared, but none at all at visible wavelengths. «This suggests that the water vapor at lower altitudes is shielded by clouds that are opaque at visible wavelengths but transparent in the infrared,» said Allart of iREx/Université de Montréal and Université de Genève, who led the study. The combined data from the two instruments allowed the researchers to narrow down the height of the clouds to an atmospheric layer with a pressure between 0.3 and 0.5 millibars.
«We don’t yet know the composition of the clouds, but we do know that they are not composed of water droplets as they are on Earth,» Allart says. It also remains a mystery why sodium is found in such an unexpected place on this planet. The team’s observations with the ESPRESSO instrument also suggest that, unlike planets in our solar system, WASP-127b not only orbits around its star the wrong way, but also orbits in a plane other than the equatorial plane, the ecliptic. «Such an orientation is unexpected for a hot Saturn in a relatively old star system and could be caused by an unknown companion,» Allart said.