Orbiting a hot F-star in only 1.27 days, WASP-121b is a highly irradiated hot Jupiter found by Laëtitia Delrez et al using the WASP-South survey. A team led by Tom Evans at Exeter has now pointed the Hubble Space Telescope at WASP-121b and found that its atmosphere shows a “stratosphere”. That is, the higher layers of the atmosphere appear to be hotter than the lower layers.
This is possible if molecules high in the atmosphere absorb radiation very efficiently. The “stratosphere” interpretation comes from finding spectral features caused by water, but seeing them in emission (as expected if the atmospheric temperature increases with height) rather than in absorption (expected if the temperature declines with height).
The data show the Hubble spectrum observed during transit using the WFC3 instrument. The red line is a model including a stratosphere. The blue lines are, for comparison, colder “brown dwarfs” which don’t have a stratosphere. The WFC3 data (circles with error bars) clearly favour the stratosphere interpretation.
NASA have put out a press release about the discovery, while the press team at Exeter have produced an illustration of the highly irradiated planet:
The story has been picked up by CNN, The Telegraph, New Scientist, NDTV, phys.org, the Mail Online, the International Business Times, Gizmodo Australia and over 40 other news and science websites.
The hot Jupiter WASP-121b, discovered recently by Laetitia Delrez et al, is a very good opportunity for learning what the atmosphere of an exoplanet is made of. Being in a close, 1.27-day orbit around a hot star makes the atmosphere hot, while being a bloated planet of 1.9 Jupiter radii makes the atmosphere puffy. That means one can observe the planet in transit, projected against its star, and readily observe spectral features caused by the atmosphere absorbing star light.
Thomas Evans et al have pointed the Hubble Space Telescope at WASP-121b. To model the resulting spectrum they find they need an atmosphere containing titanium oxide, vanadium oxide, and iron hydride. In the plot below, models with these molecules are plotted red and yellow, and fit the observations, while models without, plotted in green and purple, do not.
The model also shows that WASP-121b has clear skies, rich in water vapour. It looks as though WASP-121b will become one of the most important exoplanets for such atmospheric characterisation work.