Using the NASA/ESA Hubble Space Telescope, astronomers have detected a stratosphere – an atmospheric region that is very important to life – on a massive and hot exoplanet called WASP-33b.
This is an artist’s concept of a hot Jupiter exoplanet. Image credit: NASA / ESA / G. Bacon, STScI.
The presence of a stratosphere can provide clues about the composition of a planet and how it formed. This atmospheric layer occurs when molecules in the atmosphere absorb UV and visible light from the star. This absorption warms the stratosphere and acts as a kind of sunscreen layer for the planet below.
Until now, researchers were uncertain whether these molecules would be found in the atmospheres of massive, hot exoplanets.
“Some of these planets are so hot in their upper atmospheres, they are essentially boiling off into space. At these temperatures, we don’t necessarily expect to find an atmosphere that has molecules that can lead to these multilayered structures,” said Dr Avi Mandell of NASA’s Goddard Space Flight Center in Greenbelt, a co-author of the study accepted for publication in the Astrophysical Journal (arXiv.org preprint).
In the atmosphere of our planet, the stratosphere sits above the troposphere – the turbulent, active-weather region that reaches from the ground to the altitude where nearly all clouds top out. In the troposphere, the temperature is warmer at the bottom and cools down at higher altitudes.
The stratosphere is just the opposite. In this layer, the temperature increases with altitude, a phenomenon called temperature inversion.
On Earth, temperature inversion occurs because ozone in the stratosphere absorbs much of the Suns UV radiation, preventing it from reaching the surface, protecting the biosphere, and therefore warming the stratosphere instead.
Similar temperature inversions occur in the stratospheres of other planets in our Solar System, such as Jupiter and Saturn. In these cases, the culprit is a different group of molecules called hydrocarbons.
Neither ozone nor hydrocarbons, however, could survive at the high temperatures of most known exoplanets. This leads to a debate as to whether stratospheres would exist on them at all.
Using the Wide Field Camera 3 aboard the NASA/ESA Hubble Space Telescope, Dr Mandell and his colleagues have settled this debate by identifying a temperature inversion in the atmosphere of the hot Jupiter WASP-33b.
The planet has about 4.5 times the mass of Jupiter and orbits the star WASP-33 (also known as HD 15082), located around 380 light-years away in the northern constellation of Andromeda.
The astronomers used the Hubble observations, and data from previous studies, to measure emission from water and compare it to emission from gas deeper in the planet’s atmosphere. They determined that emission from water was produced in the stratosphere at about 6,000 degrees Fahrenheit (3,300 degrees Celsius). The rest of the emission came from gas lower in the atmosphere that was at a temperature about 3,000 degrees Fahrenheit (1,650 degrees Celsius).
They also presented evidence that WASP-33b’s atmosphere contains titanium oxide, one of only a few compounds that is a strong absorber of visible and UV radiation and capable of remaining in gaseous form in an atmosphere as hot as this one.
“Understanding the links between stratospheres and chemical compositions is critical to studying atmospheric processes in exoplanets. Our finding marks a key breakthrough in this direction,” said study co-author Dr Nikku Madhusudhan from the University of Cambridge, UK.
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Korey Haynes et al. 2015. Spectroscopic Evidence for a Temperature Inversion in the Dayside Atmosphere of the Hot Jupiter WASP-33b. ApJ, accepted for publication; arXiv: 1505.01490
