Discovery of hot Jupiter's stratosphere could help search for life on other planets

For the first time, astronomers have discovered an exoplanet 900 light-years away from our Solar System with a stratosphere. The discovery will help astronomers hone the techniques that could lead to the discovery of life on other exoplanets.

The study, published in Nature, confirms a theory that dates back to the early 2000s: super-hot gas giants outside our Solar System can have a stratosphere — a layer of the upper atmosphere where the temperature increases the higher up you go. "This result is exciting because it shows that a common trait of most of the atmospheres in our solar system, a warm stratosphere, also can be found in exoplanet atmospheres," study co-author Mark Marley, of NASA's Ames Research Center in California's Silicon Valley, said in a statement.

The planet is called WASP-121b. It belongs to the so-called ‘hot Jupiter’ exoplanet class, and it closely orbits a star that is very bright.  These types of planets are so big and hot that they are quite easy to study, thus allowing astronomers gain some insights on exoplanets and answer questions about their formation.

The planet’s mass is 1.2 times that of Jupiter, and its radius is about 1.9 times Jupiter's. But while Jupiter orbits our sun once every 12 years, WASP-121b has an orbital period of just 1.3 days. This exoplanet is so close to its star that if it got any closer, the star's gravity would start ripping it apart. It also means that the top of the planet's atmosphere is heated to 2,500°C.

WASP-121b

The top of the planet's atmosphere is heated to 2,500°C, hot enough to boil some metals. Image & Caption Credit: NASA, ESA, and G. Bacon (STSci)

To study the stratosphere of WASP-121b, scientists analyzed how different molecules in the atmosphere react to particular wavelengths of light, using the Hubble telescope. "The emission of light from water means the temperature is increasing with height," said Tiffany Kataria, study co-author based at NASA's Jet Propulsion Laboratory, Pasadena, California. "We're excited to explore at what longitudes this behavior persists with upcoming Hubble observations."

In Earth's stratosphere, ozone gas traps ultraviolet radiation from the sun, which raises the temperature of this layer of the atmosphere. While in solar system planets the change in temperature within the stratosphere is typically around 56°C, on WASP-121b the temperature rises by 560°C. Scientists do not yet know what chemicals are causing the temperature increase in WASP-121b's atmosphere, although they suspect it could be vanadium and titanium oxides.

"This super-hot exoplanet is going to be a benchmark for our atmospheric models, and it will be a great observational target moving into the Webb era," said Hannah Wakeford, co-author.

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