How the discovery of a hot Jupiter’s stratosphere could help the search for life on other planets

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For the first time ever, an exoplanet located 880 light-years away was found to have a stratosphere — a layer in the upper atmosphere where the temperature increases the higher up you go. The discovery may help astronomers refine the techniques that could lead to the discovery of alien life on other exoplanets.

The planet, described in a study published today in Nature, is called WASP-121b. It belongs to a class of exoplanets called hot Jupiters — worlds that are so big and hot that they are fairly easy to study, thus allowing astronomers to hone their skills and confirm their theories. Today’s discovery confirms what astronomers suspected: super hot gas giants outside our Solar System can have a stratosphere.

WASP-121b is massive — nearly twice the size of our Jupiter. And because it orbits much closer to its host star than Mercury orbits around the Sun, its atmosphere heats up to more than 4,500 degrees Fahrenheit. (At that temperature, you could boil iron.) Using NASA’s Hubble Space Telescope, the researchers were able to detect glowing water molecules in the planet’s atmosphere, a clear signal that WASP-121b has a stratosphere. What exactly is causing the stratosphere is more of a mystery, but it could be gases like vanadium oxide and titanium oxide, which are believed to act like the ozone on Earth, the study says.

On Earth, the lower atmosphere is divided into two regions: the troposphere, which is closer to the surface, and the stratosphere, which is the upper layer. The stratosphere contains ozone, which absorbs harmful ultraviolet radiation from the Sun and heats up the stratosphere in the process. That means that, while the temperature decreases the higher up you go in the troposphere, in the stratosphere, the higher up you go, the temperature gets warmer. Most other planets in our Solar System — like Mars, Jupiter, Saturn, and even moons like Saturn’s Titan — have stratospheres. (On Jupiter and Titan, for instance, methane is responsible for them.) So astronomers have long thought that planets outside our Solar System would also have one.


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How Earth’s stratosphere works.
Image: News & Views by Kevin Heng / Nature

On hot Jupiters, scientists believe that stratospheres are created by chemicals like vanadium oxide and titanium oxide, which absorb radiation from stars and exist as gases only at the highest temperatures. But up until now, scientists hadn’t been able to find conclusive evidence that stratospheres on exoplanets exist. “The fact that we hadn’t seen any [stratosphere] where we expected we would was challenging our expectations,” Michael Line, an assistant professor in the School of Earth and Space Exploration at Arizona State University, who wasn’t involved in the study, tells The Verge. So finally finding one is very exciting.

To study WASP-121b, the researchers looked at how the planet’s brightness changed at different wavelengths of light. They saw that water molecules in WASP-121b’s upper atmosphere were glowing and emitting light instead of absorbing light. That means there’s a layer of hot water gas up there, not a cooler layer that’d be expected if there’s no stratosphere. The change in temperature within WASP-121b’s stratosphere is extreme: about 1,800 degrees Fahrenheit. For reference, on planets in our Solar System, the change in temperature is usually less than 212 degrees Fahrenheit. “It’s some of the best evidence to date of a stratosphere in an exoplanet,” lead author Thomas Evans, a research fellow at the University of Exeter, tells The Verge.


An illustration of WASP-121b.
Image: NASA, ESA, and G. Bacon (STSci)

There’s still a lot we don’t understand about WASP-121b and its atmosphere. For starters, we don’t know exactly what’s causing the stratosphere, Evans says. The researchers were able to detect vanadium oxide and some titanium oxide, but the data was not conclusive. The discovery also raises questions about other similar hot Jupiters that were found to have no stratosphere, says Thomas Beatty, a postdoc at the Center for Exoplanets and Habitable Worlds at Penn State, who did not take part in the study. Why does WASP-121b have a stratosphere, and others don’t? What’s really going on in WASP-121b’s atmosphere?

The field of exoplanet research is just at the beginning, and part of it focuses on finding traces of life outside of our Solar System. Worlds like WASP-121b are way too hot to possibly host any kind of life as we know it — whether or not they have a stratosphere. But studying them is still key: it allows researchers to test their theories and learn more about the underlying physics. “They are a first step towards honing our skills, developing our tools, getting ready for the things that are more Earth-like,” says Kevin Heng, the director of the Center for Space and Habitability at the University of Bern. “This is the first step in a long road.”

It’s a bit like playing Super Mario Bros. 3, says Line at Arizona State University. In the game, you can use a Warp Whistle and skip levels to get to the game’s final world. But without going through all the levels, learning skills and collecting tools, you have a slim chance of beating Bowser at the end. Studying planets like WASP-121b is like going through all those levels: it allows astronomers to build their knowledge, so when we have an Earth-like planet that could host life, we’re better prepared to study it. “You don’t want to skip [hot Jupiters] along your journey because you want to build your tools,” Line says. “So when we think we’re measuring biosignatures on maybe a terrestrial planet, we’ll have a better understanding of the underlying physics that could possibly trip us up.”

Next, Evans wants to keep observing WASP-121b for longer periods of time. Eventually, he hopes to study the planet using NASA’s James Webb Space Telescope, which will launch next year and will be the most powerful space telescope ever built. JWST will allow astronomers to make even more precise measurements, and hopefully it’ll solve the mystery of what’s causing the stratosphere on WASP-121b. “James Webb is going to be the so-called game-changer,” Line says. “The amount of data per planet is going to be orders of magnitudes greater.”

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