Hubble spots exoplanet with glowing water atmosphere

Hubble spots exoplanet with glowing water atmosphere:

This artist’s concept shows hot Jupiter WASP-121b, which presents the best evidence yet of a stratosphere on an exoplanet. Image & Caption Credit: Engine House VFX, At-Bristol Science Centre, University of Exeter

Researchers working with data from NASA’s Hubble Space Telescope have found the strongest evidence to date for the existence of a stratosphere – the layer of an atmosphere in which temperature increases with altitude – on an exoplanet (a planet outside of the Solar System). The new study was published in the August 3, 2017, issue of the journal Nature.

“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,” said Mark Marley, the study’s co-author who is based at NASA’s Ames Research Center. “We can now compare processes in exoplanet atmospheres with the same processes that happen under different sets of conditions in our own solar system.”

The researchers studied WASP-121b, an example of a type of exoplanet called a “hot Jupiter”. The planet’s mass is 1.2 times the that of Jupiter and its radius is 1.9 times Jupiter’s. Wasp-121b is much closer to its star than Jupiter is to the Sun. While it takes Jupiter 12 years to revolve once around the Sun, WASP-121 orbits its star once every three days. If the exoplanet were any closer to its star, the star’s gravity would rip it apart. WASP-121’s atmosphere is heated to 4,600 degrees Fahrenheit (2,500 degrees Celsius), hot enough to boil some metals.

An earlier study found possible signs of a stratosphere on the exoplanet WASP-33b and other hot Jupiters. The new study provides the strongest evidence yet because scientist observed the signature of hot water molecules for the first time.

“Theoretical models have suggested stratospheres may define a distinct class of ultra-hot planets, with important implications for their atmospheric physics and chemistry,” said Tom Evans, lead author and research fellow at the University of Exeter, United Kingdom. “Our observations support this picture.”

The scientists studied the atmosphere of WASP-121 by using Hubble’s spectroscopy capabilities to analyze how different molecules react to specific wavelengths of light. For example, water vapor in the planet’s atmosphere behaves in predictable ways depending on the temperature of the water.

The top of the planet’s atmosphere is heated to a blazing 4,600 degrees Fahrenheit (2,500 Celsius), hot enough to boil some metals. Image & Caption Credit: NASA, ESA, and G. Bacon (STSci)

A star’s light can penetrate deep into a planet’s atmosphere, raising the temperature of the gas there. The gas then radiates its heat into space as infrared light. If there is cooler water vapor at the top of the atmosphere, the water molecules will block certain wavelengths of light from escaping into space. If, however, the water molecules at the top of the atmosphere have a higher temperature, they will glow at the same wavelengths.

“The emission of light from water means the temperature is increasing with height,” said Tiffany Kataria, the study’s 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, raising the temperature of this layer of the atmosphere. Other bodies within the Solar System also have a stratosphere. For example, methane is responsible for heating the stratospheres of Jupiter as well as Saturn’s moon Titan.

In planets of the Solar System, the change in temperature within a planet’s stratosphere is approximately 100 degrees Fahrenheit (about 56 degrees Celsius). On WASP-121b, the temperature in the stratosphere rises by 1,000 degrees Fahrenheit (560 degrees Celsius). Researchers do not yet know which chemicals are responsible for the temperature in WASP-121b’s atmosphere. Vanadium oxide and titanium oxide are possible candidates because they are commonly found in brown dwarfs – “failed stars” that share some characteristics with exoplanets. Compounds such as these are expected to be found on only the hottest of hot Jupiters because high temperatures are required to keep them in a gaseous state.

“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, the study’s co-author who worked on this research while at NASA’s Goddard Space Flight Center, Greenbelt, Maryland.

Video courtesy of NASA

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