WASP-121b is about two times bigger than Jupiter and 1.2 times more massive. Discovered in 2016, the planet is located 881 light-years away in the constellation Puppis. Its atmosphere is so hot that metals such as iron and magnesium are vaporizing and escaping the planet’s gravitational pull.
This artist’s illustration shows WASP-121b, an alien world that is losing magnesium and iron gas from its atmosphere. Image credit: NASA / ESA / J. Olmsted, STScI.
Hot Jupiters are gas giants with orbital period less than 10 days. The short period means that these planets are very close to their stars, usually less than 0.1 AU, one tenth of the distance between the Earth and the Sun.
Normally, they are still cool enough inside to condense metals into clouds. But that’s not the case with WASP-121b, which is orbiting so dangerously close to its star that its upper atmosphere reaches a blazing 4,600 degrees Fahrenheit (2,538 degrees Celsius) — about 10 times greater than that of any known planetary atmosphere.
Johns Hopkins University’s Dr. David Sing and colleagues used the Space Telescope Imaging Spectrograph (STIS) aboard the NASA/ESA Hubble Space Telescope to search in ultraviolet (UV) light for the spectral signatures of magnesium and iron imprinted on starlight filtering through WASP-121b’s atmosphere as the planet passed in front of, or transited, the face of its home star.
“Hot Jupiters are mostly made of hydrogen, and Hubble is very sensitive to hydrogen, so we know these planets can lose the gas relatively easily,” Dr. Sing said.
“But in the case of WASP-121b, the hydrogen and helium gas is outflowing, almost like a river, and is dragging these metals with them. It’s a very efficient mechanism for mass loss.”
“Heavy metals have been seen in other hot Jupiters before, but only in the lower atmosphere,” he added.
“So you don’t know if they are escaping or not. With WASP-121b, we see magnesium and iron gas so far away from the planet that they’re not gravitationally bound.”
“UV light from the host star, which is brighter and hotter than the Sun, heats the upper atmosphere and helps lead to its escape. In addition, the escaping magnesium and iron gas may contribute to the temperature spike,” he said.
“These metals will make the atmosphere more opaque in UV, which could be contributing to the heating of the upper atmosphere.”
The sizzling planet is so close to its star that it is on the cusp of being ripped apart by the star’s gravity.
This hugging distance means that the planet is football shaped due to gravitational tidal forces.
“We picked this planet because it is so extreme,” Dr. Sing explained.
“We thought we had a chance of seeing heavier elements escaping. It’s so hot and so favorable to observe, it’s the best shot at finding the presence of heavy metals.”
“We were mainly looking for magnesium, but there have been hints of iron in the atmospheres of other exoplanets. It was a surprise, though, to see it so clearly in the data and at such great altitudes so far away from the planet.”
“The heavy metals are escaping partly because the planet is so big and puffy that its gravity is relatively weak. This is a planet being actively stripped of its atmosphere.”
The results were published in the Astronomical Journal.
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David K. Sing et al. 2019. The Hubble Space Telescope PanCET Program: Exospheric Mg ii and Fe ii in the Near-ultraviolet Transmission Spectrum of WASP-121b Using Jitter Decorrelation. AJ 158, 91; doi: 10.3847/1538-3881/ab2986
