There are thousands of confirmed exoplanets in the cosmos, and many of them are members of solar systems very different than our own. As missions like Kepler and the Transiting Exoplanet Survey Satellite (TESS) have highlighted more distant worlds, astronomers have been surprised how many of them have so-called “hot Jupiters.” WASP-121b is the hottest of these close-orbiting gas giants. How hot is it? It’s so hot that heavy metals leak out as it rockets around the star.
WASP-121b made headlines in 2017 when scientists used Hubble to characterize its stratosphere. That was a first for any exoplanet, and it showed that the planet’s temperature increases along with the altitude, just like planets in our solar system. This is a hot Jupiter with 1.2 times the mass of Jupiter itself. It orbits a star about 900 light-years away that’s slightly larger and warmer than the sun, but it’s so close that its year is only 30 Earth days long.
Even by the standards of a hot Jupiter, WASP-121b is absolutely scorching. At 4,600 degrees Fahrenheit (2,500 Celsius), it’s 10 times hotter than any other exoplanet yet discovered. While it’s only a little more massive than Jupiter, it’s diameter is almost twice as large because the intense heat from WASP-121 has caused it to swell. The new Hubble observations demonstrate what that intense heat means for the planet.
The fluffed up outer layers of WASP-121b are under less gravity than the inner layers, so they fall away from the planet as it orbits. In most gas giants — even hot Jupiters — that would be largely hydrogen and helium. However, Hubble indicates that WASP-121b is losing heavy metals like magnesium and iron. Astronomers hypothesize the incredible heat is enough to lift heavy metals from the lower layers of the atmosphere upward where they can be lost to space.
WASP-121b is too far away to image the atmosphere or trailing metallic gas directly, but Hubble can track the planet as it transits in front of its host star. The changes in light allow them to determine what’s happening to the exoplanet’s atmosphere via spectroscopy. Perhaps the most interesting aspect of the new analysis is what the hellish heat is doing to the planet’s shape. So much of the atmosphere is being pulled away that the planet probably looks a bit like a football.
Scientists hope to learn more about this extreme planet in the future. The upcoming James Webb Space Telescope should be able to characterize its atmosphere even more accurately.
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