WASP-121b is an extraordinary planet. It’s a hot Jupiter, slightly bigger than the gas giant in our Solar System but so close to its star that its temperature is in the thousands of degrees. The planet is the first one discovered to have a stratosphere, but unlike our planet, it has a layer of titanium oxide instead of ozone.
Its upper atmosphere is much hotter than its lower layers, passing beyond 2,500°C (4,600°F). It is so hot that most metals melt, and now Hubble has witnessed some of these metals escaping from the exoplanet – something never seen before.
As reported in The Astronomical Journal, the planet is so close to the star that it’s being stripped of hydrogen and helium, the lightest elements. As these gases are siphoned by the star, other elements escape too, including iron and magnesium in gas form.
"Heavy metals have been seen in other hot Jupiters before, but only in the lower atmosphere," lead researcher David Sing of Johns Hopkins University said in a statement. "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."
The planet is only 3.8 million kilometers (2.4 million miles) from the star, 2.5 percent of the distance between the Earth and the Sun. Its high temperature makes it bloated, and combined with the strong pull from its sun, it is deformed. Instead of being sort of spherical, the planet looks more like an American football.
"We picked this planet because it is so extreme," Sing said. "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 observations were part of the Panchromatic Comparative Exoplanet Treasury (PanCET) survey. Astronomers focused on 20 exoplanets and observed them in ultraviolet, visible light, and infrared to expand our knowledge and understanding of them. The WASP-121 system is located roughly 900 light-years from Earth.