Sizzling exoplanets have been found before, but WASP-121b continues to surprise astronomers. In 2017, they discovered a water vapor atmosphere that glowed because it was so hot.
New observations with the Hubble Space Telescope show that the exoplanet – the name for a planet outside our own solar system – is sending heavy metals streaming into space because of the heat it faces. The intense gravity of the star it orbits is also deforming WASP-121b into the shape of a football.
The Hubble observations were published Thursday in the Astronomical Journal.
WASP-121b, 880 light-years from Earth, is considered a hot Jupiter-like planet. It has a greater mass and radius than Jupiter, making it puffier.
If WASP-121b were any closer to its host star, it would be ripped apart by the star’s gravity. The top of the planet’s atmosphere is heated to 2,500 degrees Celsius – so hot that iron can exist only as a gas, instead of a solid.
“This planet is a prototype for ultra-hot Jupiters. These planets are so heavily irradiated by their host stars, they’re almost like stars themselves,” said study author Drake Deming, a University of Maryland astronomy professor. “The planet is being evaporated by its host star to the point that we can see metal atoms escaping the upper atmosphere where they can interact with the planet’s magnetic field. This presents an opportunity to observe and understand some very interesting physics.”
Heavy metals like iron and magnesium are vaporizing and releasing in a stream from the planet. This is the first observation of a hot Jupiter releasing heavy metal gases into space.
“Heavy metals have been seen in other hot Jupiters before, but only in the lower atmosphere,” said lead researcher David Sing of Johns Hopkins University. “With WASP-121b, we see magnesium and iron gas so far away from the planet that they’re not gravitationally bound. 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.”
Other known hot Jupiters contain clouds of heavier elements in their atmosphere because they’re still cool enough to condense the elements. The upper atmosphere of WASP-121b reaches 4,600 degrees Fahrenheit.
Because the star and its planet are so close to each other, the exolanet’s round shape has become more like an ovoid football as it faces the star’s gravity.
“The 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,” 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.”
Planets are made from disks of dust and gas that initially formed the star they orbit. This gas creates the planet’s early atmosphere as it’s forming. Hydrogen and helium are the most abundant elements throughout the universe, so they’re common building blocks for the young planet atmospheres.
Once a planet like WASP-121b moves closer to its star, the early atmosphere is destroyed and burned away. Yet the hot exoplanet still has an atmosphere.
“Hot Jupiters this close to their host star are very rare. Ones that are this hot are even rarer still,” Deming said. “Although they’re rare, they really stand out once you’ve found them. We look forward to learning even more about this strange planet.”
WASP-121b is a great candidate for follow-up observations with NASA’s James Webb Space Telescope, which launches in 2021. The telescope will be able to peer into atmospheres of exoplanets and determine their chemical makeup.
