Artist's illustration showing an alien world that is losing magnesium and iron gas from its atmosphere.

The exoplanet WASP-121b is moulded into a rugby-ball shape by the proximity of its parent star, which also spurs the escape of the planet’s metallic gases into space. Credit: NASA/ESA/J. Olmsted

Planetary science

A heavy-metal planet spits iron into space

The gassy exoplanet loses its hold over the heavy elements in its atmosphere.

Scorched by ultraviolet light from its parent star, an exoplanet named WASP-121b is surrendering iron and magnesium gas to space.

Astronomers led by David Sing at Johns Hopkins University in Baltimore, Maryland, used the Hubble Space Telescope to probe the atmosphere of WASP-121b, which lies about 275 parsecs (900 light years) from Earth. By scrutinizing patterns in the star’s light, the researchers spotted telltale signs of iron and magnesium escaping from the planet’s wispy outer atmosphere.

Such heavy elements would normally occur as clouds of liquid droplets in a planet’s atmosphere. But WASP-121b is in very close orbit around its star. This allows UV radiation to heat the planet’s upper atmosphere to roughly 2,500 ºC — hot enough to vaporize the metals in the region.

Because WASP-121b is a gassy planet, similar to Jupiter, its gravitational pull is relatively weak. As a result, it can’t prevent its outer atmosphere — including the heavy metals — from escaping into space.