Star caught consuming planet

According to new data collected by Hubble's Cosmic Origins Spectrograph (COS), the hottest known exoplanet in the Galaxy, WASP-12b, may also be the shortest lived, for it is being mercilessly consumed by its host star.

Astronomers already knew that WASP-12b was an oddity, orbiting its star so tightly that its atmosphere is superheated to some 2,200 degrees Celsius and the whole planet is contorted into an elongated rugby ball shape. Its swollen atmosphere has ballooned to three times Jupiter's radius and is swirling catastrophically into the star such that it may only have 10 million years left before it is completely devoured.

Artist concept of WASP-12b spilling into its host star. Gravitational forces due to the planet's proximity to the star have stretched the planet into an egg shape. Image: NASA, ESA, G. Bacon (STScI) and C. Haswell (The Open University, UK).

The exchange of matter between two stars is not unusual, but this is the first time it has been seen so clearly for a planet. “We see a huge cloud of material around the planet which is escaping and will be captured by the star,” says team leader Carole Haswell of The Open University. “We have identified chemical elements never before seen on planets outside our own Solar System.”

WASP-12b was discovered in 2008 by the SuperWASP (Wide Area Search for Planets) automated survey that searches for tiny dips in a stars' brightness as a planet passes in front of it. WASP-12b is so close to its star that it completes one orbit in just 1.1 days.

The sensitivity of Hubble's new COS instrument permitted measurements of the star's light at ultraviolet wavelengths that revealed absorption lines from elements such as aluminium, tin and manganese that become more pronounced as the planet transits the star, meaning that the elements exist in the planet's atmosphere as well as the star's. Furthermore, the fact that COS could detect these features at all suggests that the planet's heated atmosphere is highly extended.

In addition, COS measurements enabled astronomers to produce a light curve to show precisely how much of the star's light is blocked out during a transit event, which can be translated into the planet's radius. The planet is 40 percent more massive than Jupiter and its exosphere is so deformed that its radius exceeds its Roche lobe – the region of space around the planet within which material is gravitationally bound, and beyond which can escape onto the star, which is exactly what the Hubble observations confirm.