A tasty snack for a hungry sun (Image: Vanderbilt University)

Distant stars that gobble up Earth-like planets are unlikely to be good hosts for life – after all, no one wants to share the neighbourhood with a world-devouring sun. Now astronomers have figured out how to identify the grisly leftovers of a sun-like star’s planetary feasting, which should make it easier to rule out planet-eaters and instead track down systems that still have habitable worlds.

Stars are mostly made up of hydrogen and helium, the fuel for nuclear fusion reactions that produce their heat and light. But they also can contain a spattering of other elements on their surfaces. Analysing starlight lets astronomers determine which elements are present in a star system, and gives clues to the kind of planets it contains.

To find out more, Keivan Stassun at Vanderbilt University in Nashville, Tennessee, and his colleagues used telescopes at the Las Campanas Observatory in Chile to look at a system containing a pair of sun-like stars called HD 20781 and HD 20782. The two stars formed from the same cloud of dust and gas, meaning they would initially have had the same chemical composition. Any differences must be down to their orbiting planets. Currently, we only see that one star has two Neptune-mass worlds and the other has a Jupiter-like planet.


Planet bullies

The team analysed the levels of 15 elements in both stars, including aluminium, silicon, calcium and iron, which make up the building blocks of Earth-like planets. Both stars had much higher levels of these elements than our own sun, suggesting they have absorbed large amounts of Earth-like material – an estimated 20 Earth masses for the first star and 10 Earth masses for the second.

This “heavy” material could not have come from dust and gas caught up by the stars as they formed, or it wouldn’t show up on their surfaces. “The only way that we are able to detect this signature is if the rocky material was sprinkled on to the surface of the star later on,” says Stassun.

It is likely that these rocky planets were bullied into the gaping maw of their stars by their bigger planetary brothers. HD 20781’s Neptune-like planets orbit at around a fifth of the distance from the Earth to the sun, while HD 20782’s Jupiter-mass planet is on an eccentric orbit that sometimes places it as close as 6 million kilometres from the star, or less than one per cent of the orbiting distance of our Jupiter. Stassun thinks that these massive planets originally formed further out but then migrated inwards, pushing any Earth-like worlds to their doom.

Rocky fingerprints

Hunting for exoplanets requires many years of careful observation, as telescopes such as NASA’s Kepler mission have to watch for a planet to pass in front of its star multiple times. Measuring a star’s chemical signature only takes one night, meaning systems unlikely to host life can be quickly ruled out. “The one that looks like it swallowed its Earths already is probably not the one to start with,” says Stassun.

René Oudmaijer at the University of Leeds, UK, says looking at chemical signatures is a good idea that could potentially speed up the search for Earth-like exoplanets. “Astronomers don’t want to waste their time,” he says, but he cautions that the technique requires difficult and precise measurements. “The challenge will be to really identify that a star has the fingerprint of a planet that has been eaten up.”

Journal reference: The Astrophysical Journal, DOI: 10.1088/0004-637X/787/2/98