Billions of stars have swallowed up a planet

At least one star in every 12 seems to be a devourer of planets. This may be because star systems are readily destabilised when outside objects such as rogue worlds or other stars fly close by, and the disturbance can shake up planets’ orbits and throw them into their stars.

Fan Liu at Monash University in Australia and his colleagues figured out how often this occurs by observing 91 pairs of stars using several of the world’s most powerful telescopes. They chose stars that were most likely born together in binaries, because these couples should have formed with identical chemical compositions. That way, the researchers could determine if one of them had swallowed up a planet in the past, because doing so would have changes its composition relative to its binary partner.

They found that about 8 per cent of the pairs contained one star that had devoured a planet and therefore showed signs of a higher abundance of heavy elements than its twin. Those stars each appear to have ingested between 1.7 and 8.4 Earth masses of material. This matches with previous predictions.

“Our estimates are conservative,” says Liu. “I would guess the actual rate might be higher, but probably still less than or around 20 per cent.” This may vary based on where in the galaxy a given star is born.

Figuring out how many stars chow down on their planets is potentially a crucial part of understanding the abundance of life in the universe and how likely we are to find it.

“It’s a question of, how many stars and planets are there that behave in a way that’s conducive to the development of life?” says Meridith Joyce at Konkoly Observatory in Hungary, part of the research team. “Knowing how many stars there are and how many stars host planets are two parts of that calculation, but we also need to know how many stars are going to eat those planets.”