When stars eat their planets, the carnage can be seen billions of years later.

The vast majority of stars have planets. We know this from observations of exoplanetary systems. We also know that some stars don’t have planets, and may never have had them. This raises an interesting question. Suppose we see an old star that has no planets. How do we know if he ever did? Perhaps the star lost its planets during a close approach of another star, or perhaps the planets spiraled inward and were consumed like Chronos devouring his children. How could we know? A recent study on arXiv answers half of that question.

It all boils down to a strange little element known as lithium.

Lithium is the third element in the periodic table. Although most of the atoms formed during the Big Bang were hydrogen and helium, small amounts of lithium were formed from the Big Bang. About one atom in ten billion, according to the current model. But it turns out that there is less lithium in the universe than you might expect. That’s because while other elements like carbon, oxygen, and iron are created in the heart of large stars, lithium is destroyed. It’s an effect known as lithium burning, and it means that older stars typically don’t have much lithium present in their atmosphere.

Astronomers use this effect to distinguish between high-mass brown dwarfs and low-mass stars. If there is a lot of lithium present in the atmosphere, then fusion is not happening and it is a brown dwarf. Not much lithium and you have a star. But some stars have atmospheric lithium. They are clearly large and hot enough to undergo fusion, and have not burned up the lithium in their atmosphere. So what gives?

The common hypothesis has been that these unusual stars must experience unusual internal mixing that somehow prevents lithium from passing into the interior of the star, where it can be consumed. This latest study proposes an alternative. Perhaps these stars consumed their young planets instead.

Since planets don’t burn lithium when a star eats a planet, its lithium is added to the star’s mix. The team simulated how that added lithium would behave inside a star and how long it would take for it to fade from the upper layers of the star. They found that the smallest red dwarf stars are quite effective at burning the new lithium. Because a small star has large convection zones that mix its interior very well, new lithium is used up in a few hundred million years. But for the largest and most Sun-like stars, lithium can last for billions of years. Long after a planet is consumed, its lithium is still present in the stellar atmosphere.

So if we see an old Sun-like star with lithium in its atmosphere, it’s very possible that it once had planets. Stellar lithium seems to be a good sign of a star sated after a planetary meal.