🪐 Red dwarfs with chocolate around their mouths: they devour their planets

The Gaia-ESO Spectroscopic (GES) survey has highlighted a puzzling observation: red dwarfs, these small, cool stars, contain lithium in their atmospheres. These stars should be devoid of it. The only plausible explanation is that they have swallowed their own planets, thereby enriching their gaseous envelope with this element.

Although less massive than our Sun, red dwarfs have extremely hot and turbulent interiors. This fiery environment quickly burns any lithium they might have at birth. Thus, detecting this element in their atmosphere is indirect evidence of a planetary feast. Astronomers have long sought this signature, which confirms that these stars can devour their planets.


In this study, the team of Robin Jeffries from Keele University analyzed data from the GES survey. They identified six red dwarfs in three different star clusters, exhibiting lithium levels far higher than expected. These stars likely absorbed the equivalent of three to ten Earth masses of planetary material. This process brought fresh lithium into their outer layers, where it is normally absent.

Red dwarfs are the most common stars in the Milky Way, accounting for about 75% of its stellar population. This phenomenon could be very common in our galaxy. This discovery opens a new window into the evolution of planetary systems, especially during their early chaotic phases.

Scientists now want to understand at which stages of their lives these stars are most likely to devour their planets. By studying other clusters of varying ages, they can trace the timeline of these absorptions. This will help better understand the formation and survival of planets around red dwarfs, stars that are nevertheless considered potential hosts for life.

These results were published in the journal Monthly Notices of the Royal Astronomical Society. They confirm a long-cherished hypothesis among astronomers: small stars can be planet-eaters. The next step will be to observe more red dwarfs to generalize this finding and refine models of planetary evolution.

Lithium, a precious chemical clue

Lithium is a light element, produced mainly during the Big Bang and by cosmic rays. In stars, it is transformed by nuclear fusion at relatively modest temperatures. Red dwarfs, with their very hot interiors, burn all the lithium they possess within a few hundred million years. Its presence in their atmosphere is therefore abnormal and signals a recent external input.

This input can come from planets or rocky fragments that fall into the star. Indeed, planets retain their lithium since their formation, because they do not undergo nuclear reactions. Thus, detecting lithium in a red dwarf is tantamount to finding evidence of a planetary meal.

Red dwarfs, stars of our galaxy

Red dwarfs are the most common stars in the Milky Way, representing about 75% of all stars. Their low mass (from 8% to 60% of the Sun's mass) gives them exceptional longevity: they can shine for trillions of years. However, their small size makes them difficult to observe directly.

These stars are often surrounded by planetary systems, as shown by the discovery of Proxima b. Their abundance makes them prime targets for the search for habitable exoplanets. Understanding their behavior is therefore essential to estimating the survival of planets around them. If absorption is frequent, the chances of finding intact worlds decrease.