Scientists have discovered "red dwarfs" with traces of "eaten" planets

Simply put, scientists didn't see the star actually "eating" a planet in real life. They found a possible clue: a substance that a red dwarf should almost never have. The authors think lithium could have entered the stars atmosphere along with rocky planets or planetary material.

The study is published in Monthly Notices of the Royal Astronomical Society. The paper talks about six early M dwarfs in three young star clusters with ages ranging from about 50 to 200 million years. These stars are distinguished from similar neighbours by their unusually high lithium content.

Details

Red dwarfs, or M dwarfs, are stars that are small and cool compared to the Sun. They are very common in the Galaxy, and small rocky planets are often searched for around them. But the interior of such stars is hot enough that lithium breaks down quickly in nuclear reactions.

That's why lithium has become an important clue. If a young star has an unexpectedly large amount of this element, it means that it could have got there later - for example, together with the matter of the planetary system. The authors call this a possible sign of planetary engulfment - the absorption of planets or planetary material by a star.

The team used data from the Gaia-ESO spectroscopic survey. Spectroscopy allows them to decompose a star's light into "chemical signatures" and understand what elements are in its atmosphere. The scientists searched for stars that are similar in age, temperature and position to other stars in their cluster, but stand out sharply in their lithium content.

There were six such objects. They are located in three scattered clusters: NGC 2451a, Blanco 1 and NGC 2516. According to the authors' calculations, these stars make up about 2-3% of similar M-dwarfs in clusters of this age.

To explain the excess lithium, the researchers considered several options. For example, it would be possible to assume that the stars are actually younger than the others or that lithium in them destroyed more slowly due to the peculiarities of magnetic activity. But the authors consider these explanations less likely: the stars look like normal members of their clusters, and most of them do not show signs of a binary system.

The most suitable version is the absorption of stony matter. The authors estimate that to produce the observed excess lithium, the stars must have absorbed about 3 to 10 Earth masses of volatile-poor material. This could be the matter of rocky planets, similar in composition to the inner planets of the solar system.

It is important not to take this too literally. It is not necessarily about each star swallowing one ready-made "second Earth". It could have been a collection of rocky bodies, debris, protoplanets or planets that in a young system approached the star and were swallowed by it.

Why it matters

The work shows that young planetary systems can be quite chaotic. The planets in them do not always quietly occupy stable orbits. In the early stages, they may collide, change trajectories, migrate closer to the star or even crash into it.

For astronomers, this is important because these events are usually hard to see directly. But the chemical composition of a star can retain a trace of what happened. In this case, that trace was lithium: it looks like a bright marker against the red dwarf's nearly "clean" atmosphere.

The study may also help us understand how often young stars destroy part of their planetary systems. The authors are careful to note that the 2-3% found may be a lower bound: the trace of lithium fades over time, so some of such events may have gone unnoticed.

Background

The idea that stars can swallow planets is not new. Such events have long been seen as a possible stage in the formation and evolution of planetary systems. A planet may approach a star due to migration in the gas-dust disc, interactions with other planets, or gravitational perturbations.

But it is difficult to find direct or reliable indirect signs of such events. In many stars, the changes in chemical composition are very small. In addition, you have to distinguish the trace of an engulfed planet from the usual differences between stars.

In this sense, young star clusters are easy to study. The stars in them were born at about the same time and from similar matter. So if one of them differs sharply from its neighbours in its lithium "signature", it is easier to notice and explain.

Source

Research: R. D. Jeffries, R. J. Jackson, I. Baraffe, "Lithium-rich M-dwarfs at the ZAMS: Evidence for planetary engulfment?", Monthly Notices of the Royal Astronomical Society, 2026.