Stellar danger: Young red dwarf stars can also devour planets – astronomers have now discovered six such “planet destroyers”. This is indicated by a clear excess of lithium in the outer shells of these dwarf stars – this distinguishes them from all their peers in their star clusters. The amount of lithium suggests that these red dwarfs must have devoured three to ten Earth masses of planetary rock material – the equivalent of an entire super-Earth or several Earth-sized protoplanets.
When our sun approaches its end in a few billion years, it will become a planet killer: it will expand into a red giant and swallow up the planets of the inner solar system – probably including our Earth. Astronomers have already discovered evidence of such entangled planets in some red giants and white dwarfs: planetary debris and accumulations of heavy elements were found in their shells.
Early end for planets
But young stars can also devour their planets – for example, when protoplanets collide with each other and move out of their orbit, or when a young planet forms in an unstable orbit or too close to its star. Until now, however, it was unclear how often planets meet such an early end. Chemical indicators such as an increased lithium content are usually difficult to distinguish from the normal range of variations.
“But there is a little-understood combination of stellar mass and age where a strong and unambiguous lithium signature can reveal the devouring of a planet: young red dwarfs,” explain Robin Jeffries of Keele University in England and his colleagues. Red dwarfs are smaller and cooler than our sun, but very hot inside. “This destroys all the lithium inside them shortly after they form,” says Jeffries. If lithium is still detected on the surface of these dwarf stars, it must have gotten there from outside – for example through entangled planetary material.
Such an excess of lithium can best be seen when the young red dwarfs are part of a star cluster – a cluster of stars of the same age that were formed together in a cloud. “Such star clusters provide us with an entire population of stellar siblings as a comparison group,” the astronomers write.
Six outliers with excess lithium
To search for such “planet eaters,” Jeffries and his team evaluated spectroscopic data from the Gaia space telescope on 63 different open, young star clusters. In these they looked for red dwarfs whose spectrum shows significantly more lithium than that of their stellar siblings. The astronomers found what they were looking for in four of these star clusters: They found six young red dwarfs whose lithium content differed by more than 3.2 standard deviations from that of their neighbors of the same age.
“The spectra of these objects are of excellent quality and leave no doubt that the lithium signature is reliable,” emphasizes the team. “These lithium-rich outliers also do not differ from the rest of their cluster members in other features such as their position, parallax and kinematics.” In other words: These 50 to 200 million year old red dwarf stars are completely normal in every respect – except for their lithium content.
Metal comes neither from the interior of the star nor from the primordial cloud
But does the lithium really come from entangled planets or planetary building blocks? To be sure, the astronomers checked two other possible explanations. On the one hand, unusually strong magnetic activity could have prevented or delayed the sinking of the original lithium into the star’s core and its destruction. But then the strong magnetization should cause lots of starspots and rapid rotation – but that’s not the case, as Jeffries and his colleagues found.
On the other hand, the lithium excess could be due to an anomalously lithium-rich natal cloud or a long-lived, lithium-rich protostellar disk. But these scenarios don’t fit either, as Jeffries and his team report. In the first case, the stellar siblings would also have to have elevated lithium levels. “It is therefore not clear why we only see these individual isolated outliers together with completely depleted objects,” said the astronomers. The argument against the protostellar disk as a lithium source is that, even in extreme cases, it does not last long enough to survive the phase of lithium destruction in the core.
Planetary material from three to ten Earth masses
According to the astronomers, only one scenario remains: “It seems plausible that these outliers are direct evidence of planetary engulfment,” they state. Accordingly, the young red dwarfs must have devoured a planet or planetary material not long ago. According to the astronomers’ calculations, each of these red dwarfs has absorbed around three to ten Earth masses of rock material – this corresponds to several Earth-sized planets or a super-Earth.
“Such near-Earth twins and super-Earths often occur around red dwarf stars,” explain Jeffries and his colleagues. Their results may indicate that the destruction of planets around red dwarf stars – and possibly other stars as well – is more common than previously thought.
Source: Robin Jeffries (Keele University, UK) et al., Monthly Notices of the Royal Astronomical Society, 2026; doi: 10.1093/mnras/stag815