One in Four Sun-like Stars May Have Swallowed Their Planets, Chemical Clues Suggest

Start with a pair of stars born together. They formed at the same time, out of the same cloud of gas, so they should carry the same mix of chemical elements. When astronomers find that one has more iron than the other, something has happened since birth. A study published in Nature Astronomy, led by Lorenzo Spina, argues that in many cases the something is a planet that fell in and was consumed.

The team examined 107 binary systems made up of Sun-like stars. By analysing the spectrum of light each star produces, they worked out which member of each pair held more planetary material than its companion. From that, they estimate that between 20% and 35% of Sun-like stars have eaten their own planets, with 27% the most likely figure. In other words, at least a quarter of planetary systems around stars like ours have had a chaotic, dynamic past.

The question behind the work is how unusual our Solar System is. In the roughly 30 years since the first planets were found around stars other than the Sun, planetary systems have proved common in the Milky Way. Many look nothing like ours. Our planets travel stable, nearly circular orbits, which suggests they have not shifted much since they formed. Elsewhere, astronomers have watched large or medium-sized planets migrate significantly, with their gravity possibly disturbing other planets or pushing them into unstable orbits. In most such systems, some planets have probably plunged into the host star. What was not known was how common these turbulent systems are compared with quiet ones like ours.

Direct study of exoplanetary systems, even with the most precise instruments, makes that hard to settle. So the team looked at stellar chemistry instead. When a planet falls into a star, it dissolves in the star's outer layer and shifts its composition, adding more of the elements that make up rocky planets, such as iron, than would otherwise be there.

The researchers describe three findings that together they call unambiguous evidence that planet-eating explains the chemical differences. First, stars with a thinner outer layer were more likely to be richer in iron than their companions. That fits the idea, since planetary material diluted in a thinner layer changes its composition more.

Second, the stars richer in iron and other rocky-planet elements also had more lithium than their companions. Stars destroy lithium quickly, while planets conserve it. An unusually high lithium level must therefore have arrived after the star formed, consistent with a planet carrying it until the star swallowed it.

Third, stars with more iron than their companions also had more than comparable stars elsewhere in the Galaxy, yet their carbon was at standard levels. Carbon is volatile and not carried by rocks, so the pattern points to enrichment by rocky material from planets or planetary debris rather than some other source.

Spina, a postdoctoral research fellow at the Italian National Institute for Astrophysics and formerly at Monash University, calls the results a breakthrough for stellar astrophysics and exoplanet exploration. They show that planet-eating can alter the chemistry of Sun-like stars, and that a significant share of such systems had a very dynamic history unlike our own.

The practical payoff is in the search for what he calls Earth 2.0. There are millions of relatively nearby Sun-like stars, and without a way to prioritise, the hunt resembles looking for a needle in a haystack. Chemical analysis could help identify the stars more likely to host true analogues of our calm Solar System, and that is where the work points next.