One in Twelve Stars Ate a Planet

That stars can eat planets is axiomatic. If a small enough planet gets too close to a large enough star, the planet loses. Its fate is sealed.

New research examines how many stars eat planets. Their conclusion? One in twelve stars has consumed at least one planet.

The evidence comes from co-natal stars, which aren’t necessarily binary stars. Since these stars form from the same molecular cloud, they should have the same ingredients. Their metallicity should be nearly identical.

But for about one in twelve stars, there are clear differences.

The new research is titled “At least one in a dozen stars shows evidence of planetary ingestion,” and it’s published in the journal Nature. The lead author is Fan Liu, an ASTRO 3D Research Fellow in the School of Physics and Astronomy at Monash University, Melbourne, Australia.

“Astronomers used to believe that these kinds of events were not possible.”

Yuan-Sen Ting, co-author, ASTRO 3D researcher from the Australian National University

“Stellar chemical compositions can be altered by ingestion of planetary material and/or planet formation, which removes refractory material from the protostellar disk,” Liu and his colleagues write in their paper. “These ‘planet signatures’ appear as correlations between elemental abundance differences and the dust condensation temperature.”

The authors explain that these signatures are elusive. The key to finding them is to locate co-natal stars, stars that were born together and are still moving together through space.

“We looked at twin stars travelling together. They are born of the same molecular clouds and so should be identical,” said lead author Liu.

The researchers started by using the extreme accuracy of the ESA’s Gaia spacecraft. Gaia’s data allowed the researchers to identify 125 co-moving pairs of stars. Of those, 34 were considered too widely separated but were still used as a control group. The researchers then examined the remaining 91 pairs spectroscopically to determine their chemistry. They used powerful telescopes to gather this data: the Magellan Telescope, the Very Large Telescope, and the Keck Telescope. The large amount of accurate data generated by these ‘scopes allowed the researchers to detect chemical differences and made the findings possible.

“Thanks to this very high precision analysis, we can see chemical differences between the twins,” said Liu. “This provides very strong evidence that one of the stars has swallowed planets or planetary material and changed its composition.”

Liu points out that their findings don’t include stars like red giants that expand when they leave the main sequence and consume nearby planets. “This is different from previous studies where late-stage stars can engulf nearby planets when the star becomes a very giant ball,” Dr. Liu said.

These results required some detailed analysis. When determining the metallicity of the co-natal stars and how planetary material could explain the different metallicities, the researchers had to account for atomic diffusion. Atomic diffusion can transport different chemicals around in stars, which can change how abundant different chemicals can appear to be. Stars from the same cluster, and co-natal stars, can show different abundances even though they’re the same overall.

However, atomic diffusion leaves a different chemical fingerprint, and the researchers were able to determine how atomic diffusion affects apparent abundance versus how the engulfment of planetary material affects it.

The results show that some co-natal stars have different metallicity, so some of them have absorbed planetary material. But the researchers point out that some of the results may not come from planetary engulfment. It’s possible that in some of these pairs, one star absorbed material from its protoplanetary disk, which would also change its metallicity.

“It’s complicated. The ingestion of the whole planet is our favoured scenario, but of course, we can also not rule out that these stars have ingested a lot of material from a protoplanetary disk,” he says.

Showing that stars can absorb planets puts another wrinkle into our understanding of stars and their planetary systems. Engulfment doesn’t happen a lot, according to these results, but the fact that it does is intriguing. It leads to questions. How and why does it happen? What situations lead to this engulfment? How does it affect the exoplanet population, and could it affect potential habitability somehow? Engulfment leaves its mark on the star; how does it affect the planetary system?

“Astronomers used to believe that these kinds of events were not possible, said study co-author Yuan-Sen Ting, ASTRO 3D researcher from the Australian National University. “But from the observations in our study, we can see that, while the occurrence is not high, it is actually possible. This opens a new window for planet evolution theorists to study.”