Astro detectives solve the murder mystery of planet-eating stars

It’s the coldest of cold cases. An interstellar murder mystery that originated so long ago and so far away that the crime scene lurks beyond the reaches of forensic science. Or perhaps not.

Can it be proven that stars in our galaxy have ripped planets from their orbits and incinerated them in their plasma infernos?

To solve the mystery, a team of researchers from around the world, including scientists from The Australian National University (ANU), scoured a section of the Milky Way galaxy 6,000 light years wide for the chemical fingerprints of stars that have eaten their planets. This was not an investigation of stars that expand in the twilight of their lives to engulf all or most of their orbiting system, as they all will eventually.

Here the question researchers set out to answer was: how frequently are planets eaten by younger stars, like our Sun?

“We thought if we could find one or two stars that have eaten planets, that would be very exciting. But then we kept finding more and more,” ANU Associate Professor Yuan-Sen Ting, who is also a world-leading expert in AstroML (astronomical research aided by machine learning), says. “Astrophysics is not experimental science; it is really a detective job.”

In a new paper, gracing the cover of the science journal Nature, Ting and colleagues laid out the evidence for a startling accusation.

They discovered seven of 91 conatal stars – stars that were born together but are no longer near each other – chosen for the study have eaten planets. But in reality, the researchers believe that number could be much higher.

The ‘original’ Three Body Problem

If you’d prefer to confront this terrifying concept as science fiction, you happen to be in luck.

For former US President Barack Obama’s favourite sci-fi novel, The Three-Body Problem, has been adapted into a Netflix series that, coincidentally, has just premiered.

The novel, the series, and this ground-breaking astronomical study originate from the same cosmic conundrum: can we predict the orbits of planets in a system where three or more large bodies orbit one another in such proximity that all three are subject to mutual gravitational forces?

The answer, thus far, is no. The orbits in such systems are chaotic, and its planets face the possibility of being ejected inward to be engulfed by their star or being ejected outward into oblivion.

In Liu Cixin’s novel, Earth is visited by an alien race that hails from an unstable, three-body system.

“In many cases, such as in our solar system, the gravitational tugs between planets are negligible,” Ting says. “But if there is a super-massive planet, or a smaller one with an eccentric crisscrossing orbit, that could create instability.”

The search for suspects

To investigate whether planets have been eaten by their stars, the research team theorised that stars that have recently ingested planets should have detectable chemical markers.

To test their theory, they began by identifying 91 sets of conatal stars — pairs of stars that originated from the same nebula (giant clouds of interstellar gas).

“In previous research, we theorised that stars that move together were born together,” Ting says. “And in a subsequent study, we proved this by showing that stars that move together have more similar chemical makeups than two stars chosen at random.”

If two stars born of the same pregalactic cloud of dust and gas contain measurably different chemical makeups, one of them is, most likely, a planet eater.

“For example, when a star has engulfed a rocky planet like Earth, the iron-to-carbon ratio on the star’s outer surface is elevated ever so slightly,” Ting says.

But the chemical fingerprint of planet-eating stars isn’t permanent. Gradually, planetary material is churned into the inner core of the star and thus, no longer observable.

“We don’t know how long it takes for the crime scene to be washed away so to speak, so the percentage of stars that have eaten planets is probably much higher than our data suggests. We may never be able to detect the ones that happened further back,” Ting says.

The unseen accomplice

Planet-eating stars require a companion that is massive enough to create a three-body problem.

This is one of the mysteries Ting and a team of scientists organised under a collaboration named C3PO (Complete Census of Co-moving Pairs of Objects) hope to solve next.

The companion might be another star. From such a great distance, it’s difficult for astronomers to know if they are looking at one star or two that closely orbit each other.

“If the stellar companion is a second star, it would create flux and  tug on the star in question, but we also don’t see that in these cases. This puts an upper limit on how massive this second largest body can be. So, we think it’s most likely a super planet,” Ting says.

C3PO is analysing all available data for the stars that show evidence of planet eating. One thing the researchers are looking for is a super planet passing in front of its star, an event known as exoplanet transit.

“We have embarked on work to tackle the planet instability theory through the lens of machine learning, trying to figure out all possible systems that can lead to engulfment and what exactly is the trigger,” Ting says.

“In the past, such theories were developed either by pen-and-paper calculation or direct simulations. With machine learning, we can consider all possibilities.”

Astronomers and astrophysicists are surprised by the fact that the signal of planet ingestion lasts long enough for humans bound by time and space to catch them.

“We’re a bit baffled right now. Either planet ingestion happens very frequently, or the chemical indicators of it linger for a very long time, or perhaps both,” Ting says.

The mystery deepens.

This article was co-published with the ANU College of Engineering, Computing and Cybernetics.