It’s the coldest of cold cases, an interstellar murder mystery that originated so long ago and so far away that it 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 scoured a section of the Milky Way galaxy 6,000 lightyears 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 was: How frequently are planets eaten by younger stars, like our Sun?
“Astrophysics is not experimental science; it is really a detective job.”
In a new research paper gracing the cover of the science journal Nature, Dr Ting, Dr Fan Liu (lead author from Monash University), Dr David Yong (ANU RSAA) and an international team of researchers laid out the evidence for a startling accusation:
Seven of 91 conatal stars chosen for the study have eaten planets, and very likely, a lot more than that.
Three-Body Problem
If you’d prefer to confront this terrifying concept as science fiction, you happen to be in luck.
The former US President Barack Obama’s favourite sci-fi novel, Three-Body Problem, has been adapted into a Netflix series that, coincidentally, premiered this week as well.
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 attractions?
The answer, thus far, is no. The orbits in such systems are chaotic, and its planets face the possibility 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,” Dr Ting said. “But if there is a super massive planet, or a smaller one with an essentric crisscrossing orbit, that could create instability.”
The search for suspects
To investigate whether exoplanets 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 interstella gas).
“In previous research, we theorised that stars that move together were born together,” Dr Ting said. “And in a subsequent study, we proved this by showing that stars that move together have much 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.
Recent milestones in advanced computing made possible a high precision analysis of the enormous flood of data being collected by the Magellan Telescope, the European Southern Observatory’s Very Large Telescope in Chile, and the Keck Telescope in Hawaii.
They studied multiple observations of the 91 star pairs, comparing the abundancies of 21 elements. Evidence of planetary engulfment was revealed in seven of them.
“We compared the observations with the expected signals from planet engulfment, and with other possible explanations,” Dr Ting says. “By calculating the statistical evidence, our research shows that planet engulfment is the best fit.”
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 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,” Dr Ting said.
The unseen accomplice
Planet-eating stars require a companion that is massive enough to create a three-body problem.
Planet-eating stars require a companion that is massive enough to create a three-body problem.
This is one of the mysteries Dr 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 is 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 doppler 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,” Dr Ting says.
C3PO is analysing all available data for the stars that show evidence of planet eating. One thing they 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 all possible systems that can lead to engulfment and what exactly is the trigger,” Dr Ting said. “In the past, such theories were developed either my 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,” Dr Ting said.
The mystery deepens.
Learn more about the use of Machine Learning in Astrophysics research (AstroML) in the article ‘Bots heard ‘round the world: AI and the future of Astronomy’.
Read the full research paper ‘At least one in a dozen stars exhibits evidence of planetary ingestion’ here.