Planet-eating stars are more abundant in the universe than previously thought, an Australian-led study has found.
The study, by the Australian Research Council’s Centre of Excellence for All Sky Astrophysics in Three Dimensions (Astro 3D), looked at “co-natal” or twin stars, born from the same molecular cloud, where one had “eaten” a planet and the other had not.
The researchers found one in 12 pairs of stars have absorbed entire planets or planetary material, changing their chemical make up.
Astronomers assumed such an event was possible, but the first planet-eating star was only caught in the act last year, and the latest study shows the cosmic consumption is more common than previously thought and can happen with younger stars.
Dr Fan Liu, a Astro 3D researcher from Monash University, said they chose twin stars travelling together – not necessarily binary stars, which orbit around each other, but stars from the same origin – because “they are born of the same molecular clouds and so should be identical”.
The team used the European Space Agency’s Gaia satellite and data collected with the European southern observatory’s Very Large Telescope (VLT), the Magellan telescope (both in Chile) and the Keck telescope in Hawaii to detect the differences in the stars’ “planet signatures”.
In 91 pairs, they found seven instances of “planetary ingestion”, meaning an occurrence rate of 8% or one in 12.
“Thanks to this very high-precision analysis, we can see chemical differences between the twins,” Liu said.
“This provides very strong evidence that one of the stars has swallowed planets or planetary material and changed its composition.
“We’re mainly looking at elements including iron, nickel, titanium, those are the major materials that form planets like the Earth … that gives us a sign that the star has swallowed the planet.”
Last year, astronomers captured a planet being swallowed by a star for the first time.
An ageing, expanding star 12,000 light years away engulfing a nearby planet was spotted. Their observations showed an “insanely bright” star, and while they initially suspected a stellar merger, after further data analysis they could see if it was “a planet crashing into its star”.
The same sort of stellar snacking will happen to Earth in about 5bn years.
Liu said it was “complicated” trying to tell if entire planets were being swallowed. “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 said.
“[That’s] when the star and planets form from the same cloud.”
Ageing stars swell up to swallow planets. However, the Astro 3D researchers’ stars were not ageing red giants, but “main sequence” stars, in their (usually) stable mid life – like our sun.
“This is very different from previous studies where late-stage stars can engulf nearby planets when the star becomes a very giant ball,” Liu said.
“It’s actually very challenging for people to detect some events... when the star’s still young.
“We know when they’re old they evolve to a giant ball and eventually swallow planets. But when they are young, they are very stable, so it’s not expected that we would see this, almost 10%, to have already swallowed planets due to instability.”
Asked if people should be worried, Liu said this solar system was “quite stable”.
“But it’s important, we need to cherish that we have this chance to live, and to live in such a stable system,” he said.
The study has been published in the journal Nature.
