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Astronomers observe evidence of dying sun-like star engulfing a planet

They caught a star 13,000 light-years from Earth in the act of gobbling up a planet.

(CN) — Scientists have had a pretty solid idea about what happens to a star as it dies — and what happens to the planets surrounding dying stars — for decades, but in a new study published in Nature, scientists describe observing the first direct evidence confirming that when stars die, they expand hundreds to thousands of times their size and engulf nearby planets in their wake. 

“Accidental discoveries sometimes make the biggest impacts,” said Kishalay De, a postdoctoral fellow at Massachusetts Institute of Technology’s Kavli Institute for Astrophysics and Space Research, and the lead author of the study published Wednesday.“I didn’t know 20 years ago this is something I would run into,” he said. 

De said that he and a group of researchers were originally studying novae, or novas, in our galaxy. Novae are astronomical events involving white dwarfs, the last state in a star’s life, that are gravitationally bound to another star, where a star increases in brightness and then slowly decreases back to its normal brightness level. Instead of an outburst getting brighter and then going back to its original brightness level, De and his team observed a star whose outburst was long and had a faint peak flux.

Using data gathered from dozens of different telescopes, including the National Science Foundation’s Gemini South telescope in Chile and their near-infrared adaptive optics camera, De and his team analyzed the low energy outburst from a star some 13,000 light-years from Earth in our Milky Way galaxy and the surrounding temperature and composition of the gas around it. If the outburst was brighter, De said the cause of that bright flare up would have indicated it was caused by a stellar merger, but because the outburst was fainter, they determined it was caused by the evolving star ingesting a nearby planet.

“We’ve never actually caught a star in the act of engulfing a planet,” De said.

De and his colleagues' study provides the first direct evidence of this process.  

“Our interpretation of ZTF SLRN-2020 as the engulfment event of a planetary mass object by a Sun-like star provides evidence for a missing link in our understanding of the evolution and final fates of planetary systems,” De writes in the published study. “It has long been known that the population of planets in short orbital periods has sufficiently low orbital angular momentum such that they are unstable to tidal dissipation and are bound to merge with their host stars. This is consistent with the lack of old planetary systems with short orbital periods, as well as the dearth of close planets around sub-giant stars. Therefore, to our knowledge, the observations reported here offer the first direct insight into the effect of planetary engulfment on their host stars to interpret common indirect techniques used to infer past planetary engulfment via its effects on long-term stellar luminosity, chemical enrichment and stellar rotation.”

The planet that was engulfed by the star is described in the study as “plausibly a Neptune- or Jupiter-like planet strikingly similar to known systems." Since the planet was probably a gas giant, and because it was so close to the star even before it was subsumed into it, De said it couldn’t have hosted any life forms. 

As the star continues to evolve and die out over hundreds of thousands of years, it will get bigger and bigger, overtaking other planets around it before it becomes a white dwarf.

Not only does this research provide direct evidence of an important concept in astronomy, but it also provides a window into what will happen to our sun, as it starts to expand and overruns first Mercury, then Venus, and then the Earth in about five billion years. 

De said that when our sun consumes our planet, the observable outburst it causes won’t be as bright as the one he and his colleagues observed, since the planet in the study was significantly bigger than the Earth.     

“I think there's something pretty remarkable about these results that speaks to the transience of our existence,” wrote Ryan Lau, an astronomer at the National Science Foundation's National Optical-Infrared Astronomy Research Laboratory, or NOIRLab, and a co-author of the study, in a statement accompanying the study. “After the billions of years that span the lifetime of our Solar System, our own end stages will likely conclude in a final flash that lasts only a few months.”

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