Distant planet-devouring star shows Earth's fate
The first direct evidence of a dying Sun-like star engulfing an exoplanet gives us a preview of the end of (our) world.
Planet Earth has been around for some 4.5 billion years, orbiting the typical low-mass star we call the Sun. What we know about such stars is that they eventually run out of hydrogen to burn in their cores, and as a result expand in size up to a thousand times, becoming red giants. In the case of an engorged Sun, the stellar expansion will take out Mercury, Venus and Earth, and the End Of The World As We Won’t Know It will occur in around five billion years from now.
Astronomers using the Gemini South telescope in Chile have observed the first direct evidence of a dying Sun-like star engulfing a planet as it happens. The “smoking gun” came in the form of a lengthy, low-energy outburst from the star, which indicates a planet skimming along the stellar atmosphere. The star in question lies 13,000 light years from Earth.
“These observations provide a new perspective on finding and studying the billions of stars in our Milky Way that have already consumed their planets,” says Ryan Lau, a co-author of a study just published in Nature1.
For much of its life, a solar-mass star fuses hydrogen into helium in the core, and the outward pressure from nuclear reactions pushes back against the crushing weight of the star’s outer layers. When hydrogen is depleted in the core, the star begin fusing helium into carbon. Hydrogen fusion then moves into a shell surrounding the core, leading the star to expand into a red giant.
Distinguishing a planet-engulfing stellar outburst from a flare or coronal-mass ejection requires high-resolution observations to pinpoint the precise location of the outburst, together with long-term measurements of its brightness uncontaminated by light from nearby stars. Gemini South made this possible thanks to adaptive optics that slightly deform the telescope mirror to compensate for light distortion.
The outburst detected by Gemini South lasted for 100 days, and the characteristics of the ejected material provide us with an estimate of both the star’s mass and that of the engulfed planet. In this case, the ejected material was made up of 33 Earth masses of hydrogen, and 0.33 Earth masses of dust. “That's more star- and planet-forming material being recycled, or burped out, into the interstellar medium thanks to the star eating the planet,” says Lau. The progenitor star is between 0.8 and 1.5 times the mass of our Sun, and the engulfed planet 1−10 times the mass of Jupiter.
The distinct signature of a planet being wiped out by its parent star should allow astronomers to search for similar events elsewhere in the cosmos.
De et al., “An infrared transient from a star engulfing a planet”, Nature 617, 55–60 (2023); doi:10.1038/s41586-023-05842-x