END OF THE WORLD: The juicy secrets of stars that eat their planets

CHENNAI: The sun has nourished life on Earth, but it will not be so hospitable forever. Five billion years into the future, our solar system’s star will grow so immense that Mercury, Venus and, possibly, Earth will be swallowed whole. It may seem like an ignominious end to our beloved home. But scientists think that this process of “planetary engulfment,” in which stars devour their own planets, is common in the life cycle of star systems.

Researchers call them “cannibal stars” (although it is planets they eat, not other stars), and they might explain tantalising mysteries in astronomy, weird orbital configurations and polluted starlight that have puzzled scientists for years. But there is a more basic appeal: Studying planetary engulfment may help us understand the very long-term fate of Earth and provide clues in the search for extraterrestrial life. What could be more human than forecasting the end of the world and pondering whether we are alone in the universe?

“For the case of the Earth, I think it’s rather unclear whether it’s going to be engulfed or not, but it’s certainly going to become impossible to live on,” said Ricardo Yarza, a graduate student in astronomy at the University of California, Santa Cruz, who studies planetary engulfment. “It’s always interesting to imagine a civilisation becoming aware of this, like us, and realising that at some point, you’ve got to leave home.” Stars come in many varieties including simmering dwarfs and brilliant hypergiants. The life expectancy and ultimate fate of a star — and, therefore, any planets orbiting it — is tied to its mass: Red dwarfs may live for trillions of years, whereas massive stars blow up within a few million.

Sun-size stars begin their death rites when they run out of hydrogen, causing their borders to expand hundreds of times over. During this “red giant” phase, many stars devour their innermost planets before exhausting their remaining fuel.

Red giants, which were discovered about a century ago, sometimes make appearances in science fiction stories as ominous backdrops to doomed civilisations or as visions of our solar system’s distant future. Though we’ve often imagined the world-ending creep of advancing stars, the actual process of planetary engulfment remains shrouded in mystery. As early as 1967, for instance, astronomers pondered the “ultimate fate of the planetary material” that is engulfed by stars. But they could only speculate, in part because scientists weren’t able to confirm that planets orbited other stars until the 1990s.

Since that time, a staggering 5,000 exoplanets have been detected by missions such as NASA’s Kepler space observatory, ushering in a new understanding of the countless ways in which star systems evolve and how they eventually die. The next generation of observatories on Earth and beyond, including the newly operational James Webb Space Telescope, will image these worlds in never-before-seen detail, shedding light on their odds of hosting life.

The sheer abundance of known exoplanets, especially those in tight orbits, implies that the lives of many worlds will end inside the bellies of their host stars. But there are many gaps in astronomers’ knowledge because it is difficult to catch stars in the act of devouring planets. Creating models of engulfment events is also tricky, partly because of the extreme disparities between the sizes of stars and their planetary meals.

Ferreira is a journalist with NYT©2022

The New York Times