Far out cosmology: when the sun dies (Introduction)

by David Turell , Wednesday, December 20, 2023, 16:43 (129 days ago) @ David Turell

What is the Earth's fate?:

https://www.quantamagazine.org/new-clues-for-what-will-happen-when-the-sun-eats-the-ear...

"When a main-sequence star like our sun — also called a G-type star or yellow dwarf — reaches the end of its life, it runs out of the hydrogen needed to power nuclear fusion in its core. As the star turns to other fuel sources and loses mass, its core gets hotter, and its atmosphere puffs up over millions of years. Eventually, our sun will grow more than 200 times as wide as its present size.

"That swelling sun will consume Mercury and probably Venus, before growing so large it approaches Earth’s orbit — a distance known as one astronomical unit, or AU. But it could expand even further. “In some models,” said Antonino Lanza, an astronomer at the Astrophysical Observatory of Catania in Italy, “it can engulf Mars.” The main uncertainty, he said, lies in how much mass the sun will lose as it ages; the more it sheds, the smaller its final radius will be. “That is poorly known,” he said.

"For now, our best estimates suggest that the sun will grow to somewhere between 0.85 and 1.5 AU. But as the star loses mass, the weaker pull of gravity will increase Earth’s orbit, meaning our planet could escape engulfment.

'To see Earth’s future, astronomers turn to a crystal ball filled with alien planetary systems. Their goal is to find sunlike stars that will soon balloon (or have just ballooned) into red giants.

"That’s why Rho Coronae Borealis, a nearby yellow dwarf star that’s thought to be reaching the end of its sunny life, caught Kane’s attention. Three of its four known planets orbit close to the star, well within Venus’ path around our sun. The outermost planet, with a year lasting 282 days, is similar in orbit to Venus.

"Kane’s analysis, published last month, shows that the growing star will engulf the three inner planets. The innermost of those worlds, thought to be rocky and nearly four times the mass of Earth, will evaporate within a few hundred years. “The plasma superheats the planet and causes it to essentially break down,” Kane said. “Even the rocks on the surface will melt away.” The next world out, a Jupiter-mass gas giant, is so large that it will spiral inward and be ripped apart by the star’s gravity, rather than evaporating. The third planet, a smaller Neptune-mass world, will likely also be engulfed and evaporated.

***

"In the past two decades, scientists have found a handful of exoplanets orbiting white dwarfs, said Mary Anne Limbach, an exoplanet scientist at the University of Michigan. These planets survived their star’s red giant phase, although it’s not clear exactly how. Some of the worlds — which tend to be larger gas giants — were probably too far from their star to be swallowed, while others may have been pushed outward as the star huffed and puffed. (Astronomers have also seen evidence that some planets were not so lucky in the form of polluted white dwarfs, which are rich in elements associated with planets, such as magnesium and iron.) Ongoing observations by the James Webb Space Telescope (JWST) are expected to turn up dozens more exoplanets orbiting white dwarfs.

"As unusual as they might seem, these planetary systems could still be habitable, said Limbach, who leads some of the JWST white dwarf observations. “There is a place around a white dwarf where you can get liquid water” on a planet’s surface, she said. But “it’s a very challenging environment.”

"More observations of evolved solar systems, and more models like Kane’s, could provide greater insight into the fate of our own. For now, the death of our planet is a roll of the dice away from certainty. Humans may be long gone from Earth’s surface, but anyone glancing in our direction 5 billion years from now might see our planet ride out our sun’s dying breaths — or, perhaps, disappear in a brief flash of light."

Comment: if we are the only life in the universe, our Sun will kill us off, and habitability won't be an issue.