General relativity might save some planets from death

Planets orbited by dead stars known as white dwarfs can remain habitable thanks to general relativity by subtly altering their motion.

When stars like our sun run out of fuel, they expand and become red giants before expelling their outer layers, leaving only their dense, hot core – known as a white dwarf. Giant planets were found Orbing of these remnants, suggesting that worlds may survive stellar expansion.

It is also possible that rocky planets could orbit near these stars within their small habitable zones, the region around the star where liquid water could exist on the planet's surface, although none have been found yet. Here they could remain habitable for long periods of time because white dwarfs cool very slowly, perhaps over trillions of years.

Welcome to an area that will be very close to the star, within a few million kilometers – tiny compared to Earth's 150 million kilometer orbit. However, previous research suggests that any large planet orbiting nearby make it impossible to live Because of the tidal heating effect: the pull of the larger planet generates internal friction, which heats the smaller one, causing a runaway greenhouse effect akin to that on Venus.

But this is not always the case, according to modeling research by Eva Stafn and Juliet Becker at the University of Wisconsin-Madison. Their work shows that, under the right conditions, Einstein's theory of general relativity could save the inner planet.

General relativity Explains how massive objects scream space time, which we can visualize as falling or “well” in a flat sheet. Essentially, the host star's gravitational well would cause the planet's orbit to overcome – or slowly rotate – and be misaligned with any companion as the planet dipped in and out of the well.

“Precession occurs, which separates the outer planet from the inner planet,” Stafn says, preventing extreme tidal effects on the planet. “Past simulations didn't include general relativity, but this tells people to include it in these nearby systems.”

Without general relativity, any outer planet that is at least the mass of Earth and in an orbit 18 times larger than the inner planet could cause this runaway greenhouse effect, Becker says. But “if you add general relativity, it's not that terrible,” she says, where the inner planet could remain habitable even if the outer planet was as large as Neptune to a similar distance.

Mary Ann Limbach The University of Michigan says the prospects for finding such a system are unclear. “We don't even know if there are habitable planets around white dwarfs,” she says, not to mention where general relativity comes into play. Telescopes such as the James Webb Space Telescope are actively searching for rocky worlds around white dwarfs.

However, the research provides an unusual set of plausible circumstances in which, under the right conditions, the inhabitants of a distant world could be alive thanks to the curvature of spacetime.

“They might have an easier time figuring out what general relativity is than we do,” says Limbach.

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