General relativity might save some planets from death

Planets orbiting dead stars known as white dwarfs might be able to remain habitable thanks to general relativity 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 behind only their dense hot core – known as a white dwarf. Giant planets have been found orbiting these remnants, suggesting worlds can survive the expansion of the star.

It is also possible that rocky planets might be able to orbit close to these stars inside their small habitable zones, the region around a star where liquid water can exist on the surface of a planet, although none have yet been found. Here they could remain liveable for long periods of time because white dwarfs cool very slowly, possibly over trillions of years.

The habitable zone would be extremely close to the star, within a few million kilometres – tiny compared to Earth’s orbit of 150 million kilometres. However, prior research suggests that any larger planet orbiting nearby might make it impossible for life to survive because of a tidal heating effect: the pull of the larger planet would generate internal friction that heats up the smaller one, triggering a runaway greenhouse effect akin to that on Venus.

But this might not always be the case, according to a modelling study by Eva Stafne and Juliette Becker at the University of Wisconsin-Madison. Their work shows that, under the right conditions, Einstein’s theory of general relativity can save the inner planet.

General relativity explains how massive objects curve space-time, which we can visualise as a dip or “well” in a flat sheet. Essentially, the gravitational well of the host star would cause the planet’s orbit to precess – or slowly rotate – and be misaligned with any companion as the planet dipped in and out of the well.

“Precession happens that decouples the outer planet from the inner planet,” says Stafne, preventing extreme tidal effects on the planet. “Past simulations have not included general relativity, but this is telling people to include it in these close systems.”

Without general relativity, any outer planet that is at least the mass of Earth and within an orbit 18 times that of the innermost planet would cause this runaway greenhouse effect, says Becker. But “if you add general relativity in, it’s not that dire,” she says, with the inner planet able to remain habitable even if the outer planet were as big as Neptune up to a similar distance.

Mary Anne Limbach at the University of Michigan says the prospects of finding such a system are unclear. “We don’t even know if there are habitable planets around white dwarfs,” she says, let alone one where general relativity is playing a role. Telescopes like the James Webb Space Telescope are actively looking for rocky worlds around white dwarfs.

However, the research does provide an unusual set of plausible circumstances where, in the right conditions, inhabitants of a distant world might be kept alive thanks to the curvature of space-time.

“Maybe they would have an easier time figuring out what general relativity was than we did,” says Limbach.

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