General Relativity Could Rescue Some Planets from Oblivion

Planets in orbit around white dwarf stars may have the potential to remain habitable due to subtle movements dictated by the general theory of relativity.

As sun-like stars deplete their fuel, they expand into red giants, shedding their outer layers, ultimately leaving behind a dense, hot core called a white dwarf. Evidence shows that giant planets can continue orbiting these remnants, indicating that life may withstand the stars’ expansion.

Moreover, rocky planets could potentially orbit close to these stars within a compact habitable zone. This zone is the region around the star where liquid water can exist on a planet’s surface, though it has yet to be observed. White dwarfs can remain hospitable for immense periods, as they cool down very gradually, possibly for trillions of years.

The habitable zone is located million kilometers away from the stars and is significantly narrower than Earth’s orbit of 150 million kilometers. Previous studies indicated that a massive orbiting planet makes survival untenable due to tidal heating effects: the gravitational pull of a larger planet generates internal friction, leading to a runaway greenhouse effect akin to that of Venus.

However, modeling conducted by Eva Stafne suggests this might not necessarily be the case. Juliet Becker, from the University of Wisconsin-Madison, found that, under certain conditions, Einstein’s general theory of relativity can provide a lifeline for the inner planet.

According to general relativity, massive objects warp space-time, which can be visualized as a dip or “well” on a flat surface. Essentially, the gravity wells of the host star become detached from the orbiting planet, slowly rotating and interacting inconsistently as the planet moves in and out of these wells.

“There’s a precession that separates the outer planet from the inner planet,” says Stafne, which prevents extreme tidal effects on the inner planet. “Past simulations did not consider general relativity, but this highlights the importance of including it in these close systems.”

Without considering general relativity, the outer planet, which would need to be at least 18 times more massive than the innermost planet, could provoke this runaway greenhouse effect, Becker explains. Yet, “factoring in general relativity changes the outcome dramatically,” she states. The inner planet can remain hospitable to similar distances, even with an outer planet as large as Neptune.

Mary Anne Limbach from the University of Michigan is uncertain about the likelihood of discovering such systems. “I’m not even sure if any habitable planets exist around white dwarfs,” she states. Telescopes like the James Webb Space Telescope are actively on the lookout for rocky worlds in the vicinity of white dwarfs.

Nevertheless, this research reveals a unique series of plausible scenarios where inhabitants of distant worlds may thrive under suitable conditions, thanks to the bending of space-time.

“We might have a better understanding of how common relativity can be than we think,” Limbach observes.

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