General relativity could save certain planets of death

The planets in orbit around dead stars known as white dwarfs could be able to remain habitable thanks to the general relativity subtly modifying their movement.

When stars like our sun lack fuel, they expand and become red giants before expelling their outer layers, leaving only their dense hot core – known as white dwarf. Giant planets have been found in orbit around these remains, which suggests that worlds can survive the expansion of the star.

It is also possible that rocky planets can orbit near these stars inside their small habitable areas, the region around a star where liquid water can exist on the surface of a planet, although no one has yet been found. Here, they could remain habitable for long periods because the white dwarfs cool very slowly, perhaps over billions of years.

The living area would be extremely close to the star, a few million kilometers – tiny compared to the orbit of the land of 150 million kilometers. However, previous research suggests that any larger planet in orbit nearby could prevent the life from surviving due to a heating effect of the tides: the attraction of the larger planet would generate an internal friction which heats up the smallest, triggering a greenhouse effect in flow related to that of Venus.

But this may not always be the case, according to an Eva Stafne and Juliette Becker modeling study at the University of Wisconsin-Madison. Their work shows that, under the right conditions, the theory of general relativity of Einstein can save the interior planet.

General relativity explains how massive objects bent the space-time, which we can visualize like a dip or a “good” in a flat sheet. Essentially, the gravitational well of the host star would cause a precession of the orbit of the planet – or a turn slowly – and would be badly aligned with any companion while the planet plunged and left the well.

“The precession occurs which disputes the external planet of the inner planet,” explains Stafne, preventing extreme tidal effects on the planet. “The past simulations have not included general relativity, but that said to people to include it in these nearby systems.”

Without general relativity, any external planet which is at least the mass of the earth and in an orbit 18 times that of the most interior planet would cause this greenhouse effect, explains Becker. But “if you add general relativity, it’s not so serious,” she said, with the interior planet capable of staying habitable even if the external planet was as large as Neptune at a similar distance.

Mary Anne Limbach of the University of Michigan said that the prospects for finding such a system are not clear. “We do not even know if there are habitable planets around white dwarfs,” she says, not to mention that where general relativity plays a role. Telescopes like the James Webb space telescope are actively looking for rocky worlds around white dwarves.

However, research provides an unusual set of plausible circumstances where, in the right conditions, the inhabitants of a distant world could be maintained alive thanks to the curvature of space-time.

“Maybe they would be easier to determine the general relativity that we have not done,” explains Limbach.

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