General relativity may preserve habitable planets around white dwarfs
A new study suggests that Einstein's general relativity could prevent tidal heating from destabilizing rocky planets in the tight habitable zones of white dwarfs. Researchers found that orbital precession caused by the theory's effects might decouple inner planets from larger companions, allowing life to persist for trillions of years. This challenges prior models that predicted runaway greenhouse effects on such worlds.
White dwarfs, the dense remnants of stars like our sun after they expand into red giants and shed their outer layers, cool slowly over trillions of years. Giant planets have been observed orbiting these stellar corpses, indicating that some worlds survive the host star's dramatic death throes. However, rocky planets in the narrow habitable zones—regions within a few million kilometers where liquid water could exist—face threats from tidal interactions with larger companions.
Previous simulations warned that an outer planet at least as massive as Earth, orbiting within 18 times the distance of the inner world, could induce severe tidal heating. This friction would trigger a runaway greenhouse effect, similar to Venus, rendering the inner planet uninhabitable.
But a modeling study by Eva Stafne and Juliette Becker at the University of Wisconsin-Madison incorporates general relativity, revealing a protective mechanism. The theory describes how massive objects warp space-time, creating a gravitational well that causes the inner planet's orbit to precess—slowly rotating and becoming misaligned with any outer companion. “Precession happens that decouples the outer planet from the inner planet,” Stafne explains, preventing extreme tidal effects. “Past simulations have not included general relativity, but this is telling people to include it in these close systems.”
Becker adds that the outlook improves significantly: “If you add general relativity in, it’s not that dire,” allowing habitability even with a Neptune-sized outer planet at comparable distances.
Astronomer Mary Anne Limbach at the University of Michigan cautions that such systems remain speculative. “We don’t even know if there are habitable planets around white dwarfs,” she says, though telescopes like the James Webb Space Telescope are searching for rocky worlds near these stars. The research, published on arXiv (DOI: 10.3847/1538-4357/ae07c6), highlights how space-time curvature might sustain distant life under precise conditions. Limbach speculates that any inhabitants could grasp general relativity more readily than humans did.