General relativity might save some planets from death

Planets that orbit white dwarfs, the remnants of stars that have died, may still be capable of sustaining life due to the effects of general relativity, which can subtly modify their trajectories.

As stars, like our sun, exhaust their nuclear fuel, they expand into red giants and eventually shed their outer layers, leaving behind a hot, dense core known as a white dwarf. There have been observations of giant planets orbiting around these stellar remnants, indicating that some worlds can endure the star’s death throes.

It is also plausible for rocky planets to orbit closely within the small habitable zones that surround these stars—regions where conditions allow for liquid water to persist on their surfaces, although such planets remain unconfirmed. They could potentially remain viable for extended periods since white dwarfs gradually cool over time, potentially taking trillions of years.

This habitable zone would be uncomfortably close to the star, at only a few million kilometers—minuscule in comparison to Earth’s average distance from the Sun, which is about 150 million kilometers. Nonetheless, earlier studies indicate that larger planets in proximity might render life unsustainable due to tidal heating effects; the gravitational pull from a larger neighboring planet could generate friction that heats the smaller planet internally, leading to conditions reminiscent of a runaway greenhouse effect similar to that experienced on Venus.

However, new findings suggest that this situation may not always hold true. A modeling investigation conducted by Eva Stafne and Juliette Becker from the University of Wisconsin-Madison demonstrates that under certain conditions, the principles of general relativity could help preserve the inner planet’s ability to support life.

General relativity provides an understanding of how massive objects warp space-time, which we can conceptualize as creating a “well” or depression in a flat surface. The gravitational well created by the host star leads to the precession, or slow rotation, of the planet’s orbit, causing it to become misaligned with any neighboring planet as it fluctuates in and out of the well.

“Precession allows the outer planet to become decoupled from the inner planet,” Stafne notes, thereby mitigating extreme tidal impacts on the inner planet. “Previous simulations have not accounted for general relativity, but our findings emphasize its importance in examining these tightly bonded systems.”

Without considering general relativity, any outer planet with a mass equivalent to Earth and within an 18-fold orbital distance from the innermost planet would likely induce runaway greenhouse conditions, Becker explains. However, “when general relativity is integrated into the model, the outcome appears much more favorable,” allowing the inner planet to remain habitable even if an outer planet is as large as Neptune located at a similar distance.

Mary Anne Limbach at the University of Michigan expresses uncertainty regarding the likelihood of discovering such a system. “We have yet to establish whether habitable planets exist around white dwarfs,” she states, much less identifying one influenced by general relativity. Telescopes like the James Webb Space Telescope are currently on a mission to uncover rocky worlds orbiting these stellar remnants.

Nonetheless, the research presents a peculiar set of realistic scenarios where, given proper conditions, inhabitants of a distant planet could owe their survival to the curvatures of space-time.

“Perhaps they would have an easier path to understanding general relativity,” Limbach speculated.