New study: Earth may escape being engulfed by the dying Sun after all

Full text

For decades, astronomers have debated whether Earth's fate was tied to the sun's. When the star exhausts the hydrogen fuel that powers it about 5 billion years, it will swell into a red giant large enough to engulf Mercury and Venus — and, several studies have suggested, Earth as well.

New research, however, suggests our planet has a better chance of escaping that fiery end than previously thought. Using updated models of how aging stars interact with their planets, researchers found that the gravitational forces drawing Earth toward the expanding sun are weaker than older models predicted. That would give the planet more time to drift outward as the dying sun sheds its outer layers into space, potentially avoiding engulfment altogether.

The finding does not guarantee Earth's survival. Instead, researchers say it shifts the biggest uncertainty from how strongly the expanding sun tugs on planets to the poorly understood variable of how much mass the star will lose during its final stages of evolution.

"The largest uncertainty no longer comes from the tidal calculations, but from how much mass the future sun will lose," study lead author Mats Esseldeurs of the KU Leuven's Institute of Astronomy in Belgium said in a statement . "Observations of sun-like giant stars currently point towards Earth's survival, but we need better observations before we can be certain."

When stars like the sun exhaust their core hydrogen and balloon into massive red giants, they trigger a cosmic tug-of-war between expanding tidal forces pulling planets inward and shedding stellar weight pushing them outward , which ultimately decides whether nearby worlds are engulfed or saved .

This planetary push-and-pull unfolds in two stages. As the sun expands, gravitational tides act like a subtle brake, slowly draining Earth's orbital energy and pulling the planet inward. At the same time, the dying star sheds vast amounts of gas through powerful stellar winds, eventually losing about half its mass. As the sun grows lighter, its gravitational grip weakens, pushing the surviving planets outward into a wider orbit that could double their distance from the star, according to NASA .

"The fate of Earth depends on a delicate balance between these two effects," Esseldeurs said in the statement. "If tidal interactions dominate, Earth is engulfed. If mass loss dominates, Earth escapes to a wider orbit."

Previous studies reached different conclusions mostly because they treated those competing processes differently, Esseldeurs and his team argue. A few of those studies neglected tidal interactions altogether, while others relied on simplified prescriptions developed decades ago that predicted a much stronger inward pull, according to the study.

Instead of relying on those older formulas, the new study uses updated calculations of tidal forces based on the shifting internal structure and dynamics of aging stars — which the team says allowed it to precisely account for both tidal friction and shifting stellar winds — before testing the results against a range of possible mass-loss rates for the sun's final giant phase.

The results suggest that even with the weaker inward gravitational pull, Mercury and Venus are unable to outpace the expanding sun and are inevitably engulfed , while Earth and Mars migrate safely through both giant phases, leaving our planet to eventually settle into a broader orbit around the white dwarf remnant the sun will leave behind, the study found.

A schematic evolution of the sun as it goes through its giant phases. The image showed the sun as it is today, going through its two giant phases, before ending its life as a white dwarf. The study showed that Mercury and Venus will be engulfed by the expanding sun, but Earth and Mars survive, ending up in a wider orbit. Sizes and distances are not to scale. (Image credit: Institute of Astronomy of KU Leuven) The picture is far from settled, however. Because astronomers still cannot precisely observe how rapidly sun-like stars lose mass late in life, "the ultimate fate of the Earth remains uncertain," the researchers write in the new paper.

By factoring in real-world mass-loss rates from L2 Pup — a red giant star roughly 183 light-years away that was used as a proxy for our future sun due to its similar mass — the researchers confirmed that Earth will drift outward just quickly enough to avoid being swallowed, tipping the scales toward survival over destruction, the study notes.

For us humans, the finding offers academic comfort rather than practical salvation. Most scientists agree that as the sun ages, it will steadily grow hotter, boiling Earth's oceans and rendering the planet completely uninhabitable in about 1 billion years, long before the sun begins to expand.

Yet even if humans aren't around to witness it, tracking Earth's ultimate survival provides important context for how planetary systems evolve as their stars age, a framework that researchers say will be further refined by future observations of dying, sun-like stars.

"This will enable us to conduct population studies of the planetary orbital evolution around evolved stars," the researchers wrote in the paper, "and help us to constrain the future evolution of the Earth-sun system."

The study was published in June in the journal Astronomy & Astrophysics.