Earth may have a better chance of avoiding physical destruction when the sun swells into a red giant, according to a study published in Astronomy & Astrophysics. That is the cheerful version of a forecast in which Earth still loses its oceans long before anyone could enjoy the technicality.
Astrophysicists have long expected the sun to exhaust the hydrogen in its core in about 5 billion years. After that, the core will contract, hydrogen fusion will continue in a surrounding shell, and the sun’s outer layers will balloon outward while its surface cools. That red giant phase has left Earth’s final status annoyingly unresolved: swallowed by the swollen sun, or left as a burned-out planet orbiting the white dwarf that remains.
The new study argues for the second outcome as a stronger possibility than earlier models suggested. The researchers say updated modeling of tidal dissipation and stellar mass loss changes the balance of forces acting on Earth’s orbit as the sun ages.
The orbital fight
Two effects decide the problem. As the aging sun sheds material through stellar winds, it loses mass. With less solar gravity pulling on Earth, the planet’s orbit can move outward. That is the escape route.
The opposing effect is uglier. As the sun expands, Earth gets closer to the star’s extended gaseous envelope. Drag and tidal forces can bleed energy from the planet’s orbit. Tides are the uneven gravitational pull across an object, and over long enough times they can act like a brake. In previous calculations, that braking effect was expected to win, sending Earth inward until the sun vaporized it.
According to the Astronomy & Astrophysics study, tidal dissipation may be weaker than those older estimates assumed. Tidal dissipation is the process that drains orbital energy and tends to make elliptical orbits more circular. If it is less efficient, Earth has less orbital energy stripped away during the red giant phase.
The researchers also point to observations of L2 Puppis, a red giant about 209 light-years from Earth, as evidence that the future sun could shed enough mass for outward orbital drift to overpower tidal drag. That does not turn the forecast into a guarantee. It moves the odds in a less apocalyptic direction, which is about as generous as stellar evolution gets.
Survival is not habitability
The same account says Earth becomes uninhabitable much earlier. While the sun remains in its current main-sequence phase, it will keep getting hotter and brighter. Within the next two billion years, that added radiation is expected to evaporate Earth’s surface water.
The study’s more optimistic orbital result also depends on messy late-stage stellar behavior. Stellar winds and thermal pulses near the end of a star’s life are difficult to pin down precisely. If the sun loses less mass than the model estimates, tidal forces could still pull Earth into destruction.
Other planets have clearer prospects in the same scenario. Mercury and Venus are expected to be engulfed by the sun’s expanded outer layers. Mars should shift into a wider orbit and avoid destruction, although the warming would vaporize its permanent ice reserves.
Farther out, Jupiter and Saturn would keep orbiting, but their moons would not be left untouched. The stronger solar radiation could reshape moon orbits and temporarily melt the icy crusts of worlds such as Europa and Enceladus, creating surface oceans for a while. The solar system’s habitable-looking real estate may move outward after Earth has already been cooked past recognition.
This story draws on original reporting from WIRED.
