Some good news and some bad news. The good news is that, contrary to earlier fears, Earth will probably never be swallowed by the sun. The bad news, of course, is that none of us will be around to find out.
Scientists have long estimated that in about 5 billion years, the sun will run out of fuel, first expanding into a red giant and eventually becoming a white dwarf that will continue cooling for tens—if not hundreds—of billions of years. Amid this dramatic sequence of cosmic events, the fate of Earth remains uncertain.
Will it be pulled into the expanding red sun and disappear forever? Or, though long since rendered uninhabitable, will it continue orbiting the white dwarf remnant of the Sun until the universe reaches its eventual heat death?
Until now, the prevailing view among astrophysicists favored the first scenario. But a new study published in Astronomy & Astrophysics overturns that expectation, presenting new evidence that Earth may survive the sun’s transformation into a red giant after all.
The Sun’s Life Cycle
To understand what lies ahead for the planets of the solar system, you need to look inside the sun itself. At present, the sun is in its main-sequence phase, a long period of stability that has lasted for about 4.5 billion years, during which it is powered primarily by the fusion of hydrogen into helium.
This phase will continue for billions of years, but the sun will gradually become hotter and more luminous. Eventually, it will grow bright enough to evaporate all of Earth’s surface water, making our planet uninhabitable within the next two billion years.
About 5 billion years from now, the sun’s long period of stability will come to an end. By then, the hydrogen in its core will have been exhausted. The helium core will contract under its own gravity, heating up and triggering hydrogen fusion in a surrounding shell. As a result, the sun’s outer layers will expand enormously while its surface cools dramatically, giving it the characteristic red color of this stage in a star’s evolution. And this is where the mystery surrounding Earth’s fate begins.
A Complex Tug-of-War
The sun’s enormous expansion will profoundly reshape Earth’s orbit through the interplay of two opposing effects. On one hand, the sun will lose a significant amount of mass through powerful stellar winds. As its gravitational pull weakens, Earth’s orbit will gradually drift outward. On the other hand, the planet’s increasing proximity to the sun’s extended gaseous envelope will produce drag, while tidal forces—the difference in gravitational pull exerted on the near and far sides of an object, which can gradually alter planetary orbits—will act as a brake on Earth’s motion.
Until now, scientists considered it overwhelmingly likely that these tidal effects would dominate. In that scenario, Earth would gradually lose orbital energy, spiral inward, and ultimately be engulfed by the expanding sun, where it would be completely vaporized.
A New Outlook
The new study, based on improved models of tidal dissipation and stellar mass loss during the sun’s transition into a red giant, points to a different conclusion. According to the researchers, tidal dissipation—the process that drains orbital energy and gradually causes elliptical orbits, such as Earth’s, to become more circular—would be less effective than previous models suggested.
At the same time, observations of the red giant L2 Puppis, located about 209 light-years from Earth, indicate that the sun could lose enough mass for this effect to outweigh the influence of tidal forces. If so, Earth’s orbit would gradually move outward, significantly increasing its chances of surviving the red giant phase.
An Uncertain Future
Despite the study’s more optimistic outlook, Earth’s ultimate fate remains far from certain. The behavior of stellar winds and the complex thermal pulses that occur during the final stages of a star’s evolution involve many variables that are difficult to predict with precision. If the sun ultimately loses less mass than the new model estimates, tidal forces could still prevail, pulling Earth inward and leading to its destruction.
While Earth’s future remains an open question, the outlook for the rest of the solar system is much clearer. As the sun expands, Mercury and Venus will be completely engulfed by its outer layers, disappearing forever under the combined effects of intense heat and tidal forces. The outer planets, however, will follow a different path. Mars, although it will experience a dramatic rise in temperature that vaporizes its permanent ice reserves, will migrate to a more distant orbit and avoid physical destruction.
Farther out, the gas giants Jupiter and Saturn will see the orbits of their moons reshaped, while the increase in solar radiation could temporarily melt the icy crusts of moons such as Europa and Enceladus, creating oceans of liquid water on their surfaces. Which means that these worlds—at least for a time—could become successors to the Blue Planet after Earth has turned into a scorched, barren wasteland.
This story originally appeared on WIRED Italia and has been translated from Italian.