When will the Sun "expand" and consume the Earth?

Recent studies focused on aging planetary systems beyond our Solar System are enhancing our understanding of what awaits Earth as the Sun embarks on its long journey to becoming a red giant.

As the Sun ages, it will expand in size and its surface will stretch. Ultimately, it may engulf Earth and obliterate it. The fate of our planet hinges on numerous random factors.

In five billion years, the Sun will become a red giant. Will Earth survive? This is a question that currently lacks an answer, says Melinda Soares-Furtado, an astrophysicist at the University of Wisconsin-Madison.

Of course, there is a chance that Earth will be consumed by the Sun and destroyed. However, there are alternative scenarios where Earth could evade this fate and be pushed to a more distant orbit.

Research into a neighboring planetary system has provided insights into the future of our planet. Located about 57 light-years away, this system comprises four planets orbiting a star similar to the Sun. This star is 10 billion years old, which is twice the age of our Sun, and is already in the later stages of its life.

Stephen Kane, an astrophysicist at the University of California, Riverside, modeled what might happen to the planets in a billion years when the star becomes a red giant. He found that most of the inner planets, meaning those whose orbits are closer to the star, will be consumed. However, the furthest planet, which orbits similarly to Venus, may survive.

Studying aging stars enhances our understanding of their evolution and helps predict the future of our planetary system. “This is a very interesting paper,” says Jonathon Zink, an astrophysicist at the California Institute of Technology. “If we can find more systems at different stages of stellar evolution, we will likely gain insight into what will happen.”

When a planet is engulfed by a star, its demise can be swift. In 2022, Ricardo Yarz, a stellar astrophysicist at the University of California, Santa Cruz, modeled the process of a planet being consumed by a red giant.

He discovered that if a planet is close enough to the star, its orbit quickly deteriorates. The gas in the star's atmosphere creates drag on the planet, causing it to plunge deeper into the star, Yarz explained. Within a few hundred years, most planets will be destroyed.

Until recently, the final moments of a doomed planet had never been observed directly, said Kishalay De, an astronomer at the Massachusetts Institute of Technology. However, in 2020, De's team saw a star 12,000 light-years away temporarily become hundreds of times brighter. The flare was too weak to arise from a merger with another star. But, as De and his colleagues reported in May, this was precisely the intensity that could result from a “bite” the size of a planet.

The team hypothesized that a planet several times larger than Jupiter was captured when the 10 billion-year-old star began expanding into a red giant. “This is the future of our Solar System,” said De.

A star in the process of evolution

A star like our Sun, which is in a stable state and known as a main-sequence star, also referred to as a yellow dwarf, is reaching the end of its life cycle. Its hydrogen, necessary for nuclear fusion, is running out.

As the star starts utilizing other fuel sources and loses mass, its core becomes hotter, and its atmosphere swells over millions of years. Eventually, our Sun will become more than 200 times its current size.

This expanding Sun will consume Mercury and possibly Venus before reaching Earth's orbit. However, it may expand even further.

“In some models,” says Antonino Lanza, an astronomer at the Astrophysical Observatory of Catania in Italy, “it may swallow Mars.” The main uncertainty lies in how much mass the Sun will lose as it ages: the more it loses, the smaller its final radius will be.

Currently, our best estimates suggest that the Sun will grow to about 0.85–1.5 astronomical units. But as the star loses mass, the weakening of its gravity will increase Earth's orbit, meaning our planet could avoid being engulfed.

To glimpse Earth’s future, astronomers are studying the universe for other planetary systems. Their goal is to locate stars similar to the Sun that will soon become (or have already become) red giants.

This is why Rho Coronae Borealis, the nearest yellow dwarf star believed to be nearing the end of its solar life, has attracted Kane's attention. Three of the four known planets orbit close to the star, within the orbit of our Venus. The furthest planet, which takes 282 days to orbit, is similar to Venus in its orbit.

Kane's analysis, published last year, shows that the growing star will consume the three inner planets. The innermost of these worlds, which is rocky and nearly four times heavier than Earth, will vaporize within a few hundred years. “Plasma overheats the planet and leads to significant destruction,” Kane said. “Even the rocks on the surface will melt.”

The next world, a gas giant with Jupiter’s mass, is so massive that it will spiral inward and be torn apart by the star's gravity rather than vaporizing. The third, smaller planet, with a mass similar to Neptune, will likely also be consumed and vaporized.

However, the furthest planet—a Neptune-mass world—may survive. As the star expands, it will temporarily engulf the planet for several thousand years. During this time, extreme temperatures will scorch the planet's surface, but the planet itself should survive since the star's atmosphere is not very dense at that distance. Then the star will contract again and expand once more, consuming the planet for several millennia. If the planet can survive being toyed with like a cat with a mouse, it may escape the atmosphere when the star finally contracts. “So in the very end, it has a chance to flee,” Kane said.

Kane is, for example, optimistic about the planet's chances and what they might mean for our world. “I suspect that Earth will move outward and survive,” he said.

Can Earth avoid its fate?

If a planet manages to evade being engulfed, its chances for a longer life will be significant. When a star like our Sun transforms into a red giant and sheds its outer layers, only a dense, white-hot stellar “corpse,” known as a white dwarf, remains. These objects contain half the mass of the original star and are about the size of Earth. They will continue to burn for trillions of years.

Over the past two decades, scientists have discovered a group of exoplanets orbiting white dwarfs, as reported by Mary Ann Limbach, an exoplanet specialist at the University of Michigan. These planets survived the red giant phase of their star, although it is unclear how this occurred. Some of the worlds, which are typically larger gas giants, likely were too far from their star to be consumed, while others may have been pushed outward as the star contracted and expanded.

However, not all planets have been so fortunate. Polluted white dwarfs rich in elements associated with planets, such as magnesium and iron, have been discovered.

Ongoing observations using the James Webb Space Telescope will help uncover dozens more exoplanets orbiting white dwarfs.

Despite their unusual nature, these planetary systems might still be habitable, according to Limbach, who leads some of the observations of white dwarfs with the JWST. “There is a region around a white dwarf where liquid water could exist on the planet's surface,” she said. But “it is a very challenging environment.”

Further observations of evolving solar systems and more models like Kane's could provide a more comprehensive understanding of the fate of our own solar system. For now, the death of our planet remains uncertain. Humans may have long left Earth's surface by the time the Sun begins to expand, but anyone looking our way in 5 billion years will witness our planet experiencing the final breaths of our Sun—or perhaps disappearing in a brief flash of light.

Conclusion

In any case, whether the Sun engulfs Earth's orbit or not, in a billion years, life on Earth will become impossible. This will occur due to the Sun's brightness increasing by 10% and the average global surface temperature of Earth rising to 47 °C (currently only 15 °C).

At that point, the concentration of carbon dioxide will drop below the critical threshold (around 50 parts per million) necessary to sustain photosynthesis in most plants. Trees and forests in their current form will no longer be able to exist.

Some plants utilizing more efficient photosynthesis (around 10 parts per million) may survive for a while longer. However, the rise in temperature will still lead to the evaporation of oceans, which will soon vanish. This will trigger a greenhouse effect and the extinction of much life.

Some microscopic life forms may persist for about another billion years and will die out when the planet's global