Our solar system has existed in its current form for billions of years. However, the Sun, like all stars, undergoes a life cycle that will profoundly transform our cosmic neighborhood. This stellar evolution will eventually determine which planets, if any, can physically persist in the distant future.
The Sun’s Stellar Evolution
The Sun is currently in its main sequence phase, a stable period where it generates energy by fusing hydrogen into helium in its core. This phase, which began approximately 4.6 billion years ago, is about halfway through its expected 10-billion-year duration. As hydrogen fuel in the core depletes in roughly 5 billion years, the core will contract and heat up. This contraction will ignite hydrogen fusion in a shell surrounding the core, causing the Sun’s outer layers to expand significantly.
This expansion will mark the Sun’s transition into a red giant star. Its size will increase up to 250 times its current radius. During this red giant phase, expected to last about 1 billion years, the Sun’s luminosity will increase by 2,000 to 3,000 times, though its surface temperature will cool, giving it a reddish-orange appearance. The Sun will also lose a substantial amount of its mass, possibly up to half, through strong stellar winds.
After the red giant phase, the Sun will shed its outer layers, forming a planetary nebula. The remaining hot, dense core will then collapse into a white dwarf. This white dwarf will be about the size of Earth but contain roughly half of the Sun’s original mass, making it incredibly dense. No nuclear fusion occurs in a white dwarf; it simply radiates its residual heat, gradually cooling over many billions of years.
The Inner Solar System’s Demise
The inner planets—Mercury, Venus, and Earth—face a particularly harsh fate during the Sun’s red giant phase. Mercury and Venus are likely to be engulfed by the expanding stellar surface, either vaporized or absorbed into the Sun’s outer layers.
Earth’s survival is less certain, but its environment will become uninhabitable long before any physical engulfment. The Sun’s increasing brightness, a gradual process that continues even in its current main sequence phase, will cause Earth’s oceans to boil away and its atmosphere to escape into space in approximately 1 to 2 billion years. This will result in a runaway greenhouse effect, transforming Earth into a scorching, dry world similar to present-day Venus.
When the Sun expands into a red giant, its outer layers are predicted to reach or even exceed Earth’s current orbit. Even if Earth is not directly engulfed, its close proximity to the bloated red giant and interaction with its expanded atmosphere would lead to orbital decay, causing the planet to spiral inward. Earth will become a molten, airless, and barren rock.
The Outer Solar System’s Transformation
The planets of the outer solar system—Mars, Jupiter, Saturn, Uranus, and Neptune—are unlikely to be engulfed by the Sun’s red giant expansion due to their greater distances. Their environments will still undergo significant transformations. As the Sun loses a substantial portion of its mass during its red giant phase, its gravitational pull on the remaining planets will weaken. This reduced gravitational force will cause these outer planets to drift outward into wider orbits; for instance, Neptune’s orbit could eventually double its current distance from the Sun.
Despite their increased distance, the outer planets will experience much higher levels of solar radiation due to the red giant Sun’s vastly increased luminosity. This shift will cause the Sun’s habitable zone—the region where liquid water could exist on a planetary surface—to move outward. Planets and moons previously too cold could temporarily warm up. Mars, for example, might become more temperate for a period, though it would still be severely impacted.
More notably, the icy moons of Jupiter and Saturn, such as Europa, Callisto, Ganymede, Enceladus, and Titan, could see their subsurface oceans thaw. These moons might develop temporary liquid water environments, potentially becoming suitable for some forms of life for an estimated billion years. The gas giants themselves, while not engulfed, will experience significant atmospheric changes from the increased radiation and heating, with Jupiter’s surface potentially reaching temperatures over 1000 Kelvin.
What Planetary Survival Entails
After the Sun’s red giant phase, its stellar core will eventually contract into a white dwarf. Some of the outer planets and their larger moons are expected to physically endure this transition, remaining as distinct celestial bodies orbiting the stellar remnant.
However, the environment of the solar system will be profoundly different from its current state. This vastly reduced energy output means the solar system will become an extremely cold and desolate place.
The temporary warming and potential habitability experienced by the outer moons during the red giant phase will cease as the white dwarf cools. While these planets and moons may physically persist, the conditions will be inhospitable for complex life. The solar system will transform into a frozen, barren expanse, with its remaining celestial bodies orbiting a dim, cooling stellar corpse.