The death of a star is an astronomical event of immense scale, yet it is a predictable part of the stellar life cycle. Our Sun, a star of modest mass, is currently in its stable main sequence phase, which has lasted for billions of years. Approximately five billion years from now, this stability will end, initiating transformations that will radically alter the solar system and determine the final fate of the planets. This evolution will unfold over hundreds of millions of years, culminating in a stellar remnant that will persist for trillions of years.
The Sun’s Transformation into a Red Giant Star
The Sun’s dramatic change begins when the hydrogen fuel in its core is consumed, leaving behind a core of inert helium ash. Without the outward pressure from hydrogen fusion to counterbalance gravity, the helium core contracts under its own weight. This gravitational collapse causes the core’s temperature and density to increase dramatically.
The rising heat ignites a shell of fresh hydrogen surrounding the core, initiating intense fusion called shell hydrogen burning. This shell fusion releases far more energy than the previous core fusion, causing the Sun’s outer layers to swell outward to immense proportions. The star will expand to over 200 times its current diameter, turning reddish-orange as its surface temperature drops.
During this expansion, the Sun’s luminosity will increase by a factor of several thousand, bathing the solar system in intense radiation. Eventually, the dense helium core will reach a temperature high enough to ignite helium fusion, known as the helium flash. This reaction converts helium into carbon and oxygen, temporarily stabilizing the core before the star begins a second, even larger expansion phase called the Asymptotic Giant Branch.
The Immediate Fate of Mercury, Venus, and Earth
The physical expansion of the Red Giant Sun will directly engulf the innermost planets, subjecting them to intense heat, tidal forces, and incineration. Mercury, orbiting closest to the Sun, will be the first to be swallowed and vaporized by the star’s atmosphere. Venus will quickly follow, consumed by the expanding stellar material.
The fate of Earth hinges on the maximum radius the Sun reaches and the gradual outward drift of the planet’s orbit caused by the Sun’s mass loss. The Sun’s outer atmosphere is projected to expand to roughly one astronomical unit, Earth’s current orbital distance. This suggests Earth will be engulfed, but even if its orbit is pushed farther out, the intense heat will have already rendered the planet uninhabitable.
Long before physical engulfment, the increased solar luminosity will cause Earth’s oceans to boil away, triggering a runaway greenhouse effect similar to Venus. If Earth avoids total engulfment, tidal forces and the solar wind would strip its atmosphere and surface layers, leaving behind a scorched, molten core. In either scenario, Earth as a recognizable body will be gone.
Changes in the Orbits of the Gas Giants
While the inner planets meet a fiery end, the outer solar system bodies will undergo a radical transformation. As the Sun progresses through its Red Giant phases, it will experience substantial mass loss, shedding between 30% and 50% of its total mass through powerful stellar winds. This reduction in mass weakens the Sun’s gravitational pull on all remaining planets.
Because the Sun’s gravity is weaker, the orbits of the surviving outer planets—Mars, Jupiter, Saturn, Uranus, and Neptune—will naturally expand and drift outward. The gas giants and their moons could see their orbital radii nearly double their current distance from the star. This shift ensures their survival from engulfment, though their orbital dynamics will be permanently altered by the less massive central star.
The increased luminosity from the Red Giant Sun will temporarily thaw some icy bodies in the outer system, potentially making them briefly habitable. Moons like Jupiter’s Europa or Saturn’s Titan, which harbor vast subsurface oceans, could become warm enough to sustain surface liquid water. This period of warmth will be short-lived, lasting only a few hundred million years before the Sun transitions to its final stage.
The Final State: A Cooling White Dwarf
The Red Giant phase concludes when the Sun ejects its outer layers of gas and plasma into space, forming a planetary nebula. This process involves the star shedding its envelope into a rapidly expanding, colorful cloud of gas. The term “planetary” is a historical misnomer, as these nebulae have no direct connection to planets.
What remains after this ejection is the Sun’s collapsed, super-dense core, known as a white dwarf. This stellar remnant is composed of the carbon and oxygen created during the Red Giant’s core fusion. A white dwarf is roughly the size of Earth but contains the mass of the Sun, making it extraordinarily dense.
This stellar corpse is extremely hot when first formed, but it has no internal energy source. The white dwarf will slowly cool and fade over trillions of years, radiating away its residual thermal energy. The surviving gas giants and their moons will then orbit this stellar remnant in a system that is dark, extremely cold, and static, marking the final, quiet end of the solar system.