The Sun, the source of all energy and life in our solar system, is currently a stable, mid-life star. In approximately five billion years, our star will begin a dramatic transformation into a red giant, a phase of stellar death marked by immense expansion and a complete shift in its structure. This metamorphosis will trigger a catastrophic sequence of events for the inner solar system, permanently altering Earth’s habitability. The red giant phase represents the inevitable end for stars like the Sun, fundamentally changing the gravitational and thermal environment for any remaining planetary bodies.
The Mechanism of Solar Transformation
The Sun’s long, stable life is powered by nuclear fusion, specifically the conversion of hydrogen into helium within its core. This process, known as the main sequence, maintains a balanced state where the outward pressure from fusion resists the inward pull of gravity. After about five billion more years, the hydrogen fuel in the core will become depleted, causing the fusion process to cease in that central region. Without the outward pressure to counteract it, gravity will cause the now inert helium core to begin contracting under its own immense weight.
The gravitational collapse of the core significantly increases its temperature and density. This heating ignites a new layer of hydrogen fusion in a shell surrounding the collapsed helium core. The energy generated by this shell burning is far greater than the Sun’s current core fusion. This massive surge of energy pushes the Sun’s outer layers outward, causing them to expand and cool, which gives the star its characteristic reddish hue and classification as a red giant. This expansion is gradual, taking about a billion years, and will eventually cause the Sun’s radius to swell by hundreds of times its current size.
Runaway Heating and Environmental Collapse
Long before the Sun’s physical edge reaches Earth’s orbit, the planet will face an environmental collapse driven by the star’s increasing luminosity. Even as the Sun remains on the main sequence, its brightness is steadily rising, and in about a billion years, this increased heat will trigger a runaway greenhouse effect. The surface temperature will climb high enough to completely boil the Earth’s oceans, transforming all surface water into atmospheric steam.
This water vapor, a potent greenhouse gas, will trap even more heat, accelerating the warming process. The intense solar radiation will eventually strip hydrogen from atmospheric water molecules, leading to the permanent loss of Earth’s water to space. The planet’s surface will become a scorched, molten wasteland, heated to temperatures capable of melting rock. By the time the Sun fully enters the red giant phase, the surface will be completely sterilized, leaving behind a dry, lifeless world similar to present-day Venus.
Earth’s Physical Interaction with the Expanded Sun
Whether Earth will be physically engulfed by the red giant Sun is complex, involving two competing forces. When the Sun reaches its maximum size, its radius is predicted to extend out to or beyond Earth’s current orbit of one astronomical unit (AU). Models suggest the Sun could expand to about 256 times its current size, easily swallowing Mercury and Venus.
However, as the Sun expands, it also loses a substantial amount of mass via a powerful stellar wind. This mass loss weakens the Sun’s gravitational pull, causing the orbits of the remaining planets, including Earth, to spiral outward. If Earth’s orbit expands fast enough, it could potentially escape the outer boundary of the expanding star. The critical factor is atmospheric drag: if Earth enters the Sun’s extremely thin, extended outer atmosphere, friction will cause the planet’s orbit to decay rapidly, pulling it inward toward vaporization. Most astrophysicists agree that the drag force will overcome the orbital expansion, leading to Earth’s engulfment and destruction just before the Sun reaches its maximum size.
The Final State: A White Dwarf and Planetary Remnants
After spending approximately one billion years as a red giant, the Sun will shed its outer layers. This ejected material forms a vast, colorful cloud of gas and dust known as a planetary nebula. The remaining core, now devoid of fusion fuel, will collapse under gravity to form an extremely dense object known as a white dwarf.
This white dwarf will be roughly the size of Earth, yet it will contain nearly half of the Sun’s original mass. The solar system will be dominated by this dim, hot stellar remnant, which slowly cools over trillions of years. The outer planets—Jupiter, Saturn, Uranus, and Neptune—will survive the red giant phase, but their orbits will be significantly wider, perhaps even doubled, due to the Sun’s earlier mass loss. Any surviving planetary bodies will exist as burnt-out, frozen remnants orbiting the faint stellar corpse.