Life on Earth has roughly 1 to 1.5 billion years left, give or take a few hundred million. The culprit isn’t an asteroid, a supervolcano, or anything humans are doing. It’s the Sun, which grows steadily brighter over time and will eventually make our planet uninhabitable. On a much longer timescale, life anywhere in the universe faces its own final deadline: the last stars will burn out in about 100 trillion years, and after that, the cosmos slowly goes dark and cold forever.
Why the Sun Will Kill Earth’s Biosphere
The Sun gets about 1% brighter every 100 million years. That sounds trivial, but it triggers a chain reaction. A hotter Sun means faster chemical weathering of rocks on Earth’s surface, which pulls carbon dioxide out of the atmosphere. Plants need CO2 to photosynthesize, and there’s a hard floor below which they simply can’t function.
Most plants on Earth use a type of photosynthesis called C3, which stops working when CO2 drops below about 150 parts per million. Some hardier plants, like grasses and corn, use C4 photosynthesis and can survive on less than 10 parts per million. A landmark study published in Nature found that a biosphere built on C4 plants could persist for another 0.9 to 1.5 billion years from now, depending on whether low CO2 or rising heat is what finishes them off first.
Once plants go, the oxygen supply collapses. Animals follow. The only survivors would be microorganisms that don’t need oxygen or sunlight, living deep underground or in hot springs. The most heat-tolerant life forms known today, a group of single-celled organisms called archaea, can grow at temperatures up to 122°C (252°F). Bacteria top out around 100°C. Complex life like animals and plants can’t complete their life cycles above roughly 40 to 60°C. So long before Earth becomes truly sterile, everything visible to the naked eye will already be gone.
Earth’s Total Habitability Window
Current models estimate that Earth’s total habitable zone lifetime is between 6.3 and 7.8 billion years. Since complex life has existed for roughly the last half-billion of Earth’s 4.5-billion-year history, we’re already about 70% of the way through the planet’s livable window. That remaining 30% is where the billion-or-so years of habitability sits.
Around 1.3 billion years from now, surface temperatures are projected to reach about 47°C (117°F) globally, triggering what’s called a moist greenhouse effect. Water vapor saturates the upper atmosphere, where solar radiation splits it apart and the hydrogen escapes into space. Earth begins irreversibly losing its oceans. How long it takes to dry up completely remains uncertain, but the process, once started, doesn’t reverse.
What Happens to Earth After That
Even after life is gone, the planet itself has a dramatic future. Around 4 billion years from now, the Andromeda galaxy will begin colliding with the Milky Way. The merger will take about 2 billion years, reshuffling stars into new orbits and eventually forming a single giant elliptical galaxy. NASA projections suggest Earth, the Sun, and the other planets would survive this collision, just relocated to a new position in the merged galaxy.
The Sun’s real finale comes about 6 billion years from now, when it exhausts the hydrogen fuel in its core and swells into a red giant. It will grow large enough to certainly swallow Mercury and Venus. Earth’s fate is less clear, but recent modeling suggests our planet probably won’t survive intact. As astronomer Dimitri Veras at the University of Warwick put it: the Sun will definitely consume the two inner planets and spare Mars, but Earth sits right in the ambiguous zone between destruction and survival.
Could Anything End Life Sooner?
The billion-year timeline assumes no catastrophic surprises. In reality, life faces shorter-term threats, though none of them are likely to cause total extinction. Natural disasters like massive volcanic eruptions (occurring roughly every 20 to 30 million years) and nearby stellar explosions (every 250 to 500 million years) could devastate ecosystems, but microbial life has survived every mass extinction in Earth’s history so far. The odds of a civilization-ending asteroid larger than 1 kilometer hitting in the next century sit at roughly 1 in 120,000.
Human-caused risks are harder to quantify. Direct extinction from climate change appears very low, according to most risk analysts. Nuclear war would be devastating but probably not species-ending on its own. The threats that worry researchers most are engineered biological agents and unaligned artificial intelligence. Philosopher Toby Ord estimated the overall chance of an existential catastrophe this century at about 1 in 6, with AI and engineered pandemics making up the bulk of that estimate. Other researchers consider those numbers too high. The honest answer is that for human-created threats, uncertainty is enormous.
None of these risks, however, threaten all life on Earth. Microbes living kilometers underground in rock, near hydrothermal vents, or in other extreme environments are effectively immune to surface-level disasters. Total sterilization of the planet requires the kind of slow, planetary-scale physics that only the Sun can deliver.
When Life Ends in the Universe
If life somehow spreads beyond Earth, the timeline extends dramatically. Stars will continue forming normally for another 1 to 100 trillion years. The longest-lived stars are tiny red dwarfs, which burn their fuel so slowly that individual ones can last over 10 trillion years. Any planet orbiting one of these dim stars could theoretically support life far longer than Earth will.
By about 100 trillion years from now, the raw material for new stars will be exhausted. The last red dwarfs will flicker out, and the universe enters what physicists call the Degenerate Era: a dark cosmos populated only by the cooling remnants of dead stars, neutron stars, and black holes. No new energy sources, no new chemistry, no plausible way to sustain biology.
Even black holes don’t last forever. They slowly evaporate through a quantum process called Hawking radiation. The largest supermassive black holes will take a staggeringly long time to disappear: roughly a googol years (a 1 followed by 100 zeros). After that, the universe enters its final state, sometimes called the Dark Era or heat death. All energy is evenly spread out, the temperature everywhere settles to just above absolute zero, and nothing meaningful can ever happen again. This era will last far, far longer than everything that came before it combined.
There is one wildcard. If dark energy turns out to be unstable and grows stronger over time, it could tear the universe apart in a scenario called the Big Rip, potentially as “soon” as 30 billion years from now. In that case, the expansion of space would accelerate until it overwhelms gravity at every scale, pulling apart galaxies, then solar systems, then planets, then atoms. Current observations don’t strongly favor this outcome, but it can’t be ruled out.