How long before the Earth becomes uninhabitable for all life? This is a reasonable question to ask in the context of our search for life elsewhere because it gives us a framework within which we might expect to discover life on exoplanets.
Nothing lasts forever, stars live and die over the course of hundreds of millions to billions, and in the case of red dwarf stars, trillions of years. Planets are born from the remnants of their parent stars and die over a shorter timescale. Life on those planets, if it exists, is presumed to be shorter still, governed entirely by the environments provided by the combination of the star and planetary characteristics. How long life lasts there ultimately depends on how long the star lives and how it dies.
Using our solar system as an example, since we know life arose here, how long will it last? Astronomers think they have an answer.
Our Sun is a star, and like all stars, it has a lifespan. The Sun is currently in its middle age, but it will eventually get old and die.
In about 5 billion years, the Sun will run out of hydrogen in its core and will begin to fuse helium into carbon. This process will cause the Sun to expand into a red giant star.
As the Sun expands, it will engulf Mercury and Venus. It may even reach Earth’s orbit, but it is not clear whether it will swallow Earth or not.
Well before the Sun reaches this stage, astronomers believe things will get bad for life, probably billions of years before the Sun even reaches the Red Giant stage. According to some new modeling done by astronomers, In the later phase of the Sun’s life, over the next couple of billion years or so, the increasing amounts of energy Earth will receive from the aging Sun will warm its atmosphere. The hotter the atmosphere, the more water vapor it will trap as water evaporates from Earth’s surface.
The Sun’s ultraviolet rays will break this water vapor down into oxygen and hydrogen, and hydrogen is a gas light enough to escape the atmosphere.
With water escaping into space, the oceans will dry up and the crust will dry out, leaving a desolate, uninhabitable world. Life on Earth will end at least a billion years before the Sun expands into a Red Giant.
As volcanoes continue throwing carbon dioxide into the atmosphere, there will be no oceans to absorb it or life forms to store it. This buildup of carbon dioxide causes what is called a “runaway greenhouse effect,” turning Earth into a hot, dry world like Venus.
In fact, Venus may have once hosted a shallow liquid-water ocean and temperatures conducive to life before a runaway greenhouse process made its environment unbearable when it was only 2 billion years old.
In the context of life on other worlds, we need to remember that even though a star might have planets that harbor life, any life that existed there might be long gone even though the star is still relatively young.
The good news is that we have a few billion years before this happens to us. Who knows, humanity might even manage to become a star traveling civilization, capable of stupendous technological marvels that would protect us from a dying sun. A billion years is a long time for humans, even if it’s a blink of an eye in a star’s lifetime, plenty of time to think and plan and dream.