This is Exoplanet Radio, I'm Tony Darnell.
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 time scale.
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 and it's not what you think.
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's not clear whether it will swallow Earth or
not.
But 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 and the later phase of the sun's life, over
the next couple 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
the 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.
Astronomers believe that 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 build-up of carbon dioxide causes what
is called a runaway greenhouse effect, turning
the 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 runaway greenhouse processes
made its environment unbearable when it was
only 2 billion years old.
So 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 and has a few billion years left to go.
The good news for Earth is that we still
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.
Billion years is a long time for human
beings, even if it is a blink of an eye in a star's
lifetime.
Plenty of time to think and plan and dream.
Exoplanet Radio is produced by Deep Astronomy.
The music is by Geodesium
and available at Loch Ness Productions.com.
Get all episodes at exoplanetradio.com or anywhere you get podcasts.