The sun won't die for 5 billion years, so why do humans have only 1 billion years left on Earth?

February 13, 2015 by Jillian Scudder, The Conversation
As the Sun matures into a Red Giant, the oceans will boil and Earth will become uninhabitable. Credit: Fsgregs, CC BY-SA

In a few billion years, the sun will become a red giant so large that it will engulf our planet. But the Earth will become uninhabitable much sooner than that. After about a billion years the sun will become hot enough to boil our oceans.

The sun is currently classified as a "main sequence" star. This means that it is in the most stable part of its life, converting the present in its core into . For a star the size of ours, this phase lasts a little over 8 billion years. Our solar system is just over 4.5 billion years old, so the sun is slightly more than halfway through its stable lifetime.

Even stars die

After 8 billion years of happily burning hydrogen into helium are over, the sun's life gets a little more interesting. Things change because the sun will have run out of hydrogen in its core – all that's left is the helium. The trouble is that the sun's core is not hot or dense enough to burn helium.

In a star, gravitational force pulls all the gases towards the centre. When the star has hydrogen to burn, the creation of helium produces enough outward pressure to balance out the . But when the star has nothing left in the core to burn, take over.

Eventually that force compresses the centre of the star to such a degree that it will start burning hydrogen in a small shell around the dead core, which is still full of helium. As soon as the sun begins to burn more hydrogen, it would be considered a "red giant".

Magnificent coronal mass eruption. Credit: NASA, CC BY

The process of compression in the centre allows the outer regions of the star to expand outwards. The burning hydrogen in the shell around the core significantly increases the brightness of the sun. Because the size of the star has expanded, the surface cools down and goes from white-hot to red-hot. Because the star is brighter, redder and physically larger than before, we dub these "red giants".

Earth's fiery demise

It is widely understood that the Earth as a planet will not survive the sun's expansion into a full-blown red giant star. The surface of the sun will probably reach the current orbit of Mars – and, while the Earth's orbit may also have expanded outwards slightly, it won't be enough to save it from being dragged into the surface of the sun, whereupon our planet will rapidly disintegrate.

Life on the planet will run into trouble well before the planet itself disintegrates. Even before the sun finishes burning hydrogen, it will have changed from its present state. The sun has been increasing its brightness by about 10% every billion years it spends burning hydrogen. Increased brightness means an increase in the amount of heat our planet receives. As the planet heats up, the water on the surface of our planet will begin to evaporate.

An increase of the sun's luminosity by 10% over the current level doesn't sound like a whole lot, but this small change in our star's brightness will be pretty catastrophic for our planet. This change is a sufficient increase in energy to change the location of the habitable zone around our star. The habitable zone is defined as the range of distances away from any given star where liquid water can be stable on the surface of a planet.

With a 10% increase of brightness from our star, the Earth will no longer be within the habitable zone. This will mark the beginning of the evaporation of our oceans. By the time the sun stops burning hydrogen in its , Mars will be in the , and the Earth will be much too hot to maintain water on its surface.

Uncertain models

This 10% increase in the sun's brightness, triggering the evaporation of our oceans, will occur over the next billion years or so. Predictions of exactly how rapidly this process will unfold depend on who you talk to. Most models suggest that as the oceans evaporate, more and more water will be present in the atmosphere instead of on the surface. This will act as a greenhouse gas, trapping even more heat and causing more and more of the oceans to evaporate, until the ground is mostly dry and the atmosphere holds the water, but at an extremely high temperature.

As the atmosphere saturates with water, the water held in the highest parts of our atmosphere will be bombarded by high energy light from the , which will split apart the molecules and allow the water to escape as hydrogen and oxygen, eventually bleeding the Earth dry of water.

Where the models differ is on the speed with which the earth reaches this point of no return. Some suggest that the Earth will become inhospitable before the 1 billion year mark, since the interactions between the heating planet and the rocks, oceans, and plate tectonics will dry out the planet even faster. Others suggest that life may be able to hold on a little longer than 1 billion years, due to the different requirements of different life forms and periodic releases of critical chemicals by plate tectonics.

The Earth is a complex system – and no model is perfect. However, it seems likely that we have no more than a billion years left for life to thrive on our planet.

Explore further: Some potentially habitable planets began as gaseous, Neptune-like worlds

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1.3 / 5 (13) Feb 13, 2015
Golly Gee, the sun has been getting hotter and hotter, the earth is getting hotter, but the sun apparently has nothing to do with it.

Why can't educated people realize global warming is something that will happen regardless of how many laws you pass or taxes you levy.
4.7 / 5 (12) Feb 13, 2015
Whew! Thanks Gimp. I hadn't realized before that "global warming" is about processes that we don't have to worry about for hundreds of millions of years yet. Glad you cleared that up! You'd think those pesky scientists would know that, wouldn't you? Maybe you should ask to be put in charge of the IPCC technical group.
5 / 5 (11) Feb 13, 2015
Golly Gee, the sun has been getting hotter and hotter, the earth is getting hotter, but the sun apparently has nothing to do with it.
Yea, that 10% difference every billion years can really affect the planet over the course of a couple of hundred years.

Do you have the capability to work out what that percentage is?

Why can't uneducated people ask questions instead of spouting out nonsense or politically motivated ignorance.

1 / 5 (10) Feb 13, 2015
I don't know percentages, I be stupid. Just saying, there are many more variables to consider, but heck, you smarter dan me, you figger it out.

Send your IPCC to Mars, it's been getting hotter as well, but I suppose we have a factory hidden somewhere spewing out tons of man made toxins there as well.
1.4 / 5 (9) Feb 13, 2015
I love the anonymity of the internet, it's so easy to insult people, insinuate you know me so well that you can question my intelligence, create a view of my political leanings then try to destroy them while being smug and self righteous.
4.7 / 5 (12) Feb 13, 2015
I love the anonymity of the internet, it's so easy to insult people, insinuate you know me so well that you can question my intelligence, create a view of my political leanings then try to destroy them while being smug and self righteous.
I love the internet too, it gives ignorant people the ability to make broad generalizations and rhetorical inaccuracies without fear of having to back up their comments with facts or evidence.

And then, of course, anonymously insult scientists and researchers and claim a dubious political inclination.

Do you understand the word hypocrisy?
4.8 / 5 (4) Feb 15, 2015
In a billion years we should have established colonies on the outer planetary moons; hopefully we will survive long enough to stop being selfish idiots.
1.5 / 5 (4) Feb 15, 2015
Humans can save the Earth by using reflectors or feedback chemicals that decrease the solar insolation. Technically we can make life last longer on Earth. By that time we may actually be able to engineer the Sun in order to produce less energy. We could inject some nuclear poisons into it that will reduce the rate of nuclear reactions. Without us life on Earth may end one day.
3 / 5 (3) Feb 15, 2015
Our descendants will have evolved into something else by then. No need to panic. Shall be ashes anyway long before.
2 / 5 (1) Feb 16, 2015
Going by this, we don't have a billion years. That's when the oceans evaporate. We will be wiped out much before that. My guess is that complex life on earth doesn't have more than 50-100 million years. Humans will suffer, if not wiped out, still before that. Before biological suffering, the heating will impact the weather patterns and economics. That alone will kill many.

On the other hand, I think we do have another million years or so. What can we do in the next 500,000 years? For now, in the next 100 years, let's first focus on getting rid of intra-human strife and conflict. We can postpone thinking about migrating to another planet by 500 years or so.

The land is cheap in northern Canada and in Siberia. I am thinking of making some investments...
5 / 5 (3) Feb 16, 2015
... Technically we can make life last longer on Earth. By that time we may actually be able to engineer the Sun ...

Great 'we can solve it' spirit, Gigel, but it would be much easier to move the earth than to engineer the sun.

To counteract a 10% increase we'd need to move the earth 5% farther from the sun. The earth averages 150,000,000 kilometers from the sun, so 5% is 7,500,000 km. So we 'only' need to stretch earth's orbit by 7.5 meters per year.

The earth only 'weighs' 6x10^24 kg * 0.006 m/s2 = 3.6 x 10^22 Newtons, so 7.5 meters requires only 2.7 x 10^23 joules, and half would come from the earth's velocity, so we'd need to add ~1.35 x 10^23 Joules per year. Humans currently use ~600 exajoules per year (6 x 10^20 joules), so that's only roughly 200 times as much energy as we use today, and 30x LESS than the sunlight energy reaching the earth (momentum is a different issue).

Today? No way!
But in a thousand years? I wouldn't bet against it...

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