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In five billion years, the Sun’s going to evolve into a red giant. That’s bad news for Earth, but exciting for some of the worlds a little farther out.

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The Earth has an expiration date and it's coming up... in about 5 billion years. That's when the Sun is going to start running out of fuel and get bigger, better, and brighter, eventually turning into a huge red giant. And while that may spell the end of life on Earth, it might mark the beginning of a real estate boom for life in the outer solar system.

As the Sun increases its energy output, its habitable zone, the cozy, not to hot, not to cold region where water can stay liquid, will start moving further out into the solar system. And where there's water there's the potential for life. It's hard to predict what kind of life might take advantage of this new chapter in the solar system's history. All we know for sure is that the habitable zone will eventually move. So here's what we think will happen when it does. 

In about 7 billion years the habitable zone will catch up to Jupiter and that's when things will start to get interesting. Jupiter itself is so huge and inhospitable that making it a little warmer won't change much. Instead the real excitement's going to happen among its neighbors, like Europa, one of Jupiter's moons. Europa's already considered one of the prime contenders for hosting life, because of its huge ocean beneath its solid icy crust. Though we aren't sure whether that ocean is more like liquid water or slush. But either way, when the Sun's supercharged habitable zone finally catches up to Europa, it's going to bring with it enough energy to melt the entire ocean, ice and all. 

The inside of Europa is already heated by tidal interactions with Jupiter and the other big moons, whose gravitational pull continually distorts its shape. All that matter shifting around generates a lot of friction which heats up the moon's insides. So when Europa starts to thaw, it will then have two potential energy sources for life to feed off of: the internal heat escaping from thermal vents at the bottom of the ocean floor, and sunlight warming the surface of the ocean. These changes will be gradual, so if there are local lifeforms, they'll have millions if not billions of years to slowly adjust to the warming temperatures. 

But Europa's not the only moon to get the V.I.P. treatment. Ganymede and Callisto are two other moons of Jupiter and we think they both have subsurface oceans too. Which means that as the whole region heats up, they could also develop balmy oceans on their surfaces that microbes could call home.

Eventually the habitable zone will reach all the way out to Saturn where Titan, its largest moon, is also going to be ready for prime time. Titan already has a huge abundance of organic compounds that can provide basic building blocks for microscopic lifeforms. And unlike the moons of Jupiter, Titan has a pretty good chance to develop an earth-like surface, with the mixture of liquid oceans and solid ground. There's a catch though. Those oceans might not be made of H2O, because Titan has lakes of liquid methane and ethane, and scientists think that life could evolve using liquid methane to help spur along chemical reactions. 

As the moon warms the amount of methane on its surface will increase from the size of lakes to the size of entire oceans. Right now the average temperate on Titan is about 100 Kelvin. In Celsius, that's nearly a hundred and seventy degrees below zero. It only has to increase to about 200 kelvin, the perfect temperature for liquid methane. 

And Titan could stay around that useful temperature for a long time, hundreds of millions, maybe even a couple billion years. For any methane dependent lifeforms, that's a pretty long party. 

Saturn's nearly twice as far from the Sun as Jupiter, so it'll take a while for it to warm up enough to melt water-ice, but eventually even Titan will reach a point where the ice starts to melt and give water-based life a chance to grow. 

Compared to the entire lifetime of the Sun, these new habitable worlds won't last very long. We're talking about a few tens to a couple hundred million years for Jupiter's moons and maybe up to 2 billion years if you include the cooler temperatures needed for liquid methane oceans on Titan. But hey! Life on Earth only moved out of the oceans and on to land about 400 million years ago, and look at all the progress we've made since. 

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