D-6: Only A While

The following was originally published on the Stellaris forums.

 

Drink up my darling
Be brave and smile
Death is for ever
Life’s only a while
Hello and welcome to another episode of the astroknowledge series! Today I’m going to talk about a place which I find utterly enthralling. This is not particularly interesting from a scientific point of view, but as a human I can’t get over how evocative it is.

In 1993, Artie Hatzes, an American astronomer living and teaching in the pleasant Thuringian mountains of Germany, developed a hypothesis that there was a gas giant orbiting the nearby giant star of Pollux. At the time there was very little way to tell (even Latham’s Planet was only confirmed in 1991), but Hatzes kept his idea in mind. Technology soon improved and in 2006 he was able to confirm that it existed. The planet has had various designations but in 2015 it was named Thestias.

[IMG]
(Artist’s impression courtesy IAU.)

Thestias is not habitable: it is a gas giant and is far too close to Pollux for its moons to be habitable either. However, it may have been within Pollux’s habitable zone back before she became a giant star. This means that if anyone was living there, they would have had to evacuate.

(For the moment, we’ll ignore the fact that Pollux is too young for Thestias to have developed intelligent life. Earth took ~600 million years to develop even single-celled life, which is more or less Pollux’s entire main sequence lifespan.)

They’d have had millions of years to evacuate, especially if their astronomers were competent enough to spot it coming, so it’s not as if this would have taken them by surprise. Entire civilisations can rise and fall in that time.

However – and here’s the interesting thing – it is likely that where there’s one planet, there’s more. This means that when Pollux became a giant and her habitable zone moved outwards, some planets might have become habitable. The new habitable zone would be somewhere around 6-10 AU, which in our Solar system places Jupiter just outside of it and Saturn just inside it.

It wouldn’t last long, of course. Pollux is 742 million years old and will not live to see her nine hundred millionth birthday. This habitable zone is only temporary But even a hundred million years is a while.

What would it be like to live there?

The evacuation of a planet sounds like a rushed affair, but remember that they have advanced warning and plenty of time. However, the expansion and first dredging of a star is sudden, and Pollux’s habitable zones would not shift gently. The two planets would not be habitable at the same time. Therefore while the evacuation of people from the moons of Thestias may have taken generations, they would have been fleeing to a planet which has no atmosphere and no liquid water. Domed cities and tunnels would be their best bet here. It would not be a pleasant life for the first few generations of colonists, and they may not be pleasant people. There’s work to do here, but most of it consists of waiting and preparing.

It’s only once Pollux swells from a white star to an orange giant that Old Thestias can truly be abandoned and the work on New Thestias can begin. Asteroids can be gathered and melted for air and water. After a few centuries, the biosphere can begin to grow. People can emerge from their tunnels and domes to walk freely under the sky of their new home.

In a few hundred thousand generations they’d have to do it again, of course, and this time they would have to take a much longer journey. All that they’ve gained is time. But time is enough, because life is good at enduring, especially when that life has radio telescopes and supercomputer simulators.

Don’t cry for the people of New Thestias, because you should be commiserating: we’re in the same situation. The Sun will give us perhaps another 5 billion years before Earth becomes uninhabitable. If we survive until then, which I think we will, we’ll have to evacuate the same way – and inconveniently our new habitable zone will fall around Jupiter, none of whose moons are massive enough to hold an atmosphere. We’ll have to take the long journey out to the stars.

We’ve got perhaps two hundred million generations left on this planet. That’s enough time to think in the long term, even if it is only a while.

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