Can there be life on a planet orbiting a red giant?
This question can mean two things:
- Can life evolve on a planet orbiting a red giant?
- Can aliens live on a planet (or whatever) orbiting a red giant?
Because I'm such a nice guy, I'll answer both for you. But first, let's see what makes a red giant.
It's getting hot in herr
Stars are basically big balls of hydrogen and helium. They're all warm and shiny because they're constantly smashing hydrogen together to form helium, a process called thermonuclear fusion. Yes, that's similar to our nuclear weapons. Our sun is a continuous mushroom cloud of radioactive fun. A star needs hydrogen to get hot, just like a party needs liquor.
What happens when the liquor dries up? The party gets a bit boring. So when a star uses up it's hydrogen, it starts to cool down. It's gravity takes over and starts pushing everything down on itself. This creates heat; a lot of heat. Things get so hot, the heat pushes left-over gases way out and the star gets much bigger than it ever was. But without all that hydrogen fusion stuff, it just ain't the same. All the cool people left the party, and now there's only a few weirdos who are breaking out the coke and heroin. It's still a party, but it's a bit different.
Thus you have a red giant. It's an old star that used up all it's hydrogen fuel in it's youth, and now it's a reddish-white shadow of its former self. It's much bigger, but it's not as strong.
Today's forcast: partly cloudy with high chance of plasma
So, can life evolve on a planet orbiting a red giant? Sort of. Life could have evolved before the sun turned into a red giant. That's what we did here on Earth. What will happen when the Sun turns into a red giant? The outer edge of the Sun will be in between Mars and Jupiter. That's right, we'd be inside the red giant. That's bad. Unless something evolved with SPF 10 million skin, the party's over.
Chances are that any life evolving around a sun that turns into a red giant would be destroyed. For life to evolve at all, it needs to be relatively close to the star or there's not enough energy to support life. But being that close means life would be engulfed by the red giant as it expands to giant status. Intelligent life might be able to survive (super air conditioning anyone?), but your alien ecosystem would go bye-bye.
It's doubtful that life would evolve after the giant-turning phase, because the star's energy is rather weak. The giant is so giant that, by the time light leaves it's center, it's lost a lot of it's zing. Not enough punch, no bam! Maybe we could find some very simple forms of life, such as microscopic or simple plant life, but that's about it.
We're aliens! We can do whatever we want!
But can aliens live near a red giant? Why not? It's still a star, so it was gravity, light, and all that. The light is rather weak, but these aliens could have their own power sources for keeping their party going. Let's put it this way:
- If life evolved around a sun that changed into a red giant, that life is toast.
- Life can evolve after the sun went giant, but it would be small and weak. We're talking microbes and moss.
- If aliens have spaceships and stuff, they can live wherever they damn well please, red giant or not.
One last nitpick. A protostar, or a bunch of gases that haven't gotten their act together to become a real star, is also a red giant technically. It's has about the same power output, but there's a big difference: age. Protostars are so young, they probably didn't have enough time to develop planets yet, let alone life. No parties for protostars. Too young.
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