Comment by btilly | Hacker News Reader

Comment by btilly | Hacker News Reader

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BS.

Filter 1. If we had no large moon, roughly every 30 million years or so our axial tilt would become too extreme for life. We do not have another example of a planet with a moon to rival ours in relative size.

Filter 2. Without Jupiter acting as a cosmic vacuum cleaner, we'd predictably be hit by enough asteroids and comets that we'd never have evolved. In thousands of solar systems that we've found, we've found lots with rocky planets, and lots with gas giants. Ours is the only system with BOTH rocky planets AND gas giants.

Filter 3. Supernovas are dangerous. For example we are currently in the Local Bubble. Which is the result of a supernova about 4 million years ago which would have wiped us out if we were this close when it happened. Luckily for us we have a weird orbit where we only pass through the galactic plane every 30 million years or so, and spend most of our time well away from potential supernovas.

Filter 4. Planets need to be in the habitable zone, around a star where that zone remains habitable for a very long period. Most solar systems have no suitable planet.

Putting these together on a back of the envelope, it is likely that we're the only solar system in our galaxy which is likely to remain habitable and stable for long enough that complex intelligent life could evolve.

Good comment. I would add that our Sun is alone, while up to 85% of solar systems are at least binary. It's possible multi-star systems make life more difficult. Similarly, Earth's magnetosphere may be uncommon, since we don't see a similar one on Mars or Venus. And maybe Earth's rotation is uncommon and many planets are tidally locked.

I wonder, though, how this stacks against the sheer size of our galaxy. There are 100 - 400 billion stars in the Milky Way, including about 4 billion Sun-like stars. Plus we keep revising estimates upward when it comes to exoplanets. Recent observations seem to indicate that 33% - 90% of Sun-like stars have rocky planets at a distance where liquid water is possible.

Are features like an unusually large Moon or a protective gas giant so rare that our planet and solar system are truly unique?

We don't know how rare either actually is. For both we have just one example - us. This suggests odds of no more than one in several thousand. But it doesn't take too many filters of that order of magnitude for a few hundred billion to become a handful, and the handful to become just one.

Your point about binary star systems is interesting. Orbital dynamics have a well-known tendency to be chaotic. In a single star system, this chaos is significantly less of a problem. But in a double star systems, the odds of moving in/out of the habitable zone over time are much higher. So that's probably a filter. But not as big of one as the ones I listed.

Mentioned elsewhere in the thread:

Filter N: Even if life is complex it might not be likely to become intelligent enough to send signals and venture into space. It took quite a long time before we showed up.

But it comes back to the number of chances. To wit, there's approximately 200,000,000,000,000,000,000,000 stars in the universe. We might be that one in a 200,000,000,000,000,000,000,000 shot where it all came together to create life but it's rather improbable. Almost certainly is other life out there.

That's just counting the visible universe. We have no idea how much farther it extends!

That said, most of the visible universe is now unreachable due to the ongoing expansion of the universe. And life somewhere unreachable, even in theory, is not of that much interest to me.

My big one is multicellular life. Mitochondria are sooo weird.

Multicellular happened over and over again.

Mitochondria are weird, but we don't know that such organelles are necessary for life to become intelligent. And we don't know how weird that capture was. It is easy to make an argument for improbability. And it is easy to make it too extreme, like the absurd arguments that Creationists use against evolution.

That's a great list of filters. Thank you.

Do you happen to know books, movies, articles or other media that expand on this?

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