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Comment by skribbj | Hacker News Reader

How common are "life-friendly" planets (Let's say: on a ratio of Life-Friendly Planet:Non Life-Friendly Planet)? And is there any way to check if life-friendly planets contain life without actually going there?

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A good question. No sarcasm, it depends on your definition of "life friendly". Right now, we can't see much more than an orbit and a mass for these things. (There's a spectrum on a few of them, but AIUI for most exoplanets we're still just getting them from star wobbles, which gives us only orbit and mass.) Combined with the stats for the stars themselves, we can say whether or not they could conceivably be in the "liquid water" zone.

But we can't say that they are definitely suitable for life. If Venus and Mars were found orbiting other stars, the press release would declare they could be "life-friendly"; if you squint a bit, both are at least on the edge of where you could get liquid water under the right circumstances. Being much closer to the real Venus and Mars, we know they are both not actually great candidates, because we can see the details that prevent them from being useful clearly.

So I'd say "life-friendly" is a continuum, and kinda relative to the amount of knowledge we have about the planet, which for most exoplanets right now, is very low, making the "life-friendly" bar correspondingly low.

Detecting life in general is impossible; we are fairly sure Earth has had life on it in the past that did not meaningfully change the spectrum of Earth. However, there are some signatures that would be very, very, very suggestive. From what I've gathered, Earth's atmosphere containing free oxygen isn't quite proof of life from a single snapshot, as there are conceivably processes that could produce it momentarily, but having oxygen in our atmosphere consistently for millenia and eons is difficult to explain with anything other than life.

There are plans for doing spectrography on the atmospheres of these planets. This would allow us to get a pretty good picture of the chemical composition. Biological life as we know it should leave a different footprint than a lifeless planet but in the end we can only extrapolate what we already know about life.

Yep! And more specifically, if we found a planet that had an atmosphere with O2 in it, that would be highly indicative of life. Although it's not the only option, O2 is pretty reactive, and you need a constant O2-producing force to maintain it. An O2-producing force would be something like plants or some life that is creating hydrocarbons from CO2 and H2O.

I don't know that exact ratio, but the number of stars that you can see that probably have planets in their "habitable zone" is pretty high... One in 5.

https://www.popsci.com/article/science/one-five-sun-stars-ha...

In our solar system, 2-3 of the 8-9 planets are in the "habitable zone". So let's just say it's maybe one in 20 or so.

As other commenters have said, we don't have the tools to see that far yet, so it's only through a miracle of math, physics, and engineering that we can deduce the size, distance from the star, and atmospheric composition of exoplanets from variations in the brightness and color of a distant star over time. The James Webb telescope should help, but because of the nature of the technique our observations are constrained to solar systems that are perfectly aligned with ours, such that the planets pass directly in front of the star relative to the observer. It's possible that we can survey enough of those stars to get a rough idea of the ratio of life friendly planets, but we'll likely never know for sure. Maybe conditions for supporting life are correlated with alignment to the galactic plane!

Over the last two decades we’ve gone from not knowing if there were any planets in other solar systems to knowing that almost all of them have planets. We now know the second factor in the Drake equation is nearly 100%.

We don’t yet know the third factor yet, the percentage of planets capable of supporting life. But we do know about 22% of planets are small enough to possibly be rocky and are within the “habitable zone” of a star.

So a possible answer to your first question is that it’s likely 0-22% of stars have at least one planet capable of supporting life like that on Earth. This doesn’t say anything about moons or asteroids or the probability of life developing on a body that could support it.

I'm curious to know if Mars would be considered life-friendly from 12 light years away.

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