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Have we observed other planets within the “Goldilocks Zone” of a star to have reasonable temperatures, that have a breathable atmosphere, water, and gravity within a healthy range for human life? Basically, have we observed another planet that could be colonized with very limited to no terraforming required? Distance from us isn’t a factor in this question. I’m also fine with answers that include planets that we think fit these metrics based off of our limited observation ability, but can’t necessarily confirm.

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    $\begingroup$ No, we haven't. Our capacities in that regard are just being built up (the next generation of ELTs, or Extremely Large Telescopes), but even while any future claim of atmospheric detections will inevitably lead to claims of habitability, those will be unprovable bogus claims. We can't know what the atmospheres of those future discoveries are with at least another or two future generations of telescopes. $\endgroup$ Apr 21, 2023 at 12:20
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    $\begingroup$ Ya, it probably does. I also got an answer here for this question that listed a bunch of potential candidates! 👍🏻 $\endgroup$
    – Kal Madda
    Apr 21, 2023 at 21:37

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As others have said, the answer is yes and no. There are quite a few planets (and, as is less often covered, gas-giants able to support moons) that could potentially host life.

Planets and moons both face different challenges: a planet will probably need a major moon like Earth's to keep its tilt stable, thus reducing the difference in seasons; a moon, meanwhile, must be both far enough from its host to not suffer extreme tidal forces, and close enough to not stray too far from said host's grasp, AKA its hill-sphere. A moon also requires enough mass, which generally depends heavily on the mass of the host. It's worth mentioning that none of the giants I'll list later on have a hill-sphere large enough for a moon going by OUR giants' moon distances to be habitable (or at least the habitable range isn't very large), so I looked at a few unconfirmed exomoons (moons beyond our solar system) and used the most certain distance ratios instead.

I've spent the past three-odd years looking at NASA's exoplanet catalogue for this very thing, though I should mention I'm absolutely just an amateur. I'm not a student or anything.

If you're as new to astronomy as I was, "±" means "give or take", so with 5±1, the number is either 4 or 6 according to estimates by the relevant study.

"Metallicity" is how much metal is in a star versus lighter elements (specifically iron versus hydrogen), and I believe it's correlated with activity -- low metallicity means a more consistent output from the star, which then means better odds for life.

Here follows my current list of favourites, all within "just" 100ly of Earth (80ly should be roughly three generations of travel when we work out fusion power).

  • Each lowercase letter in bold is a planet, and a '?' means it's not confirmed.
  • "Cool-factor" is just how interesting each system is to me.

47 UMa b: 9.4/10 life, 7/10 cool-factor. Star properties slightly rough, but otherwise, b is almost exactly in the middle of the HZ. This one just keeps proving itself a fantastic candidate. It may, though, be dangerously close to subgiant age.

HD 45364 b,c, d?: 8.6/10 life, 8/10 cool-factor. Age unknown (4.9 - 11.6gy?). b is probably too small for a habitable moon, but it's not too far off. c's hill is only okay with the Neptune ratio or higher. I'm not sure where I got d from...

HD 114783 b 8/10, 6/10 cool-factor. 1.1Jm gas-giant right in the middle of the HZ; star's age (7 +-7) is fairly good for low activity, BUT its metallicity is around 0.17. 68.7ly from Earth.

HD 4208 b: 7.6 life, 7/10 cool-factor. 1.02jm slightly cold gas-giant, barely any eccentricity. Being the only (known) planet in its system might mean more (and bigger) moons too, but then... only having one planet is a bit boring. The star's age isn't very certain: 3.813±2.970. Its metallicity is low, at least.

55 Cnc A f: 7.3/10 life, 7.5/10 cool-factor. Going by Sol's standards, it'd be fairly unlikely to have a moon around f that's both massive and distant enough for life. f is also closer to the inner HZ than I'd like.

GJ 876 b, c: 7.2/10 life, 8/10 cool-factor. ST ±157. Flare star? X-ray source?

Tau Ceti f: 6.6/10 life, 7/10 cool-factor. Mf 3.93 ±1, PE slightly rough. Many uncertain worlds, but f (a possible ocean-world) looks promising.

HD 82943 b, d?: 6/10 life, 8/10 cool-factor. One 1.68jm gas-giant, one POSSIBLE 0.29 one (d), both within the HZ. d is right in the middle. Unfortunately, 943 would only be old enough for complex life at a stretch. It's most likely just barely 4 billion years old, though the archive says its 2.68 at best.

HD 34445 f, b: 5.7/10 life, 6/10 cool-factor. Eb ±0.7, star may be too active due to age.

GJ 667C c,f,e: 5/10 life, 8.5/10 cool-factor. Age uncertain, possibly 2 - 5gy? Taking the rot p from Wikipedia (that most accurate of sources) and comparing it to K2-146, it might be 5gy... ALL EXCEPT b AND c ARE PROBABLY FALSE. https://arxiv.org/pdf/1307.6984.pdf.

HD 40307 g: 5.3/10 life, 6.8/10 cool-factor. VERY close to mid HZ, but likely a water-world; density uncertain. Eg ±0.3.

GJ 3293 b,c: 5/10 life, 7/10 cool-factor. Age uncertain, but properties indicate <3gy; might be old enough at a stretch. SL ±0.11/-0.04. The only planet with a decent moon orbit is probably too small for a moon of sufficient mass.

GJ 1061 d: 4.6/10 life, 5/10 cool-factor. ST ±98. Eccentricity might help retain atmosphere for d, which is right in the middle of the HZ, but very uncertain. One to watch!

HD 20794 f: 4.5/10 life, 7.3/10 cool-factor. f is on the rocky/ocean border, and it's somewhat close to the inner HZ limit... It's also not confirmed.

a Cen A b: 3.6/10 life, 5.6/10 cool-factor. Unconfirmed, probably too small for a habitable moon even at its highest mass estimate; really close to the inner HZ too.

HD 180617 A b: 3/10 life, 3/10 cool-factor. May be too small for a decent moon, and very cold. Only works with the most generous Hill ratio.

Trappist-1 d,e,f,g,h: 2.7/10 life, 8.5/10 cool-factor

GJ 3323 b: 0.7/10 life, 1/10 cool-factor. SL ±0.5/-1.4, PM ±2.5. If E is 0.32 not 0.23, may retain atmosphere

Teegarden's b,c,d?: 0.08/10 life, 5/10 cool-factor

GJ 180 d: 0.05/10 life, 4/10 cool-factor. ST ±230/-81

GJ 357 d: 0.01/10 life, 4.7/10 cool-factor

Luyten's b: 0.01/10 life, 3.3/10 cool. SL ±0.24/-0.6

p. Cen b: 0.0001/10 life, 5/10 cool-factor

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Yes and no.

But mostly no. While it is true we have found other planets with atmospheres, experts at NASA (or if you're in Europe, ESA, Japan JAXA, et cetera et cetera) will all tell you that an atmosphere doesn't necessarily mean something is alive there. While the Kepler mission did find planets in the "Habitable" zones of the stars they orbited, that doesn't mean anything is living on said planets.

As @AtmosphericPrisonEscape said in a well said comment, we can't prove exactly what those atmospheres are made of. For all we know, they could be exactly like Earth, thick and poisonous like Venus, or paper thin like Mars. But those are just rough guesses that can't be proven juuuust yet. Or there is a possibility they (the planets) are gas giants like Jupiter that don't have a surface at all and are therefore impossible to inhabit.

And so, until we can analyze the atmospheres of exoplanets, don't believe everything you hear.

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