If they found the those planets in the habitable zone, are they planning ahead for human settlement of them?

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  • $\begingroup$ This seems related: space.stackexchange.com/q/5851/10234 (but I wouldn't say it's a duplicate because the other question is asking about now and this question is asking about foresight). $\endgroup$
    – duzzy
    Commented Jul 24, 2015 at 17:20
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    $\begingroup$ As it stands, this looks like it might be an interesting question, but I'm not quite sure exactly what it is asking. Can you try and explain that a bit better? $\endgroup$
    – PearsonArtPhoto
    Commented Jul 24, 2015 at 17:54
  • $\begingroup$ Such planets would make interesting candidates for further study, but only via instruments located on Earth or in Earth or perhaps solar orbit. Just getting to relatively nearby Mars is such a big, expensive, complex, risky, outright dangerous exercise that a human presence won't happen any time soon. Visiting a planet many orders of magnitude further away would be that much more of a challenge. We can imagine what it might be like or how we might get there, but that's about all we'll have the capacity to do for the foreseeable - imagine. $\endgroup$
    – Anthony X
    Commented Jan 23, 2016 at 22:06

3 Answers 3


There's a few different definitions of circumstellar habitable zone (CHZ), and it goes by a few names, but in a nutshell, no, planets orbiting within this zone are not necessarily habitable for humans (Mars isn't, and it's within the CHZ), thus no "planning" to even visit, let alone colonize any specific ones yet either;

What we mean with it is that these planets orbit within the Goldilocks zone (another term for it, referring to the Goldilocks' "just right" from the Goldilocks and the Three Bears), so the zone that permits liquid surface water and is "just the right distance" from the star the planet orbits that it seems it could be far enough from it that its surface isn't too hot, and close enough that its surface isn't too cold. Additionally, potentially habitable exoplanets within CHZ are also suspected to be rocky bodies and not too massive (it's easier to establish their mass, even their radius than their composition), so their surface gravity would be Earth-like.

But, more often than not, we don't know nearly enough about them to say if they even have any water at all, or a breathable atmosphere, or any other conditions for life as we know it. We don't even know for sure their surface temperature, since we'd require direct imaging to establish that to any precision, and the bodies in question could be geothermally active, have an atmosphere in a runaway greenhouse effect like Venus does, which is just barely outside of most definitions of CHZ, or other surface activity completely changing reality from our estimates based on limited data.

So definitions for circumstellar habitable zone, whichever you apply, also aren't the best indication of exoplanet's habitability. For example, on top of not guaranteeing any water, breathable atmosphere, even suitable temperature, and so on, it also rules out as potentially habitable other bodies where conditions for life might be suitable below the planet's surface (there's a few candidates in our own Solar system, like Europa, Ganymede, Enceladus, even Ceres, all outside of CHZ), planet's possible moons that might be habitable even if the planet itself isn't (same examples as before, except Ceres they're all moons to rather inhospitable planets), or have habitable regions, e.g. at its poles, but the most of the planet otherwise isn't suitable.

OK, so being within the CHZ isn't the best indication of potential habitability on its own, so we had to create additional habitability indexes. And, like you might have suspected by now, there's really many of those too:

  • Earth Similarity Index (ESI)—Similarity to Earth on a scale from 0 to 1, with 1 being the most Earth-like. ESI depends on the planet's radius, density, escape velocity, and surface temperature.
  • Standard Primary Habitability (SPH)—Suitability for vegetation on a scale from 0 to 1, with 1 being best-suited for growth. SPH depends on surface temperature (and relative humidity if known).
  • Habitable Zone Distance (HZD)—Distance from the center of the star's habitable zone, scaled so that −1 represents the inner edge of the zone, and +1 represents the outer edge. HZD depends on the star's luminosity and temperature and the size of the planet's orbit. Note that even though many planets have an HZD value similar to Venus (−0.93), including Kepler-438b, the HZD is not used to rule on whether a planet has suffered a runaway greenhouse effect or not, and therefore, Kepler-438b is currently assumed to be a mesoplanet rather than a hyperthermoplanet.
  • Habitable Zone Composition (HZC)—Measure of bulk composition, where values close to zero are likely iron–rock–water mixtures. Values below −1 represent bodies likely composed mainly of iron, and values greater than +1 represent bodies likely composed mainly of gas. HZC depends on the planet's mass and radius.
  • Habitable Zone Atmosphere (HZA)—Potential for the planet to hold a habitable atmosphere, where values below −1 represent bodies likely with little or no atmosphere, and values above +1 represent bodies likely with thick hydrogen atmospheres (e.g. gas giants). Values between −1 and +1 are more likely to have atmospheres suitable for life, though zero is not necessarily ideal. HZA depends on the planet's mass, radius, orbit size, and the star's luminosity.
  • Planetary Class (pClass)—Classifies objects based on thermal zone (hot, warm, or cold, where warm is in the habitable zone) and mass (asteroidan, mercurian, subterran, terran, superterran, neptunian, and jovian).
  • Habitable Class (hClass)—Classifies habitable planets based on temperature: hypopsychroplanets (hP) = very cold (< −50 °C); psychroplanets (P) = cold; mesoplanets (M) = medium-temperature (0–50 °C; not to be confused with the other definition of mesoplanets); thermoplanets (T) = hot; hyperthermoplanets (hT) = very hot (> 100 °C). Mesoplanets would be ideal for complex life, whereas class hP or hT would only support extremophilic life. Non-habitable planets are simply given the class NH.

Yes, it appears we're really that complicated beings, requiring that complicated set of conditions to thrive. And there's more, if you look at the list of potentially habitable exoplanets, you'll be hard-pressed to find any for which you could say with any degree of certainty that they fit our requirements. Uncertainty of data we work with is simply too great to depend with your life on it.

So no, nobody would be planning human settlement of any specific exoplanets that we lack crucial data on, despite that we might refer to them as potentially habitable. It's simply not good enough to send a hundred year starship or anything similar towards it, then later realize there's nothing to colonize.

The closest one we currently know of, Tau Ceti e, is 11.9 light years (ly) away. To put that in perspective, that's 5,701 times the distance traversed by Voyager 1 (132 AU) in nearly 38 years, and it's not even close to being out of the Solar system's sphere of influence, so it's still being decelerated by its collective gravity and it will be for the next 30,000 years or so when it will be gravitationally closer to Gliese 445 that's a star of between 0.15 - 0.3 Solar masses and about 17.6 ly away.

But, not wanting to leave you with a bitter taste for interstellar colonization, some scientists and groups are trying to prepare for that time early. And when you think about it, it makes a lot of sense even though we don't yet have a single suitable target we're certain of, or indeed means of getting there in reasonable time, since it's not exactly as simple as planning a trip to Venice and it will take centuries to do it, and many decades of developing better remote surveying technologies to decide where to go. If you're interested in finding out more, and I haven't lost you by now, see for example videos of the 2013 Starship Century Symposium, and also 2013 Starship Congress. Also check our other threads tagged as and .


Are the planets in the "habitable zone" actually habitable for humans?


We did discover exoplanets in the goldilock zone around some planets, but we know almost nothing about these planets. We know their approximate mass and orbit and for some we know some very unreliable data about the upper atmosphere from their transmission spectrum (subtle color change of the light of the star they orbit when they pass in front of it). But we don't even know if they are solid. They could even be very small gas giants.

Do not let those pretty pictures fool you. They are "artist impressions", i.e. how artists with lots of creativity and not much scientific background imagines how those planets could look.

If they found the those planets in the habitable zone are they planning ahead for human settlement of them?


With our current knowledge of physics and the resources available to space exploration, there is just no feasible method to bring humans to another solar system. There aren't even serious plans for unmanned interstellar missions, except in form of a couple thought experiments which never went further than the whiteboarding stage. Interstellar travel is still very, very far in the realm of science fiction.

The next big milestone for manned space exploration is Mars. And the plans for Mars are already very vague.

  • $\begingroup$ Is a small gas giant in the habitable zone possible? $\endgroup$
    – Uwe
    Commented Jun 14, 2020 at 11:09
  • $\begingroup$ @Uwe That might be a good question for astronomy.stackexchange.com. But hot jupiters are a thing, so "not too hot not too cold jupiters" are quite plausible, imo. $\endgroup$
    – Philipp
    Commented Jun 14, 2020 at 11:25

No, humans cannot survive on any other planet than Earth.

Everything we encounter physically in our everyday lives is biological or of biological origin. The ground, the things, the air, everything. Earth surface and atmosphere is like a cell membrane which "we" have both created and are dependent on. And only in the configuration which "we" have given it for the time being. "We" is everything you get in contact with. We are all relatives. Even if all humans die now, 99.5% of our genes are out there anyway. We are our own environment, and that makes us alone.

The pure math of combinatorics proves that there can never exist a world in the visible universe where we can survive, other than the one we have created here on Earth. Just take the number of amino acids to the power of the protein length times the number of proteins, and you'll see that there aren't enough numbers in the visible universe to describe it. We only live once.

  • $\begingroup$ ... rocks? Rocks aren't biological. And I don't see how the amino acid combinatorics prevents us from seeding a planet with Earth life and living on it. This sounds pseudoscientific. $\endgroup$
    – ikrase
    Commented Jun 14, 2020 at 17:08

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