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Recently, I realized a lot of factors must contribute toward making a planet habitable, other than just the ones that come to mind like having oxygen, the right temperature, and so on.

What makes a planet habitable? Is there an official set of criteria? I don't see how one could say we have $x$ number of official potentially habitable exoplanets unless there was a standard set of criteria.

EDIT: I'm not sure if the correct term is "habitable" or "inhabitable", but what I am obviously asking is what factors determine whether humans and other organisms could live on a given planet.

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    $\begingroup$ I think this is a great and on-topic question. Good places to start may be en.wikipedia.org/wiki/Planetary_habitability and en.wikipedia.org/wiki/Terraforming. $\endgroup$ – milancurcic Dec 23 '14 at 18:46
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    $\begingroup$ Call me skeptical. IMHO, a better name for those lists of habitable exoplanets is "list of possibly habitable exoplanets based on incomplete/inconsistent knowledge of the exoplanets and an incomplete, Earth-centric concept of what constitutes habitability." $\endgroup$ – David Hammen Dec 23 '14 at 18:48
  • $\begingroup$ @DavidHammen I agree. Until it actually happens, we can only talk about possibly or theoretically habitable, while the term habitable may take different meanings in different contexts. $\endgroup$ – milancurcic Dec 23 '14 at 18:53
  • $\begingroup$ @DavidHammen you are quite correct. My pre-edited question mentioned that I follow an earth scientist on twitter who's always tweeting about inhabitable exoplanets and that's why this question came to mind. but you are right. Now that i think about it, he always says POTENTIALLY inhabitable exoplanets. I have changed the question to reflect that. $\endgroup$ – Stan Shunpike Dec 23 '14 at 19:27
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    $\begingroup$ Not a big deal, but it strikes me that 'habitable' is a better word than the synonym-that-sounds-like-an-antonym 'inhabitable', which (like inflammable) is apt to be misunderstood. $\endgroup$ – kwinkunks Dec 29 '14 at 21:22
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Circumstellar habitable zone

There are a few pages that explain the concept in detail, but the gist is that the circumstellar habitable zone is (my definition)

The region around a star inside which a planet similar to Earth can have liquid water on it surface.

However, with more detailed observations of Europa, with its possible underground ocean, and Titan, with its hydrocarbon lakes, the definition may be modified:

The region around a star inside which a body can have liquid water, or where other compounds suited for the formation of life can arise; alternatively, the region around a gas giant where moons can be heated by tidal forces such that the compounds can exist.

The latter section, of course applies to moons only. The galactic habitable zone (see here and here) is another - slightly less-well-defined - area where a habitable planet should lie.

You can calculate the habitable zone around a star based off of what defines it: the luminosity of the star, primarily. There's a poorly-explained formula here, although Planetary Biology gives a much more explicit derivation. The two important equations are these: $$r_i = \sqrt{\frac{L_{\text{star}}}{1.1}}$$ $$r_o = \sqrt{\frac{L_{\text{star}}}{0.53}}$$ where $r_i$ and $r_o$ are the inner and outer radii, and $L_{\text{star}}$ is the luminosity of the star. Note, though, that these are for the conventional definition of the habitable zone, neglecting non-$\ce{H2O}$-based compounds and tidal heating of moons orbiting gas giants. This pre-print, by Kopparapu et al., does give another interesting formula: $$d=\left(\frac{L/L_{\odot}}{S_{eff}} \right)^{0.5} \text{ AU}$$ where $S_{eff}$ is a parameter determined by the effective temperature $T_{eff}$ of a planet at that distance and some coefficients, as well as the Sun's $S_{eff}$. But those findings are rather new, so I'd stick with the older formulas.

So the habitable zone is determined primarily by the star's luminosity.

Mass

An atmosphere is generally considered a must for planets with Earth-like lifeforms. Low-mass bodies, such as the Moon, can't hold on to one, and that's one of the reasons that moons have not been of as much interest as planets have been. Atmospheres, among other things, can keep the planet at a nice temperature and allow distinct climates to form. As this site elaborates on, they also help protect the planet from radiation like UV rays. Our ozone layer is really helpful in that regard.

Mass isn't the only thing that helps a body keep its atmosphere. For example, Titan is relatively low-mass (although relatively high-mass for moons), yet it still has an atmosphere. This is because the solar wind - which can hurt atmospheres - is so weak at Titan's distance from the Sun.

To expand on what gansub mentioned: Magnetic fields are important because they are extremely useful when it comes to helping a planet retain its atmosphere. As Luhmann and Russell explain in "Mars: Magnetic Field and Magnetosphere", Mars lost its magnetic field long ago, and so the solar wind is gradually stripping it away, albeit at a really slow rate.

Rotation

This section is based a lot on this paper, by Yang et al. It, too is recent, so keep that in mind.

Before I get into the paper, I'll say this: Rotation helps because it keeps one side of the planet from being baked while the other side freezes. Tidally-locked planets aren't the greatest places for life. A decently-fast rotation can really help.

Anyway, Yang et al. came to this conclusion (as voiced in their opening sentence):

Planetary rotation rate is a key parameter in determining atmospheric circulation and hence the spatial pattern of clouds. Since clouds can exert a dominant control on planetary radiation balance, rotation rate could be critical for determining mean planetary climate.

In other words, rotation helps clouds, and since clouds help govern the climate of the planet, a proper rotation rate can make the climate less extreme.

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Typically when searching for possible life on other worlds (aka inhabitable), scientists look for terrestrial worlds with liquid water or some other basic liquid. If a world has a solvent of some sort and some type of geologic activity (which is a source of heat and molecular diversity), then it would be considered possibly "inhabitable" for the types of life we are familiar with. However, if a planet has some strange characteristic that would cause periodic extreme events (e.g. highly elliptical orbit), it's environment would be less stable and therefore less likely to support life. Check out this site on the subject: http://lcogt.net/spacebook/what-are-requirements-life-arise-and-survive or refer to wikipedia as mentioned in the comments http://en.wikipedia.org/wiki/Planetary_habitability

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Few other conditions in addition to liquid water and plate tectonics are global magnetic field and distance of the planet from the host star.

For a recent comprehensive review on the subject please refer to this article -

http://www.geology.wisc.edu/~astrobio/docs/Lammer_et_al_2009_Astron_Astro_Rev.pdf

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