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一颗行星宜居的标准是什么?

最近，我意识到，要使一颗行星适合居住，必须有很多因素，而不仅仅是那些浮现在脑海中的因素，比如是否有氧气、合适的温度等等。

是什么让一颗行星适合居住?有官方的标准吗?我不明白为什么有人会说我们有x个可能适合居住的系外行星，除非有一套标准。

编辑:我不确定正确的术语是“宜居”还是“可居住”，但我显然想问的是，是什么因素决定了人类和其他生物能否在给定的星球上生活。

回答

**星周宜居带**有[a](http://www.universetoday.com/32622/habitable-zone/)[少数](http://www.britannica.com/EBchecked/topic/1674268/habitable-zone)[页面](https://en.wikipedia.org/wiki/Circumstellar_habitable_zone)详细解释了这个概念，但要点是，星周宜居带是(我的定义)>恒星周围的区域，在该区域内，类似地球的行星表面可以有液态水。然而，通过对[木卫二](https://en.wikipedia.org/wiki/Europa_(moon)(可能有地下海洋)和[土卫六](https://en.wikipedia.org/wiki/Titan_(moon)(可能有碳氢化合物湖)的更详细的观察，这个定义可能会有所改变:>恒星周围可能有液态水的区域，或者可能出现适合生命形成的其他化合物的区域;另一种可能是气态巨行星周围的区域，在那里卫星可以被潮汐力加热，这样化合物就可以存在。当然，后一部分只适用于卫星。银河系宜居带(见[这里](http://astro.unl.edu/naap/habitablezones/ghz.html)和[这里](https://en.wikipedia.org/wiki/Galactic_habitable_zone))是另一个——稍微不那么明确的——宜居行星应该存在的区域。你可以计算出一颗恒星周围的宜居带，这主要是根据恒星的亮度来决定的。这里有一个解释不清楚的公式(http://www.astro.sunysb.edu/fwalter/AST101/habzone.html)，尽管[行星生物学](http://www.planetarybiology.com/calculating_habitable_zone.html)给出了一个更明确的推导。两个重要的方程是:$$r_i = \sqrt{\frac{L_{\text{star}}}{1.1}}$$ $$r_o = \sqrt{\frac{L_{\text{star}}}{0.53}}$$其中$r_i$和$r_o$是恒星的内外半径，$L_{\text{star}}$是恒星的光度。不过，请注意，这些都是针对可居住区的传统定义，忽略了非H2O基化合物和围绕气态巨行星运行的卫星的潮汐加热。[这个](http://xxx.lanl.gov/pdf/1301.6674v2.pdf) Kopparapu等人的预印本确实给出了另一个有趣的公式:$$d=\left(\frac{L/L_{\odot}}{S_{eff}} \right)^{0.5} \text{AU}$$其中$S_{eff}$是由行星在该距离上的有效温度$T_{eff}$和一些系数以及太阳的$S_{eff}$决定的参数。 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](http://triplehelixblog.com/2012/06/criteria-for-habitable-planets-and-implications-for-earth/) 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](https://en.wikipedia.org/wiki/Atmosphere_of_Titan). 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"](http://www-ssc.igpp.ucla.edu/personnel/russell/papers/mars_mag/), 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](http://arxiv.org/pdf/1404.4992v1.pdf), 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](https://en.wikipedia.org/wiki/Tidal_locking) 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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