| Habitable zone |
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The Goldilocks zone
Not too hot, not too cold
Around every star there is a band of distances where a planet like Earth could keep liquid water on its surface. Too close and water boils away; too far and it freezes. Like the porridge in the fairy tale, the middle is "just right". Astronomers call it the habitable zone.
How we draw it
The band depends on how bright and how hot the star is. These pages use the widely cited limits of Kopparapu and colleagues (2014): the inner edge is where a planet would lose its oceans like Venus, the outer edge is where even a thick atmosphere could no longer keep early Mars warm.
What it does not mean
Being in the zone does not mean a planet has water, air or life. It is a first filter. A planet's size, atmosphere and its star's flares all matter, and most of that is still unknown for these worlds.
Dim stars, tight zones
Around small red dwarfs the zone sits very close in, so planets there orbit in days, not years. TRAPPIST-1's habitable-zone planets have "years" of 6 to 12 days.
Habitable zone here is an estimate from the star's brightness and temperature (Kopparapu et al. 2014, optimistic limits). Planet data: NASA Exoplanet Archive, Planetary Systems Composite Parameters. This research has made use of the NASA Exoplanet Archive, which is operated by the California Institute of Technology, under contract with NASA under the Exoplanet Exploration Program.
← Systems within 100 light years