It can’t be too much on either extreme - it has to be just right. That’s why we sometimes call the habitability zone the Goldilocks zone. The star itself has to be of a certain spectral type that is not too weak nor too powerful, and falls within an age range that is amenable to stability as well. Planets that exhibit these kinds of characteristics are almost exclusively limited to a certain orbital range that is neither too close nor too far from a star. It needs to be a rocky planet, of course - and it can’t be too big or too small. It can’t be too hot or too cold or else the water will simply evaporate or freeze out. Apologies for the cliche, but when it comes to the search for aliens, water is life.įor liquid water to pool up in a stable form on the surface of a planet, the temperatures must fall into a favorable range - provided there is sufficient atmospheric pressure. That’s the reason discovering liquid water on the surface of Mars was so exciting - and that’s what explains what we look for in the Goldilocks zone. When you peel back all the layers and you get right down to what is needed for single-celled microorganisms to survive and reproduce, the single most important factor for life is water. Here’s the deal: for reasons that are plainly obvious (unless you’re a conspiracy theorist, which in that case, we have a presidential candidate you may want to take a look at), we only have Earth to look at for context about what’s essential for life. The 21 Kepler-discov.ered exoplanets in what NASA calls the 'Small Habitable Zone,' aka the Goldilocks zone NASA Ames/N.

Goldilocks galaxy