The habitable zone

Life as we know it needs liquid water. Too near a star, water boils away. Too far, it freezes. In between is a band where a planet with the right sort of air could keep seas on its surface. That band is the habitable zone. By one widely used estimate the Sun’s runs from 0.99 to 1.70 times the Earth’s distance.

Venus is well inside the inner edge, and it shows what happens there. It is nearly the same size as the Earth and may once have had water. Now its surface is at about 464 °C, hot enough to melt lead, under air 92 times as heavy as ours. The white in this picture is the top of its clouds.

Mars is inside the zone, and it is a frozen desert. It averages about −65 °C. The zone only says a planet could hold water if it had enough air to keep warm, and Mars has almost none: less than a hundredth of the Earth’s. Being in the zone is not enough.

The Earth sits only just inside the inner edge, by that estimate. And its place is not the whole reason it is comfortable. With no blanket of air at all, the Earth would average about −18 °C and the seas would freeze. The air adds about 33 degrees. A planet’s own air matters as much as where it is.

Every star has its own zone. A dim star gives little heat, so its zone lies very close in. Round the small red star TRAPPIST-1, three planets lie in the zone, and all three are more than eight times closer to their star than Mercury is to the Sun.

And water turns up outside the zone altogether. This is Enceladus, a small moon of Saturn, nearly ten times further from the Sun than we are. The bright jets are water, spraying out of a sea hidden under its ice. That sea is kept liquid by the squeezing of Saturn’s gravity, with no help from sunshine. The zone is the first place to look. It is not the only one.