The Habitable Worlds Catalogue

The simplest question is also the most human: how much is this world like ours? The Earth Similarity Index answers with a single number between zero and one — one is a perfect twin, zero is nothing alike.

It blends four ingredients: how big a world is, how dense, how hard you would have to push to escape its gravity, and how warm its surface runs. The catch is in the blend — the four are multiplied, not averaged, so a single hostile ingredient drags the whole score down. A world can match Earth on three counts and still score low if the fourth is wrong.

Pick a world below and watch which ingredient betrays it.

Habitability does not have to live on the surface. An eirenesphere — named for Eirene, the Greek goddess of peace — is the region inside a world where temperature and pressure alone would let microbes survive.

It can be a thin skin at the surface, like Earth's, or a hidden ocean kilometres beneath ice, like Europa's. It even includes a calm layer high in the scalding clouds of Venus. But a habitable room is not the same as an inhabited one: Venus has a larger eirenesphere than Earth, yet it is unstable and bone-dry.

Choose a world and look inside.

Earth is the only living world we know, so we treat it as the standard. But it may not be the best. A superhabitable world would be more welcoming to life than Earth itself — a little larger, a little warmer, a little older, circling a calmer, longer-lived orange star that buys billions of extra years for life to unfold.

Shallow seas scattered with islands would multiply the sunlit coastlines where life thrives. Earth is habitable; these worlds might be more so.

Toggle each trait to rebuild Earth into something better than home.