Some extrasolar planets might be Earth-like. This means that they have conditions very similar to that of the Earth. Planets are ranked by a formula called the Earth similarity index or ESI for short. The ESI goes from one (most Earth-like) to zero (least Earth-like). For a planet to be habitable it should have an ESI of at least 0.8. For comparison, the four solar terrestrial planets are included in this list.