Scientists have outlined which moons and planets are most similar to earth, according to the journal Astrobiology.

In their paper, the authors propose two different indices: an Earth Similarity Index (ESI) and a Planetary Habitability Index (PHI), to figure out which moons and planets are most likely to harbour extra-terrestrial life.

The ESI rates planets and moons on how Earth-like they are, and the factors as size, density and distance from the parent star are considered.

The PHI includes the factors of whether the world has a rocky or frozen surface, and whether it has an atmosphere or a magnetic field.

It also takes into account the energy available to any organisms, either through light from a parent star or via a process called tidal flexing, and in which gravitational interactions with another object can heat a planet or moon internally.

Whether organic compounds are present and whether liquid solvents might be available for vital chemical reactions are also included in PHI.

The following are the 10 moons and planets which are most similar to earth.

 Gliese 581e（格利澤581e） ESI: 0.53

Gliese 581 e or Gl 581 e is an extrasolar planet found around Gliese 581, an M3V red dwarf star approximately 20.5 light-years away from Earth in the constellation of Libra. It is the fourth planet discovered in the system and the first in order from the star. (From Wikipediapedia)

The planet was discovered by an Observatory of Geneva team led by Michel Mayor, using the HARPS instrument on the European Southern Observatory 3.6 m (140 in) telescope in La Silla, Chile. The discovery was announced on 21 April 2009. Mayor's team employed the radial velocity technique, in which the orbit size and mass of a planet are determined based on the small perturbations it induces in its parent star's orbit via gravity. (From Wikipediapedia)