Scientists have outlined which moons and planets are most likely to harbor extra-terrestrial life, 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).

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 likely to harbor extra-terrestrial life.

 Enceladus （土衛二） PHI: 0.35

Enceladus is the sixth-largest of the moons of Saturn. It was discovered in 1789 by William Herschel. Until the two Voyager spacecraft passed near it in the early 1980s very little was known about this small moon besides the identification of water ice on its surface. The Voyagers showed that the diameter of Enceladus is only 500 kilometers (310 mi), about a tenth of that of Saturn's largest moon, Titan, and that it reflects almost all of the sunlight that strikes it. (From Wikipedia)

Enceladus is one of only three outer solar system bodies (along with Jupiter's moon Io and Neptune's moon Triton) where active eruptions have been observed. Analysis of the outgassing suggests that it originates from a body of sub-surface liquid water, which along with the unique chemistry found in the plume, has fueled speculations that Enceladus may be important in the study of astrobiology. The discovery of the plume has added further weight to the argument that material released from Enceladus is the source of the E ring. (From Wikipedia)