U.S. scientists have come up with two new tools to determine which moons and planets in the universe are most likely to harbour alien life.

And they have used those tools to rank many other planets and moons in the solar system, as well as some planets in other parts of the universe. Their results are published in the journal Astrobiology.

Plugging in parameters such as a planet's mass, radius, and average temperature generates a series of measures in the Earth Similarity Index, which "provides a quick screening tool with which to detect exoplanets most similar to Earth," said Dirk Schulze-Makuch, an astrobiologist at Washington State University in Pullman, Wash., in a statement.

He added that scientists know empirically that Earth-like conditions can support life.

But the researchers noted that some conditions that aren't Earth-like — such as those found in oily lakes on Titan, a moon of Saturn — could theoretically also be conducive to life — albeit not the kind we are most familiar with.

In order to flag planets with those types of conditions, the scientists came up with the Planetary Habitability Index, which assumes a more general set of conditions are needed for life:

  • A stable and protected substrate, but not necessarily the rocky surface found on Earth.
  • Energy, although the main source doesn't need to be light from a Sun-like star.
  • Chemistry that allows large, chain-like molecules to be built, but not necessarily the carbon-based chemistry on Earth.
  • A liquid medium, but not necessarily water.

"The disadvantage of this approach is that it is intrinsically more speculative," the researchers wrote. "The alternative, however, to miss habitable worlds due to restricted assumptions would be self-defeating."

The only known planet or moon either inside or outside our solar system that is truly "Earth-like" using the Earth Similarity Index was found to be Gliese 581g, a planet announced discovered in 2010 that other researchers have since suggested does not exist, based on a reanalysis of the data.

Meanwhile, a number of planets scored "surprisingly large" values on the Planetary Habilitability Index, including Jupiter, Venus and Titan, which scored higher than more Earth-like Europa, a moon of Jupiter that is believed to be covered in water and ice.

The researchers suggested that both Venus and Jupiter could support life in their clouds. Such organisms could have originally evolved from ground-dwelling life on Venus's surface when conditions were less hostile or from one of Jupiter's moon's respectively, they said.

As for Titan, it is deemed more likely than Europa to support life because it has a confirmed atmosphere and liquids on its surface, including some that can undergo organic chemistry similar to that used by life on Earth.

"Any life that might exist on Titan, however, would most likely differ more greatly from life as we know it than would any life that might be found on Europa," the researchers wrote.

They added that their tools should be updated as new and better information is made available, and could incorporate other parameters deemed conducive to life.

Whether the "anomalies" in the results generated by their own version of the Planetary Habitability Index "call into question the underlying assumptions on which our calculations are based or point to new ways of thinking about the distribution of life in the Universe is a matter to be decided ultimately by more and better data," they said. " In the meantime, seeming anomalies such as these can serve to question our intuitive opinions about habitability."

Planetary Habitability Index Rankings

The following are listed in order from most habitable to least habitable, with their relative PHI score in brackets. Planets outside our solar system are indicated as exoplanets:

 1. Earth (0.96)

  2. Titan, moon of Saturn (0.64)

  3. Mars (0.59)

  4. Europa, moon of Jupiter (0.49)

  5. Gliese 581 g, exoplanet (0.45)

  6.Gliese 581 d, exoplanet (0.43)

  7. Gliese 581 c, exoplanet (0.41)

  8. Jupiter (0.37)

  8. Saturn (0.37)

  8. Venus (0.37)

11. Enceladus, moon of Saturn (0.35)

12. Io, moon of Jupiter (0.30)

13. Gliese 581 b, exoplanet (0.29)

13. HD69830 b, exoplanet (0.29)

15. 55 Cancri c, exoplanet (0.26)

15. Neptune (0.26)

15. Uranus (0.26)

18. Ceres, asteroid, dwarf planet (0.23)

19. Triton, moon of Neptune (0.23)

20. Pluto, dwarf planet (0.22)