Most liveable alien worlds ranked

[article from BBC Science]

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

Among the most habitable alien worlds were Saturn’s moon Titan and the exoplanet Gliese 581g – thought to reside some 20.5 light-years away in the constellation Libra.

The international team devised two rating systems to assess the probability of hosting alien life.

They have published their results in the journal Astrobiology.

In their paper, the authors propose two different indices: an Earth Similarity Index (ESI) and a Planetary Habitability Index (PHI).


  • Earth – 1.00
  • Gliese 581g – 0.89
  • Gliese 581d – 0.74
  • Gliese 581c – 0.70
  • Mars – 0.70
  • Mercury – 0.60
  • HD 69830 d – 0.60
  • 55 Cnc c – 0.56
  • Moon – 0.56
  • Gliese 581e – 0.53

“The first question is whether Earth-like conditions can be found on other worlds, since we know empirically that those conditions could harbour life,” said co-author Dr Dirk Schulze-Makuch from Washington State University, US.

“The second question is whether conditions exist on exoplanets that suggest the possibility of other forms of life, whether known to us or not.”

As the name suggests, the ESI rates planets and moons on how Earth-like they are, taking into account such factors as size, density and distance from the parent star.

The PHI looks at a different set of factors, such as whether the world has a rocky or frozen surface, whether it has an atmosphere or a magnetic field.

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

And finally, the PHI takes into account chemistry – such as whether organic compounds are present – and whether liquid solvents might be available for vital chemical reactions.

The maximum value for the Earth Similarity Index was 1.00 – for Earth, unsurprisingly. The highest scores beyond our solar system were for Gliese 581g (whose existence is doubted by some astronomers), with 0.89, and another exoplanet orbiting the same star – Gliese 581d, with an ESI value of 0.74.


  • Titan – 0.64
  • Mars – 0.59
  • Europa – 0.49
  • Gliese 581g – 0.45
  • Gliese 581d – 0.43
  • Gliese 581c – 0.41
  • Jupiter – 0.37
  • Saturn – 0.37
  • Venus – 0.37
  • Enceladus – 0.35

The Gliese 581 system has been well studied by astronomers and comprises four – possibly five – planets orbiting a red dwarf star.

HD 69830 d, a Neptune-sized exoplanet orbiting a different star in the constellation Puppis, also scored highly (0.60). It is thought to lie in the so-called Goldilocks Zone – the region around its parent star where surface temperatures are neither too hot nor too cold for life.

The highly rated worlds from our own solar system were Mars, with a value of 0.70, and Mercury, with 0.60.

The Planet Habitability Index produced different results. The top finisher here was Saturn’s moon Titan, which scored 0.64, followed by Mars (0.59) and Jupiter’s moon Europa (0.47), which is thought to host a subsurface water ocean heated by tidal flexing.

The highest scoring exoplanets were, again Gliese 581g (0.49) and Gliese 581d (0.43).

In recent years, the search for potentially habitable planets outside our solar system has stepped up several gears. Nasa’s Kepler space telescope, launched into orbit in 2009, has found more than 1,000 candidate planets so far.

Future telescopes may even be able to detect so-called biomarkers in the light emitted by distant planets, such as the presence of chlorophyll, a key pigment in plants.



  1. Great post, Jazakallah for this! I have a question, when we say were are ‘20.5 light-years away’, what does this mean?

    1. When we say 1 light year, it is a measure of distance and it means that it would take light 1 year to cover that distance. So the statement “a planet 20.5 light years away”, implies it would take light 20.5 years to get there from earth! To show how crazy that distance is – it would take Apollo 10 (which currently holds the high speed record for a manned vehicle) just around 548 692 yrs to reach Gliese 581g. According to our current understanding, humans haven’t even been around that long on Earth!

      Lets hope faster than light travel is possible, otherwise we are glued to the Earth forever! I am banking my money on wormholes… maybe warp drive….

    1. Chlorophyll has an absorption spectrum. That means that we know from experiments done here on earth, what type of wavelength of light Chlorophyll molecules absorb. Below is a graph of that absorption spectrum,

      Chlorophyll absorbs blue and red light, whilst reflects green and a specific type of infra-red light. So, if the detected light shows the same absorption patterns as seen here on earth we know that there is chlorophyll present on that planet or at least some molecule that is very much like chlorophyll in properties.

      I hope this answers your questions.

      1. This sounds like a promising technique to detect extra-terrestrial photosynthetic organism. However it seems similar to the SETI program, which sent radio signals out into space hoping to detect advanced extraterrestrial civilizations with the ability to send radio waves back. To many scientists today however, that is pseudo-science as we have not established the existence of extraterrestrial life (except for a bunch of fossils on meteorites which look eerily similar to cyanobacteria). Then, there is also the theory of panspermia – that is, that the biomolecules that led to life on earth as we know it came from a meteorite of extra-terrestrial origins. However, the Qur’an and Promised Messiah (as) very expressedly say that extraterrestrial life does indeed exist, so it’s only a time of when and not if. Drake’s equation (probability of intelligent life in the universe) should possibly be testament to that (though people call that pseudoscience as well, go figure).


Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s