The chances of anything coming from Mars are 0.70 (or 0.59) they said

Sorry to those of you without access to the BBC but i'm sure it's accessible elsewhere on the net but this list of "Planets most likely to have or sustain life" was pretty interesting. The actual paper is in the journal of Astrobiology but it's a subscription journal (boo) but i will see if my uni has access to it as i'd like to read the actual paper. Anyway it comes up with two criteria; the Earth Similarity Index and the Planet habitability Index.


The Earth Similarity Index (ESI) which, as the name suggests is how similar to the earth the other planets/moons are. This seems like a fair assumption given that this is the only place where we know life exists. The factors considered here are things like size, density and distance from parent sun. In this chart Earth has a score of 1 and the closer other places get to 1 the higher the chance of their being the possibility of life.

the top ten for ESI are

• 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

You may want to wiki some of the less obvious names as they make fairly interesting reading (especially considering one of the Gliese's existence is debated)

The second method for scoring a planet's chance of having/sustaining life is the Planet Habitability index. This one isn't so strongly biased towards being earth like because it seems there could be other conditions that could make life possible rather than being on a lump of rock that's like Earth. The energy source is one of the key factors here, usually in the form of a sun, or whether it has the chemistry to support life. The other large factor is the likelihood of liquid water - which then means the body has to be within in a zone from its star that would mean the water is in a liquid form. This is called the habitability Zone. This then ties in with the atmosphere of the planet as this can have a huge effect on the temperature. Using these qualifiers the top 10 comes out like this;

• Earth - 0.96
• 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

I like how Earth doesn't get a perfect score here. I'd like to think there's somewhere out there that has a better score.The Gliese world's still do pretty well here and it stirs my Ewok-loving heart to see two moons in the top 4.

What I find fascinating about these stats is that it makes it feel like life is relatively "easy" to turn up. If we can ever prove the existence of life having existed (or even more mind-blowing, still existing) within our own solar system then surely the chances of life existing in other start systems is almost inevitable? I'm not talking about little green men or galactic empires - I'd settle for organic chemistry or at a push some RNA. A bacteria would have me learning how to do jazz-hand back-flips. Hopefully those various probes in the solar system will give us some more clues in the next decade or so. I imagine the planet habitability index will become more accurate once we have some non-earth criteria to play with too. If we ever decide to venture further afield it would be good to make an educated guess as to which world we should head towards too.




And for those wondering what the title is a tribute to, you're in luck - I'll give you the song!