Exobiology: Life in the Outer Solar System: The Gas Giants and Pluto

Exobiology was the original term given to the sciences central to the question of life-in-the-Universe. It’s now been largely replaced by Astrobiology, but I’ll stick with the original. To investigate life-in-the-Universe perhaps we should start a bit closer to home and look to our own outer solar system, starting with the gas giants (Jupiter, Saturn, Uranus and Neptune) and Pluto. Apart from Planet Earth, we have no absolute proof positive to date that any other life, albeit relatively simple forms compared to terrestrial life forms such as ourselves and associated companion animals and plants, exists within the confines of our solar system. However, the odds are fairly high, almost certain in fact; those other, extraterrestrial life forms do exist in our immediate cosmic neighbourhood. 

Taking each of the four gas giant abodes (the Jovian planets) in the outer solar system followed by Pluto in turn…

Jupiter (The Giant Planet): Jupiter is the largest planet in our solar system, but composed mainly of gas. In fact, Jupiter has been insulted by being compared to our Sun, but a failed Sun. If Jupiter had been a fair few masses larger, it would have ignited in a ball of thermonuclear fusion and become a second stellar object in our solar system, turning it into a binary star system. Jupiter however is still solar enough such that it emits more energy than it receives from the Sun. Jupiter, because of the intense gravity, compresses its own stuff, and compression produces heat. Important point number one: Jupiter has its own internal energy source.

Important points two, three and four: Secondly, Jupiter’s atmosphere is composed of the right sorts of chemicals that one identifies with origin of life events – hydrogen, methane, ammonia, water vapour, etc. Thirdly, Jupiter’s atmosphere is turbulent such that there is a lot of mixing of those elements and compounds. Fourthly, the atmospheric bands of Jupiter are highly coloured, an indication that there’s lots of complex chemistry including organic chemistry going on within.   

The upshot of all of this is that it is not implausible that within the upper reaches of Jupiter’s atmosphere, as per the case of Venus, simple life forms couldn’t exist, survive and thrive. You have the chemistry – you have the energy. And maybe Carl Sagan was right and that something more complex than just a unicellular ecosystem could exist in Jupiter’s atmosphere. But it would have to be an atmospheric ecology.

Saturn (The Ringed Planet): Saturn is a quasi twin of Jupiter. Although slightly smaller and farther away from the Sun than Jupiter, the same general arguments that apply to Jupiter apply to Saturn. That is to say, Saturn has the right sorts of chemistry – and an internal energy supply. It’s however slightly less dense than the other gas giants, such that if you could find an ocean big enough, Saturn would float! That however has no bearing on the issue of finding an atmosphere-based ecosystem there.

Uranus: Uranus is a poor cousin compared to the likes of Jupiter and Saturn. It’s mainly a gas planet, albeit way smaller, but so far out from where it’s all happening that it isn’t too likely that even simple life could flourish in the atmospheric depths, although the chemistry isn’t dissimilar to that of the closer-in Jovian planets of Jupiter and Saturn.

Neptune: The same sorts of arguments apply in general to Neptune as to Uranus, with one slight exception. Although farther out, Neptune, like Jupiter and Saturn, radiates out excess energy. It’s solar independent, at least as far as any life forms might describe their environment and energy supply.

Pluto: The planetoid Pluto was recently officially demoted from strict planetary status, and is no longer acceptable to refer to it as the ninth planet. However, when I was growing up, it was the ninth planet, and so I say buggers to astronomical officialdom. That rant aside, Pluto is hardly top rock for vacation seekers. It’s cold. I mean it’s really cold. It makes Antarctica seem absolutely tropical in comparison. I mean polar bears would freeze to death on Pluto, not that there’s anything reasonably resembling an atmosphere as we know it for them to breathe. Again, Pluto is too cold to allow for the high temperature chemistry we associate with life-as-we-know-it. If you’re looking for life in our solar system, Pluto wouldn’t be your first port of call.