Planetary Life: Does size matter?

Michalec got in touch on the forum, Chris Smith put this sizeable situation to Astrophysicist Matt Bothwell...

Matt - So I really like this question. I think we should start with the caveat right at the beginning which is that we only know of life on one planet, which is earth, so as much as we are going to be trying to make educated guesses they are still kind of hypothetical situations. That being said the laws of physics do let us make a pretty educated guess which is that size does matter. And that the direction it goes in is bigger planets should have smaller animals. The reason for that is just because of gravity, so the bigger animal is the more massive it is the more it has to fight against gravity to hold itself up and so on a big planet with really strong gravity a really massive animal is going to be doing really really badly. You can see an example right here on planet earth: the biggest land animals that ever existed were dinosaurs but in the ocean where the buoyancy of the water slightly counteracts the force of gravity. It kind of simulates a low gravity environment almost. And so ocean creatures like whales can be much much bigger than any land creatures so we would definitely expect big planets would have small animals and vice versa.

Chris - There's also another possible effect to superimpose on this isn't there which is you get this phenomenon here on Earth called island dwarfism. Not Ireland as in where Helen comes from, as in an island surrounded by ocean, which is where you have a small landmass where the resources are limited. You tend to have smaller animals that are better able at not exhausting those resources than say a big animal which would munch its way through all the vegetation very very quickly and so in the same way that we think certain other animals have shrunk over evolutionary time and they've been dwarfed in that way. There's evidence that humans or human ancestors; this has happened with the island of Flores where the Homo floresiensis - these hobbit people - actually were thought to have evolved and are in a limited area and therefore they may have been forced to become small.

Matt -  That's really interesting I think I guess on a planetary scale a planet would have to be very very small indeed for that kind of effect to kick in and so I think a complete lack of gravity might be a problem before limited resources.