Expert: Philip Stahl - 4/6/2008

Question
Most scientists' speculation re life on other planets is, IMO, too anthropocentric, in that they automatically assume that a yellow, Sol-like star is needed for life along with an Earth-sized planet, same gravity as Earth's etc. In what range of conditions re type of sun/gravity/atmospheric pressure/amount of water etc. do you think life on other planets could exist in? For example, is it feasible for life to exist on planets orbiting a blue supergiant sun?

Thanks,
Geoff

Answer
Hello, Geoff

I don't think it's so much a case of being "anthropocentric" as "carbon-centric". In other words, having the (very reasonable) expectation that carbon-based life dominates in the universe as opposed to silicon, lead or any other type.

There are good sound reasons for this, particularly the remarkable bonding power of carbon. It is truly the basis of most "organic molecules". The thing is, however, that for any carbon-based organisms to flourish (via evolution as they do on Earth) stringent conditions are required. This is why G-type stars are favored - especially low in the spectral sequence (e.g. G1, G2, G3).

They will more likely be the stars to support habitable "life spheres" based on temperature, and also last long enough for evolution to deliver results.

The corollary to this is that hot blue giants won't allow adequate time for any carbon-based evolution to occur for their planets (even if at the right distance ). It simply isn't going to happen for a star that will only last 20 million years!

Thus, I come down along with the majority of others, on the carbon-based side. Just call me a carbon-based chauvinist!

If at such time someone can show a basis for silicon life I will be all ears.

Until then, the ranges are limited, and the temperatures etc. are as well, and this is for carbon-based organisms.

Btw, two good books to get to read more on why such limits make sense are:

"Intelligent Life in the Universe" - by Sagan and Shlovskii

"Origins" - by Irwin Shapiro