Expert: Philip Stahl - 4/25/2007

Question
There's a new habitable, relatively Earth-like planet mentioned in Yahoo news:-

http://news.yahoo.com/s/ap/20070425/ap_on_sc/habitable_planet

Previously, we've all been told that life could not exist except under conditions very close to that of Earth(ie 1g,  average 21(?) degree Celsius temperature, presence of water etc., Yellow star like our Sun etc.) Yet this planet circles a Red Dwarf star. Do you still think the planet unlikely to be habitable, despite those scientists' claims(in which case I'd be interested in your reasoning), or do you think that there is a much wider range in which life could develop on an extrasolar  planet (in quantity)? If you hold the latter view, I'd be most interested in the details of that range(maximum temperature-range, highest(or lowest) planetary gravity, lowest or highest air-pressure etc.)

Thanks

Geoff

Answer
Hello,

The home star Gliese 581 is designated as a "red dwarf" which means much cooler and less luminous than the Sun (or other Main Sequence stars). It's also designated a variable star and has an apparent magnitude of +10.55.

From this one can work out a more precise idea of its relative luminosity using the expression for the absolute  magnitude (M):

M =  m  - 5 log (d) + 5

where m denotes the apparent magnitude, d is the distance in parsecs (20.5 LY ~  6.29 pc).

Thus,

M =  +10.55 - 5 log(6.29) + 5

= +10.55 -5 (0.798) + 5

= +10.55 - 3.99 + 5  

M ~  +11.55

This means the luminosity is barely five ten thousandths of the solar value.  (This is using the H-R diagram provided in the Smithsonian High-Energy Astrophysics Handbook).

To arrive at the temperatures estimated, this means the planet would have to be *extremely* close to the star, Gliese 581. Indeed, from the article link you provided, it is said to be "14 times closer" to its star than Earth to the Sun. This would place it at roughly 6.64 million miles away.

This is way TOO close to presume any life might develop, except perhaps in the most primitive form. Even then, it is hard to foresee how it would endure if the star suddenly altered luminosity (according to its variable star nature) or unleashed a flare.

My own computer calculations show the planet would have to be even closer to achieve a solar constant (insolation) value similar to Earth at its surface. Thus, to get 1360 W/m^2 (giving the temps. referenced) my computations show the planet would need to be 45 times closer (assuming the luminosity given earlier).

This would place it at a precarious distance of barely 2.06 million miles from the star!

Hence, my POV is that the odds of any life on 581 c are slim and none.

There are undoubtedly many inhabited worlds out there, I just don't think Gliese 581 c is one of them.