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View Full Version : Milky Way may bear 100 million life-giving planets



A.DIM
2014-Jun-12, 03:09 PM
So say the authors of a paper "Assessing the Possibility of Biological Complexity on Other Worlds, With an Estimate of the Occurrence of Complex Life in the Milky Way Galaxy (http://www.sciencedaily.com/releases/2014/06/140609100725.htm)."

Is it all mere guesswork or can such a Biological Complexity Index tell us something meaningful?

Van Rijn
2014-Jun-13, 10:26 AM
Contrary to the title, the article (found here (http://www.mdpi.com/2078-1547/5/1/159/htm)) seems to be making an argument about number of planets that might be able to *support* complex life, based on known exoplanets, not the number of planets where complex life *occurred*. The second would necessarily be guesswork, of course, though for the first we do have at least some information on exoplanets.

But from a quick look, they appear to be assuming we have much more information about the exoplanets than we have in reality. For instance, they say this:


Gliese 581c, orbits slightly inside the inner limits of a habitable zone around a star one-third as massive as the Sun. Its mass is six times greater than Earth’s but with a density 31% lower. Its equilibrium temperature is estimated to be 39 C, with an atmospheric pressure of 4.3 bar.

But on Wikipedia:

http://en.wikipedia.org/wiki/Gliese_581_c#Habitability_and_climate

it discusses arguments that the planet may be like Venus with a runaway greenhouse, high atmospheric pressure and a Venus-like temperature (possibly 730 C compared to their claim of 39 C). On the other hand, it may have a surface covered in ice. We don't have enough information to say.

Paul Beardsley
2014-Jun-13, 12:07 PM
Would it be fair to say we know less about these worlds than Schiaparelli knew about Mars?

eburacum45
2014-Jun-13, 03:40 PM
Would it be fair to say we know less about these worlds than Schiaparelli knew about Mars?
Oh, much less. Schiaparelli had a very good idea of the radius, mass, albedo, density and surface gravity of Mars, if nothing else. We can only estimate all of those qualities for all of the known exoplanets. A goodly fraction of them only exist as probablities, and might vanish when we get better data.

But Schiaparelli still drew conclusions about Mars that the data did not warrant, as your post seems to imply. We should try to avoid that trap.

Paul Beardsley
2014-Jun-13, 04:15 PM
Oh, much less. Schiaparelli had a very good idea of the radius, mass, albedo, density and surface gravity of Mars, if nothing else. We can only estimate all of those qualities for all of the known exoplanets. A goodly fraction of them only exist as probablities, and might vanish when we get better data.

But Schiaparelli still drew conclusions about Mars that the data did not warrant, as your post seems to imply. We should try to avoid that trap.

Thanks for the answer - very well put.

Van Rijn
2014-Jun-13, 11:57 PM
I've often felt a bit of deja vu with reporting on exoplanets. It reminds me of the speculation about other planets before the space probes flew. Somewhere I think I still have a very early NASA brief that shows two speculative pictures of Venus under the clouds, one a desert, the other a tempestuous ocean surface. If I ever find it again, I'll scan it in and show it on the board.

Paul Beardsley
2014-Jun-14, 03:27 AM
Exactly. The universe absolutely doesn't care what we hope or expect to find, and it has no interest in being constrained by what we can imagine. I think nearly every world we've explored has thrown up at least one massive surprise (in the case of Jupiter's major moons, the fact that they are a highly diverse selection of worlds at all was more surprising than their individual surprises), and Venus and Mars are completely different to how we "knew" they would be.

The confounding of our expectations happens so often it's as if the universe is doing it on purpose. Of course it isn't really, but that doesn't mean it's going to stop.

Looking for other Earths among the stars is like... me, 30 years ago, looking for a woman just like my girlfriend who had dumped me. Eventually I moved on and wised up and learned to appreciate individuals as individuals.

ravens_cry
2014-Jun-23, 03:20 PM
I've often felt a bit of deja vu with reporting on exoplanets. It reminds me of the speculation about other planets before the space probes flew. Somewhere I think I still have a very early NASA brief that shows two speculative pictures of Venus under the clouds, one a desert, the other a tempestuous ocean surface. If I ever find it again, I'll scan it in and show it on the board.

I perused, damn me if I can remember where, an old book from around just after the first quarter of the 20th century that included these lovely colour plates (you know, where every picture is on thick, slick stock, and protected by a piece of tissue paper) that showed depictions of waterfalls of lava of some kind on Jupiter. Yes, on Jupiter.