A just published study (actually still in preprint) suggests that Earth like planets may be quite uncommon. I'm a little skeptical about the reasoning (based on the discussion in the link: I haven't read the paper). It's quite possible that the reason we haven't found system's like Sol's is just that we don't yet have the capability. The existence of systems which evolved in an entirely different way doesn't really bear on the number of solar systems like our own except very indirectly.

My answer to all the Drake equation/Fermi paradox variations is: Too many variables, not enough data. Maybe the earth is rare. Then again, a few years ago, based on then current planet formation models, it was thought that giant planets were extremely rare.

I'm old enough to remember all the "scientific" and extremely off-base speculation about other planets in our own solar system. The picture of our own neighborhood has changed radically.

When I was a kid, (among other things) textbooks still said that Mercury didn't turn, we didn't know what was under the clouds of Venus, Mars and Jupiter's moons were dead balls of rock like Luna, Saturn was the only planet with rings, Titan was the largest moon in the solar system, Triton didn't have volcanoes, Pluto was much larger and didn't have a moon, and there was no Kuiper belt.

When we can detect and gather spectrographic information on earth sized extrasolar planets, then we will be able to start answering the "rare earth" question. I've seen far too many hypothesis superseded by fact to have any opinion here.

Andrew -

Just outa morbid curiosity, has there ever been a study/plan published (or at least available on the web) of what would be needed to "colonise" it for humanity? For example: Supplies, Power Generation, Transportation, Farming strategies, etc?

Assume a "Earth-like" planet is discovered 10 light-years away and we've solved the FTL limitations and can send a bigass freighter full of cargo to orbit around said planet with shuttle capability to get it to the surface. What would you take? Would you go the "Earth 2" route with an advance team or send the whole enchilada at once? Maybe you could toss this out as an exercise for your readers to suggest options for when the big day finally arrives?

Its studies like this that get hung up on the assumption that we can only deem life to be 'as we know it'. Not trying to take into account that organism may evolve to overcome higher radiation levels, more/less gravity, and longer day/year cycles, etc etc.

I never really like seeing any of the aliens in movies that always have two eyes, two hands, one head, and two legs walking upright. Except for maybe Alien v. Predator which looks like that movie could rock when it comes out. I do like the occasional Star Trek episode where they came across intelligent life in a gaseous form. The one about the large space slug that was nursing off the warp core was interesting. But the Star Wars Calamari just get on my nerves. Yes in a galaxy far far away are humanoids with fish heads . . . . . give me a break.

The problem with the Drake equation is not only that everyone neglects to include the errors in the numbers they supply, but they also tend to pick the best possible numbers in every case. (An example-- The habitable zone is wide enough to support three planets? Then assume every solar system has three planets in that zone.)

For a better discussion, go to the CONTACT web site and get some of the discussion, especially by the late Poul Anderson, on the probability of Earth0like planet.

When a scientist says 'life as we know it', the only real criteria is liquid water somewhere in the lifecycle. From boiling mud pits, to pressurized water below the normal freezing point, anywhere we have liquid water on Earth, we have life. There's a long list of 'types' of symmetry even here on Earth, from spheres, flat circles, bilateral, 4-way, 5-way, 6-way, whole-lot-of-way symmetry and most anything in between. (Ocean critters, octopus, squid, starfish, jellyfish, and on and on.)

One of the inherent biases in the current 'wobble' technique favors gas-giant-near-sun. Duh.

There's another line of reasoning about the rarity of Earth that holds up as far as I'm concerned. Our solar system is _not_ at the local median H/Fe ratio. For some reason, we're 2 sigma off the median - meaning _something_ drastically increased the local heavy metal concentration. (Passed through a supernova cloud/who knows what.) Nothing _too_ unusual, but worth noting.

Second: The formation of Earth itself. There's a fair body of scientists that claim a Moon-Earth impact in the early formation period. The _Moon_ has an _extremely_ low metal content - and it would make sense that in the collision (which was strong enough to make most everything at least molten) the heavier elements stayed with the heavier body. _And_ the remoltenization would leave a higher-than-average metal content in the crust.

Third: When you are studying chemical reaction rates, you _always_ focus on 'the limiting reactant' or the '_rate_limiting reactant'. People call us 'Carbon based life'. And water seems the essential ingredient. But... the _trace_ elements are really the limiting element. Trace elements _require_ creation in a supernova. This is key not only for speeding up chemical compound formation in 'pre-evolution', but also for _drastically_ speeding up evolution by allowing for, say, bones. And haemoglobin, and the copper based blood of lobsters. And so on.

When you fire up the Drake equation and start guessing... the 'optimistic' numbers run afoul of the (very few) hard facts from SETI. That is 'So... why haven't we bloody well heard from anyone yet?!?' My answer is "Because Earth had three or so major breaks that were uncommon _when_our_solar_system_formed". (This isn't true for solar systems that form later - each H/Fe tends upwards as time passes)

IMNSHO, when 'science' broke from the church by insisting 'There's nothing special about Earth, it's just a rock orbiting a pretty normal sun.' as Copernicus, Galileo, Kepler et al dismissed the humanocentric view of _everything_, the non-humanocentric view became scientific _dogma_. Tough to now go out on a limb and say 'Hmm. Well. Perhaps we _are_ more unusual than we thought'.

Don't forget the continuing existence of the Moon as well. Many theorists now believe it was critical in the development of complex multi-cellular life. You can have all of the the other factors in place and still lack a planet that will produce what we would recognize as animals if you don't heavy the big thingy adding to the tidal movement.

The rare earth guys do make a fairly convincing argument. The key points are 1) right location in the Galaxy, which determines the metallicity of the solar system and, 2) Earth-moon double planet system which seems to account for many things. The moon appears necessary for stablizing the Earth's axial tilt, makes plate techtonics due to tidal stress, and the process of making the moon (collision with Mars-sized planet) made the Earth such that it had a heavy core (for magnetic field) and partial continental crust (for ocean basin formation).

These things suggests that the Earth is, in deed, a rarity. Of course the proof is in the pudding.

By 2015 or so, we will have several satellites that will be able to scan the skys and detect terrestrial-sized planets out to about 100LYs. This will give us the numbers for the number of Earth-sized planets that happen to be in the right orbits.

Me thinks the rare earth people are right and that N (in the drake equation) is around or less than one (thats us). The question I am interested in is how many "garden worlds" are there.

And there is the historical fact that Baha'u'llah, who may in fact be the Intermediary between humankind and The Creator, for this Day and Age (as foretold by Jesus of Nazareth) states clearly:
"Every fixed star hath its planets..." and -I can't find His statement on other creatures created in the image of God with the capacity to Know and Love- He indicates they are there, spread among the stars, and all in more- or less-appropriate obedience to the Will of God.

And the problem is, with the current lack of real information, that last comment makes about as much sense as any. The Drake equation is a "How many Angels can fit on the head of a pin?" type issue right now. It's easy to assume we have more information that we do.

The "Rare Jovian" mass planet argument made sense with the computer models at the time. But it was proved wrong once the evidence became available.

Arguments are great, but a single example, without counterexamples, is not evidence. I can argue just as well for the "Rare earth/rare life" as the "Earths/Life is everywhere" positions with what little we know now. It is going to take evidence, not arguments, to close this issue.

We actually have the evidence for H/Fe ratios for nearby solar systems. (Spectroscopic emission/absorption data.)

That's only a 100-fold (or so) increase in rarity, but in the papers I've read (not _that_ many, it isn't my field) I've never seen someone point out how important that might be to the formation of life.

I happen to think that liquid water + sufficient metals to start the chemical diversity is enough. Shrug.