The comparison to Mercury seems to be based on the planet's proximity to its star. The star is much colder that the Sun, so a closer-in planet like Mercury would not be nearly as hot as Mercury finds itself.

In terms of size of the planet, this one is much more like the Earth. Mercury is really very small in comparison, and does not have much gravity to retain any atmosphere even if it were located where Earth is. So here the comparison to Mercury really doesn't work well.

It isn't clear that life only happened on Earth once. In particular, there are life communities around thermal vents in the deep ocean that very well may be the result of a completely different spark of life. For the rest of the Earth, once life gets going, it seems to become rather dominant, not leaving much opportunity for a second genesis, though how would we know if it happened?

In chemistry labs, we find that if the right basic elements are collected and put in early-Earth conditions, some of the complex molecules of life are assembled quite naturally. This is encouraging, at least.

But, 60/40 is just a complete guess, you are correct.

I'm not sure which comment of his this refers to, but almost certainly it is (b). The planet in question does not transit its star as view from Earth, which would have been an unlikely geometry, but if it had happened would have allowed very detailed follow-up study of the planet.

Good point!

There is some controversy here. GJ 581 doesn't seem to be to dramatically variable. But others are. The lead of SETI wrote a recent paper claiming M dwarfs are not so active as to prevent life or even advanced life. However, this was in response to papers claiming the opposite. It's uncertain, but it seems GJ 581 is stable enough for long enough periods that life can evolve. Even our Sun isn't super stable, yet life exists. Thus ice ages, the Maunder Minimum and Mini-Ice-Age, and the like.

The spectrum of the star wouldn't necessarily tell us about the composition of planets. Some planet-star spectrum correlations have been seen as far as whether stars have planets, but these have not necessarily been tied to causation, and certainly not to composition of the planets. We would certainly need to calibrate any such tracer first, anyways.

The composition-age relationship for stars that you mention has more to do with the generation of stars. Stars today are made out of the waste products from the exploded material from previous stars. That material is enriched by the nuclear processes from those previous stars, meaning they start with more heavy elements. The current generation includes stars today and those from at least as long ago as 10 billion years. Beyond that you start to get to the beginnings of the universe and earlier generations of stars. So no big changes are really expected here, and the phenomenon you cite isn't currently believed to be planet-related, but rather just evolution-of-the-universe related, a very different topic.

I don't think anything about the spectra of the star could identify water at this level of precision. Planets are a billion times fainter than their stars. The spectra had signal-to-noise ratios of order 300:1, which is impressive enough, but nowhere close to enough to see features of the planet. (If Bill Gates, the man of $60 billion, woke up tomorrow with $60x300 = $18,000 to his name, he might need to be put on suicide watch. That is the level of change we are talking about.)

I answered this above, but probably after you posted this. Just for completion my answer is as follows. The RV-of-the-star itself data didn't imply the tidal locking, but rather extrapolations based on gravitational interactions, as below:

I believe they determined it as follows:

The planet is close to its star.

The planet has a fairly well known size.

The gravitational force on the near vs. far side can be calculated based on the planet-star distance and the planet size.

Guessing the planet is mostly rock (a very safe guess based on lots of planetary science information), we can guess how much frictional energy is lost in that differential stretching.

Based on the elements observed in the star, we can estimate the age as billions of years old.

The frictional forces would slow down the planet rotation much faster than billions of years (I forget the exact value, but less than 1 billion years; if you really want me to spend a few hours doing the calculation for a better estimate, let me know, but it wouldn't really matter). Thus, by now, it would be tidally locked.

The key is that the planet is closer to its star than the Earth. For example, Mercury (which isn't even as close to the Sun as GJ581g is to its star) is in a 3:2 tidal lock between its orbit and rotation. The full 1:1 lock is expected for closer planets. This is the case for the Earth's Moon, which is why we always see the same side of the Moon. This tidal locking is extremely well established with the Earth's Moon.

I believe they determined it as follows:

The planet is close to its star.

The planet has a fairly well known size.

The gravitational force on the near vs. far side can be calculated based on the planet-star distance and the planet size.

Guessing the planet is mostly rock (a very safe guess based on lots of planetary science information), we can guess how much frictional energy is lost in that differential stretching.

Based on the elements observed in the star, we can estimate the age as billions of years old.

The frictional forces would slow down the planet rotation much faster than billions of years. Thus, by now, it would be tidally locked.

The key is that the planet is closer to its star than the Earth. For example, Mercury (which isn't even as close to the Sun as GJ581g is to its star) is in a 3:2 tidal lock between its orbit and rotation. The full 1:1 lock is expected for closer planets. This is the case for the Earth's Moon, which is why we always see the same side of the Moon. This tidal locking is extremely well established with the Earth's Moon.

Certainly life as we know it has evolved to day-night cycles. Life here would be different. Raccoons (night-animals) would be as confused as deer (day-animals). But there isn't reason to believe they couldn't have evolved differently.

As far as the narrow bands of tropics, this actually helps us determine that there are temperate zones. I posted the following above, but after your post, I just don't want to retype:
"However, the great thing about this planet is that there is almost certainly a "too-hot" part, and a "too-cold" part, for humans, due to the tidal locking that you point out. However, somewhere between there, there must be a "just-right" part. This helps confirm that there is a habitable zone on the star."

The gravitational dynamics are rather well studied, for orbital stability. This is a rather robust part of the study (which, as someone interested in many-body dynamics, a very complex subject, is always surprising to me).

There might be some bizarre weather patterns, but there will be a region of what would be, to us humans, a comfortable region. This strongly suggests a nice region for life as we know it.

Could life exist as-we-do-not-know-it in a different extreme environment? Maybe. But a simpler jump is to life-as-we-do-know-it being elsewhere, since we have evidence such life does exist here, so that is why finding a human-suitable environment is so promising.

The weather might not be fun, that's for sure. But ask people in Alaska and the Mojabe---life exists nonetheless. It might be fun (or not) to be a weatherman there.

Honestly, that conclusion was a bit premature. The other coauthors (including my coworkers) avoided speculating on this point.

His conclusion was based on the idea that where liquid water can be present, so far we have always found life to within out ability to identify it. Thus, since there seems some high probability that liquid water *could* exist on this planet (though no evidence thereof, yet---it just seems likely due to the temperature and because water is such a simple and universally common molecule), and where we've found water we've found life (even in circumstances that would be considered unpleasant), he jumped to saying life was likely.

I personally think that it is premature to speculate on life in this system, since so little evidence is available. If pushed to make a call by Vegas, I'd have to say life was more likely than not on this planet, but my line would not be near 100%. Probably closer to 60/40.

Though a big fan of sci-fi (I would have to be as someone who studied astronomy), I'm afraid I'm not familiar with this one.

However, the great thing about this planet is that there is almost certainly a "too-hot" part, and a "too-cold" part, for humans, due to the tidal locking that you point out. However, somewhere between there, there must be a "just-right" part. This helps confirm that there is a habitable zone on the star.

I actually work quite closely with 2 of the authors of the paper that reports these results. Any questions? I'll try to respond to posts between now and 2 October.

I suggest you tattoo the inverse of the dimensionless fine structure constant (1/alpha) to as many digits as it is currently measured (137.035999). As a dimensionless quantity that pops up quite naturally in quantum mechanics, it is truly universal*, as it is the same regardless of one's system of measurements (SI, cgs, Imperial, CowboyNeal Units, etc.).

Alternatively, you could go with the Arecibo Message, the Pioneer Plaque, or the Voyager Golden Record.

*Note: universality not guaranteed at exceptionally large red shifts.

I'm a 1:4 food:shelter. I have 4 people in my household (wife and two kids), so many mouths to feed and a house with a mortgage. However, there are circumstances that could offset this either way:

1. My wife hyper-coupons for our food bill. She is part of a network that couples coupons to store sales, and saves at least 50% every time she goes to the store. We eat whatever is advantageous at the time (or in season, which means it is freshest and tastiest as a bonus), but always high quality food.

2. We bought a house a little more than we needed, because we bought last year when the housing market was very down. Eventually our family might grow to fill it, but investment-wise, it was a good time to stretch our buying power.

I am an astronomer, and I can tell you that, in most of our opinions, Adaptive Optics is a complete load of crap. A lot of money has gone into that technology for very little science return. The price per scientific paper for Adaptive Optics is far higher than for other technologies.

Yes, the technology can make images a bit sharper, but due to its intrinsic properties, results in those images being completely impossible to calibrate. Making a measurement is only 1% of science; 99% of the challenge is understanding the context of the measurement, its reliability, and uncertainties. Without the ability to calibrate the image, it is useless. With adaptive optics, there are image artifacts that vary in time and can mimic details for which you are searching. If you see a new point of light around the star you are imaging, is it really a distinct object, or just an artifact of the image?

Also, adaptive optics requires bright stars as guides, because the system must operate faster than the atmosphere varies (generally 1 kHz). Very little of the sky has a bright enough star nearby for this to be useful on many objects.

You can use lasers to make artificial guide stars. You still need an actual star for one stage of the correction that the laser isn't sensitive to (because it goes round-trip), but it can be fainter, and this opens much more of the sky. However, it's still far from complete sky coverage.

Finally, background light is higher for ground-based observatories. Hubble can still see fainter than ground telescopes because of this.

For versatility, large sky coverage, and faintness in high resolution imaging, you just can't beat going to space.