Yesterday it was announced that an earth-sized planet has been discovered circling the nearest star, Alpha-Centauri (around the smaller of its two main stars, actually, Alpha-Centauri B). The planet, Bb as it's currently called, has a six-day year and a surface temperature of 1500C. Not very hospitable, perhaps--but I'm about to argue that it's just fine. If we can get to this star system, we can settle it.

Let's look at two scenarios, a worst-case and a best-case, and see what's possible with each.

The Worst Case

This is a boiling hot planet. Actually, far hotter than boiling. At 1500 degrees, it's hot enough to melt rock. In a worst-case scenario, Bb has the kind of rotational resonance that Mercury does: it is not fixed with one face pointed forever at its star, like our Moon is to the Earth, but rotates so that the whole planet is regularly bathed in the blowtorch heat of the star. If there is an atmosphere, it's mostly composed of evaporated rock.

In this case, much or all of Bb's surface is a lava sea. Oh, and since this is a worst-case scenario, let's say that there are no other planets in the system, not even any asteroids. Bb is it.

If your idea of habitability is finding a more or less exact copy of the Earth and settling down on it to farm, then things are looking kinda bleak. But, if we have the technology to get to Bb, then we have the technology to live and thrive there.

Not on the surface, of course. Not even in a nearby orbit. But even if Bb is uninhabitable, it is still a great source of building material. If we have the technology to get to it, we'll have the technology to mine it, if only by dangling a skyhook down from the L2 point (or from a heliostat) to dredge the magma ocean. Haul the magma up, render it in the terrible light of the star, and ship the refined goods to a higher orbit where the temperature's a bit better. There, we can build habitats--either O'Neill colonies or, if we can harvest enough material, the coronals I describe in my novel Lady of Mazes.

With unlimited energy and (nearly) unlimited building materials, we can construct a thriving civilization around Alpha Centauri B, even if all we have to work with is this one piece of melted rock. (In terms of details, it would be a bootstrapping operation, with an initial small seed of robot miners constructing more or bigger skyhooks, more miners, etc. until exponential growth sets in, by which time it's safe for the human colonists to show up.)

The Best Case

 Even for the best case scenario, I'm going to assume that Bb is the only planet in the system. It's more likely than not that Bb actually will be tidally locked to its star--i.e., it has one face permanently aimed at its sun, and the other permanently in darkness. The point that's under a permanent noon (the 'solar pole') will indeed be a lava hell. What's interesting, though, is that some simulations show that the temperature in the twilight zone around the 'equator' and further into the night side could be quite cool. Cold, even, if you go far enough. If there's an atmosphere, there might be water and a zone of permanent rain around the mid-latitudes of the dark side, in a kind of hemisphere-wide hurricane with its eye at the anti-solar pole. And there, we might settle.

I doubt there'd be any oxygen to speak of, but we can generate that ourselves. What I find interesting, though, is that this 'dark side' is not really dark at all. Because Alpha Centauri is a binary star system, Centauri A will be visible in the 'night' sky of Bb during half its year. ...Which is only three days long. So A will cross the sky in about 75 hours, and then there'll be true night for 75 hours. This has been the pattern on Bb now for more than four billion years; it's pretty stable.

Centauri A appears very dim from Bb compared to our sun, but it's still too bright to look at and has a visible disk. It's dimmer than daylight, but much, much stronger than Earthly moonlight. Granted the luminosity range at which photosynthesis happens on Earth, I'd think plant life would do quite well on Bb's 'dark' side.

If the rain's not too bad, much of the 'dark' hemisphere might be settled. Remember that Earth is mostly covered with water; if there's no significant oceans on Bb, but enough water for rivers and lakes, then the habitable land area of Bb might be greater than Earth's. Gravity is the same as Earth's, and in fact the only major difference will be atmospheric composition/density, and the length of the day. And who knows? Maybe we can game those too, by geoengineering the atmosphere, and using a combination of distant orbital sunshades and orbiting mirrors to generate a 24-hour diurnal cycle. Ultimately, Bb could be very earth-like indeed.

The Happy Medium

I expect the reality of Bb's habitability lies somewhere in between the two extremes I've just described. In all likelihood, Bb is not alone; at the very least, there should be asteroids or planetoids of Ceres-size or larger. Bb itself might have a safe spot where industrial operations can be set up, even if it's not a place where you could live. It can export vast quantities of raw materials to colonists elsewhere in the Centauris.

All of which means one thing: Alpha Centauri is now a viable destination. If we can get there, we can live there. And knowing this makes real possibilities that, until yesterday, we could only dream about.

How many planets are there within 20 lightyears of our sun? Even five years ago we couldn't have answered this question. Today, without actually having spotted any, we can give a fairly confident estimate of how many there should be, and what they should be like. Interested in finding out? Then read on.

The Studies

There's been a lot of commentary in the news in the past year or so about the Kepler mission's cataloguing of distant planets. Kepler has allowed the number of known exoplanets to balloon up past 700 at latest count. Of course, since Kepler is watching a vastly distant patch of sky, it can't tell us how many planets there are in our local neighbourhood. 

A lot has been written about the significance of Kepler's technique, which involves watching for the mini-eclipses that happen when a planet crosses the face of its star. Very little has been written about a parallel hunt that uses microlensing to accomplish a similar end. Microlensing looks for the distortions in the image of a star made by a planet's gravity. These surveys have been going on for ten years now and the results are staggering. 

For instance, did you know that by some estimates there are up to 100,000 nomad planets--planets without a home--for every star in the galaxy? In my 2002 novel Permanence I boldly proposed that there might be one or two brown dwarfs for every star, and that seems to be true; but even in my wildest dreams I couldn't have imagined there might be tens of thousands of planets Pluto-sized or larger drifting between Earth and Alpha Centauri! I still can't really believe it.

These nomads are interesting, because sufficiently large ones (many will be of super-earth size, 2 or more earth-masses) can sustain a trickle of heat from their interiors for billions of years. Though their surfaces may be frozen, they can easily support sub-surface oceans like the one thought to exist in Jupiter's moon Europa. In other words, they can support life. There should be some thousands of these worlds for every star in the galaxy.

This is just the beginning of what the microlensing survey data is showing us. There's enough data now to begin to estimate how many orbiting planets your average star has, and what kind of planets they are. And the combination of microlensing survey data and Kepler data lets us be really precise.

The Numbers

 Kepler's preliminary data seems to indicate that one third of main sequence F, G, and K stars (sunlike stars) have at least one earth-sized planet within the star's habitable zone. There are nineteen such stars within 20 lightyears of us, so this indicates, conservatively, that there are six earth-sized planets in the habitable zone of sunlike stars within 20 light years of Earth. 

These numbers don't include habitable moons of gas giants that might orbit within the zone. So the actual number could be higher by one or two.

The microlensing survey lets us be precise for the whole population of stars. Here, survey says that the average number of planets per star in our galaxy is 1.6. This leads to the number of bound planets in the galaxy being close to 200 billion, and the number of total planets (including nomads) being ten quadrillion. (There are thus trillions of nomadic super-earths, many of which will have sub-ice oceans capable of developing life.)

The microlensing survey data is so far limited to planets in the super-earth to Jupiter size range, and between .5 and 10 AU distance of their stars. Within those limits, it suggests that 17% of stars have a Jupiter-like planet; 52% have a Neptune-sized planet and 62% have a super-earth. Since the smaller the planet, the more likely it is, we can continue this trend-line to say that in all likelihood, each star will have at least a 62% chance of having an Earth-sized planet. This puts the number of Earth-sized planets in the galaxy at 60 billion or so. The absolute number within 20 light years is at least 42. There's 51 stars outside the main sequence (giants or dwarfs) within 20 light years; another study suggests that the absolute probability for all stars of having a planet within the habitable zone is about 12% (which looks highly conservative). That would add six to our local total, meaning that within 20 light years, there should be at least 12 habitable earth-sized planets. This doesn't count marginal planets, exomoons and Europan worlds. Or, of course, nomads.

To zoom in on a couple of famous local stars, we can say that it's highly unlikely that Alpha Centauri has no planets, given that it is a triple system all of whose stars could support planets. We know Alpha Centauri has no gas giants, but that's consistent with the numbers; but the odds that either Centauri A or B have at least one earth-sized planet within the habitable zone are very high. The Centauris are close to our sun in age, so their planets may still be able to support life.

Tau Ceti, a very sunlike star only 12 light years away, probably has a couple of planets. It's an older star, however, and any earth-sized planets are probably getting arthritic: their plate tectonics will be shutting down somewhere around now. They'll be more like the Barsoom of Edgar Rice Burroughs' Mars novels: ancient and dying.

There's a lot more being discovered and theorized; for instance, one new study suggests that having a Jupiter-like massive planet in your solar system doesn't protect your planet from massive impacts, but on the contrary is a actually bad for you. Another suggests that at least 12% of earth-sized planets have a moon large enough to stabilize their axial tilt (a supposed necessity for planetary habitability) and another suggests that axial tilt won't affect climate all that much anyway. The prospects for life look good around the nearest stars.

The galaxy is literally overflowing with planets, far more than can be crammed into the orbits of its stars. Many of these planets could support life. The question now is, do they?

And if so, where are our nearest neighbours?

Okay, Keith B. Wiley's new paper does have a somewhat daunting title: The Fermi Paradox, Self-Replicating Probes, and the Interstellar Transportation Bandwidth. But it's a pretty easy read and hugely well worth it--because in this paper Wiley provides what may be the clearest discussion yet of the core puzzle Fermi first proposed sixty-two years ago: if alien technological civilization is even possible, then they should be here; at the very least, such civilizations should be visible to us. That we are instead faced with 'the great silence' is one of the most troubling and, yes, paradoxical, results of modern science.

I addressed the Paradox in my novel Permanence, coming up with a possible new solution for it; although Milan Cirkovic and other astrophysicists haven't disproved my central contention, they've since shown that it's not a show-stopper. As Wiley points out in this paper, even if the lifetime of an interstellar civilization is short; even if they're all doomed; there is no credible argument as to why they couldn't create self-reproducing probes (SRPs) to investigate the entire galaxy that, collectively, outlive the originating civilization. This is the very scenario I paint in Permanence. SRPs are a cheaper solution than one-off expeditions. In fact, SRPs are so efficient a solution to exploration and colonization that, plugging in some highly conservative numbers of how many civilizations there might be out there, Wiley shows that hundreds to billions of such probes should actually be here, in our solar system, right now!

Wiley blows up some of the keystone explanations for the Paradox, including Geoff Landis's percolation model, which previously I'd considered a pretty solid argument. Wiley is so good at demolishing easy explanations, in fact, that he brings us almost all the way back to square one, where Fermi had us in 1950. Where are they? We haven't a clue.

The mystery deepens almost by the day, because we've now identified 700 extrasolar planets and the count is increasing rapidly. We should shortly be racking up lists of Earthlike worlds, and we're closing in on good estimates of how many there must be in our galaxy. And the number is in the billions. So one central argument against the existence of alien life--the 'rare Earth' argument that environments to host it must be rare--has been more or less disproven. And that, just this year.

As possible explanations dwindle, we are being drawn inexorably toward the one explanation that is no explanation: that we really are alone. Why should this be? As Wiley shows, all it would take would be one alien species with our capabilities appearing, sometime in the past couple of billion years, and for that species to surpass where we are now technologically by, oh, say, a couple of hundred years... and the evidence for their existence should be present right here in our own solar system. It's an astonishing conclusion.

So are we alone? Well, there is one other possibility, at this point. I've lately been trumpeting my revision of Clarke's Law (which originally said 'any sufficiently advanced technology is indistinguishable from magic'). My revision says that any sufficiently advanced technology is indistinguishable from Nature. (Astute readers will recognize this as a refinement and further advancement of my argument in Permanence.) Basically, either advanced alien civilizations don't exist, or we can't see them because they are indistinguishable from natural systems. I vote for the latter.

This vote has consequences. If the Fermi Paradox is a profound question, then this answer is equally profound. It amounts to saying that the universe provides us with a picture of the ultimate end-point of technological development. In the Great Silence, we see the future of technology, and it lies in achieving greater and greater efficiencies, until our machines approach the thermodynamic equilibria of their environment, and our economics is replaced by an ecology where nothing is wasted. After all, SETI is essentially a search for technological waste products: waste heat, waste light, waste electromagnetic signals. We merely have to posit that successful civilizations don't produce such waste, and the failure of SETI is explained. 

And as to why we haven't found any alien artifacts in our solar system, well, maybe we don't know what to look for.  Wiley cites Freitas as having come up with this basic idea; I'm prepared to take it much further, however.

Elsewhere I've talked about this particular long-term scenario for the future, an idea I call The Rewilding. Now normally one can't look into the future; in the case of the long-term evolution of technological civilization, however, that is precisely what astronomy allows us to do. And here's the thing: the Rewilding model predicts a universe that looks like ours--one that appears empty.  The datum that we tend to refer to as 'the Great Silence' also provides the falsification of certain other models of technological development. For instance, products of traditionally 'advanced' technological civilizations, such as Dyson spheres, should be visible to us from Earth. No comprehensive search has been done, to my knowledge, but no candidate objects have been stumbled upon in the course of normal astronomy. The Matrioshka brains, the vast computronium complexes that harvest all the resources of a stellar system... we're just not seeing them. The evidence for that model of the future is lacking. If we learn how life came to exist on Earth, and if it turns out to be a common or likely development, then the evidence for a future in which artificial and natural systems are indistinguishable is provided by the Great Silence itself.

Check out Wiley's paper. And just think: the Great Silence may turn out to be no paradox at all, but positive data about what our own future will look like.

The August, 2011 issue of Scientific American has an article by George F.R. Ellis about whether we can prove that a multiverse exists.  I did a double-take when I saw this, because it reminded me that back in 2003, SciAm had published an article by Max Tegmark claiming that it does exist.  At the time I wrote this blog entry pointing out that Tegmark's article wasn't based on science at all, but was pure speculation.  Nice to see somebody agrees with me.

It's almost time to name Gliese 581d.  

Two billion years or so before our own solar system coalesced, this planet was formed around a dim red star that's now about 20 light years from Earth.  Gliese 581 d is therefore an ancient world, orbiting around a cold star.  But it may be habitable.

That's the conclusion of the latest study, by R. D. Wordsworth, F. Forget1, F. Selsis, J.-B. Madeleine, E. Millour, and V. Eymet (the paper is Is Gliese 581d habitable? Some constraints from radiative-convective climate modeling; you can find it on archiv.org).  They ran simulations based on what we know about the planet and its star, and conclude that if d has a sufficiently thick atmosphere of CO2, it could have liquid water at its surface.  Other studies of so-called "super-earths" like d hint that many or most of them will be water planets, global oceans.  And, when you factor in a recent study of habitable zones that indicates they could be much broader than first assumed, it seems that if this world has any sort of an atmosphere at all, then it's likely habitable.  So, here's what we know about d:

• It's between 7 and 13 times the mass of the Earth, but its radius is unknown (however, likely around 1.15 times Earth's radius).  If it's as dense as the Earth, then its surface gravity is about 2 times Earth's; but Earth is the densest of the solar system's rocky planets.  If d is an ocean world, it's likely a lot less dense and its surface gravity may be the same as Earth's.  In that case, though, it is almost certainly an ocean world, with no accessible land at all.
• It's may be tidally locked to its star, meaning that the sun stays fixed in one spot in the sky, and one whole hemisphere is in permanent darkness.  This is a condition usually taken to mean that the planet in question would be lifeless because the atmosphere would all condense on the cold side; but numerous studies have now shown that tidally-locked planets can retain their atmospheres quite well.  They do, however, tend to be windy.
• It may well have a thick CO2 atmosphere (researchers suspect these are common) in which case, provided minerals are able to leach up from the depths of the planetary ocean, it may have been capable of hosting life for six billion years now.

There's a really good chance that d could support life--though you and I wouldn't want to live there, since we'd weigh twice what we do on Earth and the atmosphere would be toxic.  But it could still be a lush world, overflowing with life.

What does it look like on this world?  The sunlight of its permanent day isn't red, though we call Gliese 581 a "red dwarf."  To us, it would appear to have about the same spectrum as a 60 watt bulb, which is to say, yellowish-white; and daylight is a bit dimmer than it is on Mars, so with the naked eye, it's visually like wearing a good pair of sunglasses.  The human eye adapts to a wide range of light conditions, so you wouldn't really notice the difference.  But, if d has an atmosphere, the sky is blue.  Old as it is, d may no longer have active plate tectonics, so, like Mars, it probably doesn't have mountains or volcanoes.  But it won't be a cratered environment, either, if the atmosphere is thick enough for water to be stable.  --And speaking of water, the weathering effects of high wind and water over billions of years suggest that it's become a very flat world lately, with either a global ocean, many shallow seas and low islands, or vast dry plains.

But this is amazing--because we're talking about a real planet here, not some speculative possible world; and not some science-fictional dream.  d does exist; we'll soon know whether it really is habitable, and within a few years, may be able to detect signatures of actual life in its atmosphere.  Already, we've learned enough to know that there are billions of other planets sailing through the galaxy with ours.  If we learn that Gliese 581 d really could sustain life, we'll be able to begin estimating (roughly, at first) how many habitable planets the Milky Way contains.  Considering how close Gliese 581 is to us, that number could be huge.

So what do we name this new world?  It is ancient, far older than our own worlds; so it would be fitting to name it after one of the Titans, who are older than the Greco-Roman gods we've named our planets after.  If it's a sterile ocean, I vote for Oceanus; if it could host life, then my favoured name would be that of Oceanus's wife, the goddess of rivers and lakes: Tethys.

Welcome, Tethys, and may you divide history into two parts:  the long age in which we wondered whether we were alone in the universe--and a new epoch in which we know we are not.