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.

Centauri Dreams is one of my favourite sites for discussions on the mechanics and romance of interstellar travel.  They've just done an article discussing my concept of the interstellar cycler, which I used as the basis for the Cycler Compact civilization in 2002's Permanence.  

I've written more extensively about cyclers here, and after Permanence, haven't gone back to the idea for a while.  I'm glad other people are still worrying away at the ideas, because as with all proposals for interstellar travel, the devil's in the details.  Surprisingly, the more you look at the idea, the easier some aspects of it become; for instance, in the description of the concept over at orbitalvector.com, they elaborate on an idea attributed to Jeremy Totten, whereby cyclers can reproduce slowly by the accumulation of donated resources from waystation stars.   There are many variations on the idea, some of which I've explored (eg. if cyclers can reproduce through resource accumulation, they can also be initially built that way), and other people continue to find more.

The big question--for me--is whether I'll ever write another Cycler Compact novel.  I hope I can, but that plan suffers from the fact that I am constantly coming up with other new ideas, and the burning need to get those written down means cyclers keep ending up on the bottom of the priority list. But you never know; if inspiration hits, I'll be happy to return to that universe.