The Nearest Exoplanets
It's time for a survey. We can't see them, but we can now calculate how many should be nearby
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.
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.
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?