Planets in habitable zones, planets orbiting twin suns, miniature solar systems, rogue planets, planets, planets, planets. If there is one single piece of information you should take away from the recent flood of incredible exoplanetary discoveries it is this: Our universe makes planets with extraordinary efficiency - if planets can form somewhere, they will.
We've been sidling up on this fact for some time now, but it's still a remarkable thing to acknowledge. Ten to fifteen years ago, as the first exoplanet detections began to come in, we understood that what we were seeing was potentially just the tip of the iceberg. These were massive objects (Jupiter sized or greater) and most of them were orbiting much closer to their parent stars than any equivalent giant planet in our solar system - hence the 'hot Jupiter' moniker that is still used today. Statistics improved, as did our understanding of how detection techniques were biased towards finding these types of planets (owing to their greater gravitational influence on their parent stars), and estimates were made that suggested only a few percent of normal stars harbored such worlds.
Of course time went by and astronomical instruments were refined, more and more data was accumulated, and longer orbital period planets and less massive planets were discovered. The figure to the left here illustrates the evolving range of planetary masses (or lower limits to planet masses) as a property of the year of discovery for confirmed exoplanets (excluding the thousands of to-be-confirmed-candidates from NASA's Kepler mission). Here in 2012 we're dipping well and truly into Earth-sized planetary terrain (about 0.003 times the mass of Jupiter on this scale).
By 2010 gravitational microlensing searches for planets were indicating that Neptune-sized objects on large orbits were at least 3 times more common that Jupiter-sized planets at similar distances from their parent stars. And hot on the heels of these measurements new Doppler, or 'wobble', detections of exoplanets indicated that at least 1-in-4 normal stars should harbor Earth-sized planets within about a quarter of the distance of the Earth from the Sun (0.25 AU).
It was becoming increasingly apparent that planets might be plentiful. Entering 2011 then the first big results from NASA's Kepler mission began to make waves. With these came the statistical inference that the most numerous types of planets orbiting within 1/2 an Earth-Sun distance (0.5 AU) were Neptune-sized worlds, clocking in with a frequency of occurrence of about 17% (i.e. around 1 in every 6 stars). Close behind came Earth-sized objects, in about 6% of all systems. With a little extrapolation, and assuming a total of 200 billion normal stars in the Milky Way galaxy, it was clear that there might be millions of Earth-sized worlds in the habitable zones of their stellar parents, across the galaxy.
But things were just starting to warm up. The next item was another statistical inference from gravitational microlensing surveys, that now indicated a very substantial population of 'rogue' planets - giant worlds perhaps ejected from their stellar nests by strong gravitational interactions with other planetary chicks. The conclusion was that free-floating, wandering, objects as large, or larger than Jupiter, outnumbered stars in our galaxy by almost 2 to 1. It's a remarkable result, but what about planets very much in the grasp of their parent stars, the equivalent of our own solar system?
Recently a new microlensing analysis by Cassan et al. appeared in Nature that explicitly targets planets orbiting between about 0.5 and 10 AU from their parent stars. The results solidify and carry forward all the measurements from before. About 17% of stars (give or take several percent) harbor Jupiter mass planets, cool Neptunes exist around about 52% of stars and Super-Earths (5 to 10 times the mass of Earth) exist around roughly 62% of stars. Even with sizable errors in these estimates (as much as 20-30%) the numbers are astonishing - there are at least 1.6 planets orbiting from 0.5 to 10 AU for every star in the galaxy. Combine this with the Doppler survey numbers (25% of stars with 'Earth-sized' planets within 0.25 AU), the Kepler numbers (17% of stars with 'Neptunes' orbiting within 0.5AU), and the microlensing estimates of 2 rogue giant planets per star in the galaxy and you have, well you have an awful lot of planets.
Of course one has to be careful in pulling these numbers together. Different detection methods and surveys have different biases, and if (for example) a giant planet orbits at 0.5 AU from its star then dynamical stability may preclude the possibility of other worlds nearby. Nonetheless, the bottom line is, I think, very clear; there really are planets everywhere, and they must number in the hundreds of billions in the Milky Way.
Despite where we find ourselves, on a small rocky world, there was no reason to believe that the universe would make planets as efficiently as it seems to. Our situation is merely one data point, and a horribly biased a posteori one at that, and our models of planet formation are, to be quite frank, struggling to keep up with the flood of new data. Nonetheless, from the point of view of astrobiology and the search for life elsewhere, planetary bodies remain the primary, critical, target. There are simply no other environments in the cosmos that offer the same potential for diverse and complex chemistry in multiple phases of matter, and the potential for such long-term equilibrium (albeit a dynamic type of equilibrium with energy and chemistry in both sporadic and cyclical flux).
Thus, the sheer abundance of planets profoundly impacts the nature of our exploration of the universe and our quest to understand our own significance or insignificance. There is nothing trivial about the discovery of planetary plentitude, because it means that we are finally on the cusp of seeing whether a statement made two and a half thousand years ago is correct or not:
"To consider the Earth as the only populated world in infinite space is as absurd as to assert that in an entire field of millet, only one grain will grow"
- Metrodorus of Chios (Fourth Century B.C.)
It's extraordinary to think how far we have come since these words were written.
(Oh, and as for moons, well don't even begin to go there. Our solar system carries over 160 natural satellites around with it, so moons might yet turn out to be the most numerous planetary-type bodies of all...)