I was asked by a magazine to review Alan Boss's new book The Crowded Universe. They asked for a review that was a much an essay on the field as a review of the book itself, which made it a very fun exercise. The following is based on the review that I got to write.

Two hundred fifty years ago, Immanuel Kant, in his Universal Natural History and Theory of the Heavens, laid out a remarkably modern-sounding account of the state of the universe. Moons go around planets. Planets go around stars. Stars go around the Milky Way. The Milky Way and other galaxies (“other Milky Ways,” he called them) go around something even larger. The solar system had an understandable origin, and inevitable consequences:

The planetary structure in which the sun at the centre makes the spheres found in its system orbit in eternal circles by means of its powerful force of attraction is entirely developed, as we have seen, from the originally distributed basic stuff of all planetary material. All the fixed stars which the eye discovers in the high recesses of the heavens and which appear to display a kind of extravagance are suns and central points of similar systems.

To paraphrase: gravity takes stuff and turns it into stars surrounded by planets, and it has done so everywhere you see a star in the sky.

For the first 240 years after the publication of Kant’s assertion, this fact could only be verified for only a single star in the sky: the sun. In 1995 Michel Mayor and Didier Queloz announced the discovery of the first planet orbiting a star other than the sun. Now, fourteen years later, almost 300 stars are known to have planets around them. It is not quite “all of the fixed stars which the eye discovers,” but it’s getting close. Kant was substantially correct. It had been accepted since the 17^th century that our sun is not special, but is, instead, but one of many stars in the universe. Now, at the beginning of the 21^st century, it is clear that our planets aren’t special either.

Except that some of our planets are still special.

It is tempting to describe the many planetary systems that have been discovered in the past decade and a half as simply weird. Rather than the orderly arrangement of planets that we have here in the solar system, with small planets close, large planets far, and everything going around the sun in satisfyingly circular orbits in a common disk (each one of these properties is “inevitable”, according to Kant, and according to most astronomers up until late 1995), we have instead found planets the size of Jupiter that orbit their stars closer than Mercury, planets with orbits as elliptical as some of the comets in the solar system, and planets with separations from their central star far beyond even the most distant objects detected in our solar system. Weird, indeed. The only type of planetary system that we haven’t found, it seems, is one like our own. Nowhere out there has there been anything quite like the solar system; nowhere out there is another Earth.

But even this special position that our home planet holds is now in jeopardy.

Alan Boss’s new book The Crowded Universe tells the story of the development and launch of NASA’s Kepler spacecraft, which was recently launched from the earth to go into orbit around the sun. Kepler’s 3 1/2 year mission is simple to state: find the Earths. Kepler, along with a similar ESO mission CoRoT, will be the first to finally have a chance to tell us whether planets like the one on which we live are as common as Kant would hope or as rare as some astronomers think.

Boss weaves the story of Kepler (surely a must-read cautionary tale for anyone contemplating a life in NASA mission development) with the larger story of the entire, now booming, field of exo-planets. As someone whose astronomical career has spanned the period Boss discusses, I’m glad someone was taking notes. It is fun to be able to go back to those days when each new planetary discovery was an exciting event with multiple teams struggling to outdo the others with firsts. First planet at the distance of the earth! First transiting planet! First multiple planet system! With the current richness of the exo-planet field it is easy to forget that almost all of this is under a decade old.

Boss gives the insider story not only of the Kepler mission development and the birth and childhood of the entire exo-planet field, but, in a stroke of luck for us all, he got to play a intimate role in the definition of planets in our own solar system, and he gives what I believe is the first account of some of the inner workings of the International Astronomical Union committee that first started trying to figure out what to do with Pluto and Eris and the things that we now call dwarf planets. The demotion of Pluto was unassailably reasonable, but the events leading up to this eventual demotion were some of the more publicly comical occurrences in recent astronomical history. Reliving these moments is an excellent reminder that for all of their command of the physics of the universe around them, astronomers, being human, have the capacity for nearly infinite folly.

But for Boss and The Crowded Universe, Pluto is just a distraction, and rightly so. The meat of his book is the race for finding something like the Earth. Sitting in the middle of the events, it would be easy to get caught up in the day-to-day (or perhaps committee meeting to committee meeting) details. But Boss, while detailing the daily work of himself and other scientists involved in the field, never ceases to forget that we’re privileged to live in such at a time when a nearly-Copernican-magnitude revolution is unfolding.

Yet even if Kepler and CoRoT find an abundance of planets, the 250 year old Kantian revolution will not be complete. The planets that these spacecraft might find could be the precise size of the Earth and could orbit their stars at the exact distance of the Earth, but while an astronomer might be willing to call such a thing Earth-like, most people will still want to know more. Does it have liquid water? Does it have a recognizable atmosphere? And, inevitably, the only thing that really matters, could it – no: does it – support life?

The answer to these questions will take decades or more to answer. Kepler and CoRot are simply first steps along the way. In the meantime, we can perhaps take solace from Kant:

I am of the opinion that it is not particularly necessary to assert that all planets must be inhabited. However, at the same time it would be absurd to deny this claim with respect to all or even to most of them.

It took 240 years to prove him mostly right the first time. With a little bit of luck and a little bit of perseverance and, as Boss shows, a lot of the day-to-day work of astronomers around the world, the final step might come just a little bit faster.