Planets: The Grand Tally

June 14, 2007
by Seth Shostak - Senior Astronomer

A month ago, there was news that washed over astrobiologists like high tide in the Bay of Fundy: the existence of a possibly habitable planet around the nearby star Gliese 581.

It was certainly one of the most encouraging discoveries in the whole planet-hunting enterprise. And it got me thinking about the big picture. In particular, how many such worlds are still beyond our telescopic ken, sequestered in the rarefied gloom of the cosmos?

The after-dinner answer to this question is “plenty”.

Sure, that’s a vague response, but it’s a radical change from the situation only a few decades earlier. As a kid, I spent a lot of time sitting in the dark at New York’s Hayden Planetarium. There, voice-of-God narrators would proclaim that the (then) nine planets encircling the Sun were the product of a stellar fender-bender. Billions of years ago, I was told, some random star happened to slide close to Sol, its gravity pulling out hot, epidermal gas that cooled and condensed into planets.

That nifty, near-collision scenario was first promoted by New Zealand astronomer Alexander Bickerton near the end of the 19th century. Perhaps because of its dramatic flair, or possibly because of its Genesis-like creation of fertile planets from the roaring, incandescent ribs of the Sun, this improbable idea enjoyed more popularity than it deserved. Had it been true (and the fact that it couldn’t be true was quickly obvious to theoreticians, since it failed to account for the enormous angular momentum of the planets), a rip-off scheme would mandate that our solar system is just about the only planetary assembly in the entire Galaxy. Such near-collisions are incredibly rare. Single stars are in one another’s neighborhood about as often as Himalayan Sherpas are in yours.

So we can rule out the parenting of planets by two stars enjoying a brief encounter. Instead, we now know that small, cold worlds emerge from a disk of gas and dust that surrounds a nascent star. There is still some controversy about the details of this process, but there’s no doubt that Nature has concocted a commonplace method for extracting scarce heavy atoms (silicon, carbon, oxygen, nickel, iron) from the thin mists of protoplanetary disks, and rearranging them into balls a few thousand miles thick. Planets o’plenty.

How numerous is “plenty”? The tally of extrasolar planets is currently about 240, a number that ticks over faster than Mario Andretti’s odometer. Of all the stars studied, roughly 5 – 10% are found to have planets. But our instruments are far from perfect, and champion planet trapper Geoff Marcy figures that the percentage of stars that are actually attended by planets is much higher.

“Virtually all single stars (stars that are not in binary systems) must have planets of some sort – rocky, gaseous, Neptune-like, and so forth,” says Marcy. “Among the binary stars, all those separated by at least the distance from us to Pluto also have planets of some sort.”

Since roughly half of all stars are binary, and half of those are widely separated, the bottom line is that Marcy suspects that roughly three-fourths of all galactic stars have planets. From an astronomical perspective, that’s as good as all of them.

Now, how many planets does each star have? Well, the Sun has eight, nine, or a few more, depending on your semantic sympathies. But from the standpoint of extraterrestrial biology, counting planets is hardly adequate, since there are at least five moons in our own solar system that are big enough, and complex enough, to tantalize us as possible abodes for life. We now know of seven other worlds (two planets, in addition to the five moons) in our back yard that might – just might – offer conditions suitable for life.

So here it is: there are a few hundred billion stars in the Galaxy, and there are maybe a hundred billion galaxies in that part of the cosmos we can survey with our telescopes. With 5 or 10 interesting orbs per solar system, the visible universe contains a hundred billion trillion worthy worlds. A hundred billion trillion.

That’s more than all the dust motes floating in all the rooms of all the buildings of Earth.

So sure, the planet around Gliese 581 is beguiling. Maybe it has the conditions for life, and maybe it even has life. Then again, maybe it doesn’t. But as I told my roommate when his girlfriend pranced off with the football quarterback, “there are other fish in the sea.”

Indeed, it’s fin city out there.