Planets: The Grand Tally

A monthago, 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 Gliese581.

It wascertainly one of the most encouraging discoveries in the whole planet-huntingenterprise. And it got me thinking about the big picture. In particular, howmany such worlds are still beyond our telescopic ken, sequestered in therarefied gloom of the cosmos?

Theafter-dinner answer to this question is "plenty."

Sure, that'sa vague response, but it's a radical change from the situation only a fewdecades 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. Billionsof years ago, I was told, some random star happened to slide close to Sol, itsgravity pulling out hot, epidermal gas that cooled and condensed into planets.

That nifty,near-collision scenario was first promoted by New Zealand astronomer AlexanderBickerton near the end of the 19th century. Perhaps because of itsdramatic flair, or possibly because of its Genesis-like creation of fertileplanets from the roaring, incandescent ribs of the Sun, this improbable ideaenjoyed more popularity than it deserved. Had it been true (and the fact thatit couldn't be true was quickly obvious to theoreticians, since it failed toaccount for the enormous angular momentum of the planets), a rip-off schemewould mandate that our solar system is just about the only planetary assemblyin the entire Galaxy. Such near-collisions are incredibly rare. Single starsare in one another's neighborhood about as often as Himalayan Sherpas are inyours.

So we canrule 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 thatsurrounds a nascent star. There is still some controversy about the details ofthis process, but there's no doubt that Nature has concocted a commonplacemethod for extracting scarce heavy atoms (silicon, carbon, oxygen, nickel,iron) from the thin mists of protoplanetary disks, and rearranging them intoballs a few thousand miles thick. Planets o'plenty.

Hownumerous is "plenty"? The tally of extrasolar planets is currentlyabout 240, a number that ticks over faster than Mario Andretti's odometer. Of allthe stars studied, roughly 5?10% are found to have planets. But our instrumentsare far from perfect, and champion planet trapper Geoff Marcy figures that thepercentage of stars that are actually attended by planets is much higher.

"Virtuallyall single stars (stars that are not in binary systems) must have planets ofsome sort ? rocky, gaseous, Neptune-like, and so forth," says Marcy."Among the binary stars, all those separated by at least the distance fromus to Pluto also have planets of some sort."

Sinceroughly 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 allgalactic stars have planets. From an astronomical perspective, that's as goodas all of them.

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

So here itis: there are a few hundred billion stars in the Galaxy, and there are maybe ahundred billion galaxies in that part of the cosmos we can survey with ourtelescopes. With 5 or 10 interesting orbs per solar system, the visibleuniverse contains a hundred billion trillion worthy worlds. A hundred billiontrillion.

That's morethan all the dust motes floating in all the rooms of all the buildings ofEarth.

So sure,the planet around Gliese 581 is beguiling. Maybe it has the conditions forlife, and maybe it even has life. Then again, maybe it doesn't. But as Itold my roommate when his girlfriend pranced off with the football quarterback,"There are other fish in the sea."

Indeed, it'sfin city out there.

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Seth Shostak
Senior Astronomer, SETI Institute

Seth Shostak is an astronomer at the SETI (Search for Extraterrestrial Intelligence) Institute in Mountain View, California, who places a high priority on communicating science to the public. In addition to his many academic papers, Seth has published hundreds of popular science articles, and not just for; he makes regular contributions to NBC News MACH, for example. Seth has also co-authored a college textbook on astrobiology and written three popular science books on SETI, including "Confessions of an Alien Hunter" (National Geographic, 2009). In addition, Seth ahosts the SETI Institute's weekly radio show, "Big Picture Science."