Is E.T.'s World a Planet?

Jul. 25, 2002

by Seth Shostak - Senior Astronomer

Freeman Dyson is betting that alien life doesnt live on a world like yours. More specifically, if you check out the tentative wager this celebrated physicist has logged at the Web site www.longbets.org, youll see that Dysons hunch is that the first discovery of extraterrestrial life will be made someplace other than on a planet or on a satellite of a planet.

In other words, if we succeed in finding some biology beyond Earth, it wont be camped out on a large ball of rock.

I have great admiration for Freeman Dysons breathtaking ideas. But I suspect hes taking a long shot here. He hasnt qualified the bet to say "intelligent extraterrestrial life." So if an upcoming mission to Mars, Titan, or Europa discovers microbes busy eking out a living deep underground or afloat in hidden oceans of water or natural gas, then Dyson will have to pay up, assuming we havent found some other biology first.

Is there something else we could find first? Theres always the possibility of breaking open a fallen meteorite to reveal some sort of metabolizing slime within. Or maybe intelligent aliens will make an unequivocal landing behind the White House rose garden. But if youre talking near-term success in uncovering life elsewhere, then SETIs a big part of the mix. SETI, of course, could find a signal at any time.

So how reasonable is Dysons hypothesis of a non-planetary home for intelligent life? After all, the usual convention in SETI as expressed in everything from the Drake Equation to the strategy of targeted searches is that complex, thinking beings will evolve on Earth-like worlds, and stay there.

The first conjecture isnt enormously controversial. But the second may be too conservative. Intelligent beings might travel, and at the very least could spread through their own solar systems. Dyson has pointed out that the fundamental problem with planets is that theres not much real estate for the mass involved. Spheres have the minimum surface area for a given volume of stuff. You can improve things by chopping the Earth in two and rolling up each of the halves like clay balls. This will increase the acreage by 26%. Do it again (now four balls), and youll win another 26%.

Reconfiguring Earth is a big job, and would probably run afoul of environmental protection agencies. But theres no need: as Dyson has noted, if more space is your thing, then the asteroids are already available as bite-size hunks of matter, close enough to the Sun to intercept interesting amounts of energy, and composed of materials suitable for supporting life. Theres at least ten thousand times as much surface area on the asteroids as on our home planet. So our future, Dyson suggests, lies in exploiting this abundant acreage, for otherwise we may crowd ourselves into a nasty situation here on Earth.

Assuming that intelligent species elsewhere have done the same, shouldnt we be broadening (in a literal sense) our SETI searches?

In fact, our searches already are broad! The Arecibo telescopes beam, as used for Project Phoenix, covers all of a 100 light-year-distant solar system out to two thousand times the Earth-Sun distance. It will encompass anyones asteroid belt. Our optical searches are similarly sensitive to wide swaths of other star systems.

But what if life has adapted to existence in a vacuum (another Dyson suggestion)? You might think that life in a vacuum sucks, but if such atmosphere-independent biology exists, it might migrate to giant molecular clouds and other interstellar feeding grounds where resources are far more plentiful than in a planetary system. If this has happened, SETI sky surveys (such as SERENDIP IV, the data-collecting component of the SETI@home effort) might turn up intelligence situated between the stars.

Is Freeman Dyson right? Obviously, no one yet knows, and its certainly possible that life even of the savvy variety might be situated elsewhere than on planetary surfaces. Fortunately, todays SETI searches have those extraplanetary bases reasonably covered.

Meanwhile, those who are optimistic about the chances for finding life beneath Mars dusty dirt or under Europas icy crust may wish to step forward and take up Freeman Dysons bet.