Of all the months to choose to finally read Rare Earth: Why Complex Life is Uncommon in the Universe by Peter D. Ward and Donald Brownlee, a book I’ve long had on my “to read” list, this was probably the most fortuitous. It was a matter of chance, really. Reader S.S. saw the thing on my Amazon wish list, realized it was in a pile of books she intended to part with, and kindly sent it along to me.
I’m grateful for that, because I’ve wanted to read it for some time. What’s more, though, is that as a result I wound up reading it in the month in which scientists announced they’d found the first-known extrasolar terrestrial planet theoretically capable of supporting life. The planet, Gliese 581 C, is thought to be about one and a half times the diameter of Earth, approximately five times Earth’s mass, and revolving around its sun Gliese 581 within the “habitable zone,” a distance from the star at which liquid water could exist on the planet’s surface. Gliese 581 is a red dwarf, much cooler than our Sol, and thus its habitable zone is much closer in. Gliese 581 C is about .07 Astronomical Units from its sun, and takes only 13 of your Earth days to complete a revolution around Gliese 581.
The scientists have been much more cautious than the media when it comes to fantasizing about another Earth out there. It’s not known for sure whether Gliese 581 C is actually a terrestrial planet. At five times the mass of Earth, astronomer David Charbonneau of the Harvard-Smithsonian Center for Astrophysics told Scientific American, “A five-Earth-mass planet “sort of looks like Earth, but it sort of looks like Neptune. So which is it?” If Gliese 581 C is indeed terrestrial, it could be completely covered with water — given the configuration of the other two known planets in the system, some astronomers speculate that the system has undergone planetary migration, meaning that the planets may have formed much farther from their parent star and moved inward. If Gliese 581 C formed on the cold side of its system, it could have amassed a large amount of solid water — a possible local parallel being Jupiter’s moon Europa — which would, on migrating inward, melt into a world-encompassing ocean.
Wait. Did I say the scientists were cautious? Team member Xavier Delfosse from Grenoble University may have let his excitement get the better of him when he spoke to New Scientist.
“On the treasure map of the universe, one would be tempted to mark this planet with an X,” says Delfosse. “Because of its temperature and relative proximity, this planet will most probably be a very important target of the future space missions dedicated to the search for extraterrestrial life.”
I did a little back-of-the-envelope figuring. If Voyager 1, the fastest human artifact, had been aimed at Gliese 581 instead of at some random destination in the constellation Camelopardis, it would be able to check out Gliese 581 C in only about 367,200 years. Better get that grant proposal written quick, guys.
But it’s the speculation about indigenous life on Gliese 581 C that made me glad to be reading Ward and Brownlee.
For those of you who haven’t read the book, the premise is that complex life could not have arisen nor survived long on Earth without a rather unusual sequence of events having occurred. As an absolute baseline, the solar system in question needs to be rich in “metals” — by which astronomers mean elements heavier than lithium — which the majority of such systems are not. The star must be relatively consistent in energy output, and the planet’s orbit close to circular, else the surface temperature will swing too widely for organic chemistry to be truly content. Other large planets in the system also need those circular orbits, lest they perturb the orbit of the test planet. It helps if one of those planets is large enough to suck up most of the interplanetary debris to reduce the number of planet-sterilizing impacts, as Jupiter kindly has for us. But one planet-breaking impact may be required, such as the one Earth probably got from a Mars-sized planet about four and a half billion years ago that tore both planets to shreds, the shreds re-coalescing to form the Earth-Moon system. That moon has kept the Earth’s axis from wobbling overmuch, meaning that the planet hasn’t gone through periods of millions of years with one pole pointed sun-ward and the other pole in permanent night.
That collision, if it happened, likely shaped later life on Earth in two other ways. The collision acted as a giant refinery. Much of the mass of the two planets made up of lighter elements went into orbit to become the Moon: the heavier stuffflew less far and formed Earth 2.0, which is now the densest planet in the Solar System. The other planet’s core is now part of Earth’s core. Most of the Earth’s core is liquid iron, which through processes not completely understood creates the Earth’s magnetic field. That magnetic field is the reason we can breathe: without it, the charged particles put out by the Sun — the Solar Wind — would have ablated the atmosphere away long ago.
The second way that Ward and Brownlee think that collision helped us is a bit more tenuous. Some of the heavy elements the Earth possesses in relative abundance include uranium and thorium, both of them radioactive, with some isotopes possessing extremely long half-lives. Along with the relatively light potassium 40, the radioactivity of these elements is thought to be what keeps the core as hot as it is. The core heat heats the mantle, which — being plastic — convects to disperse that heat. That convection drives plate tectonics. And if not for plate tectonics, argue Ward and Brownlee, there likely would have been no continents on Earth, and no shallow seas around them. Life on Earth would have been solely of the deep-ocean variety, and biodiversity extremely limited, quite likely enough that it could have been wiped out by any one of the early mass extinctions. No other planet in the Solar System exhibits plate tectonics, which may be rare among planets in general.
The book is admittedly Panglossian, with events that may only have influenced Earth life (such as continental weathering and its role in the carbon cycle) being offered as possible necessary preconditions for complex life. And despite Ward and Brownlee defining complex life as multicellular eukaryotic organisms, it’s clear that what they really mean is Earth-type animals. To read Rare Earth, you might decide that plants had nothing to do once their single-celled ancestors oxygenated the Precambrian atmosphere, and that space explorers finding a planet full of forests without animals would have failed to find extraterrestrial life. (In fact, in a discussion of bacteria on Martian meteorites, they explicitly say that no complex life could long survive in space, which might be news to those familiar with fungal spores, or for that matter the spores of a number of higher plants.)
But the book contains a number of important points relevant to the hubbub about Gliese 581 C. That close orbit, for instance. Planets that orbit close to their stars, or moons that orbit close to their planets, often become “tidally locked” — they, like our moon, mainly point one face at the body they orbit. Such an fate is being discussed as a likely possibility for Gliese 581 C. Scientists are talking about the blazing bright side and the cryogenic dark side being separated by a thin habitable zone, which is relevant if you’re fantasizing about colonizing the planet as an uncomfortable way station, but not so much if you’re expecting complex life to evolve and hang on over a couple billion years. Even if the planet could maintain an atmosphere somehow: can you imagine the windstorms?
And speaking of wind, we’d better hope that Gliese 581 C has an Earth-style magnetic-field-generating molten nickel-iron core. Red dwarf stars, despite being cooler and thus dimmer than our sun, apparently tend to generate significantly stronger stellar winds than does Sol. Gliese 581 C is about one-fourteenth as far from its sun as we are from ours, and its sun kicks up more plasma: unless Gliese 581 C has one heck of a planetary magnetic field, its atmosphere would have been scraped away long ago like so much balsa wood under a wire brush.
I have suggested here on previous occasions that the nerds of the world are unlikely, anytime soon, to meet buxom Andorran wenches unfamiliar with your quaint Earthling feminism. It made people unhappy. I expect the same people will be impatient with my suggestion that even if you discount the more handwavy parts of Ward and Brownlee’s book, the possibility of multicellular life on Gliese 581 C — let alone sentient life of a sort that SETI astronomers might find — depends on a couple big, less than likely “if"s. Hopeful hype aside, we still haven’t found a place nearly as suited to complex life as Earth.
