n the heady science--and science fiction--of the 19th and early 20th centuries, astronomical objects came in exactly four flavors: stars, which were white-hot and diffuse and very large; planets, which orbited stars, and were smaller and denser and had obvious "planetary" features such as clouds and weather; moons, which circled planets but were too small to hold atmospheres of their own. And finally asteroids--star-orbiting lumps of rock unmolded by their own meager gravity, distinct from planets and moons by their decidedly nonspherical shapes. A handful of particularly large asteroids, such as Ceres and Vesta, turned out to be round after all, and were sometimes classified as small planets just to keep things tidy. A couple of misshapen moonlets were classified as "captured asteroids" for the same reason.
There was also this Einstein guy, talking about something called "black holes," but he had funny hair and an accent, and was Jewish besides. The important thing was that solar systems formed as these whopping-great whirlpools of hydrogen gas and assorted impurities. The stuff that spiraled in toward the center got to be part of the star. The stuff in the whirlpool's outer disk got concentrated into ringlike bands, which eventually collapsed into smaller whirlpools which became the planets and their moons.
In between the planets--most especially in the gap between Mars and Jupiter, where Bode had pointed out another planet would be numerologically amusing--the gas and its impurities simply accumulated into little clumps--asteroids. Or comets, sure, if you were very far away from the parent star. Comets were exactly like asteroids, but made of colder stuff like methane ice. Incidentally, nothing would live on comets, or asteroids, or moons, which lacked not only atmosphere but also the chemical building blocks crucial for life. As for the planets, well, they were probably full of dinosaur jungles and green, naked women.
Such an orderly universe must have been both aesthetically and educationally pleasing: astronomy in four easy lessons, science fiction in five. Alas, like other simplified cosmologies of the ancient and classical world, it suffered the unfortunate defect of not being true.
Durable and versatile
First off, the chemical building blocks of life (such as water, amino acids and organic molecules such as sugars), once thought to be rare and delicate, turn out to be among the commonest substances in the universe, found not only in comets and asteroids and occasionally moons, but also in the interstellar gas clouds from which solar systems are built in the first place. In addition, Earth's most primitive lifeforms--"archaea," a class of microorganisms unknown and unsuspected until a few decades ago--are enormously more durable and versatile than any neo-Victorian biologist could imagine. Archaea are found not only in the boiling waters of deep-sea volcanic vents, but in the hot rock and metal of the Earth's deep crust, and in the ice of our polar caps. The spores of many archaea could even survive the heat an cold and radiation of outer space, perhaps for millions of years.
"Panspermia," an extraterrestrial origin for Earthly life, used to sound far-fetched and silly, but the fossil record certainly does indicate that life on Earth appeared not slowly and painfully, but with blinding suddenness, just as soon as environmental conditions permitted it. Indeed, ever since the possibly-microbe-bearing Martian meteorite ALH84001 made the idea fashionable, it's increasingly easy to speculate that interplanetary space--perhaps even interstellar space--is hopelessly contaminated with the spores of life. Stranger things have turned out to be true.
Another sabot in the gears of classical astronomy is in its assumptions about planets; most of them turn out to be a lot less hospitable than Earth, owing mainly to inclement temperatures and the escape or chemical entrapment of oxygen and hydrogen, resulting in a dearth of liquid water.
There also turns out to be a big distinction between the rocky or "terrestrial" planets which form close to a star and the much larger gaseous ones which form farther out. While terrestrial bodies are made of heavy elements such as oxygen and silicon and even iron, the composition of the gas giants is pretty much the same as that of stars: mainly hydrogen and helium, spiced with a bit of carbon. And their moons, far from being sterile balls of rock and ice, often support volcanoes, atmospheres and, most surprising of all, even oceans. The tidal forces of our own tiny moon are enough to drag water up and down our beaches in a steady rhythm.
Similarly, the much stronger tides of a Jovian (Jupiter-like) planet are sufficient to heat some moons' interiors to the melting point and beyond, making these tiny worlds literal hotbeds of geothermal activity.
Jupiter's second moon, Europa, has a cold, icy crust and a warm, rocky core. In between, there appears to be a layer of liquid water several hundred kilometers deep, probably warmed at the bottom by volcanic vents similar to those in Earth's own deep oceans. This warmth has nothing to do with sunlight, which is 27 times less intense than it is at Earth, and by itself could not heat the little world above liquid nitrogen temperatures.
Still, geothermal heat is as good as any other sort. In fact, recent geological evidence indicates that half a billion years ago, the Earth's surface froze from pole to pole, completely decoupling its oceans from both sunlight and atmosphere, leading to a period of rather extreme chemical and biological evolution which may have paved the way for advanced multicellular lifeforms. So, strangely enough, the most "Earthlike" environment in our solar system may be a distant, icy body slightly smaller than our own lifeless moon.
To confuse matters even further, the discovery of dozens of alien solar systems has confirmed that gas giant planets, instead of lingering in the cold and dark where they've formed, often spiral in toward their parent stars, finally settling into scorching orbits much tighter and hotter than that of our own planet Mercury (which itself is hotter than molten tin).
Such "hot giants," with superheated atmospheres and no solid surface, don't bear much resemblance to "planets" in the usual sense.
There are even hotter giants as well, gaseous bodies which are less massive than stars, but much weightier than typical gas giant planets. Large enough, in fact, that the heat of their formation is enough to ignite nuclear fusion. Not the fusion of hydrogen--the preferred fuel of stars everywhere--but of deuterium and tritium, heavier and much rarer materials which ignite at one-third the temperature (3 million degrees Kelvin, vs. 10 million for hydrogen). Such objects, termed "brown dwarfs," are thought to cool off over a period of hundreds of millions of years, eventually assuming the same ambient temperatures as any other gas giant. Interestingly, this does mean they pass through a long "autumn" phase during which their temperatures are Earthlike, and their atmospheres rich with water vapor.
It also means, unfortunately, that only the very young dwarfs are hot enough (and therefore bright enough) to observe directly. Still, brown dwarf companions are being discovered for many of our neighboring stars, and our own sun's suspected 10th planet, far beyond the orbit of Pluto, may be one as well. What's more, a recent survey of the Trapezium Cluster's "stellar nursery," where thousands of new stars have formed in the past million years, indicates that free-floating brown dwarfs, unattached to any larger star, may be common as well.
In fact, free-floating gas giants may also be common, as evidenced by the recent discovery of 13 of them in, again, the Trapezium cluster. These are really young planets, still glowing from the frictions and collisions of their own formation, so that even from 1500 light years away we can see them clearly. In another million years they'll be cold, dark, and even more invisible than the elusive brown dwarfs. This is tantalizing; we can guess at the frequency and distribution of these dark stars and darker planets, but we have no way of knowing if any are close by. Not with today's technology, anyway.
Littered with failed stars
Ever since the dawn of spaceflight, visionaries have imagined trips to the stars, with "generation ships" on centuries-long ballistic courses, or with "Bussard Ramjet" or antimatter-annihilation starships accelerating to very nearly the speed of light, or even, more fancifully, with "warp drives" capable of exceeding that universal speed limit. Of course, none of these ships could be built by any present Earthly society -- not even by all of them working together in miraculous harmony. But each of these designs presumes a vast, empty journey across light-years of open vacuum.
What if the truth is simpler? What if space is littered with these failed stars, scattered between the bright ones like a stellar Polynesia, making interstellar travel a series of short hops, rather than a single gigantic one? What if a simple fusion reactor carried just enough fuel to push a spacecraft to our solar system's Planet X in reasonable time? What if it could refuel there, harvesting just enough hydrogen or deuterium or helium to limp along to another dark neighbor, and another, and another?
Granted, it would take a long, long time to get to Alpha Centauri that way, and probably a much, much longer time to find a planet somewhere that looked even remotely like our rain- and sun-drenched Earth. But given the likelihood of tidally warmed moons, and the obvious possibilities for life there, we may just find that the cold, dark spaces are where most of the action is anyway.
Wil McCarthy is a rocket guidance engineer, robot designer, science fiction author and occasional aquanaut. He has contributed to three interplanetary spacecraft, five communication and weather satellites, a line of landmine-clearing robots, and some other "really cool stuff" he can't tell us about. His short fiction has graced the pages of Analog,
Asimov's, SF Age and other major publications, and his
novel-length works include Aggressor Six, the New York Times Notable
Bloom, and The Collapsium.
