Throughout the genre of science-fiction, the emphasis has largely been placed upon Earthlike worlds. This is natural enough, since many stories revolve around living characters and their environments. Since these characters (often together with an assortment of hostile wildlife) need a benign environment in which to live, most adventures take place on worlds that could easily be mistaken for Earth itself, or at least its environment. At its extreme, this attitude has come up with some ridiculous stereotypes (possibly the worst offender is the "Class M asteroid" of Star Trek; It's a rock with the constituency of builder's rubble; how the hell did it get an atmosphere and Earthlike gravity?).
Still, if one looks beyond the stereotypical Earthlike terrestrial planet, benign environments may come to exist in the most unusual places, as astronomers have recently come to understand. Needless to say, this is very encouraging news for GMs who want to expand their players' horizons.
Recent discoveries of planets around other stars have confirmed what many astronomers considered a possibility. Gas giant planets have been detected orbiting sunlike stars at distances that would approximate Earth's position around the Sun. If such planets were orbited by Earth-sized moons, they would probably give rise to earthlike biospheres, with possibly one or two important differences. If the gas giant were a massive one like Jupiter, its magnetic field would create deadly radiation zones, which may include the orbit of nearby moons. Life on the moon would evolve to cope, of course, but it may preclude visitors from other worlds paying a visit. Also, the rotation of a moon is "locked" to its primary. Just as the Moon keeps one face turned to the Earth, so an alien world would do the same. As a result, its "days" may range from a few days to several weeks in length, depending on how far from the gas giant it orbits. In far orbits, these could become extreme environments, with the midday sun beating down for weeks on end, while the world shivers in a night that could last for months. Such environments have existed on Earth in the past, when the polar regions were still warm enough for forests to grow. Nothing like it has ever been seen by Human eyes, though, and any civilisations on such a world would evolve somewhat differently from our own, due to the extreme conditions.
Similarly, extreme conditions like those mentioned above could exist on a world with an extreme axial tilt. While this is mentioned in passing in the GMG, it would be quite a thing to see the sun spin in circles in the sky, steadily climbing and descending as the year wears on. Mythology on such a world would likely be very colourful indeed, especially when coupled with the extreme seasonality of the planet. It's likely that only the equatorial regions would be able to support any kind of civilisation.
When speaking of planetary systems, many astronomers refer to so-called "goldilocks" orbits. That is, not too hot, not too cold, but just right. In our own system, for instance, Earth is positioned towards the inner edge of the "ecozone" of the Sun. Mars, unfortunately, is a little way outside the ecozone. Even if its chemistry and size were the same as Earth, it would still be too cold for liquid water oceans. Since the presence of liquid water appears to be the single most important ingredient for the development of life, worlds which cannot support it must remain barren.
However, following recent data from telescopes and space probes, astronomers have had to revise their ideas of where such life-bearing environments can exist.
One of the most important discoveries of the nineties has been the Galileo probe's close-range pictures of Jupiter's moon Europa. Its smooth but fraactured surface makes tantalising hints that there may very well be an ocean beneath the icy crust. While this ocean would be more saline than that of Earth, and the radiation environment so close to Jupiter is enough to kill a human, the environment would still be conducive to life. The heat source comes from Europa's core, which is constantly pulled and kneaded by the tidal forces of Jupiter. As a result, the core is still molten, creating enough heat to create a worldwide, deep ocean capable of supporting life. Furthermore, very strong evidence has been discovered to support the idea of subterranean oceans beneath the surfaces of two other Galilean moons, Ganymede and Callisto. While these would be colder, there is also far less radiation to wory about. With some luck, any of these three worlds may well host life, weither microbial or maybe something more complex.
Life on such a world would vary depending on the nature of the ocean. If the ocean is worldwide (like that on Europa is believed to be), then the vent-sytems and their ecologies can interact by using the ocean currents, driving evolution strongly while producing a worldwide population of related species. If, however, the vent-systems only melt sea-sized areas around themselves while leaving the rest of the ice solid, then life will be cut off from other populations, limiting the scope for growth while preserving diversity between populations (they would be as divided as ecosystems on other planets, in fact). In the latter case, perhaps cracks would form between closely-located vent systems, creating convoluted cave-networks through which the two or more populations can interact. Such times would be very dramatic for the lifeforms involved, sparking possible conflicts and driving evolution harder in these areas.
As well as moons orbiting massive gas giants, subterranean oceans may well exist on water-rich worlds that are a little beyond their sun's ecozone and would otherwise be frozen and lifeless.
Castles in the Air
Much discussion has taken place about the possibility of life within the atmospheres of gas giant planets like Jupiter. Frigid at the cloud-tops, the atmospheres of these worlds rapidly become hotter and denser as one descends. With the odd exception, such as Jupiter's South Equatorial Belt, the large-scale structure of the systems remains fairly stable for long periods of time. The clouds themselves are rich in water, carbon dioxide and a whole range of simple and complex organic compounds. Although the radiation environment is so deadly that it could kill an unprotected human several hundred times over, there are terestrial organisms that could survive even such terrible radiation doses; life that has evolved in situ would be readily able to withstand its effects.
As to what such life would be like, opinion is divided and arguments rage to this day. Many maintain that the long-term environment is too unstable to allow life to evolve, while other maintain that microbial life could withstand such evolutionary pressure. A few still maintain that there could actually be complex life as well, perhaps in the form of dirigible-like creatures the size of airships (or even larger). While this environment would be useless to humanoid heroes, less radiation-intensive worlds may still harbour such ecosystems (maybe with greater eficiency), while allowing suitably-equipped heroes to pay these places a visit. How would they cope with an environment that is never still, with no solid place to land on? Should make for quite an interesting game!
Perhaps the most bizarre idea is one put forward only a year or two ago. It is now understood that early solar systems are composed of literally a hundred or more smallish planet-sized bodies called planetesimals. When first formed, they go careering about in all kinds of orbits, some as eccentric as comets, some as stable as Earth's. After a few million years, collisions have destroyed the more rambunctious members, leaving only those planets whose orbits can live with each other. However, not all the more eccentric worl;ds will have been destroyed in collisions. a large probportion would be involved with close-approaches to massive worlds, and their orbits would be radically altered. Some would be cast into the central star itself, while others would be flung into deep interstellar space. These orphaned planets may well have quite an unusual destiny.
At this stage in the system's formation, the planetesimals have a thick, hydrogen-rich atmosphere. If the planet is a rocky, terrestrial type, its young atmosphere is many times denser than the one it would normally end up with, since solar-wind pressure strips away the hydrogen after a few million years. Deep in interstellar space, however, this atmosphere can remain intact. Although the planet receives no illumination from a sun, its own internal heat is as active as ever, and the thickened atmosphere would work as a very effective thermal blanket. As a result, liquid water may well be stable at the surface, giving rise to oceans like those on Earth. With oceans, life can evolve, and even become every bit as complex as that of a starbound planet.
If the atmosphere of this planet is opaque, then the surface will be utterly dark; the inhabitants blind. If, however, the atmosphere is transparent, then the world will be bathed in starlight. Big-eyed, nocturnal-looking creatures would populate the world (even humans can see clearly enough in starlight). If they became intelligent, what would they make of the stars? What would they make of a visitor who claimed to come from a star?
Another interesting quality of an orphaned planet is that the lack of a star does away with stellar vagaries. Our own Sun is getting steadily brighter, at a rate of 1% per hundred million years. In a billion years it will be 10% brighter than today, which will be bright enough to boil away the oceans of Earth. At this point, the story of life on Earth will come to an end, barely halfway through the planet's lifetine (it will likely be destroyed as the Sun becomes a red giant). An orphaned planet's internal heat reserves will last for at least tens of billions of years, allowing biological evolution incredible amounts of time to evolve all manner of lifeforms. It's hard to imagine what life could achieve given that amount of time.
Â© Mark Peoples 2000.