Well, well. Here we are again.
There were times I despaired of ever revisiting these pages, but life's a funny old thing. In keeping with the spirit of rebirth, the first of this new irregular series of articles (no promises, this time!) takes another look at the subject last visited; life-sustaining planets that are out of the ordinary.
Days Without End
Under certain circumstances, a planet may develop that maintains a tidal lock to its stellar primary. Although often cited in old SF novels, the real circumstances surrounding such a planet are rarely explored. In the simplest sense, a tidal lock occurs because an orbiting body is very close to its primary. The difference in gravitational attraction between the nearest and furthest points on the orbiting body serves to keep "tugging" the planet into facing the same way. Over a long period of time, this tugging serves to rob the planet of its rotation, this momentum usually transferring to its orbit, raising it. Such a process is going on even now between Earth and the Moon. The Moon has already locked onto the Earth, but eventually the Earth will lock onto the Moon, as well. Earthâ€™s liquid water oceans exacerbate this process.
To develop a lock to its sun, however, a planet must be very close indeed. So close, in fact, that any planet so close to a Sunlike primary would be a raging inferno fit to put Mercury to shame. A life-bearing planet, close enough to be locked to its sun would have to orbit a mid- to low-range red dwarf star. These stellar misers can be as light as 0.1 Solar masses, and emit as little as 0.1% the luminosity of the Sun. However, even these chill stars can warm a planet if it's close enough, every bit as well as a brighter star.
So, what would such a planet be like? Well, although it would receive, on average, the same radiation as a more conventional Class1 planet, its environment is anything but conventional. With unending sunlight on one side and eternal dark on the other, it is a world of extremes. An airless world would experience extremes of temperature, similar to Mercury and the Moon, but perhaps even more so. Classic SF has often called these planets "ribbon-worlds", maintaining that only the barest line of habitability exists at the twilight edge between the hemispheres. The presence of an atmosphere, however, changes everything. Even under these circumstances, it works to mitigate the extremes of climate between the hemispheres, and in these conditions life can thrive. Air heated at the stellar zenith point will expand and rise, pulling in colder air from the nightside to replace it, creating a system of weather cells. Looked at simply, we end up with a set of climate poles at stellar zenith (the point at which the star is directly overhead) and stellar nadir (the point at which it's directly below) that create their own air-masses, with a changeable zone dominated by front systems around the transition area. The "hot pole" will certainly be extremely hot, and the "cold pole" extremely cold whatever the atmosphere, although oceanic currents may again mitigate the extremes. To accomplish this, the atmosphere needs to be about 1.5 bar or higher, which intimates a planet comparable in mass to the Earth. The danger with M-class dwarfs is that many of these stars have a nasty tendency to flare, pouring large amounts of deadly UV that would be very strong at the planetâ€™s surface. Surface-based life would have to be pretty hardy to survive these outbursts, or else would be confined to the seas and caves of the planet. On a more positive note, however, if the flares occur relatively often, the increased UV flux will actually promote the formation of ozone at high altitudes, protecting life at the surface from the worst of the UV.
Slightly smaller planets, with atmospheres less than 1.5 bar, will have an unusual climate. On the far side from the stellar primary, the thinner air would be too thin to transfer enough heat to prevent the air from freezing out onto the surface. Such a planetâ€™s farside would resemble Neptuneâ€™s moon Triton, with a thick layer of nitrogen â€œsnowâ€. Toward the edges of the frozen zone, within perhaps 10-15 degrees of the terminator (the dividing line between night and day), the edges of these nitrogen glaciers would boil off into the vacuum in spectacular fashion, probably creating short-lived clouds of dilute nitric acid as the frigid gas meets the warmer air of the stellar nearside. Any living things near the nightside freeze-out zone would probably have to be chemically as well as thermally hardy in the extreme!
It's the plant life that will be most different from our own, however. Unlike our own poles, the star will never seem to move. It will never rise, or set, or change in any way. Photosynthesising plants will surely develop to exploit this, with cut-throat competition to get at the light. A plant in shade will be forever in shade. Their leaf-structures will probably angle to catch the light directly as on Earth, so that those plants beneath the star will have broad, horizontal foliage, while those on the rim of daylight would have their leaves standing perpendicular, to catch the near-horizontal sunlight. If there are plants or plant-like lifeforms on the nightside, they will have to do without sunlight of any kind; feel free to let your imagination run wild with this idea!
So, how do people keep time on such a world? Spacefaring colonists will no doubt bring their own timekeeping devices along with their own circadian rhythms, but the natives will never experience time the way we do. With no day/night cycle, there will be no split between diurnal and nocturnal animals. Creatures must rest, but it will not be to any externally imposed rhythm. In particular, the dayside will never have or need any kind of calendar. Itâ€™s hard to imagine- an existence without days, months, seasons or years, just an unending present. On the nightside, however, things may be a little more normal. The planet is orbiting its star, after all, so the stars will rise and set like they do on Earth. Well, not exactly the same way. It will take a whole "year" for the stars to turn full circle. Still, any intelligent nightside natives will probably use these to mark development stages in their life cycle. Without seasons, however, these calendars will never have the same importance they did on Earth.
Under the Sea
The term â€œgas giantâ€ is used to refer to the four largest planets of our solar system, namely Jupiter, Saturn, Uranus and Neptune. However, theyâ€™re not all alike. Jupiter and Saturn, as well as being far more massive than the outer giants, are different in their chemical makeup, too. Primordial hydrogen and helium, with smatterings of other gases dominate their makeup, whereas Uranus and Neptune have atmospheres dominated by methane and ammonia, with a fair bit of water. That chemical difference is important, and has prompted some astronomers to propose another classification of the outer planets. If Jupiter and Saturn are gas giants, then Uranus and Neptune are ice giants.
In our solar system, the giant planets are fairly far from the Sun. In other systems, however, there are giants living in very close proximity. Often referred to as â€œHot Jupitersâ€, their formation has perplexed scientists, since there simply isnâ€™t enough material to build a large planet so close to its star. The general consensus is that the original stellar discs from which the system formed were thick enough for friction to be a major force. As a result, the planets actually slow down as they orbit, and start to fall slowly toward the star. Once the star ignites, its radiation pressure scours the inner system clean of most of the dust and debris, halting the process, and the infalling planets remain at their final positions. Weâ€™ve already noted that a Jovian planet that halts in its starâ€™s habitable zone might just have an Earth-like moon, but what about an ice giant? What happens to a planet made of methane, ammonia and water ice when it moves to a sunnier clime?
What you end up with is, in fact, that wonderful SF trope: the ocean world. Weâ€™ve often seen such worlds in SF, but not these new kinds. Bluefall is a classic oceanic planet, essentially an Earth with a little more water. Europa, Ganymede and Callisto are a more recent version, with oceans sealed by a global ice-sheet. These planets are rather different. For a start, theyâ€™re considerably more massive. These planets would average around ten times the mass of the Earth, and be about twice as large. A mantle of water many thousands of kilometres deep would surround their rock/metal cores. However, this doesnâ€™t mean that their oceans would be thousands of kilometres deep. When we think about the effects of high pressure, we tend to see that increased pressure causes the melting point of a substance to fall. However, under extremes of pressure equalling more than 11,000 bar, ice becomes stable as its melting point rises again. Water achieves this point at a depth of about 100km, with an ocean-floor pressure of 11,000+ bar. As a result, these planets would have oceans of water ten times deeper than the very deepest ocean trenches, sitting above a layer of ice thousands of kilometres deep (about 4-5,000km to be precise).
So what about life? Well, the chemical systems of such a planet would echo those of Earth, although on a rather grander scale. We can expect a full range of bottom-adapted and surface life, perhaps even communities living deep in volcanic fissures cut by superheated water through the ice from volcanic outlets far below.
Finding decent real estate would be a different matter. These planets would have incredibly deep oceans but no rocky surface. Not so much as a volcanic island would poke its head above the waves. Any dry land would have to be imported, artificially floated and kept in place by active motor systems. One could imagine artificial pumice being created from asteroidal materials and used as a buoyant base material, with other rocks (possibly including mature corals) providing the surface. Eventually topsoil could be created and seeded. Alternatively, engineered kelp-like plants and algae could be used to create living rafts.
A Place in the Sun?
As astronomers have come to catalogue the vast array of objects in the Universe, some have been rather problematical with regards to classification. One such group of objects is the so-called brown dwarfs.
Itâ€™s uncertain as to whether they are super-massive planets or really under-massed stars. They are large (from a little larger than Jupiter to maybe a fifth the diameter of the Sun), and appear to form during normal stellar formation. For this reason, they are tentatively classified as star-like objects. However, in many important ways, they are unlike stars. For a start, they are much, much less massive than the smallest known red dwarfs. Secondly, they are not massive enough to fuse hydrogen in their cores, a process normally considered to be the very definition of a star. Instead, these dark, mysterious, massive objects emit only infrared light, the residual heat o their formation, and are very difficult to detect except by their gravitational effects.
The cooler of these objects may, surprisingly, prove to be clement places for life to exist. Their upper atmospheres are marked by two substances, titanium oxide and carbon. These (particularly the carbon) form a very effective thermal blanket, insulating the upper reaches of the starâ€™s atmosphere. In this area, the â€œterrainâ€ would resemble the massive cloud belts of Jupiter, although surprisingly, the wind speeds could be considerably lower.
It has been postulated in the past that Jovian planets like Jupiter may possibly have strange lifeforms living amongst their clouds. If floating, blimp-like life can exist on a Jovian planet it could conceivably exist amongst the vast clouds of a brown dwarf, too.