Orbital patterns

You're making a lot of assumptions here which are, frankly, rather naïve. The only life we know of to a certainty is on this planet. Photosynthesis may take place elsewhere, but we have no way of knowing if it does. Life may exist elsewhere, but we have no way of knowing if it does. On our planet, life arose more than four billion years ago. (Our star system is about five billion years old.) That life form is known as archaea. You might look up that word, because they are a unique life form. Some of them photosynthesize sugars by breaking down carbon dioxide into carbon and oxygen. The oxygen is give off as a waste gas. Oxygen was so rare, as a molecular gas, in our early atmosphere, that the oxidation of minerals on the surface of the planet above sea level or of minerals dissolved in water routine "scrubbed" the molecular oxygen out of the atmosphere. But after more than two billion years of this, archaea had become so successful and wide-spread that they were producing oxygen as a waste gas faster than minerals were oxidizing and taking oxygen out of the atmosphere. This lead to the first great extinction event of which we know, called the great oxygenation event (you might look that up, as well) which took place about one-point-eight billion years ago. Some cells had evolved into eukaryotes (look it up), which differ from archaea and bacteria because they have membrane-bound organelles, such as the cell nucleus, where genetic material resides, and the mitochondria, which resemble bacteria, and have their own RNA. The eukaryotes (you are a eukaryote) survived the great oxygenation event, while most archaea did not. Archaea survive to this day, but they ceased to be the dominant life form at the time of the great oxygenation event.

The point of all of that is that life arose here in specific conditions of climate, atmospheric composition and insolation (the relative amount of sunlight--starlight, really--which reaches the surface of the planet. Additionally, the earth's core is still very, very hot, and spins faster than the planet as a whole does. This creates a sort of dynamo which gives our planet a strong magnetic field. That magnetic field, called the magnetosphere, deflects or traps a good deal of the radiation from the sun in the non-visible ranges. That serves to protect the planet from the possibly damaging portions of solar radiation.

All of this means that life arose here because of a crucial combination of conditions. We have no way of knowing if life can or ever has formed under other conditions, but we do know that most of the life on this planet would not survive if the climate were a good deal colder or a good deal hotter. Life would not survive here if the atmospheric conditions were significantly different. That's a no brainer--life forms and evolves to survive in the conditions that exist at that time. We just don't know enough to know if it could form and survive under other conditions.

Your question about the orbits of bodies in the solar system--our star system--is perceptive. I don't believe there are any bodies in this star system the orbits of which are perfectly circular (i could be wrong). The orbit of Mercury is the most eccentric. At aphelion (farthest point away from the star) it is almost a half a million kilometers away from the star. At perihelion (the nearest point to the star), it is less than 70,000 kilometers away. (When i write "the star," i'm referring to our star, Sol, the "sun.")

Venus has the least eccentric orbit of all the planets. It's not circular, but it doesn't miss it by much. At aphelion, it's about 109 million kilometers away from Sol. At perihelion, it's about 107 million kilometers away. Venus is well on the way to being tidally locked to the star. A day on Venus, the time it takes to turn once on its axis, is longer than its year, the time it takes to go once around the star. Venus is not a very nice place. If there was ever any life on Venus, it didn't last very damned long. Carl Sagan, whom many people admired, and whom conservatives loved to hate, wrote his doctoral dissertation on the atmospheric conditions on Venus, among other things about the conditions of planets. His predictions of those conditions came very close to what has since been discovered. The Russians sent several Venera probes to Venus, and had confidently stated that one off them had reached the surface. Sagan said that it had not, and that it had been crushed by atmospheric pressure before reaching the surface. The Soviets said that Sagan was full of horsie poop, but their actions belied their words. Later probes were made much more sturdy, eventually reaching five tons. Venera 8 and Venera 9 reached the surface and sent back images. Sagan's estimation of atmospheric conditions was vindicated. The surface temperature of the atmosphere exceeds 450 degrees centigrade. The atmospheric pressure is in excess of 90 bar. One bar is what scientists used to believe was the atmospheric pressure at sea level on our planet. They had a big "Doh" moment, though, are realized that sea level changes constantly. Currently, the atmospheric pressure at mean sea level is 1.013 bar--one bar, 13 millibars. The atmospheric pressure of Venus is about 90 times greater than that of our planet. No life, no way, Jose.

I'm going to post this, and post more later, if and when i get the time.

Sorry for the language errors in that post--i was having connection problems with the interwebs when i was trying to write and post it.

Both Terra (the earth) and Mars have significantly elliptical orbits. Another important characteristic of both planets is their axial tilts. The axial tilt of a planet is based on how far from perpendicular to the plane of the ecliptic they are. The plane of the ecliptic is an imaginary plane which extends outward in all directions from the equator of the star. The planet we live on tilts twenty-three and a half degrees from the perpendicular, which is, by the way, the reason we have seasons. Mars tilts twenty-five degrees from the perpendicular, it's orbit is also more elliptical than the orbit of our planet, which means that its seasons are somewhat more extreme in terms of climactic conditions than ours are. I'll write more about Mars in another post.

These tilts are the result of impacts with other bodies in the solar system. In the case of our planet, the probable cause is the impact which created our satellite, Luna, the moon. The cause of Mars' axial tilt is problematic, largely because planetologists don't want to sign-on to the only comprehensive hypothesis so far advanced to explain it.

Jupiter's axial tilt is only three degrees--which is the same as that of Venus, and has little effect on the planet. Saturn is tilted, too, and more than Terra or Mars. Saturn is also seasonal. Neptune has a similar axial tilt. Uranus is the real odd-ball, though. The axial tilt of Uranus is more than 97 degrees, almost 98 degrees. It must have really got pasted at one time. If you were to thihk of the other planets within the orbit of Uranus as spinning tops, Uranus is like a ball rolling over a flat surface. Pluto's axial tilt is said to be sixty degrees, or almost one hundred twenty degrees, depending on who is telling the tale. We just had a probe fly by Pluto, and obtained more information, about five times as much information as we previously had. Pluto was previously thought to have one moon, Charon. Pluto has been demoted to a dwarf planet, and Charon is so close to it in size, that many scientists now consider it a binary dwarf-planetary system. Charon has got pasted like nobody's business. Maybe that's from living out there in comet alley.

But as the New Horizons probe approached Pluto, the mission control team discovered four more moons. They can bad-mouth Pluto all they like, she's got a respectable retinue.

More later--when i'll go back to Mars. Mars is the one planet i know most about, excepting Terra.

An object in orbit around a star can only be in complete darkness due to a blockage in synchronous orbit between the object and the star, and this is extremely unlikely to the point of being virtually impossible. Such orbits would be unstable and would not last long (even for a single cycle around the star).

Some objects are tidally locked to their star in such a way that only one side of the object ever faces the star. In those cases only 50% of the object would be exposed to light. The other side would be in perpetual darkness. I believe such a planet was discovered not long ago.