ScienceDaily (2007-11-22) -- The next time you take a moonlit stroll, or admire a full, bright-white moon looming in the night sky, you might count yourself lucky. New observations suggest that moons like Earth's -- that formed out of tremendous collisions -- are uncommon in the universe, arising at most in only five to ten percent of planetary systems.
A Large 5 Percent
Though 5 to 10 percent of billions of planets is still a large number of planets with large moons (the size of the Moon is so large in comparison with the Earth, it leads some to speak of the Earth and the Moon as a Earth-Moon system, or a double planet).
However, this is an important issue when discussing, speculating, on alternative extrasolar biological, psychological, theological and societal realities that could exist.
Firstly, how important is a large moon to the rise of, and sustainability of, life on a planet? Some say the moon, by its size, helps protect the Earth from large meteor bombardment. That is debatable. One could actually argue that the Earth protects the Moon more than the Moon protects the Earth since the Earth is larger than the Moon, and hence a bigger target and a larger "attractant" due to its larger gravity. Also, one could also argue that the Moon also increases the chances of meteors coming close, by adding its gravitational pull (think of the Earth and Moon as one system: add up the gravitational attraction it would have on passing meteors). Additionally, while the Moon is large, it is separated by 250,000 miles. The further away you hold a large shield from you, the less of a shield it becomes. When a meteor heads towards the Earth, the Moon would have to be pretty much in the direct path. Remember, the gravity pull of the Earth is several times larger than the Moon's - in a tug a war between the Earth and the Moon over a meteor, I would lay bets on the Earth "winning."
So while the Moon may give the Earth some protection, it may not give us as much as we think. But, it may have been enough. Let us say that without the Moon's protection, we would've experienced only one additional large meteor impact over the history of the planet to date. That one additional impact would easily change the course of sentient development on Earth - most likely delaying it by millions of years. And quite possibly changing the final face of the sentient creature that did eventually evolve.
Secondly, the size of the Moon does help keep the Earth from being too wobbly on its axis. This helps keep the seasons from being overly dramatic, which would make it more difficult for complex life to arise (though, I would argue, not impossible, but probably would prolong its rise, and thus delay the rise of sentient life).
Thirdly, the Moon affects the tides on Earth. A smaller moon would have a much smaller effect, and without a moon, there would be an even small effect (the Sun would have an effect, but only 1/3 of the effect the Moon presently has). The tides have profound effects on the Earth - mainly from erosion which helps mix chemicals, especially for the young Earth (when the Moon was closer and had a much more dramatic effect on the tides).
By the way, did you know that this tidal affect is also responsible for pushing the Moon slowly away from us? The Earth is rotating faster (essentially 29 times faster) than the Moon orbits around it. This causes the high tide to move ahead of the Moon. Recall that a high tide is a bulge in the oceans of the Earth facing the Moon (and directly opposite of it). This bulge has mass, and thus gravity; this added gravity tugs on the Moon - but since it is slightly ahead of the Moon (due to the Earth rotating faster than the Moon orbits) - this tiny gravitational tug pulls on the Moon, accelerating it. The result of this constant tug is that the Moon is slowly accelerating, and thus spiraling away from the Earth. As time passes, the Moon will get further and further away (though as it gets further away, the Moon will have lesser effect on the tides, which will thus mean that the resulting acceleration would decrease - but not to zero, the Moon will continue to spiral away). So for those that remark at how miraculous it is that the Moon is the same apparent size in the sky as the Sun need to recall that it hasn't always been that way, nor will it stay that way (plus the Moon isn't exactly the same apparent size as the Sun - but it is very close to it).
Another result of the above is that the energy for the acceleration of the Moon comes at a cost to the Earth: it's angular momentum decreases. The result for this is a lengthening day. Some calculate that billions of years ago the Earth spun much faster, and without any moon, a full day today would be around 8 hours instead of 24. This faster spinning Earth could also have much stronger winds as well. Life would look a bit different on a planet that had stronger winds, a shorter day, and smaller (but more frequent) tides.
Not having a moon would definitely have affects on the culture and theology of a sentient race. Think of how strongly our Moon plays into many of our primitive theologies and myths. But another consideration is how the dark skies would affect the alien race as well: darker skies would mean more attention paid to the stars, but I think an even stronger effect is that having a large moon so close to us may have encouraged us as a species to think about exploring space sooner than we would've otherwise. Without a moon, there is nothing close enough to explore - for thousands of years, the planets were just thought of as wandering stars. The only other physical body was the moon. Without the Moon, it would be quite some time before we would realize there are other physical bodies besides the Earth. Without a moon, there would not be that stepping stone that we enjoy - going to the Moon is far easier than going to Mars. It gives us a chance to learn, experiment, and gain experience before heading off to more arduous and more difficult explorations.
One way a habitable planet may have some of the benefits of a large moon without having a moon orbiting it, is for the planet to be a large moon itself, and circling a gas giant. While that gas giant would surely attract more than its share of meteors, the orbiting moons are so very small compared to the parent planet, that most would probably miss the moons. Not all, of course, but no planet or moon is totally safe from meteors.
Let's say a planet about the size of the Earth was circling a planet the size of Jupiter. There would definitely be tides! And as we see with our own system, giant planets are like a solar system themselves: ringed by many captured moons. A habitable "moon" circling such a gas giant would have many near by physical neighbors to visit, explore, and colonize (or base stations on). Such a civilization may be more even encouraged to explore space than our own.
Gas Giant Theology
And what a god the gas giant would probably play in their primitive theologies! Would they look up in the sky filled, at times, completely by the gas giant, and think "is that heaven" or "is that hell?"
Actually, that may be a downside - orbiting such a large planet would mean periods where the sun would be blocked by the gas giant, and day and night would both be dark (with part of that darkness without even any nighttime stars). Though the times the moon was in front of the gas giant (between the planet and the sun), the night would be ruled by the reflected sunlight off of the planet's cloud tops. Life has adapted to life above the arctic and antarctic circles here on Earth, with their months long days and months long nights, so life could easily adapt to orbiting a gas giant - though the orbiting moons may well have such fast orbiting periods (for one thing, as big as Jupiter is, it is still much smaller than the sun) that these periods of full light and full darkness would be short - Io orbits Jupiter in under two days, while Europa orbits in under 4 days. Leda and Himalia have the longest orbiting periods for a Jovian moon: just over 238 days for Leda and 250 for Himalia. Nights (where the sun is totally blocked by Jupiter) on those moons are probably around one or two months long (haven't worked out the exact numbers; the distance from Jupiter is a factor - a theoretical moon with an orbit of 250 days at 600,000 Km from Jupiter would have a longer day long nights than another moon with the same orbit, but at 11,480,000 Km away. The shadow of a planet is not a cylinder, but a cone, it gets more narrow the further out it goes).
The Phase Lock Waltz
Another thing to consider is if the orbiting moon is in phase lock with the planet - the rotation period the same as its orbital period, thus ensuring one side always facing the planet it is orbiting. Our Moon is like this - we only see one side of the Moon from Earth. Those living on the far side of the orbiting moon would never see the gas giant (well, if they stayed near the equator. If they moved some distance north or south of the equator, they would begin to see the top, or bottom, of the gas giant over the horizon). They would have a long day, followed by a long night. Those on the near side would always face the gas giant. Most of their daylight would be the reflected light off the gas giant, though as the moon orbited the planet, those living on the near side would get short glimpses of the sun - it wouldn't rise high in the sky, however. And then they would experience a starless night (though for the same reason they would get a short glimpse of the sun before or after night, they would get a short glimpse of a star filled night sky after or before night).
Phase Lock Theologies
It would be interesting to entertain what kind of mythologies would arise on the near side and on the far side of such a world. For the near side, a massive god before them, and a bright god that flirtatiously appears once during the short year, and a star filled sky half a year later. For the far side, the bright god of the day, but this looming god just over the horizon, peaking over. And then there's the many small moons whizzing by, some below, some above the world - what to make of them? Also, would such a world figure out sooner than we did that the universe doesn't orbit around the habited world?
Another downside to orbiting a gas giant could be the intense radiation belt surrounding the giant planet. Io, a moon of Jupiter, orbits closer to the Jupiter's cloud tops than the Moon orbits the Earth. Jupiter has massive radiation belts, and Io cuts through them, causing Io not only to be bathed in high levels of dangerous radiation, but creating huge currents of electricity that flow along Io's own magnetic field (bathing the moon in auroral glow).
Another downside is that such a "moon" would have greater tides. The closer it circles the gas giant, the greater the tidal forces. It is thought that the tidal energies experienced by the moons orbiting Jupiter and Saturn are keeping the cores of the moons heated. There is direct evidence of geological activity at work on many of the moons. For instance, it is thought that tidal forces are what are causing the volcanic activity on Io, and is one of the mechanisms thought to create liquid oceans under the frozen surface of Europa (also a moon of Jupiter).
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"Solar System." Jet Propulsion Lab. 25 November 2007. <http://www.jpl.nasa.gov/solar_system/>
University Of Arizona. "Astronomers Say Moons Like Ours Are Uncommon." ScienceDaily. 22 November 2007. 25 November 2007 <http://www.sciencedaily.com/releases/2007/11/071121184530.htm>