|The odds of extraterrestrial life existing...
||[Feb. 7th, 2011|02:20 pm]
There's a famous equation that is supposed to determine how likely it is that intelligent life that can communicate with us exists in the galaxy. It's called the "Drake Equation", and here's how it's defined:
All of those variables are outlined on the wiki page, but basically, you take the total number of stars in the galaxy, multiply it by the number of planets around those stars, multiply THAT by the fraction of planets that can potentially support life, then by the fraction of planets which actually do develop life, then by the fraction of planets with life that evolve into intelligent life, then by the number of planets who actively try to communicate, then by the amount of time the civilization exists for.
Basically, we're pulling numbers out of our collective scientific butts.
However, it occurred to me today that there's a much more important question that may go into the Drake equation...you can see that it calls for "planets which can potentially support life". What that means is, are planets in the "habitable" zone, also known as the "Goldilocks" zone, because it's not too close to the sun to burn up the atmosphere, nor too far away such that it receives too little heat.
It gets complex, though, because the habitable zone of stars is variable, based on the size of the star and its output (huge stars like Betelgeuse have amazingly huge habitable zones, but don't last very long, and probably not long enough to develop life, let alone intelligent life).
So basically, we live near an "average" star, Sol. If we assume that its habitable zone is average, too, then we know that it stretches from just inside the orbit of Venus to just inside the orbit of Mars (or maybe outside the orbit of Mars, depending on who you ask). We're smack dab in the middle.
So say we get three "good" orbital "spots" for planets. Does that mean you could say we can plug in that number to the Drake equation? Well, sure, you /could/, but it might be disingenuous...check this out.
There are 8 planets in our star system. There are, however, 130+ known moons. Now granted, lots of these moons are nothing more than glorified asteroids, but a great many are quite large (Ganymede, a moon of Jupiter, is twice as massive as our moon, and larger than the planet Mercury).
A great many of the exoplanets we've been finding have been gas giants, probably because for a long time, we've used gravitational shifting to determine the existence of these planets, but our techniques are getting better. The smallest exoplanet we've found so far is a "superearth" (in other words, most likely made of rock and metal, like our planet) twice the mass of Earth. Everyone gets really excited when it comes to finding superearths, especially if they're in the habitable zone, because it's believed that they're most likely to harbor life.
While that's exciting to me, it's more exciting when I hear of a super-massive gas giant in the habitable zone. While a star like ours gives the ability to have a few good candidate planets, one gas giant the size of Jupiter gives the ability to have several candidate moons, and a planet 10 times the size of Jupiter? How many rocky moons could that support? Dozens, probably.
The idea of a single planet providing dozens of spots for life is exciting. The issue then, is, is the average rocky moon as capable of supporting life as the average rocky planet?
That's definitely not established. We have exactly one data point on life being established on a rocky planet (that's us, by the way). We have absolutely no data on life existing on rocky moons.
The biggest difference between most moons (that we've seen) and most planets (again, that we've seen) is that the majority of moons are tidally locked. What that means is that because of the gravitational stresses on the moon from the planet (and vice-versa), moons tend to always have the same side of the moon facing the planet. That's the way our moon is (The side of the moon facing us is always the same). That's also the way most other moons in the Solar system are, too.
The popular idea that the moon has a "dark side" is wrong. The "back" of the moon is lit half the time, and is dark half the time, and the day / night cycle (which we see as the phases of the moon) is around 28 days. That means that where-ever you are on the moon, it's day for around 14 days and night for around 14 days. As far as we can tell, that's not ideal circumstances for life to develop (but again, only one data point).
In any tidally locked moon, the length of the day is equivalent to the orbital period (assuming there's only one sun in the star system). For a moon like ours, as we said, that's 28 days. Ganymede flies around Jupiter in 7 days, though, so it's not a given that extreme periods for the day/night cycle are necessary.
The question that, in my opinion, the Drake Equation hinges on is whether or not life can evolve on a tidally locked moon. If so, the opportunities for life are vastly improved.