Tuesday, February 5, 2008

Habitable Moons - Are They Common?

image credit: digitalblasphemy.com
"Thetis Moon" © DigitalBlasphemy.com
Earlier posts discussed the possibility of habitable moons, including how they may increase the habitable zone of a solar system, which would then affect the Drake Equation, giving it a larger number for N (the number of civilizations we can communicate with at this time in the galaxy).

Recent work by Caleb Scharf, Columbia University's Director of Astrobiology, points to the possibility that habitable moons may not be rare - they may even be as common as habitable planets.

Take a look at our own solar system as an example. Our solar system has several moons that, if orbited the sun instead of a planet, would be large enough to be considered planets themselves. Several of them have atmospheres, and at least one, Europa, is almost certain to have liquid water - though recent articles, which will be discussed in later posts, suggest that life can exist in ice, and that, thus, liquid water may not be necessary for life to exist (though it may be necessary for sentient life to evolve).

If our system is not unusual, then it should be common for extrasolar systems to have many moons as well, some of them large enough to have atmospheres and to retain water. As we've seen in previous posts, water is found to be rather common in planetary discs and systems (1).

However, a heat source is needed. And to the rescue comes tidal forces: tidal forces caused by the moon's parent planet which will create internal heat for the moon. This is due to the fact that gravity decreases with distance and the gravitational pull on the near side of the moon is greater than the gravitational pull on the far side. For a moon in a circular orbit, the moon will adjust its shape to adapt to this gravitational differential, and no tidal heating will occur. But for a moon in an eccentric orbit, the gravitational differential will change rhythmically, and the moon will be kneaded like a lump of bread dough (OK, a bit of an exaggeration). This will heat a moon even if it is outside of the solar system's main habitable zone. This increases the areas in a solar system where life can form.

So when we are looking for extrasolar life, we need to look at large moons as well as planets. Right now our technology allows us to detect only large gas giants. However, rapid advances will (possibly as early as this year) allow the ability to detect terrestrial planets. The ability to detect water planets is on the horizon as well. I can not say if the ability to detect habitable moons will exist in the near future, but I would not rule it out.


1. Many chemicals are found in space, including interstellar gas clouds of sugar and of beer!


Browne, Malcom W. "Alcohol-Laden Cloud Holds the Story of a Star." New York Times. 30 May 1995. 5 February 2008. <http://query.nytimes.com/gst/fullpage.html?res=990CE7D81531F933A05756C0A963958260>.

Scharf, Caleb A. “The potential for tidally heated icy and temperate moons around exoplanets.” Astrophysical Journal. 648 (2006) 1196-1205.


Vaks said...

What percentage of intelligent life on Earth is capable of building radios, rockets, and computers?

Defiantly not 1%. That would mean 1 out of every 100 semi-intelligent species could build a radio.

I think it is more like 1 out of a few million/billion.

By simply looking in our own back yard, Earth, we can quickly deduce that highly intelligent life is a very rare phenomenon. Try plugging in .00001% into Drake’s equation and a very different bleak result will exist.

If humans didn't exist how many other species on this planet could build a radio?

I have always been very concerned with this flaw in Drake’s equation.

I believe there could be plenty of life and even basic intelligent life around our galaxy but in regards to highly intelligent life I am a bit concerned in putting a number based on 1%.

I know it is not very intriguing to think that we could be alone (or but a few) out there but perhaps we are truly a blessed species and shouldn’t take for granted our existence.

Mr. David Merchant said...

Good questions! The Drake Equation is rather controversial.

One possible argument is that you only need one intelligent species capable of building the radio on the planet. So out of billions of species on this planet, you only need one to rise up and build that radio. If humans didn't exist, another highly intelligent species would, eventually, arise. It may take millennia, sure, but that's OK as far as Drake's Equation is concerned.

If we look at our own Earth we will see that on this habitable planet the chance of an intelligent species arising is 100%, and the chance of that species building a means of communicating across space is 100%.

Some argue that given the right conditions (water, energy source, somewhat "boring" astronomical environment, and basic elements like carbon) life will arise - that it is a natural process. The building blocks of life are very common in space. Water is being found to be rather common itself.

The main question is intelligent life a natural result - is intelligence a logical result of life evolving over millenniums? Are there evolutionary pressures for intelligence to arise? Or does high intelligence arise only by pure random chance? A fluke? Those are the harder question to answer.

Whatever the answer, I fully agree that we shouldn't take for granted our existence. Over the long span of life on this planet, highly intelligent life has existed for only a small moment. There is some debate that intelligence was beginning to arise with the dinosaurs before they were wiped out and the process toward intelligence began again.

And even if intelligent life was somewhat common, it would most likely be different from us in many ways - culturally, emotionally, psychologically, and socially different. We, as an intelligent species, may be unique.

Finally, we need to keep in mind that the Drake Equation doesn't take into account seeding, "terraforming"/colonization or contamination from space probes: that is, aliens could artificially increase both the instances of life arising on planets, and of intelligent life eventually arising on those planets.

Since it would be difficult to find planets with exactly the same characteristics, the lifeforms artificially created, seeded, inserted, and/or modified would not be the same as the aliens' own lifeform. But the end result would be a higher result for Drake's Equation.