The Recipe for Life-bearing Planets Gets More Complex

The creation of life on our planet was a long, drawn-out affair, taking more than a billion years of chemical and biochemical processing to accomplish after the planet formed some 4.5 billion years ago. Technically, life began some 3.8 billion years ago in some life-friendly oasis on the planet, meaning that the correct conditions were there for some chemically-rich soup to react to something like an influx of heat or a zap of lightning, producing the first living things.  There’s enough ambiguity in there that just about anybody can come up with a theory about what happened (including some pretty jaw-dropping ones about LGMs, travelling deities, and so on), but the scientific consensus (based on verifiable research) is that the primal ooze finally combined in ways that led to the first life forms, and from there it was evolution all the way, baby.

Before all this happened, though, the planet had to form, and it had to do it in the right place. There’s the rub.  If a planet forms too close to its star, its surface gets broiled. Mercury’s a good example here — its surface is alternately flame-roasted and then chilled as it rotates on its axis only 69 million kilometers from the Sun. Get too far away from the Sun, say out in the realm of the gas giants, and it’s too cold for a hard-body planet (i.e. rocky) to form life.

Distance isn’t the only characteristic you have to consider, however.  There’s also a little thing called “tides” — and I’m not talking simply about the ocean tides we experience here on Earth, although they’re part and parcel of the same phenomenon.

When two bodies interact with each other, gravitational interactions can push and pull on their surfaces, creating tides — and that also heats them.

Jupiter’s moon Io shows an extreme case of tidal heating — gravitational interactions between Jupiter and this tiny moon and its sibling moons Europa and Ganymede cause the surface to bulge up and down. This also heats Io’s interior, and the end result is a volcanic moon.

Tidal heating between a star and its planet (or even a planet and its moons) can drive plate tectonics. Earth has plates, is heated from within, and also has a “tidal” relationship with the Moon. Our planet’s “basement” is basically made up of seven major plates (and several smaller ones) and the continents and oceans ride along on top of them. (For more about plate tectonics on Earth, go here or here.).  Among other things, tectonics  keeps excessive carbon dioxide from accumulating in a planetary atmosphere. If it hadn’t performed this service on Earth, we might have a deadly greenhouse atmosphere like the one at Venus.

A group of scientists at University of Arizona is looking into the role that such tides play on planets and what influence they may have on whether life could evolve on rocky planets around other stars. Brian Jackson, Rory Barnes and Richard Greenberg of UA’s Lunar and Planetary Laboratory gave a paper at the Division of Planetary Sciences meeting in Ithaca,  New York, and in it they say that tides can play a major role in heating terrestrial planets. Such tides could create scenes of unbelievable hellishnesson rocky alien worlds that would be livable if conditions were better. And tidal heat can work in reverse, creatiing conditions favorable to life on planets that would otherwise be unlivable.

What this means is that as astronomers search out worlds on other planets, they might need to examine exoplanets in great detail to see if tidal heating (from their stars or interactions with possible moons) is playing a role in their livability factors. Recently there have been so-called “super Earths” discovered around other stars. These planets are somewhere between two and ten times as massive as Earth. If they really ARE Earthlike (meaning that they’re rocky bodies around the size of the Earth or bigger) then it’s possible that tidal heating from interactions with their star or nearby moons may be great enough to melt them, or at least produce volcanism at a level that we see at Io. This would make them pretty poor prospects for being life-bearing planets, and they’d be more like  “super-Ios.”

The more massive a planet is, the greater the effects of tidal heating will be on its surface and interior.  This means that the most easily detectable super-Earths could be dominated by volcanic activity, which is one of the big conclusions that the University of Arizona team came to in their research.  So, the first Earth-like planets found are going to be the most easily spotted, and thus they’ll be big. This means they’ll probably going to be strongly heated and have big volcanoes.

And as astronomers find Earth-like planets in what they cal,l the “habitable zone” around other stars, those planets may well NOT be habitable if they’re gobsmacked by tidal heating.

On the other hand, if a planet is smaller than it should be, or maybe lies outside the habitable zone, it could still support life if it is heated by tidal interactions that could cause outgassing of volatiles (gases, ices) that enrich a planet’s atmosphere with the right stuff needed for life. Tidal heating also can generate sub-surface liquid oceans on water-rich rocky planets that would otherwise be frozen, just as tidal heating is believed to warm a sub-surface liquid water ocean on Jupiter’s moon Europa.

Also, tidal heating could produce enough heat to drive plate tectonics for billions of years, long enough for life to appear and flourish.

So, for those of you keeping score at home, the ingredient list for life is getting more and more refined. And, when we look at other planets in our search for life, we need at where the planet exists in relation to its star, how long it’s been around, whether it can supply the water, warmth, and “food” for life, and now, whether or not it is subject to the correct application of tidal force.