Sunday, October 10, 2010

A 100 percent chance of life?

Karl has written asking about the recent discovery of Gliese 581g, a potentially Earth-like planet:
Planetary Astronomer Mike, are there friendly aliens on that planet waiting for us to greet them?
In particular, he was referencing this article and highly optimistic quote:
"Personally, given the ubiquity and propensity of life to flourish wherever it can, I would say that the chances for life on this planet are 100 percent. I have almost no doubt about it," Steven Vogt, professor of astronomy and astrophysics at University of California Santa Cruz, told Discovery News.
I'll try to put this all in a little perspective. I've been in Pasadena this past week at the largest planetary conference of the year. This single quoted statement has been the source of a whole lot of jokes this week during the conference's off-hours, e.g. "I'd say there's about a 100% chance I'd like fries with that."

Now, acting on sources that are entirely a combination of hearsay and ad hominem arguments, I know some people here who know Vogt, who tell me that he is absolutely the kind of guy who would never, ever say something like this. So, if you're feeling even a little bit generous, it's not too much of a stretch to say that the press fumbled this one. This problem is rampant enough throughout astronomy that NASA actually offers seminars on how to speak to the press in a manner that, among other things, teaches you how to keep them from misquoting you.

A prime example of such an offense is the whole meme about "back in the 70's, scientists used to think we were about to have an ice age!" In truth, of 49 papers published between 1965 and 1979 which predicted global climate change, 42 predicted global warming while only 7 predicted a coming ice age. Why the ice age theory caught on in the mainstream press is not immediately obvious. Perhaps the authors of the 7 ice age papers were just more vocal (notably, 4 of those 7 papers were from just a single author who in later years became a very vocal global warming skeptic), but just as likely is that an imminent ice age simply sounds more sensational than a little spot of toasty weather. Either way, it's hurt the long-term credibility of climate science...but I'll save that for another rant.

As for the whole idea that the planet exists within the "habitable zone", I generally try to avoid that term. The whole concept of such a zone rests around the idea that there's a narrow range of locations at which liquid water can form. While location is one factor which plays into the equation, the properties of the planet tend to be much more important.

Planetary scientists talk about this in terms of the "equilibrium temperature" of a solar system body. Essentially, given the rate at which a body is absorbing heat from the Sun while also dissipating heat back out to space, you can find the temperature you'd expect it to be. Generally this simple equation only works well for big rocks which have no internal energy source or heat redistribution mechanisms. i.e. asteroids, moons, and dead planets like Mercury. Even this very simplified form of finding the temperature has an inherent dependence on the properties of the body, though - namely, its "albedo".

Albedo is just the reflectiveness of an object. Earth's average albedo is 36%, which means it reflects 36% of the incoming sunlight out to space, while 64% is absorbed and actually goes into heating the planet. Using only the equilibrium temperature, we'd expect Earth's average temperature to be -18 degrees Celsius, which is below the freezing point of water. What's been neglected in this formulation is the importance of the infrared absorbers in our atmosphere, better known as the greenhouse effect. Although they're relatively minor constituents by abundance, water vapor, carbon dioxide, and methane are enough to raise Earth's average temperature 33 degrees to a happy 15 degrees Celsius. This is warm enough for liquid water to exist and life to flourish.

The matter gets worse for the case of Venus. You may have seen it hanging low in the Western sky lately, bright enough to tell you it has a very high albedo. In fact, its albedo is 72%. Although Venus is 30% closer to the Sun, working with only equilibrium temperatures we'd actually expect Venus to be a few tens of degrees *colder* than Earth because it reflects so much more of the incoming sunlight. This makes the idea of a habitable zone very difficult to swallow since a planet closer to the Sun is actually predicted to be colder. This too, however, breaks down when you consider the greenhouse effect. Venus' atmosphere, almost 100 times thicker than Earth's and made of 95% carbon dioxide, is enough to raise the temperature nearly 500 degrees Celsius.

On the other side of the matter, there's Jupiter's moon Europa. At a distance 5.2 times greater than the Earth to the Sun, we'd expect this to be a very, very cold place - and at it's surface it most certainly is, with an equilibrium temperature of -170 degrees Celsius. At this location, we'd never expect Europa to be inside the habitable zone. Drill through its approximately 100-km thick ice shell, however, and you'll find an ocean so deep that it contains more liquid water than all the oceans on Earth. Again, the neglected heating term here is the internal energy of Europa generated from the massive tidal forces the moon feels from Jupiter's gravity. The case of Europa has opened astrobiologists to the possibility that Earth-like planets may not be the only place to look for E.T., but moons of large gas giant exoplanets. as well. The field of "exomoons" is fertile ground which has only just recently been opened up.

One more consideration: the method used to detect this specific extrasolar planet - "radial velocity" - can only tell us its minimum possible mass, not its actual mass. Radial velocity is based on the planet gravitationally tugging on its parent star, and we observe the doppler shifts of the star only along our line of sight. In other words, we only see the parent star's movement towards us or away from us. Given the specific amount of doppler shift that we observe, if the planet's orbit were oriented edge-on to our line of sight (i.e. a 90-degree inclination), then its mass should be three times that of Earth. If, however, the orbit's orientation is closer to face-on (i.e. a zero-degree inclination), then most of the tugging would be in the "plane of the sky", with only a small percentage being the to-and-fro motion we observe with doppler shifts. In this case, the total amount of tugging is quite a bit more than what we can observe in the doppler shifts, meaning the planet's mass would be substantially larger than three Earth-masses. (Non-trivial problem for the math-heads out there: find the expectation value of the inclination of a random planet's orbit. It's a neat answer.)

A final note here concerning the timing of this announcement: the Kepler mission results are expected soon. This spacecraft uses a very different technique than the radial velocity method outlined above. Rather, Kepler uses the technique of "transits". It stares at a set field of stars, watching for a specific kind of subtle dip in the brightness of one of them which can only be caused by an orbiting planet eclipsing its parent star. There are several nice things about this technique. First, we know its orbit must be nearly edge-on to produce an eclipse, which it turn means knowing its exact mass using a follow up radial velocity technique, not just its minimum mass. Second, by studying exactly which colors of light are absorbed more than others, we have a spectral fingerprint which allows us to identify the exact constituents of the planet's atmosphere, which in turn gives us a better handle on the planet's true temperature rather than just its equilibrium temperature.

Up until now the Kepler science team has been very hush-hush about the results coming in, but the rumor mill has been churning with whispers that several Earth-like planets are expected to be announced in a matter of months, and most astronomers I know are waiting with bated breath. With such an announcement looming on the horizon, announcing an Earth-like planet now is good opportunity to tap into this building excitement.