A Habitable Zone by any other name…
… Is still not guaranteed to support life. That’s one thing that people need to keep in mind, as NASA’s Kepler Mission begins its long vigil. Kepler will examine around 100,000 stars in one region of the sky continuously for at least three and a half years, and using that accumulated data astronomers will get a much better sense of what solar systems are really like in this part of our Galaxy. Kepler data help settle a major question in astronomy and geology… how common are Earth-like planets? Right now we’d simply don’t know, because most extrasolar systems found so far are not like ours. Not even remotely. Many have hot gas giants the mass of Jupiter or greater orbiting very close to their sun. Many have gas giants that spiral in long, elliptical orbits that would have long since thrown any inner Earth-like planets into their sun or into deep space. A few systems have smallish, probably rocky planets orbiting dim, feeble suns where the Habitable Zone – the region around the star that receives enough light for liquid water to exist on a planet’s surface – almost grazes the tiny sun’s surface.
Part of the problem with detecting other Earths, up until now, has been an issue of detection limits. The best techniques available to astronomers up until now – now being the age of Kepler – were not technologically capable of finding Earth-sized planets around stars of the Sun’s mass. Kepler has that capacity, so in a couple of years we’re likely to have some better answers to the question of how many rocky worlds there are in the Galaxy.
But being in a Habitable Zone (HZ) does not mean a world is inhabited. Our own system teaches us this lesson. Venus is in our Sun’s HZ, technically, but Venus’ bone-dry surface is hot enough to melt lead. Earth, not that much further from the Sun than Venus, is a biospheric paradise. In the case of Venus, too much atmospheric CO2 early in its history led to a runaway greenhouse effect that dessicated and hellified the planet… but if Venus had a somewhat lower atmospheric CO2 content to start with, it might today have oceans. That detail wouldn’t be obvious from the distance of a few light years. Observed from another star system, Venus would appear to be a planet of slightly less than one Earth mass, in a circular orbit, with an atmosphere bearing strong spectroscopic fingerprints of CO2. The temperature at Venus’ cloud tops isn’t nearly as infernal as its surface, so from several light years away we wouldn’t be able to tell that Venus is dead and searing. That’s a lesson we need to keep in mind, as Kepler data begins to roll in: be cautious about raising the “life” flag.
Chris Rowan, who writes the Highly Allochthonus blog, makes this point very well.
Kepler can identify rocky planets with Earth-like masses that orbit at the right distance from their parent stars that liquid water can potentially exist on their surfaces*. However, what it can’t do is tell us whether they have actually developed into life supporting worlds; and in our own solar system, Venus (about 25% closer to the sun than the Earth, about 80% of its mass, and yet a heat-sterilised volcanic hell-hole) provides a cautionary tale about getting too carried away if and when we start finding extra-solar ‘Earths’.
Kepler will be able to tell us if life is obviously present (if the angles of observation are just right) because light filtered through a planetary atmosphere can carry absorption spectra of gases indicative of life, such as molecular oxygen or ozone. Such traces, coupled with infrared planetary emissions indicative of atmospheric water vapor in a not-too-hot atmosphere, would constitute a very strong case for life. But that’s an ideal scenario… and most planets are not going to be ideal by those criteria. Which raises the question… are those criteria too narrow?
In Rare Earth, by Peter Ward and Donald Brownlee, the authors posit a hypothesis that complex life is probably very rare in the universe. Ward and Brownlee argue that life itself may be common at a microbial level, but complex life such as plants and animals – that is, big multicellular life forms – might end up being very unusual. This view rests on the premise that complex life can only occur where conditions are very much like Earth’s: our type of planet, our type of sun, a moon like Earth’s, and many other criteria. The Rare Earth hypothesis is a kind of null hypothesis – a relatively sound bet – based on what we know so far about the universe. That knowledge base is likely to change in the near future, as Kepler and later missions such as the Terrestrial Planet Finder assimilate more and more data.
We could find, as we peer further, many new categories of world where life is possible… in fact I think that outcome is likely. For example, life doesn’t need to live on a planetary surface – it could develop in the cyclopean sea-depths of an Ocean Giant, a planet like Neptune but with low enough levels of hydrogen to allow liquid water to condense. Such a world could possess deep oceans of warm water, yet circle very far from its sun… far outside the local HZ. Similarly, Earth-sized worlds orbiting gas giants bigger than Jupiter could support life outside the local HZ, if they possess sufficient tidal and radiogenic heat to keep water liquid and drive internal volcanic activity. Fundamentally, life appears to need the right combination of chemicals, enough energy to drive a metabolism, enough elbow room to develop genetic diversity, and sufficient time to allow evolution to run its course. It’s likely there are many ways to get those features together… including ways that will surprise us.