Kepler and the Rare Earth hypothesis

In this week’s geology podclast, we discussed the recently launched Kepler mission, and how the media brouhaha surrounding it has often failed to distinguish between ‘Earth-like’ in the sense of planets that share Earth’s mass and orbital period, and ‘Earth-like’ in the sense of ‘life-supporting’ – and the fact that Kepler is geared to finding the former rather than the latter. Kepler is designed to look for the signs of planetary transits – dips in a stars’ brightness that occurs when any extra-solar planet moves between us and its parent star. The smaller the planet, the smaller the effect of its transit, and the harder it is for ground-based telescopes to filter out any signal from twinkly atmospheric distortion. By escaping this noise by deploying outside the Earth’s atmosphere, and by observing the same section of the sky continously for several years, Kepler (and the European Space Agency CoRoT mission, which works along similar principles) is well-equipped to find planets with Earth-like masses in hundred-day orbits, rather than the gas giants that dominate the catalogue of known extra-solar planets.
So, 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’. In reality, we’ll have to wait for the likes of the Terrestrial Planet Finder or Darwin to directly identify possible signatures of life on any candidates that Kepler tracks down, by looking at the spectral characteristics of their atmospheres (although big Earth telescopes may also be able to contribute). However, this shouldn’t be seen as a shortcoming. Kepler is designed to fill in a big gap in our present knowledge of solar systems outside our own: by the end of its mission, we’ll have a much better idea of the average number of planets around a typical star, and the distribution of their sizes and orbits. We’ll know whether our solar system, with its inner rocky planets and outer gas giants, is typical or unusual.
Of course, knowing these things does, in a broader sense, have an impact on the question of the rarity or abundance of life (or at least, our type of life) elsewhere in the Universe. As in any question where any answer basically boils down to guessing from a virtually non-existent dataset, opinions on this question range right from ‘it’s everywhere’ to ‘it’s just here’. Advocates of the latter talk of the ‘Rare Earth’: they believe that the emergence of life (or, at least, complex life) is the end product of a sequence of highly improbable events (such as the impact that formed the moon) that effectively make the Earth a rather favoured place in the Universe. Philosophically, I find this a rather unconvincing argument; a last remanent of mankind’s inner child, still determinedly insisting that we are, in some way, the centre of the Universe. But scientifically, I must admit our ignorance: we presently lack the knowledge to even begin to calculate the odds. Once Kepler is done, however, we’ll finally have some relevant information on the rates of small rocky planet formation in reasonably close, stable, orbits. As it turns out, any number much above zero is a blow against the ‘Rare Earth’, thanks to the cumulative effect of mind-boggling numbers. There are maybe 300 billion stars in the Milky Way: even if only 1% of them have rocky planets in the right place, that’s still 3 billion potential Earths. In the light of that number, the possibility that the journey from rocky planet to life-supporting rocky planet depends on a convergence of other low probability events seems much less intimidating; when you have several hundred million goes at the roulette wheel, you’re probably going to luck out more than a few times.
Of course, we’ll have to wait until the numbers are actually in; but whilst Kepler is not going to spot any new Earths out their beyond the Final Frontier, it should provide some solid hints about if, and how often, we eventually will.
*the so-called ‘habitable zone’, although – since this automatically classifies anywhere else as uninhabitable – it’s not a designation I’m particularly fond of.