Kepler and the Rare Earth hypothesis

kepler.jpgIn 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.

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Comments (13)

  1. Darren S. A. George says:

    When I hear about the “Rare Earth” hypothesis, I think of praeseodymium.

  2. andy says:

    These transit searches offer potential for other kinds of discoveries: for example, once long-period planets start being detected in transit, the whole area of “exomoons” (at present limited to theory and a possible detection of a moon-forming disc around the planet Fomalhaut b) begins to open up.
    Then again, maybe I’m just weird in finding the diversity of planetary systems more interesting than the attempt to discover an Earth-clone.

  3. Hugh Troy says:

    I fervently hope that when the data from Kepler comes in that ward & Brownlee’s hypothesis is well and truly blown out of the water.

  4. David Marjanović says:

    Do be careful to separate “life” and “complex life”. Ward & Brownlee write that the former may well be even more common than generally suspected, while it’s the latter that requires a long and growing list of rare conditions and events.

    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.

    Well, you could also see it as a continuation of the Copernican Revolution: not only are we not special, we are not even normal! We’re a misbegotten fluke!
    But, seriously, we’re talking about science here. I don’t see where philosophy even enters the question. :-|

    a possible detection of a moon-forming disc around the planet Fomalhaut b

    Wow!!! This ought to have been all over all media!!! I’ll check out — oh, here it is. Well, we’re talking about a Jupiter-size-class planet the temperature of Neptune, so life is highly unlikely, even though probably not impossible if one of the moons is similar enough to Europa or so.

  5. Frederic Hould says:

    How can the volume of a planet be deduced from a dimming of the star it travels in front of, if one considers that a large planet may travel on a path not exactly between the Kepler observatory and its parent star, then dimming its parent star partially, (while a smaller planet traveling exactly in front would dim its star more)? Is it all to do with the orbit period?
    On the same topic, are all planetary systems in the Milky Way on the same orbital plane, or do they differ much, and if so, we must assume a portion of all planets are undetectable by the Kepler Observatory because these out-of-plane planets will seldom cross the direct line between the Kepler Observatory and its related star. What portion of planets do we estimate are thus undetectable, or do we know at all?

  6. Christopher Gwyn says:

    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.
    Uh….no; and for the same reason. There are a huge number of factors (insolation, impacts, and so on) that go into whether a planet ‘like Earth’ is able to support life, and a huge variation in the habitats that life will prosper in, or at least survive in, on Earth. But most of the life on Earth is single-celled, and single-celled life is the most ubiquitous. Planets like Earth, in that they support complex multi-cellular lifeforms in complex ecologies, can be very very rare in a universe where single-celled life (as well as viruses, etc.) is pervasive. Ward and Brownlee refer to an Earth with complex multi-cellular lifeforms in complex ecologies as ‘Rare’ because of the large number of things that have to ‘go right’ in order for those ecologies to occur, but they fully recognized that any planet with intermittent liquid water would almost certainly have native life.
    Those planets with single-celled life and no multi-cellular life may be very good candidates for a ‘helping hand’….I wonder how many ‘fixer-uppers’ there are in our part of the galaxy….

  7. Chris Rowan says:

    Ward and Brownlee refer to an Earth with complex multi-cellular lifeforms in complex ecologies as ‘Rare’ because of the large number of things that have to ‘go right’ in order for those ecologies to occur
    So they say. But currently, there is no way of knowing if the events that they consider important to the development of complex life are, in fact, as pivotal as they claim. Correlation does not equal causation, especially when your sample size is one.

  8. Nick says:

    Frederic:
    Planetary systems are in all sorts of random orientations; this is why only a very small percentage would line up with our line of sight, and this is why most exoplanets thus far have not been discovered using this method, and this is why Kepler is looking at a huge swath of stars – with enough stars, some are guaranteed to be lined up with our line sight.

  9. rickflick says:

    Fascinating stuff. I think Im going to need another lifetime or two to get to the bottom of this. 8-)
    Scientists have to be patient, but others don’t. My brother says he believes that ets routinely visit us, my sister insists she saw et’s spaceship when she was a kid. I’m the only one in the family who’s willing to wait. Another 62 years might be nice.

  10. grolby says:

    But, seriously, we’re talking about science here. I don’t see where philosophy even enters the question. :-|

    This complaint doesn’t make a lot of sense to me. As scientists, I see everything we do as being deeply philosophical, not in the sense that we are ‘doing’ philosophy, but in the sense that what we do, how we deal with data, how we interpret models, how we consider some conclusions to be appropriate and others inappropriate, even how and why we make certain arguments, should be guided by an internally consistent set of philosophical principles. Some of these are universal, such as the principles that guide the scientific method and the urge to be skeptical. In other philosophical issues, there is considerable variation and argumentation, and that’s where a lot the back-and-forth and disagreement between scientists on big questions appears. The major breaking point on the question of complex extrasolar life right now IS philosophy. Philosophy guides our hypotheses and influences the direction in which we search for answers. In cases like this, it is enormously influential, and fuels a lot of debate. What we want, as scientists, is for the breaking point to be the data (though you can never escape philosophical differences). The plan is for Kepler to reduce the philosophical dimension to the question.

  11. Lab Lemming says:

    The kepler FOV is about one millionth of the stars in the galaxy, and the geometric detection probability for an Earth around a sun is around 0.2%, so multiply the total detections by 500 million for a whole galaxy estimate- assuming the galactic core and rim behave the same as our suburbs.

  12. Great summary of Kepler’s significance to the Rare Earth hypothesis. Ward and Brownlee’s proposition that complex life is probably rare is a reasonable null hypothesis, given our current dataset of N = 1. But as we gather more detailed information about the occurrence of worlds that can support liquid water and possibly life, considerations like Rare Earth will hopefully move from free speculation to something more constrained and predictive.
    In my own research I try to use Rare Earth as a starting point for investigations that might better inform our perspective of where complex lift could physically develop… with a goal being elucidation of potential non-Earthlike settings where prebiotic synthesis and metabolism are thermodynamically and physically feasible. Basically, to try and understand whether other situations can in principle support complex ecosystems, or if Earthlike conditions really are as exclusively optimal as Rare Earth suggests.
    In some ways the Rare Earth Hypothesis could be dismissed as an argument from ignorance, in that it depends on our lack of data concerning other types of biospheres. But I think it’s actually a very useful proposition, because it forces us to consider more fully the potential for complex biospheres under more exotic – but quantitatively describable – prevailing conditions. It forces us to think harder about the issue, basically, and to put our nickels down on whether some planetary outcomes are more plausible than others.