Punch up the numbers, add more than fifty planets to the count of those that potentially could harbor life, bringing the total almost to 700. These new exoplanets were discovered by the HARPS (High Accuracy Radial Velocity Planet Searcher) at the La Silla Observatory in Chile. The announcement, at the Extreme Solar System conference in Grand Teton National Park, Wyoming (USA), includes 16 so-called ‘super-earth’ planets – those that are Earth-like in rocky composition but are larger in size. This is the largest batch of exoplanets yet discovered.
HARPS is the most successful planet hunting device to-date, with more than 150 credited to its analysis. It searches among 376 Sun-like stars, those relatively near to our own Sun, to detect as many low-mass (that is, smaller) planets as possible with the radial velocity technique. This approach uses the Doppler effect, which displaces a star’s spectral lines (color spread), to detect the presence of planets in orbit around a sun. At the moment, with relatively limited sensitivity, HARPS can only detect fairly large planets – roughly the size of Neptune in our own Solar System. Even with that limitation, HARPS has so far revealed that at least 40% of the suns it has observed have at least one Saturn-size planet. If this number holds up, it will mean yet another expansion of the estimate for planets – billions in just this galaxy alone. What is the probability that some of these planets have life?
Many probability calculations, like tossing a coin, are quite straightforward because initial conditions are known. For example, a coin has two sides and the probability of a tossed coin coming to rest standing on edge approaches zero. No such luck with exoplanets and life. The knowns, such as they are, are based solely on our own experience. As far as we know, life and water are inextricable. Life requires some kind of energy source, which is as far as we know comes from solar energy, chemical energy, or geologic energy. Pretty much every planet in a solar system has one or more of those energy sources, so that doesn’t do much for the probabilities. There is this thing about a ‘Goldilocks Planet’ – not too warm, not too cold, but just right for life. That’s probably valid, but the bandwidth of success for life could surprise us (either way). Finally there is the size of the planet, with the supposition that too big means too much gravity and no life, or conversely too small implies no atmosphere or, well we just can’t see the small planets at light-years distance anyway.
What we know about conditions for life is limited to what we know from Earth. Predictions about what kind of exoplanets will have life are, for the time being, limited to extrapolating from that knowledge. With the discovery of fifty new potential life-holding planets, the odds for the life elsewhere in the universe increase. On exactly what kind of planets – other than a correspondent to Earth – we don’t know. HARPS and its descendents will continue to refine that knowledge. One of these fine days…or years…