Cylons and Smelloscopes: False Positives and False Negatives in the Search for Extraterrestrial Life

Are there planets outside of our solar system? Is there life on other planets? Is life on other planets like life on Earth? These are questions that astronomers, astrobiologists, chemists, and geologists are trying to answer with current experiments. In order to answer these questions we must observe distant planets and we must determine what life on those planets may be like. Detecting extrasolar planets is tricky enough, but imaging what alien life is like may well be stranger than science fiction. Yesterday evening I attended a lecture sponsored by the Seattle Astronomical Society given by Shawn Domagal-Goldman titled Cylons and Smelloscopes: False Positives and False Negatives in the Search for Extraterrestrial Life. It was an excellent lecture and filled with interesting topics. Shawn touched on the philosophical problem of defining life in the broadest of senses (is Number Six alive?) and he pointed out that the verification of life on distant planets faces technical challenges and basic scientific limitations (a smelloscope sure would help!).

Dimitar Sasselov set off minor shock waves of gossip and rumors in the media and astronomy communities when claimed that the NASA Kepler mission had found 140 Earth-like planets a few weeks ago during a talk he gave at the TED Global 2010 meeting in Oxford. The media thought we had found earth's twin, but astronomers knew that Sasselov had exaggerated the situation. Sasselov had to post a redaction of sorts on the Kepler blog in order to clarify what he said. What he should have said is that the Kepler mission will find and verify the presence of potentially habitable planets and that Kepler currently had 140 candidate extrasolar planets. The candidates are not confirmed and so a pessimistic outcome could be that half of the candidates will be false. The difficulty in finding extrasolar planets or life is fraught with false positive and false negatives. A false positive is a detection that seems like exactly what you were looking for, and maybe it is, but the detection was either bad data or you were looking for the wrong thing. A false negative is a detection which you conclude is not what you were looking for, but either your data was fouled or your detection threshold was too constrictive.

How do we find planets outside of our solar system? There are at least five methods to find planets: Doppler shift, astrometric measurement, transit method, gravitational microlensing, and direct detection. Shawn discussed in depth the Kepler mission that is currently monitoring more than 150,000 stars in the direction of the Cygnus constellation for any signs of extrasolar planets that may be orbiting those stars. So, what method does Kepler use to find  planets? It watches for eclipses! When a planet orbiting a distant star crosses in front of the star some of the light from the host star is blocked. The planet will transit (astronomers often use the world transit not eclipse for exoplanets) in front of the the star once an orbit and thus the period of orbit can be determined. A secondary eclipse also occurs when the day side of the planet is blocked by the star. The video below illustrates the whole process.


Yes, there are planets outside of our solar system. The current exoplanet detection count is 473 and counting; you can watch that count go up over at Planet Quest. Kepler may double that number, but more importantly it has the ability to find earth size planets. Most of the planets found to date have been large, hot, and inhospitable to most kinds of life anyone can fathom.

How do we detect signs of life on other planets? Astronomers look for bio-markers in the planet's atmosphere. Bio-markers are molecular signatures of certain compounds that could not be produced by non-biological process; bio-markers indicate that dynamic non-equilibrium chemistry is present on the surface of that planet. Astronomers can measure the light emitted as a function of wavelength, the spectra, that a planet emits to determine the molecular species present in the atmosphere. For example the Earth's atmosphere has the spectral signature of water which means it has conditions in which life as we know it can thrive. If we found an earth size planet that had water in its atmosphere which wasn't too hot we would say we had found a habitable planet. If we found oxygen or ozone (03) in an atmosphere it would almost certainly mean life was present on the planet because 03 is quickly removed from atmospheres through standard geological processes such as oxidation of iron, but it may remain present in an atmosphere if it is continually replenished by the photosynthesis mechanism of algae and plants. One of the topics Shawn talked about in his talk and a focus of his research was the problem of being certain that non-biological processes are not creating the oxygen rich atmospheres. The runaway greenhouse effect combined with the photo-disassociation of carbon dioxide can produce oxygen in a similar way to biological life. This is where the smelloscope would be useful: ozone along with other non-equilibrium species such as nitrous oxide and methane in specific ratios would be the scent we are looking for. Bio-signatures were not present on the early Earth. In fact the Earth probably looked a lot more like Venus. The diagram above shows that Venus, Earth, and Mars all have distinct spectral features that tell us about their atmospheres. The hardest part of looking for bio-signatures is that we do not have a telescope that is sensitive enough. Trying to take the spectra of a planet orbiting a bright star is like trying to tell the color of the wings on a gnat hovering around a spotlight on the moon. Like a baseball player holding up one hand to block the sun from his eyes as he focuses on the ball an occulter or star shade working with an existing telescope in space would do the trick. The current funding situation in astronomy is dire, but there is hope that a mission called New Worlds will one day work with the James Webb Space Telescope to allow us to take a closer look at planets which Kepler is finding.

Is there life on other planets? We don't know and it may be a more complicated question than is suspected. There is a bias towards looking for life that is similar to what life on Earth is like. There is a bias towards looking for life that alters its host planet's atmosphere significantly enough to detect it with telescopes on earth. There is a bias towards looking for life that is alive as we define it. These biases may lead to false negatives in the search for life, but as Shawn pointed out the possibilities for life to exist are much grander than our imaginations so we do the best we can. Also, despite the difficulties for finding life on other planets and the gulf between the public's perception of aliens and reality scientists are taking this as a serious venture. Scientists from diverse fields are coming together to forge a path forward. One such project is the Virtual Planet Laboratory which employs scientists in fields such as geology, chemistry, biology, and astronomy. The Virtual Planet Laboratory is a team of scientists who are building computer simulated planets to discover the likely range of planetary environments for planets around other stars so we can better look for habitable planets and distinguish between planets with and without life. However, we can't even discern with certainty the presence of life on Mars or Europa at this point, what hope do we have for finding life on distant planets?

I think there is a lot of hope and I am not alone in that sentiment. I don't search for planets or life in my research, but I think that the search for life, particularly intelligent life, is a fundamental question. It is natural to wonder about the Universe on the grandest of scales, but it is wise to be concerned with what happens on the smallest of scales because that is where we will find life. We expect to find the unexpected in the search for life.


References:

Beichman, C. A., Woolf, N. J., & Lindensmith, C. A. (1999). The Terrestrial Planet Finder (TPF) : a NASA Origins Program to search for habitable planets JPL publication