The era of complex planetary systems is here. The solar-type star HD 10180 is just 39 parsecs away and hosts at least 5 extrasolar planets; this is the richest planetary system yet discovered. The ESO 3.6 meter telescope in La Silla, Chile made the observations of HD 10180 with the precise High Accuracy Radial velocity Planet Searcher (HARPS) spectrograph for six years to confirm their findings.
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
Physicists are planning to create a laser so powerful that it will tear apart spacetime, well, it won't destroy spacetime, but it will tear particles out of the vacuum with dire consequences for the laser. I first made the statement that 'lasers will tear apart spacetime' when referring to future ambitious projects planned by the Extreme Light Infrastructure (ELI) when I was writing for Lindau Nature on 50 Years of Lasers. It is a bold claim, perhaps a colorful interpretation of the physics, but none the less recent experimental and theoretical work indicates that there is a fundamental limitation on the attainable intensity of lasers.
The vacuum that makes up spacetime is teeming with virtual particles that are inconsequential to low energy phenomena. Particles and their antiparticles, such as electrons and positrons (e- and e+), can be produced in pairs under certain conditions when energy is converted into matter. When enough energy is focused with laser pulses the peak electromagnetic field strength of the laser is enough to pair produce e- e+ pairs which will cause an avalanche-like quantum electrodynamics (QED) cascade which will instantly disrupt the laser pulse.
A paper recently submitted to the arXiv (this paper hasn't been peer reviewed yet) by A Fedotov, N. Narozhny, G. Mourou, and G. Korn, Limitations on the attainable intensity of high power lasers, outlines how there is critical QED field strength that the authors state is unattainable and it is creeping up on experiments very fast. The idea that lasers could create particles or that there is limit in nature on the magnitude of the electromagnetic field is not new. Neils Bohr first suggested that a maximum field of Es=2Ï€m2c3/eh was physically unrealizable from theoretical considerations and the vacuum production e- e+ pairs by a massive electromagnetic field was hypothesized in 1950 by J. Shwinger (who later received the Nobel prize for fundamental work in quantum electrodynamics). On the experimental front the limits to the laser was hinted at some time ago. In 1997 the Stanford Linear Accelerator (SLAC) collided what was then the worlds most powerful laser with electrons from the Stanford accelerator. The photons from the laser were boosted to produce backscattering gamma-ray photons which interacted with the oncoming laser beam. The energy of the laser and the gamma-ray photons was so high that real particles of matter and antimatter were created from the vacuum.
In this recent paper the authors argue that simultaneous pulses of lasers could reduce the maximum Es field that may occur by two orders of magnitude to a mere ~1025W/cm2. The new analysis relies on the production of e- e+ pairs at the Shwinger limit, but also takes into account the effect of secondary effects which the SLAC experiment did not have enough energy or speed of pulses to observe. Optimistically the ELI project or the XFEL project could reach the maximum laser intensity within the decade. A super high power facility is planned by the ELI with intensities of ~1029 W/cm2 and the European XEFL, pictured above, will create extremely short and intense X-ray laser flashes they may also reach this limit by 2014.
The authors point out that the critical difference with future experiments and previous analysis of electromagnetic field strengths produced by lasers is that the most powerful lasers will play not only the role of the target, but will also be responsible for the acceleration of any new particles created. Thus at high laser intensities electron and positron pairs will be created and will immediately be accelerated to relativistic energies and emit hard photons, which will in turn produce new e- e+ pairs. Thus a back-reaction, an avalanche of new particles, will develop from the vacuum by short focused laser pulses. The authors show that creation of even a single e- e+ pair may result in complete destruction of the laser field.
This year is the 50th anniversary of the first successful laser built by Theodore Maiman and so it is rather fitting that we may have come full circle from the first laser to a theory of the ultimate laser. Yet, hurtles remain in the theory with respect to actually calculating the back-reaction of particles within the laser field (my hunch is that the particle avalanche may act to defocus some energy thus restoring the maximum Es QED field to a an immense energy...) and in experiment with respect to actually building the ultimate laser.
Colliding Particles is a series of films following a team of physicists involved in research at the new Large Hadron Collider (LHC). It is a creative documentary series (catch more episodes here) which is really well done and worth watching. Colliding Particles follows Gavin, Jon and Adam and their project, code name Eurostar, in their attempts to find the elusive Higgs Boson. One of the main aims of the the LHC is to discover once and for all whether the Higgs actually exists or not, and ‘Eurostar’ might just hold the key to finding out:
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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.
