Richest Yet Planetary System Discovered: HD 10180

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.

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

Limits on Lasers

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 E_s=2πm²c³/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 E_s 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 E_s QED field to a an immense energy...) and in experiment with respect to actually building the ultimate laser.

Thirty Five Images of Space Helmet Reflections

Cool. Thirty five images of space helmet reflections via 3 Ton Gallery.

Colliding Particles

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:

Hubble Bubble

The Copernican principle holds that humans are not privileged observers of the Universe. Copernicus stated that the Earth is not at the center of the solar system or at any particularly special position in the heavens. Modern cosmology has extended this idea to reason that the earth does not occupy any unique position in the Universe. Modern philosophy of science pushes the principle even further to conclude that every observer (even if they be they little green men) should reason as if they were the most standard observer. However, despite all these humble and rational thoughts it is still tempting to explain certain aspects of modern cosmology that seem finely tuned as consequences of observer selection effects. Namely I am speaking of dark energy or the accelerated cosmological expansion which supposedly could be explained if we occupy a privileged position near the center of a large, nonlinear, and nearly spherical void in mass density. The idea that the region of the cosmos around us could be a void is colloquially known in astronomy as the Hubble bubble. Technically a Hubble bubble is defined as a region of space wherein there is an observed departure of the local value of the Hubble constant from its cosmologically averaged value.

Lets speculate a little further on what it would be like to live in a Hubble bubble. In the standard cosmological model of the Universe the structures we see today like galaxies and clusters of galaxies (and similarly the structures we don't see like the massive dark matter halos the visible matter is embedded in) formed from tiny primordial quantum fluctuations in the early universe. The fluctuations were random variations in density such that locations which were over-dense formed galaxies and those which were under-dense formed voids. It is possible, in fact statistically quite acceptable that there are voids of various sizes in the Universe. These voids would become increasingly under-dense as the Universe evolved and equivalently over-dense regions of the Universe became increasingly over-dense. Inside the void matter would expand outward due to the gravitational pull of matter in surrounding dense regions and thus an observer at the center of the void would see an accelerated expansion of matter outward. Now it is also possible that our entire observable Universe is a Hubble bubble, but that really flies in the face in all of cosmology. It is unfounded, absurd, and really the whole idea of a Hubble bubble may explain dark energy, but is hardly a very good explanation.

The Hubble Bubble is wildly speculative and precision cosmology has almost completely defeated it as a credible explanation. First, as the framework of cosmology has been successful resting on the Copernican principle it seems odd to throw it out now. It is odd and largely misguided. First, the probability of producing a void of necessary magnitude; to mimic aspects of dark energy is extremely small in the standard structure formation models. Second, the probability of an observer being at the center (the only location where the expansion effect would be noticed) is extremely low. Finally, the void would need to be close to spherical to match the observed spatial smoothness (or isotropy) of the universe. These qualitative arguments and many more quantitative arguments from precision cosmology data are laid forth in a recent paper by A. Moss, J. Zibin, and D. Scoot titled Precision Cosmology Defeats Void Models for Acceleration. The abstract follows:

The suggestion that we occupy a privileged position near the center of a large, nonlinear, and nearly spherical void has recently attracted much attention as an alternative to dark energy. Putting aside the philosophical problems with this scenario, we perform the most complete and up-to-date comparison with cosmological data. We use supernovae and the full cosmic microwave background spectrum as the basis of our analysis. We also include constraints from radial baryonic acoustic oscillations, the local Hubble rate, age, big bang nucleosynthesis, the Compton y-distortion, and for the first time include the local amplitude of matter fluctuations, σ₈. These all paint a consistent picture in which voids are in severe tension with the data. In particular, void models predict a very low local Hubble rate, suffer from an "old age problem", and predict much less local structure than is observed.

The paper makes several quantitative arguments against the plausibility any kind of void model for cosmic acceleration by drawing together an impressive amount of cosmological data and technical expertise, however, they don't ever mention the term Hubble Bubble. A 2007 paper by Conley et al. takes the Hubble Bubble paradigm head on: Is There Evidence for a Hubble Bubble? The Nature of Type Ia Supernova Colors and Dust in External Galaxies. In Conley et al. they explore how dust effects the colors of type Ia supernovae because they reason if the dust can be modeled as a purely local Milky Way effect then the supernovae data would actually favor the Hubble Bubble. Of course, despite difficulties the analysis, they find that in their parametrization there is evidence for more than the simply effect of local Milky Way dust implying doom for the Hubble Bubble. So the Hubble Bubble has been burst.


References:

Adam Moss, James P. Zibin, & Douglas Scott (2010). Precision Cosmology Defeats Void Models for Acceleration arXiv preprint arXiv: 1007.3725v1

Conley, A., Carlberg, R., Guy, J., Howell, D., Jha, S., Riess, A., & Sullivan, M. (2007). Is There Evidence for a Hubble Bubble? The Nature of Type Ia Supernova Colors and Dust in External Galaxies The Astrophysical Journal, 664 (1) DOI: 10.1086/520625

Physics & Music

I enjoy very much the intersection of classical music and physics (for example see my posts on the phenomena of lightning and on Quantifying Goethe) so if I was in the UK I would definitely be checking out the ongoing performance lecture about the legacy of Albert Einstein the scientist, the man, and the musician. Music was an important part of Einstein's life and his passion for music is what has inspired me to continue to learn the Viola after a fifteen year hiatus. The show is called Einstein's Universe and it is put on by particle physicist Brian Foster and British musician Jack Liebeck. The lecture tour will be a fusion of science communication and classical music. You can read a bit more about the show over at Physics World here and see a video about it below.

You can catch a Foster and Liebeck perform an arrangement of the Mozart violin sonata in C Major k.296 here.

Upper Bound on Neutrino Masses from Galaxy Surveys

Cosmology not only probes the absolute mass scale of the neutrino but is a completely independent method to test against. In any case, it is imperative to include an accurate prescription for the neutrino in cosmology, as any failure to do so can bias the other cosmological parameters. A cosmological constraint on the sum of the neutrino masses is primarily a constraint on the relic big bang neutrino density Ω_ν. One can relate this density to the sum of the mass eigenstates ∑m_ν as given by Ω_ν= ∑m_ν/(93.14 h² eV). The direct effects of the neutrinos depend on whether they are relativistic or nonrelativistic and the scale under consideration. Neutrinos have a large thermal velocity as a result of their low mass and subsequently erase their own perturbations on scales smaller than the free streaming length. This subsequently contributes to a suppression of the statistical clustering of galaxies over small scales and can be observed in a galaxy survey. The abundance of neutrinos in the Universe can also have a direct effect on the primary CMB anisotropies if nonrelativistic before the time of decoupling (i.e., when sufficiently massive). However, one of the most clear effects at this epoch is a displacement in the time of matter-radiation equality. All these cosmological effects can be used to impose bounds on the neutrino mass. Previous studies have capitalized on these signatures and have started to place sub eV constraints on the absolute mass scale . We utilize the new Sloan Digital Sky Survey MegaZ luminous red galaxy (LRG) DR7 galaxy clustering data  to provide the first photometric galaxy clustering constraint on the neutrino and, combining with the CMB, examine the complementarity of these early- and late-time probes. With an almost comprehensive combination of probes this renders one of the tightest constraints on the neutrinos in cosmology and therefore physics.

Cosmological observations provide independent constraints on the neutrino mass scale provided that a few assumptions (a flat universe with Gaussian and adiabatic primordial fluctuations and a constant spectral index for example) can be made. Compared to the prospects of current-to-next generation particle neutrino experiments (like KATRIN) it may be that astronomical surveys of the cosmic microwave background anisotropies or optical surveys of the large scale structure of the Universe will place the tightest constraints on neutrino masses for some time. Continue reading from the excerpt above written by Shaun Thomas, Filipe Abdalla, and Ofer Lahav on their invited viewpoint article in Physical Review Letters (freely available):Upper Bound of 0.28 eV on Neutrino Masses from the Largest Photometric Redshift Survey.