The "Learning Curve"...
Science has long known the study of light waves traveling from a distant star can reveal some amazing things - like the discovery of distant planets. But can this same science be applied to discover artificial transitting bodies? According to exciting new research - It can. As a body transits a star, the "curve" of the light wave helps to determine its shape and size, eventually revealing their number. We know that there are many large, natural spherical objects orbiting distant stars, but this same "curve" could help us to determine constructed devices. Is it possible that these were placed into orbit by advanced civilizations as a "signal" to others?
The answer lay in study...
In April 2004, Luc F. A. Arnold, (Observatoire de Haute-Provence CNRS 04870 Saint-Michel -
l'Observatoire, France) was working on a transit generated by a saturn-like planet when he had an idea. Could this same principle be applied to look for transitting bodies that were extra-terrestrial in nature? Says Luc, "I discussed of it with several colleagues who found the idea interesting." This method of detecting planets by a transit was originally proposed by Otto Struve in 1952 in a paper on spectroscopy. When a planetary body passes in front of a star, we can observe a luminosity drop, determine the orbital period and garner the orbital distance. The radius of the planet - relative to the star radius - is deduced from the luminosity drop, but what would happen if it was not a orbiting planet? A collection of artificial bodies would produce lightcurves easily distinguishable from natural ones. If only one artificial object transits, detecting its artificial nature becomes more difficult, but if the shape is not spherical - the light curve will differ. For example, the triangular object, 2-screen or louvered 6-screen satellite will show an entirely different signature. If multiple artificial objects were detected transitting - this could possibly be a form of signaling the presence of other intelligent life - one with a sky coverage equal to the range of the laser pulse method.
Right now, (SETI) programs focus on the search for radio or optical laser pulses. An alternative is to look for artificial planet-size bodies which may exist around other stars. Since they would always pass in front of their parent star for a given remote observer, there is a strong possibility they can be detected and characterized using the transit method. Perhaps they might be huge structures built by advanced civilizations, like very lightweight solar sails - or massive very low density objects constructed for the purpose of interstellar communication by the means of transit. If only one orb-like body is involved, spectroscopy and future high angular resolution imaging will provide our answers. What we do know is a planetary transit lightcurve contains fine features due to the object shape - such as planet oblateness, double planets or ringed planets. "The sphere is the equilibrium shape preferred for massive and planet-size bodies to adapt to their own gravity, one can consider non-spherical bodies, especially if they are small and lightweight, and transitting in front of a dwarf star to produce a detectable signal. Non-spherical artificial objects - like triangles or more exotic shape - have each a specific transit lightcurve. We also must consider the case of multiple objects: A remarkable lightcurve would be created by free-flyers transitting their star successively in a distinguishable manner. At each period, we would observe a series of transits whose number and timing would claim its artificial nature and will of communication."
The bulk of Luc Arnold's work - just accepted for publication in the "Astrophysical Journal" - has been to prove through computer simulation the effects of different and multiples shapes and show these differing light curves. During a simulation, the stellar flux is zeroed out in pixels and compared to the normal flux in a strip (treated as an ellipse) containing the transit area only. This non-planetary object transit is then fitted with a planetary transit using a Powell algorithm. For example, the equilateral triangle produces a "priori" - a transit lightcurve far different than an orb. It generates a symmetrical lightcurve, as opposed to an oblate object whose curve is asymmetrical. Should it rotate, the residual lightcurves will show additional modulation when set against a gradient - such as the limb - showing
sudden slope variations during ingress or egress. Says Luc, "I computed the theoretical lightcurve of an artificial object (a triangle for example) and then try to fit it with a planetary -natural- transit, through an iterative 'trial and error' algorithm. The triangle lightcurve cannot be exactly be fitted by a planetary transit, and the algo ends with non-zero residuals, i.e. a non-zero difference between the two lightcurves. This difference is the 'personal' signature of the artificial object."
For a symmetrical two-fold object, the lightcurve would be phase-shifted, a total deviation for that of a sphere. The residuals would be three times as great as in the case of the triangle. Of far more impact would be a louvered object, an elongated structure, which would produce undulation in its longer period of ingress and egress. It would, in effect, cause multiple "transits" making it far more detectable. The nature of these oscillations could very well be considered a sign of intelligent device. If several objects were spatially arranged in groups to ingress a star in a mathmatically constant manner, these drops in the lightcurve could clearly represent a type of message - the language of science.
With the computer simulations perfected, Luc knows what a natural or artificial transitting body should look like in a lightcurve - but has science observed a transit on a particular star? "Up to now, there is only one transit lightcurve obtained with a very good accuracy - the transit observed by Tim Brown et al. 2001 for HD 209 458b with Hubble Space Telescope." says Luc, "they found the lightcurve could be fitted with a spherical body to within the measurement accuracy." This type of information provides Luc Arnold with the model he needs to compare with non-spherical bodies, but "If more observations of HD 209 458b transits are collected with HST in the coming month, then the overall photometric accuracy will be increased and a new best fit search could be done."
In June 2006, Luc's vision may be realized. COROT (a space mission approved by the French Space Agency CNES, with a participation of Austria, Belgium, Brazil, Germany, Spain, ESA and ESTEC) will be dedicated to stellar sismology and the the study of extrasolar planets - the first approved space mission dedicated to these subjects. The spacecraft will consist of a ~ 30 cm telescope with an array of CCD's as detectors an will monitor the lightcurves of well chosen stars. The incident beam after the main mirror will be two-fold, with one side dedicated to sismology, and the other to planetary transits whose sector will be two CCD's. The lightcurves of tens of thousands of stars will be monitored in two colors and the overall potential of COROT is to detect several tens of Earth sized planets.
What does this kind of new technology mean to researchers like Luc Arnold? "I consider two kinds of artificial objects. If you have a collection of free flyers, then just observe thousands of stars (like COROT will do) and wait for transit. Probably you will discover a lot of 'natural' transits. These space missions will give an accuracy of down to 0.01% but 1% could be sufficient if objects are big enough to reach this level of occultation of the parent star." According to his research a single transit of an artificial body would require that kind of accuracy, but a multiple transit would have a more relaxed photometric accuracy. "1% photometry is within the capability of thousands of amateur astronomers equipped with CCD." Chances are far greater that a communicative civilization would favour a series of objects over a single non-spherical one for signaling their presence. Transits of opaque objects are achromatic, putting them within detectability of CCD over the entire spectrum - while a laser pulse would require a dedicated instrument. It's a powerful argument that artificial transits could be every bit as effective as laser pulse.
As Luc points out, this type of research may well be within the realm of the contributing amateur astronomer. The search for signs of extra-terrestrial intelligence and the confirmation of other planets continues on. "For the moment, there is no project to apply this idea. If it the idea turns into a specific (SETI) observing program, a number of collaborations would be welcome! But in fact, several planetary transit searchs are already operational, and the multiple transit case could be discovered within the course of these
programs ! -- maybe tomorrow!" While tomorrow might seem like an impossible dream, Luc knows differently: "Maybe tomorrow in the case of multiple 1%-deep transits: Several programs looking for extrasolar planetary transits are already in operation (OGLE, etc.), After 2006 when COROT will be launched for the detection of shallower transits, we'll have the possibility to analyse in detail the shape of the transit lightcurve."
Luc Arnold's work has already been submitted to the SETI institute. For the rest of the citizens of planet Earth, we await the results. Will tomorrow show us a possible energy collection, communication or study device put into orbit by another sentient species? If we consider what we know of astronomy to be a basic "truth" throughout the Cosmos, then a discovery of this magnitude could be the biggest news of them all... "Assuming we are sure to have detected an alien artefact in a transit lightcurve, my opinion is that we should consider it as a clear 'Hello world... We are here!' adressed to the whole Galaxy!"
And we ought to be considering waving back....
I would like to thank Luc Arnold for his constructive advice as well as exciting research. We wish him success in the future! Light speed... ~Tammy Plotner