Using the Transit Method for SETI Detections

Every study using transit methods to detect objects around other stars is looking for planets. But a paper by Luc Arnold (Observatoire de Haute-Provence, France), soon to be published in The Astrophysical Journal, suggests that the same methods could be employed to find artificial planet-sized objects in orbit around stars. Arnold sees this as a possible SETI ploy, for transits of multiple objects could be used to emit signals that might be detected by other civilizations.

What would such objects be? Giant solar sails, perhaps, or huge low-density structures of other configuration built purposely as a means of interstellar communication. Arnold’s work inevitably recalls Freeman Dyson’s 1960 Science article “Search for Artificial Stellar Sources of Infrared Radiation,” which developed the idea that would later be known as a Dyson Sphere, an artificial cluster of rotating objects the size of a planetary orbit that would collect almost all the solar energy available and create a vast habitat for life.

But Arnold falls back on Jill Tarter’s suggestion that advanced technologies might try to send signals that would be discovered by other civilizations in the course of their normal astronomical observations. So he is developing a new spin on ‘optical SETI,’ while noting that the light-curves of objects from spheres to triangles and even more exotic shapes will have their own distinctive signature, even as multiple objects could send a ‘message’ whose timing and number would announce the willingness of their makers to communicate.

Upcoming missions like Kepler and the European Corot may be able to detect such objects as they look for planetary transits. “Transit of artificial objects also could be a mean for interstellar communication from Earth in the future,” Arnold concludes. “We therefore suggest to future human generations to have in mind, at the proper time, the potential of Earth-size artificial multiple structures in orbit around our star to produce distinguishable and intelligent transits.”

A preprint of Arnold’s paper “Transit Lightcurve Signatures of Artificial Objects” can be accessed at the ArXiv site.

The study of Dyson Spheres remains intriguing. None has as yet been observed (the paper to consult is Bradbury, R.J. “Dyson Shells: A Retrospective,” which appeared in The Search for Extraterrestrial Intelligence (SETI) in the Optical Spectrum III, 2001 Proc. SPIE Vol. 4273, pp. 56-62). But it is also true that some stars do display an excess of infrared that has not been explained. The most likely cause is a hitherto unknown natural process, but the data also fit a possible Dyson signature.

Dyson sphere visualized

Image: A Dyson Sphere would consist provide a vast amount of habitable space, while taking advantage of almost all the solar energy available. Credit: Steve Bowers.

That work was done at the University of California at Berkeley. Charles Conroy (working with SETI@Home chief scientist Dan Werthimer) determined that a Dyson Sphere would radiate with an excess temperature of about 300 degrees Kelvin, which would translate to surplus radiation at the 12 micron wavelength. Using a list of candidate stars, each one billion years of age or older (those whose protoplanetary disks would have dissipated, thus eliminating a possible source of excess infrared), Conroy found 33 stars whose infrared radiation seemed excessive in the 12 micron range.

Followup studies using the SETI resources at Berkeley yielded no unusual radio emissions or light signals, leaving the mystery of the excess infrared unsolved. You can read more about Conroy’s work with Dyson sphere candidates in this article by Amir Alexander.

The original paper on these objects is Dyson, F.J. “Search for artificial stellar sources of infrared radiation,” Science 131, pp. 1667-1668 (3 June 1960). The Bradbury paper mentioned above, “Dyson Shells: A Retrospective,” offers refinements to the Dyson concept, an analysis of earlier work, and extrapolations on new signatures for optical SETI study. Bradbury is particularly valuable in discussing the distinction between an all-encompassing Dyson ‘sphere’ and a Dyson ‘shell.’

Maybe the term ‘swarm’ is even better: In a later letter to Science, Dyson noted that his concept of a ‘sphere’ had been misunderstood: “The form of ‘biosphere’ which I envisaged consists of a loose collection or swarm of objects traveling on independent orbits around the star. The size and shape of the individual objects would be chosen to suit the inhabitants. I did not indulge in speculations concerning the constructional details of the biosphere, since the expected emission of infrared radiation is independent of such details.”

A New Take on Artificial Intelligence

An intelligent computer that can operate autonomously is the heart of any interstellar robotic probe. In an important sense, it would be the probe, running its systems, adjusting its course, repairing damage, conducting experiments and choosing the future direction of its own research. We’re a long way from such autonomy, but a new company called Numenta may be laying some of the groundwork. The firm plans to use the theories of Jeff Hawkins, inventor of the PalmPilot and co-founder of Palm Computing and Handspring, to create hardware that can think and learn like a human brain.

Jeff HawkinsHawkins’ theories appeared in his recent book On Intelligence, and surfaced again at the PC Forum conference in Scottsdale AZ, where he explained his plans in some detail, as discussed in a recent story by Erick Schonfeld in Business 2.0. Hawkins believes the brain’s basic function is to store patterns, creating a model of the world that is constantly being used as a reference that can predict what will happen in the future. From the article:

The key point for Hawkins is that the brain does not just store static patterns. Rather, it stores sequences of patterns over time. Just as important is that there’s a constant feedback loop passing information in both directions between the higher regions of the brain, where past sequences are stored, and the sensory regions, where new sequences of patterns are coming in. So the brain is always comparing the old sequences to the new sequences to predict what will happen next. In a sense, it is always calculating probabilities based on this constant flow of sensory data and stored memories.

Hawkins calls this memory system Hierarchical Temporal Memory. Translated into software, such a system is not programmed in the traditional sense but trained — the HTM system discovers the underlying patterns in its sensory input over time.

Numenta is an attempt to turn theory into software, using a formalism called Belief Propagation. “Belief propagation,” says the company’s Web site, “explains how a tree of conditional probability functions can reach a set of mutually consistent beliefs about the world. By adding time and sequence memory to each node of the tree, belief propagation can be morphed to match Hawkins’ biological theory.”

Centauri Dreams‘ take: Hawkins’ Redwood Neuroscience Institute has developed a software program that allows computers to recognize line drawings even when they are reversed or otherwise altered. What is exciting here is that the software relies not on brute computing power but a theory of how the human neocortex works that, if workable, could lead to artificial intelligence breakthroughs in relatively short order. Applications in machine vision, language translation and robotics may flow from all this, though Hawkins cautions that commercial products are still several years away.

Photo credit: Asa Mathat

Looking for Life Around Red Giants

Should we narrow the search for life-bearing planets to Sun-like stars? The answer may be ‘no’ if we take age into account, according to a new study from an international team of astronomers. Stars well into their red-giant phase may have actually revived outer, icy planets to offer them a chance at developing living ecosystems of their own. This happens because stars become brighter as they get older, pushing their habitable zones deeper into any planetary system they possess.

The study considered the aging process of stars having the same mass as the Sun, and also considered stars with 1.5 and 2 times its mass. “Our result indicates that searches for life-giving worlds outside our solar system should include planets around old stars,” said Dr. Bruno Lopez of the Observatoire de la Cote d’Azur, Nice, France. Lopez is lead author of a paper on this research that is to appear in The Astrophysical Journal.

Solar habitable zoneThat puts more than 150 red giants within 100 light years on a list of possible targets for study by missions like Terrestrial Planet Finder, which will search for life signatures in the atmospheres of exoplanets. And it extends in interesting ways our recent discussion of the Circumstellar Habitable Zone (CHZ) within which life is able to form. That zone has usually been defined as the area within which water can exist as a liquid. The new research shows how flexible the habitable zone concept can be as the host star changes.

Image: A sun-like star grows into its red giant phase, increasing in size and luminosity. Energy in the form of heat can now reach a once-frozen and dead moon. The icy surface quickly melts into liquid water, filling in old craters with warmer seas. The stage is now set for the possible formation of new life. Credit: NASA.

In order for life to form on worlds around red giants, a lengthy period of stability is required. The earliest fossils on Earth are some 3.5 billion years old, although life probably evolved earlier, its traces obscured by the planet’s changing geology. But assume one-half to one billion years for the emergence of life and the numbers look promising. Planets between 2 and 9 AU from a solar-mass star may have a window of up to two billion years for life to emerge as their sun’s habitable zone grows.

“The temporal transit of the habitable zone does not appear incompatible with the possible duration for the development of life,” said co-author Dr. Jean Schneider of the Observatoire de Paris, France. And Dr. William Danchi of NASA’s Goddard Space Flight Center, a co-author of the paper, speculated that microbes from inner worlds might make their way to outer planets in such a system, allowing life to take hold even when the red giant phase proved too swift for its normal development.

More on these speculations can be found at this GSFC page, which also contains interesting animations of changing habitable zones. The paper in question is Lopez, Schneider and Danchi, “Can Life develop in the expanded habitable zones around Red Giant Stars?” available in preprint form at the ArXiv site.

‘Deep Impact’ Mission in Cruise Phase

NASA’s Hubble, Spitzer and Chandra space telescopes will all be watching in July when the Deep Impact spacecraft releases its impactor module (about the size of a coffee table) into the path of onrushing comet Tempel 1. Deep Impact’s flyby module will be watching, too, as the impactor creates a crater that may be anywhere from two to fourteen stories deep, releasing cometary dust and ice and exposing underlying materials that have remained unchanged since the formation of the Solar System.

Now in the cruise phase of its flight, Deep Impact has been through a test of its autonomous navigation system, and its high gain antenna is operating nominally. A mission status report provides some details about the early stages of the flight, when critical subsystems were put through their paces:

Another event during commissioning phase was the bake-out heating of the spacecraft’s High Resolution Instrument (HRI) to remove normal residual moisture from its barrel. The moisture was a result of absorption into the structure of the instrument during the vehicle’s last hours on the launch pad and its transit through the atmosphere to space.

At completion of the bake-out procedure, test images were taken through the HRI. These images indicate the telescope has not reached perfect focus. A special team has been formed to investigate the performance and to evaluate activities to bring the telescope the rest of the way to focus. Future calibration tests will provide additional information about the instruments’ performance.

Deep Impact impactorAboard the spacecraft are a high-resolution camera and infrared spectrometer, a medium resolution instrument (MRI), and a duplicate camera on the Impact Targeting Sensor, all of which will record the collision between the impactor and Tempel 1. The two will collide at approximately 37,000 kilometers per hour (23,000 mph).

Image: This is an artist’s rendition of the flyby spacecraft releasing the impactor, 24 hours before the impact event. Pictured from left to right are comet Tempel 1, the impactor, and the flyby spacecraft. The impactor is a 370-kilogram mass with an onboard guidance system. The flyby spacecraft includes a solar panel (right), a high-gain antenna (top), a debris shield (left, background), and science instruments for high and medium resolution imaging, infrared spectroscopy, and optical navigation (yellow box and cylinder, lower left). The flyby spacecraft is about 3.2 meters long, 1.7 meters wide, and 2.3 meters high. The launch payload has a mass of 1020 kilograms. Credit: NASA/JPL.

Dr. Michael A’Hearn of the University of Maryland, College Park, Md., added, “We are very early in the process of examining the data from all the instruments. It appears our infrared spectrometer is performing spectacularly, and even if the spatial resolution of the High Resolution Instrument remains at present levels, we still expect to obtain the best, most detailed pictures of a comet ever taken.”

Out onto the Wine-Dark Sea

“From time to time, alarm has been expressed at the danger of a ‘sensory deprivation’ in space. Astronauts on long journeys, it has been suggested, will suffer the symptoms that afflict men who are cut off from their environment by being shut up in darkened, soundproofed rooms.

“I would reverse this argument: our culture will suffer from sensory deprivation if it does not go into space. There is striking evidence of this in what has already happened to the astronomers and physicists. As soon as they were able to rise above the atmosphere, a new and often surprising universe was opened up to them, far richer and more complex than had ever been suspected from ground observations. Even the most enthusiastic proponents of space research never imagined just how valuable satellites would actually turn out to be, and there is a profound symbolism in this.

“But the facts and statistics of science, priceless as they are, tell only a part of the story. Across the seas of space lie the new raw materials of the imagination, without which all forms of art must eventually sicken and die. Strangeness, wonder, mystery, and magic — these things, which not long ago seemed lost forever, will soon return to the world. And with them, perhaps will come again an age of sagas and epics such as Homer never knew.”

— Arthur C. Clarke, from “Space and the Spirit of Man,” an essay that first appeared in Voices from the Sky (New York: Harper & Row, 1965) and is now available in the author’s Greetings, Carbon-Based Bipeds! Collected Essays 1934-1998 (New York: St. Martin’s Press, 1999). The latter is indispensable for anyone seriously interested in interstellar flight and the broader cultural issues it raises.

On a more current note, it was fitting (and true to Clarke’s point) that David Southwood, the European Space Agency’s director of science, should read a poem when he needed to sum up the Huygens team’s exhilaration at landing on Titan. The poem was ‘On First Looking Into Chapman’s Homer’ by John Keats, from which this excerpt:

Then felt I like some watcher of the skies
When a new planet swims into his ken;
Or like stout Cortez when with eagle eyes
He star’d at the Pacific—and all his men
Look’d at each other with a wild surmise—
Silent, upon a peak in Darien.