What might make a star particularly interesting from a SETI point of view? Bruce Cordell looks at the question in a post in the latest Carnival of Space, drawing on a JBIS article by Martin Beech (“Terraformed Planets and SETI,” February 2008). The method seems to be to examine the ratio of a star’s age to its Main Sequence lifetime.
Beech does this for 123 stars with known exoplanets, making the interesting point that terraformed planets might throw a particular observational signal in systems with the right ratio. Three are particularly promising for future study: HD4308, HD190360, and 70 Virginis. Pondering all this, Cordell writes:
If habitable planets are discovered near these or similar stars, ebullient Earth-bound astronomers contemplating interstellar voyages will check their spectra, to see if ‘the lights are on’ just in case any ETI’s are home.
A star of a certain age, in other words, may have been around long enough to allow an extraterrestrial civilization not only to emerge but to make its presence known to other observers, either intentionally or through evidence of planetary engineering. Cordell is right that we’ll be ebullient to find such a planet, but at this stage in the game, finding a potentially habitable planet around any star, regardless of age or possible inhabitants, is going to be cause for celebration.
More on Dyson Spheres: Tibor Pacher writes with a pointer to an older proposal to search for Dyson-style engineering using the German ISOPHOT instrument, an imaging photo-polarimeter that was built for the ESA’s Infrared Space Observatory satellite. Here’s the essence of the proposal:
The program will be the first attempt to perform active SETI in the infrared using a spacecraft. A photometric survey, covering 3–60 microns, of several old main sequence stars will be performed in order to assess infrared excesses compatible with the presence of large astro-engineering products like Dyson spheres that emit a blackbody temperature of several hundred K. This survey shall identify candidates for Dyson spheres. In addition, a few objects which are known to show infrared excesses in the 12 or 25 micron IRAS measurements are considered for a detailed photometric investigation. The usage of the ISO satellite is crucial for the success of the program as only ISO currently offers, with its infrared photometer, the high sensitivity that is needed to detect the radiation of cold artificial structures superimposed on the several thousand K blackbody background spectrum of the host star.
As far as I know, the Infrared Space Observatory’s operational phase ended before any observations could be implemented — does anyone have further information? The paper is Tilgner and Heinrichsen, “A Program to Search for Dyson Spheres with the Infrared Space Observatory,” Acta Astronautica Vol. 42 (May-June, 1998), pp. 607-612.
Centauri Dreams readers know all about Greg Laughlin’s hopes to create a dedicated radial velocity search for planets in the Alpha Centauri system. The UC-Santa Cruz planet hunter notes the key factors — brightness, age, spectral type, metallicity, orientation, and sky position — that make Alpha Centauri b “…overwhelmingly best star in the sky for detecting habitable planets from the ground and on the cheap.”
So I’ve been wondering for some time how Laughlin reacted to recent work by Philippe Thébault and his collaborators, work that notes how unfavorable the environment around both Centauri stars is for planets to form. We know there are stable orbits around Centauri b, for example, and the right number of planetary embryos should produce terrestrial-class planets there. But if Thébault’s team is correct, the perturbations produced within this binary system, coupled with gas drag on the planetesimals, create a situation where the planetesimals don’t hold together after they collide because of high collision velocities.
The end of our hopes for the Centauri stars? In a recent post, Laughlin remains cautiously optimistic:
Even when confronted with these results, I’m still cautiously long Alpha Cen Bb. It’s not that I think the simulations are wrong or that there is any problem with the outcomes that they produce. Rather, I don’t think a high gas density in the inner AU of the Alpha Cen B disk is cause for alarm.
Why? Thébault uses a model consistent with the disk that produced our own Solar System, a set of conditions here referred to as the ‘minimum-mass solar nebula’ (MMSN). Adjust the parameters for Centauri b, though, and things begin to change, with embryos forming much further out from the star (Thébault saw areas outside 0.5 AU as hostile to planet formation, making habitable planets all but impossible). Laughlin again:
In a nutshell, I don’t see evidence that the MMSN is of any particular utility for explaining the extrasolar planetary systems that we’ve found so far, and hence I’m not depressed that high gas densities were required for Alpha Cen B to have fostered an accretion-friendly environment. Reconstitute, for example, the HD 69830 protoplanetary disk or the 55 Cnc protoplanetary disk. I’m plain skeptical of the validity of a fiducial MMSN scaling for the disks that orbited the Alpha Cen stars. The Alpha Cen binary has twice the total mass of the Solar System, and more than two thousand times the total angular momentum.
Which gets us back to radial velocity observations, and the compelling need to make them over the long observing runs that will tell us what’s really around the Centauri stars.