Notes & Queries 11/24/08

by Paul Gilster on November 24, 2008

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.

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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.

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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.

tacitus November 24, 2008 at 12:42

I asked Greg a couple of questions in the comments about the prospect of planets at Alpha Cen. He put the odds at about 60% — i.e. slightly better than even — and he estimates that we should know, one way or the other, within fives years, whether there is an Earth-sized planet or larger in orbit.

The term “planetary engineering” might be a bit misleading to some — it doesn’t have to mean building something on a planetary scale. An alien civilization examining Earth’s spectrum within the past century or so (allowing time for the light to travel to their sensors, of course) would show signs of civilization, namely atmospheric pollutants and, of course, billions of street lights on the night side of the planet.

ljk November 24, 2008 at 14:36

Just to pick a nit: Isn’t Active SETI another term for METI, which is
Messaging to ExtraTerrestrial Intelligences? Unless ISO was sending
infrared beams into the galaxy, it was probably performing (if this
happened at all) only Passive SETI, which is just listening or looking
depending on what devices one uses.

Can Spitzer do some Infrared/Optical SETI?

By the way, ISO was not the first spacecraft to conduct SETI.
Several other satellites have in the past, including the Soviet Union’s
Mars 7 probe in 1974.

In any event, I am just glad to see that our modern technology is
allowing SETI to happen more often in more refined ways. I am
also glad to see that SETI is being conducted beyond the radio
spectrum, especially when it comes to Dyson Shells.

Now if we can get SETI to get past their justifiable issues with UFOs,
we might even see some serious searches for alien probes in our
Sol system.

tacitus November 24, 2008 at 19:22

Now if we can get SETI to get past their justifiable issues with UFOs, we might even see some serious searches for alien probes in our
Sol system.

That’s an interesting idea, and certain has some scientific merit (even as the longest of longshots) but I really don’t think it’s realistic to expect anyone in the astronomy community, let alone SETI, to spend any time or money looking specifically for alien probes.

Our best chance of finding one is through serendipity and astronomers’ innate desire to find something new and newsworthy. For example, one of them detects an asteroid in an unusual orbit and it’s unusually dark/bright or it seems to be spinning far faster than normal. Should something about an observed object pique their interest like that, then I would bet there’s an excellent chance that if it is an alien probe, it will eventually be confirmed as such. It’s all about getting that first, lucky break.

kurt9 November 24, 2008 at 20:33

Why is Alpha Centauri B considered better than Alpha Centauri A? Also, is there a list of the nearby stars (within 20-30 light years) that lists metallicity and hypothesized age?

Adam November 25, 2008 at 4:12

Hi kurt9

Alpha Cen B is a better target for a radial velocity search because it has less photospheric noise and a better gravitational response to orbitting planets due to its lower mass (0.9 Sol vs 1.1 Sol for Alpha Cen A).

And yes there is a list like you describe. Do a Google search and then trawl around – I’ve got a text file of that sort of data from just such a webpage, but I don’t have the URL handy.

andy November 25, 2008 at 7:55

kurt9: Alpha Centauri B is probably a better bet for detecting low-mass planets because it is likely a more stable star. Early K-type stars are good for RV detections: the low mass planetary systems around HD 69830 and HD 40307 both have K star hosts. G-type stars like Alpha Centauri A are more jittery which makes detecting low-mass planets harder.

Ronald November 25, 2008 at 9:58

“Three are particularly promising for future study: HD4308, HD190360, and 70 Virginis”.

I am a bit puzzled and surprised by that: HD190360 is in the process of leaving the main sequence and becoming subgiant, 70 Virginis is probably also in that stage, but also extremely bright (over 2.9 times solar luminosity) and variable. I could think of better stars to invest in.

“A star of a certain age, in other words, may have been around long enough to allow an extraterrestrial civilization (…) to emerge ” True, but by this age it may be time for a civilization to consider moving.

Question: “Alpha Centauri b “…overwhelmingly best star in the sky for detecting habitable planets from the ground and on the cheap” Fascinating, but if so, why isn’t this being done yet?

@Kurt9: “Also, is there a list of the nearby stars (within 20-30 light years) that lists metallicity and hypothesized age?”

I have been looking for reliable metallicity and age data as well. For metallicity, try http://vizier.u-strasbg.fr/
For age, for nearest sunlike stars I found a great recent publication (PDF): ‘Improved Age Estimation for Solar-Type Dwarfs Using Activity-Rotation Diagnostics’, by Eric E. Mamajek and Lynne A. Hillenbrand.

Anybody else?

ljk November 25, 2008 at 10:03

There are a few scientists like Scot Stride of JPL who have
looked into how we might search for alien probes in our
Sol system:

http://www.astrobio.net/news/article919.html

http://cic.setileague.org/cic/v1i2/s3eti-ata.pdf

http://www.sunstar-solutions.com/SETV/SL_editorial4a.pdf

Here are some earlier papers on SETI projects looking for
ETI visitors in our neighborhood:

Freitas, R. A., and Valdes, F., A Search for Natural or
Artificial Objects Located at the Earth-Moon Libration Points,
Icarus, vol. 42, pp. 442-447, 1980

Freitas, R. A., and Valdes, F., A Search for Objects Near the
Earth-Moon Lagrangian Points, Icarus, vol. 53, pp. 453-457, 1983

Freitas, R. A., and Valdes, F., The Search for Extraterrestrial
Artifacts (SETA), Acta Astronautica, vol. 12, no.12, pp. 1027-1034, 1985

There is also Allen Tough’s Invitation to ETI Web site, which
assumes ETI might be tapping into our Internet to study humanity
(which may explain why we have not heard from them yet) and
may even contact us through it some day:

http://www.ieti.org/

ljk November 25, 2008 at 10:40

Two more references on earlier papers analyzing the possibilities
of finding alien probes in our Sol system:

Papagiannis, M. D., Are We Alone or Could They be in the
Asteroid Belt?, Q. J. R. Astro. Soc., vol. 19, p. 277, 1978

Papagiannis, M. D., An Infrared Search in Our Solar System as
Part of a More Flexible Search Strategy, in The Search for
Extraterrestrial Life: Recent Developments, M. D. Papagiannis
(Editor), Reidel Pub. Co., Boston, Massachusetts, 1985

Dave Moore November 25, 2008 at 16:10

“The Alpha Cen binary has twice the total mass of the Solar System, and more than two thousand times the total angular momentum.”

This got me thinking: has anyone done a simulation of the formation of Alpha Centauir A & B? I asume that considerable work has been done on the formation of binary stars, and if someone has done a run on Alpha Centauri, the results would give us a much better idea on such things as the gas densities and masses of the nebulas around the individual stars and hence a much better
idea of what to expect in the way of planets.

Since we are likely to characterize the limits of planetary occurance around both stars to a very high degree in the next few years, it would at least be a good test for the binary star formation models.

ljk November 25, 2008 at 18:13

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