mangoe@umcp-cs.UUCP (Charley Wingate) (08/06/85)
[Post follow-ups to net.astro, please] In article <1363@uwmacc.UUCP> demillo@uwmacc.UUCP (Rob DeMillo) writes: >> As I understand it, the things that make the earth stand out are the >> following: >> 1) It's very small and obviously associated with a star. This makes it >> clear that whatever it is, it's a planet. > >There's a couple problems with this argument: first, the earth is >EXTREMELY close to the sun. Even though the sun is a standard so-so >star, it would take extremely fine resolution on a radio scope >to seperate the angular distance. Even if they could do >that, the earth radiates so little heat (virtually its only radio >source, the stuff we generate ourselves doesn't count, I'm afraid...) >that it would be lost in the heat of the sun...again, seperation is >a problem. You don't have to resolve it out separately. Assuming for the moment that Earth's radio emissions (human-generated) are at all visible, what will be seen is a star with a wierd secondary which is occulted once a year (in the right direction, of course). This alone, I admit, would not make it evident that the secondary was a planet. But wait.... >> 2) In radio frequencies, it is analomously hot, and NOT on spectral lines. >I'm afraid this isn't quite right. The earth, in fact the solar system, >is amazingly boring. It is a rock that generates its own internal heat >(slowly) by nuclear decay of material in the core. The rest of the >heat is reflected. If you are refering to any radio noise that >humans make, we aren't very spectacular either. Our signals, even if >they weren't hampered by the atmosphere, solar winds, noise from >the sun, etc, would attenuate before they got very far away from >us at all. Now wait a minute. This last line is totally off base. We seem to be quite capable of tossing useful radio signals around the solar system, in spite of using inferior equipment and in spite of solar noise. Once you get clear of the atmosphere, the attenuation works against the noise sources too. >> 3) At certain precisely defined frequencies, it is quite bright-- >> sometimes. >> Certain radio telescopes, when operated as radars, are very bright. >? I missed your point... ? The point is that along the line of sight of these beams, the earth appears VERY bright. Bright enough to be used as a radar at interplanetary distances (where the range degradation is FOURTH power). Probability is against detecting such a beam, but, hey, I never said detection was LIKELY. >> If you look at the solar system from the right directions, there are three >> radio sources: two thermal ones, and something substellar which has a >> really wierd radio spectrum: it has lines that are not emission lines, >> and it is really variable. If your detectors are sufficiently >> sophisticated, you should be able to "see" the earth. But you have to >> look at it exactly right. It occults over a very long period, and you >> have to be looking off of emission lines. This makes it difficult to >> find similar sorts of objects, compounded by the fact that we have >> only been doing this for about 20 years, so that only our very nearest >> neighbors could have noticed this. Someone on Sirius, however, wouldn't >> have too much trouble noticing that our system had something >> really strange in it. >Sorry, Charley, but I really have to "stick by my guns" on this one. >Detecting planets from even NEARBY solar systems is, at best, a painstaking >long complicated process. (You could always argue that advanced civilizations >have really nifty, spifo technology that could pick us out in a sec, but >that is a moot point, since what we are talking about is whether or >not the earth is an OBVIOUS object - at least that's what I'm talking >about...) If it were that easy, we would have done it...the fact is, >after studying a star a mere 6 lys away (Barnard's star), the best anyone >could come up with is a "maybe." There may be 2 large gas bodies in orbit >nearby, but it may just be an error in the way the plates were taken. >And even these "gas bodies" which should be hot thermal objects, cannot >be resolved from the glare fo a pathetic star like Barnard's. At IR wavelengths, yes. But my point is that you have to look at the Sun IN THE RIGHT WAY. It's not technologically very difficult-- it does require a lot of luck. Radio telescopy as we practice it now would never find such an object, because we concentrate on emission lines. Human radio transmissions lie off such lines. >The closest we have ever come to finding other planets thermally, was >with the IRAS satellite. It detected "bodies" moving around the >star Vega. (26 lys distance.) However, that is probably a solar system >in FORMATION, since those "bodies" are glowing at amazingly >high temperatures. (Much higher than Jupiter...) Actually, it was considered to try and look for objects fitting the appropriate description, about ten years ago. (Remember SETI?) As I recall, what killed the thing was the immense improbability of it. Assuming 1 trillion resolvable stars, with one tenth having civilizations generating the right kind of emissions for 1000 years each, you'ld have to examine about 100,000,000 before you found one. >I guess my point is, unless someone out there has some pretty sophisticated >technology...we are quite invisible...at least, ordinary...you couldn't >even get a good Master's Thesis out of us... All you need is a bigger radio telescope, and luck. This discussion started out from the question of why we haven't been contacted by another civilization. Even granting that the Earth is detectable at interstellar distances, it's clear that probability is heavily against our ever being detected. C Wingate
mangoe@umcp-cs.UUCP (Charley Wingate) (08/07/85)
[followups to net.astro, please] I knew I had sources somewhere.... In the May 1975 _Scientific American_ there is a nice article on searching for extraterrestrial intelligence by Sagan and Drake. A summary of their articles (or parts of it anyway): Arecibo Observatory, when transmitting, is at least a million times brighter than the sun. This signal can be detected by a similar receiver at a distance of about ten thousand light years. A number of other sites have similar capabilities. In the FM and VHF tv bands, the earth is quite bright. A receiver system to detect such signals was conceived of at the time of the article, to be called 'Cyclops'. Employing 1500 antennas of 100 meters each,it would be able to detect such signals out to several hundred lightyears. Such a system would not be beyond our current technology-- but it would be very expensive (~$10G). These observations do not rely on resolving the earth as a separate body. C Wingate