For hundreds of years, people have speculated on whether there are intelligent beings on other planets. During the 1800s, a rather famous and well-respected man wrote a popular book on Astronomy which even described the population on each of the other planets then known, and on the Moon and on the Rings of Saturn! I reprinted a chart from a page in his book in a related web-page, at Population of the Planets.
But it was eventually found that the conditions which exist in other locations in our Solar System are not suitable for any kind of living animal or plant, due to a variety of different issues. And so, for about the the past fifty years, various attempts to find some suitable planet orbiting some other star have been tried.
Such SETI experiments are virtually certain to be doomed to failure. The experimenters make many very weak assumptions in setting up their experiments, several of which are very likely to be wrong. They are aware of a very noisy universe, where one frequency seems quieter than others, which happens to be around 1420 MegaHertz in frequency, and which is a natural frequency of ionization radiation of hydrogen gas. They look for MODULATION-STYLE radio signals at that frequency. And they assume that the sender would be sending out enormous amounts of radio power in all directions (omni-directional).
Consider the history of the Earth. It has existed for around 4,800,000,000 years, but it has only been the most recent 100 years when we had invented radio-telescopes and other powerful transmitting antennas to be able to try to SEND such messages. Remember, we are ASSUMING that a sender would AMPLITUDE MODULATE a CARRIER WAVE with some simple modulated message. So we LOOK FOR 'artificial modulation of incoming radio waves' (what we call AM Radio).
But even we seem to have already moved onward, where we now have laser and maser technologies, where we would likely precisely aim a VERY narrow beam of radiation toward whatever specific destination we were interested in. We would NOT use the frequency of hydrogen ionization, but a very different frequency of one of the types of lasers or masers. Are WE likely to want to spend the next million years, listening for a crude and inefficient technology that only had our attention for maybe 100 years out of the entire existence of our planet? Probably not!
So the chance that some distant planet might happen to have RECENTLY discovered how to create and send microwave radio waves, in a very narrow 100-year interval of their planet's multi-billion year existence, where their very wasteful onmi-directional radiation might arrive while we might still be listening for such a crude and wasteful message, seems immensely unlikely.
There are three other possible ways by which we might become aware of the existence of a distant planet, which we now call exo-planets. They are each mechanical effects. The first is where the gravitational attraction of a fairly massive planet might PULL a star back and forth SIDEWAYS during an orbit. This HAS been detected regarding some faint visible and not-visible stars which orbit brighter stars. A number of these double stars were found in the late 1800s, and specific orbital paths have been charted for entire orbits of the pair of stars. But that effect requires that the smaller object be rather massive, in order to be able to pull the main star back and forth sideways enough to be able to detect that the main star is not moving smoothly across the sky! It is also ONLY attractive when the massive planet happens to be in a VERY SMALL ORBIT, where it causes REPEATED sideways motions of the star. Our planet Jupiter might be MASSIVE ENOUGH to cause such sideways movements of our 1000-times more massive Sun, but for this perturbatory movement to only occur once every TWELVE YEARS makes it experimentally a poor approach to try to use. It is NOT suitable for potential planets which are only a tiny fraction of the mass of a central star. Consider that in our Solar System, the Sun's mass is about 330,000 times that of the Earth
The second mechanical effect is closely related to the above. It turns out that our equipment can monitor the FREQUENCY of the light radiation given off by stars, rather precisely. If that same exoplanet discussed above was sometimes on our side of the star, and at other times, on the opposite side, the star can be pulled TOWARD us and AWAY from us at opposite times during an orbit. The sensitivity of spectrographs is great enough that this method is much more sensitive in trying to detect small orbiting stars than the first method mentioned above. But, unfortunately, even it can only detect an orbiting mass which is much greater than Jupiter's is. So some even smaller and fainter orbiting binary stars have been detected, and are not otherwise visible or detectable, and these are called Spectroscopic Binary stars.
But that method is still not nearly sensitive enough to detect planet-sized bodies orbiting stars. Note that the effect is also REALLY slow! If some distant observer was watching our Sun in order to try to detect the Earth, the frequency of the spectroscopic radiation from the Sun would only SLIGHTLY increase and then decrease OVER A PERIOD OF AN ENTIRE YEAR! It would require constantly watching the Sun for at least a hundred years in order to collect enough data to be statistically credible in that a tiny variation of the frequency of the light from the Sun was increasing and decreasing very, very slightly on a YEARLY pattern!
So the spectroscopic binary approach also is not very suitable for trying to detect exo-planets.
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This certainly DOES occur regarding some much larger objects, such as fainter stars that are not optically visible, and such stars are called eclipsing binaries. If the orbiting object is very large, where a significant part of the brighter star is eclipsed, yes, these eclipsing binary stars have been known for more than a hundred years. But for a Venus- or Earth-sized planet which would only block 1/12,000 of the brightness of the central star (as described above) the overall brightness only reduces by maybe 0.0001 Magnitude in brightness, and as also discussed above, such minor fluctuations in observed brightness is more likely to be due to atmospheric or weather changes in OUR atmosphere, or even due to Starspots on that star changing its overall brightness.
Yes, if an orbiting object was extremely large, larger than our planet Jupiter, a noticeable and measurable change might be observed with a precise photometer. But again, if that object was at any actual planetary distance from its star, such stellar transits would only occur once each orbit (at most), meaning that such a slight darkening would only occur once every 'year' and again with the peculiar brightness pattern discussed above. A hundred orbits and transits might be necessary to provide sufficient data to establish that this was an orbital transit phenomena, meaning we would need to make precise observations of that star's brightness for many dozens of years, and no one has yet done that. Tune in in about another hundred years and we might know if this is a valid experimental method!
However, IF an extremely large object is EXTREMELY near the star, then an orbit might only take a day or two and it would be so large that a transit might be seen on a daily basis. More, such a large and rapidly orbiting object would pass over the surface of its star in just seconds or minutes, where the distinctive flat-bottomed dimming pattern might be seen, and it has been detected.
The first few claims of exoplanets, during the 1990s, were all regarding such huge objects which were orbiting extremely closely to their own stars. For such situations, the photometer approach can be valid, where the brightness change is very significant, where it occurs for a short interval and where it repeats every day for months or years. But such super-large objects which are so extremely close to stars can hardly be called planets. IF they actually exist, they are essentially borderline small stars (as total mass develops the immense gravitational pressure which can initiate nuclear fusion. It is believed that Jupiter is nearly large enough to have started fusion, where we would then be in a double-star system, and where the surface temperature on the Earth would not have been stable enough such that life might ever have started, billions of years ago. If Jupiter had ignited, then each synodic year of around 400 days, the Earth would have gotten relatively near Jupiter to receive much more total radiation, and then gotten relatively far from Jupiter to receive much less total radiation. Therefore the surface temperature of the Earth would have wild fluctuations which would make survival of plants and animals somewhat questionable.
But there has been enough solid photometry evidence gathered where we are pretty sure that a handful of extremely large objects, which we choose to call exoplanets, orbit very closely around distant stars. But since humans are actually most interested in whether there are Earth-like planets that exist as exoplanets, it seems that wild optimism has taken over the search for exoplanets, where total silliness seems to now reign.
THAT is why the experiment where we might have 'discovered Venus by photometry means during a Transit', might have been very valuable. Such an experiment would have either given credibility to such a photometry approach being capable of detecting an Earth-sized exoplanet, or it would have shown that the approach is wasted effort and foolish. I personally suspect that the latter is far more likely.
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