ExoPlanets - Questions Regarding the Search

  • Recently, in 2012, astronomers have had a wonderful chance to confirm or deny the validity of the recent popular methods being used to try to detect tiny planets orbiting distant stars, the so-called exo-planets.

  • The planet Venus orbits the Sun in an orbit which is within the Earth's orbit. Venus happens to be nearly the identical size as Earth. In principle, we should see a SHADOW of Venus each time it passes between the Sun and Earth. But Venus is relatively TINY in the enormity of our Solar System, and both Venus and the Earth have orbits which are slightly TILTED (called Inclination). This USUALLY results in the POSSIBLE SHADOW of Venus passing above or below the image we see of the Sun. Only when both the Earth and Venus happen to be near locations in their orbits called NODES, does it actually happen that Venus only rarely passes in transit across the face of our Sun. In recent years, there have been two transits of Venus, in the year 2004 and in 2012. Prior to that, the most recent Transit of Venus had occurred in 1882, long, long ago! The value of this event regarding exoplanets is that during a Transit, Venus blocks off a portion of the Sun's radiation and light from us.

  • In other words, the Sun necessarily seems less bright to us during a Transit. THIS is the recently popular method being used to claim the hundreds and thousands of exoplanets which various astronomers say they have discovered. Extremely sensitive Photometers are used to try to discover such changes in brightness of a star, to therefore claim that a planet had blocked the light from getting to us.

  • Considering Venus, that planet is about 1/110 the diameter of the Sun, so when Venus is Transiting the Sun, it blocks off only about 1/12000 of the visible area of the Sun. Stars' brightnesses are described by Magnitude, and the very bright Sun is described as being about -26.700 in Magnitude. When Venus blocks off about 1/12000 of the light of the Sun, that changes the total brightness of the Sun to us, but by an extremely small amount. The Sun's total Magnitude drops to about -26.6999 Magnitude due to that reduction of the tiny area which is blocked off from us.

  • Therefore, if someone had tried to 'detect Venus' as though it was an exoplanet, the star (our Sun) would only (suddenly) get fainter by a factor of one part in 12,000; then it would STAY fainter like that for about the six hours of a Transit; and then the brightness would suddenly rise back up to its normal value (for the next hundred years! The observed brightness curve would be very unique normal level, then suddenly slightly dropping where it would stay for six hours and then suddenly rising back up afterwards.

  • We have also noted that nearly always, Venus passes above or below the image of the Sun during its INFERIOR CONJUNCTIONS so the chance of seeing a Transit is extremely rare. Would someone spend 120 years in constantly trying to monitor the precise brightness of the Sun to try to detect such a subtle reduction for a few hours? It seems rather foolish.

  • It happens that our Sun rotates rather slowly, abbout once every thirty days or so. We all know that groups of sunspots appear on the Sun's surface. Some groups of sunspots are large enough to block off even one percent of the Sun's surface. That is about A HUNDRED TIMES GREATER BRIGHTNESS REDUCTION than the effect of a Venus transit. We have no idea if other stars rotate as slowly as our Sun does, but some sunspot groups on our Sun last long enough to be seen during two or more rotations of the Sun. If someone was monitoring the Sun from a long distance away, they might easily see the brightness reduction due to a sunspot group, but then seeing the Sun at 'normal brightness' while the sunspot group was passing across the 'back side of the Sun'. Such people watching our Sun might not detect a Transit of Venus every hundred years (for a few hours), and even rarer Transits of Jupiter, a much larger planet which takes twelve years to orbit the Sun and very rarely would be at an Orbital Node such that the Sun might appear to have dimmed a tiny but. It seems far more likely that a distant observer of the Sun might think that there was ONE planet orbiting around the Sun, in an orbit that took around thirty days to complete. It wouldn't be an exo-planet at all, and merely groups of sunspots circling the face of the Sun.

  • More significantly, the Earth's atmosphere has weather disturbances all the time, and the VIEWED precise brightness of the Sun constantly has small fluctuations due to many different weather events. In order to look for a variation of brightness of just one part in 12,000, which then would need to appear constant for six hours in the reduction, is essentially impossible due to fluctuations in our Earth's atmosphere and its weather. There are too many mundane reasons for why the brightness of the Sun might vary, for minutes or hours, to try to detect a CONSTANT reduction for a six-hour period.

  • The Sun itself also has many different processes where its total brightness varies by far greater variations than the tiny brightness change which would be necessary to detect Venus in this way. A group of Sunspots might occur, which are a cooler area of the Sun's surface, which might exist for a few hours or even a few days. Unless long-term observations showed a repeated reduction of brightness, on a time pattern of weeks or months (or hundreds of years), to show an exact repeatability of a phenomena that might be due to orbital motion, brightness changes due to the appearance and disappearance of a Sunspot group cannot be neglected. Combining this with the countless weather and other atmospheric effects, trying to establish a valid change of brightness pattern which might be due to an exoplanet transiting its own star, seems very unlikely. Testing such a theory by trying to prove the existence of Venus as an exoplanet during the recent two Transits of Venus, seems like the best way to confirm that the approach might have validity, but I cannot find any evidence that any astronomer did this experiment during either of the two recent Transits of Venus. Without that evidence, the potential validity for using precision photometers to try to discover exoplanets seems rather questionable.

  • There is another enormous problem. It is well established that the MAJORITY of stars anywhere near us are DOUBLE STARS and even triplets. In order for any planet to endure for billions of years, it is nearly certain that the star involved needs to be a single star, as the perturbative effects of two stars is too disruptive to any consistent orbit.

    Our Moon is a good example. When President Kennedy announced that the United States intended to send a spacecraft to land on the Moon, and only less than a decade later, no one realized how complex the orbit of our Moon is, primarily due to gravitational perturbations due to the Sun and the Earth, and its elliptic, tilted orbit. Engineers needed to know WHERE the Moon was going to be, to make sure that we would not miss it with the spacecraft. Around 1960, there had been several failures in this regard, when rockets were sent which were meant to crash into the Moon, but they MISSED THE MOON! Thousands of mathematicians and physicists worked for the entire decade of the 1960s to try to figure out where the Moon would be when we needed to know! Few people are aware that that mathematical problem had NOT YET been completely solved by 1969, but we then knew the location of the Moon to about ten-foot accuracy, and NASA was confident that they would be able to land on the Moon. Even now, fifty years later, that mathematical problem is STILL not precisely solved regarding the Moon. We can now predict where the Moon will be one week later to better than one-foot accuracy.


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.

Beginning in the 1960s, many SETI (Search for Extra-Terrestrial Intelligence) experiments have been tried, some of which are still in operation. Such experiments generally use huge radio-telescopes to listen for some radio message which might have been sent out by some civilization on a distant planet.

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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The third method, the one that generated great excitement during the 1990s, uses the assumption that a planet happens to be in an orbit which includes us in that orbital plane, such that we might then see transits of that planet over the surface of that star.

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.


This presentation was first placed on the Internet in April 2012.

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C Johnson, Theoretical Physicist, Physics Degree from Univ of Chicago