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Space Forum / SETI / October 2004Locating exoplants using the wobble method is wrong.
When the Nova program explained abt the way the wobble is used to locate the exoplanets, I was aghast. It is plain crazy and wrong. If we assume our own sun as one such sun for which we find the exoplanets, how our sun is seen by others in this wobble method? The center of gravity of all the planets at a particular time need to be computed and then the effect of this gravity on the sun needs to be computed. The wobble in the position of our sun will be seen by the other ets as the indication of the planet nearby. Obviously the center of gravity of all the planets to gether with the combined weight of all the planets will show that it is giant jupiter that is closely revolving the sun. So does this mean that the earth and mars are not there? This is how they have found the exoplanets using the wobble. First the wobble does indicate that there are planets. But the size and circularity of the orbit of the exoplanet calculation is wrong, imho There needs to be another way of locating these exoplanets, >When the Nova program explained abt the way the wobble is used to >locate the exoplanets, I was aghast. [quoted text clipped - 21 lines] > >There needs to be another way of locating these exoplanets, I don't see your problem. When a system has one planet, then the radial velocity curve that we detect is something like a sin wave, just distorted by the ellipticity and inclination of the orbit. When a system has more than one planet, then the radial velocity curve looks like several such waves superimposed on each other, because the planets orbit with different periods. Here's a plot of the radial velocity curve of upsilon Andromedae. It's easy to see that the curve is composed of two waves, one with a period of about 1284 days and the other with a period of about 241 days. What you can't see on this image is the third component, but if you could zoom in really closely, you'd see that the curve also has a ripple with a period of 4.6 days. These components correspond to the movements of three planets in the system. <http://exoplanets.org/esp/upsandb/outer_planets_sm.jpg> If there were a system identical to our own, with current technology we would have so far only detected Jupiter. That's not because we lack the sensitivity to detect the other gas giants, but because we've only had the technology to detect planets as light as Saturn for a small fraction of Saturn's 29-year orbit and we need to observe at least one complete cycle to be certain of the pattern.  SignatureMike Williams Gentleman of Leisure > When the Nova program explained abt the way the wobble is used to > locate the exoplanets, I was aghast. > It is plain crazy and wrong. Nova is popular science not science. It is supposed to be entertaining and, if necessary, informative. Nova SUCKS if you know science. If you are entertained by science, enjoy it. It is SciFi without the Ferengi. > If we assume our own sun as one such sun for which we find the > exoplanets, how our sun is seen by others in this wobble method? Extremely sensitive measurements, far beyond what we can do today and decades of data collection. > The center of gravity of all the planets at a particular time need to > be computed and then the effect of this gravity on the sun needs to be > computed. The wobble in the position of our sun will be seen by the > other ets as the indication of the planet nearby. The wobbles caused by each planet have to be measurable and there has to be enough data to establish the periods of all the sub-wobbles due to each planet. Obviously there have to be several revolutions of Jupiter and likely of Saturn to be certain what is being observed. And then Bode's law works on both distance and length of each planets year so it is good to have a multiple of the longest measurable wobble. Which is why we only have reports of very massive planets very close to the star so the planet's year is extremely short. I don't think we have one report of planet as far away as Mercury. > Obviously the center of gravity of all the planets to gether with the > combined weight of all the planets will show that it is giant jupiter > that is closely revolving the sun. So does this mean that the earth > and mars are not there? > This is how they have found the exoplanets using the wobble. > First the wobble does indicate that there are planets. But the size > and circularity of the orbit of the exoplanet calculation is wrong, > imho Let us assume we had the accuracy to measure the wobble caused by a system exactly like ours and have had it for the 12 or so years we have been able to measure it. Jupiter has an orbit of 9 years. That would be 1.33 wobbles. No one would publish based upon that, except me and thee ;) but no one would publish it. But I doubt we have the ability to measure such a small wobble today. I don't read the literature do I can't say for sure. Which means were are stuck with only having results from very large and very close planets because of the limits of measurement accuracy and the number of years we have had the ability. Anyone drawing conclusions about the types of planetary systems based upon what has been found so far is suffering from science writer syndrome. > There needs to be another way of locating these exoplanets, Feel free to invent it and promote it. SignatureBeheadings work better than a pro-American TV network. Improvised munitions work better than smart bombs. And Americans think they can win Iraq. The Iron Webmaster, 3263 > Which is why we only have reports of very massive planets very close >to the star so the planet's year is extremely short. I don't think we >have one report of planet as far away as Mercury. Several, in fact. The longest semi-major axis detected so far is 5.9 AU. That's further out than Jupiter. The planet is 4 times as massive as Jupiter. There are over 50 ESP candidates with periods >= 1 AU. See <http://c3po.barnesos.net/egpdb.cgi?orderby=semimajoraxis&order=DESC>. SignatureR.G. "Stumpy" Marsh Timaru, New Zealand <http://marsh.orcon.net.nz/> S@H 500WU ret. >> Which is why we only have reports of very massive planets very close >>to the star so the planet's year is extremely short. I don't think we >>have one report of planet as far away as Mercury. > Several, in fact. The longest semi-major axis detected so far is 5.9 > AU. That's further out than Jupiter. The planet is 4 times as > massive as Jupiter. There are over 50 ESP candidates with periods >= > 1 AU. Correct of course. I was trying to finish it and get on with life. I was avoiding going into a discussion of mass vs revolution as a limitation of detectable wobble. I don't do this sort of thing for a living so I have to discuss these things on the fly without rehearsed explanations. That does lead to errors on the side of simplification. However I point out the term candidate rather than confirmed in the sense of the wobble being observed by someone else. In line with what I did post, we have not been using this method long enough to do a proper confirmation by seeing a second cycle of the wobble for candidates Jupiter distances and out. There may be enough confidence in the method at this point in time to announce partial wobbles as detections. I would cautious in making too much of that. SignatureThe purpose of a debate is not to determine fact. The purpose is to learn to sway short term opinion. -- The Iron Webmaster, 3256 > However I point out the term candidate rather than confirmed in the >sense of the wobble being observed by someone else. Confirmed exoplanets are called "candidates" even those that have been observed by several different methods and by several different teams. Objects that have only been observed by a single team are listed as "unconfirmed exoplanets". I've no idea why such confusing terminology is used. There have been papers published on the 55 Cancri system by 24 different teams. I've not checked to see how many of those performed their own observations, but none of them disagree about the existence of the planet at 5.9 AU. SignatureMike Williams Gentleman of Leisure > Confirmed exoplanets are called "candidates" even those that have been > observed by several different methods and by several different teams. [...] Possibly, they are only "candidates" because their presumed existence has merely been inferred from hypothesis and indirect observations. I've not followed the fine detail of the measurement techniques used. I guess that the link between exoplanets and the measurements has not yet been 'proved'. The 'rigorous' nature of real science can sometimes seem other-worldly when compared to 'TV-science'! Regards, Martin Signature---------- OS? What's that?! - Martin - To most people, "Operating System" is unknown & strange. - 53N 1W - Mandrake 10.0.1 GNU Linux - An OS for Supercomputers & PCs ---------- http://www.mandrakelinux.com/en-gb/concept.php3 > When the Nova program explained abt the way the wobble is used to > locate the exoplanets, I was aghast. [quoted text clipped - 13 lines] > that is closely revolving the sun. So does this mean that the earth > and mars are not there? No, they'd merely be insignifigant compared to the effect of Jupiter, which out-masses all the other planets put together. Jupiter would tend to be noticed first. But careful observation ofer extended periods would let one produce curves that tease out the effects of some smaller masses. How long, depends on how large those masses are, and how long their orbital periods. This is why the massive, close-orbiting 'hot-Jupiters' (starting with 51 Pegasi)stood out first. This method is most sensitive to such worlds. For someone at (say) Alpha Centauri, it would take time to see the wobble generated by Jupiter, more to find Saturn in one's data, etc. Earth-mass planets might be lost in the noise for some time. Mercury or Pluto might never be discovered this way. It's a game of patience. At least until you develop the means to image these worlds directly. > This is how they have found the exoplanets using the wobble. > [quoted text clipped - 3 lines] > > There needs to be another way of locating these exoplanets, This one seems to be working fine. One can also look for the very slight change in starlight *if* the planet's orbital plane is such that it passes in front of the star, from Earth's point of view. IIRC, at least one planet's presence was verified this way, after it had already been deduced through the wobble method. Its transit was consistent with the previously observed wobble. But again, big-a.s, off-Earth telescopes would still be desirable. One could also learn something of local conditions by examining its reflected spectra.... SignatureYou know what to remove, to reply.... >> There needs to be another way of locating these exoplanets, > [quoted text clipped - 4 lines] >been deduced through the wobble method. Its transit was consistent with >the previously observed wobble. There are currently four methods that have been successfully used to detect confirmed exoplanet candidates. Two other methods have been used on previously detected planets, which helps to assure us that the radial velocity wobble really is caused by planets. This information may well not be up to date, it's a few years since I last thoroughly researched this stuff. Methods successfully used to detect planets Radial velocity: the main method Pulsar timing: detects planets around pulsars Occultation: observes the decrease in starlight as the planet crosses in front of the star Gravitational microlensing: A remote star is brightened as the planet crosses our line of sight. Methods used to observe planets that have been previously detected Transit spectrum: observes the change in the star's spectrum as some of the light passes through the planet's atmosphere as the planet crosses in front of the star. Planet spectrum: by subtracting the average spectrum of the star from the observed spectrum of star plus planet, it's possible to observe the changes due to light reflected from the planet as its phase changes. Methods that have not yet led to confirmed exoplanets Gravitational lensing: A planet in a remote unnamed galaxy was observed to pass across our line of sight to quasar Q 0957+561. This is an unrepeatable event, so it can't be confirmed. Protoplanetary divisions: When we observe divisions in a protoplanetary disk (rather like the Cassini division in Saturn's ring) we strongly suspect that it is caused by resonance with a planet. Protoplanetary warp: When we observe a protoplanetary disk that is not flat, we strongly suspect that it is being warped by the gravity of a planet which has an inclined orbit. Methanol Maser: Most natural masers originate in large molecular clouds, but there's one which comes from a pointlike source. It is conjectured that there is a small body holding the methanol cloud together. Astrometry: Attempting to observe the sideways motion of a star caused by gravitational wobble by accurately measuring changes in its position. Direct observation: satellites are being planned which may be able to use nulling interferometry to directly observe the light from exoplanets. SignatureMike Williams Gentleman of Leisure |
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