Orbital Paths

[nissasa]

If you're looking for extra-solar planets by calculating the wobble of a star, how can you differentiate between one planet going solo vs two planets with very close orbital paths or even a twin planet (two planets orbiting eachother while orbiting a star) ??

 

[Saiph]

the wobble, if created by two planets in very similar orbits, will be hard to distinguish.<br /><br />The method used, however, is pretty sensitive. The star's wobble is periodic, and can be plotted as a function of time, with it's ups and downs and such.<br /><br />Then you use the Fourier therom, which says that any wave (no matter how complicated) can be recreated by a superposition of simpler, sinusoidal waves of varying amplitudes and phases (even if the wave to be modeled has sharp corners!).<br /><br />There are programs written to do this Fourier analysis, and can pick out what set of simple waves creates the one observed.<br /><br />As such the minor blips, squiggles and other irregularities can be distinguished as seperate "sinusoidal waves" each do to a planet in a particular orbital configuration. <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>

 

[nissasa]

Any guesses on how long till the first "moon" is discovered out there? <img src="/images/icons/smile.gif" />

 

[remcook]

my hopes are with SIM<br /><br />http://planetquest.jpl.nasa.gov/SIM/sim_index.html

 

[thalion]

Answers:<br /><br />1.) Twin planets.<br />We could not determine from the doppler curve alone if an exoplanet was a double, because both planets would act as a point mass on their primary star. If they eclipsed the star, detecting double planets would be much easier.<br /><br />2.) Two planets.<br />Each planet induces its own reflex motion on the star. Depending on the eccentricity of the orbits, and their orbital ratios and periods, the doppler motions will algebraically interact with alternating constructive and destructive interference, revealing multiple gravitational interactions. A single planet curve would be fairly simple. However, the further a curve diverges from a simple one-planet curve, the more likely that there are at least two planets involved.<br /><br />Here is a great page from exoplanets.org showing how these multiple planet solutions work out:<br /><br />http://exoplanets.org/esp/55cnc/55cnc.shtml

 

[Saiph]

1) we could still tell. However the wobble variation wouldn't be the same as those planets "year" but the same period as thier orbits about eachother. Each creates a tidal bulge pointed towards them. The net effect is to the center, however as the planets spin about eachother, the individual tidal bulges will shift, causing the wobble intensity to increase (the planets line up) and then decrease (they're "tangential") <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>