Planets on other stars

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invisi

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Why is it that we have data on the planets that are over 15 ly away and we don't know if A and B Centauri have any<br /><br />heres a picture of Antares and the sun at the same distance <br /><br /><i>**Your image was too wide for the normal post window. Please resubmit it no wider than 800 pixels per UPG7**</i>
 
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dragon04

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I guess the best answer I could give is that the Centauri System (and you left out Proxima, btw) is a much more complicated model in trying to detect exoplanets with the methods available.<br /><br />I'm not sure, but I think that all of the 238 exoplanets discovered to date are in single-star systems.<br /><br /> <div class="Discussion_UserSignature"> <em>"2012.. Year of the Dragon!! Get on the Dragon Wagon!".</em> </div>
 
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h2ouniverse

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Actually there are a few spotted in multiple system. <br />HD 188753A-b is in a triple star system. It orbits close to one iof the three stars.
 
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h2ouniverse

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To spot a planet nowadays you need:<br />* either a detectable doppler effect on the light of the centralstar, due to the toggle exerted by the planet of the star (similar to the fact that Earth rotates about the CoG of Earth+Moon, i.e. about a point more than 4000km from Earth's center. With this method, you detect big planets, often very close to the star because you observe the star over a relatively short period. <br />* or a transit, but that requires the system to be seen "edge-on" (i.e that the Sun is in the plane of the star's ecliptic) = /> occurs sometimes, but seldomly<br />* or by imaging the planet = /> achieved for now on 2 or 3 of them, because they are very big, very young (hence very hot, i.e. very bright in infra-red) and far from the star (>50 AU typically)<br /><br />So we may still detect planets in the alpha centauri A-B + proxima system. But probably relatively small ones or very far ones (*) (**)...<br /><br />(*) unless you see the star above its pole. Then the radial velocity method does not work even for a big planet. I do not know whether centauri is in that scheme.<br /><br />(**) if you have a Neptune at more than 100 AU from alpha centauri A+B, with a period of about 1000 years, you will find it very hard to spot by radial velocity.<br /><br />You can see there are many ways for an alpha-centaurian planet to have escaped our detection methods.
 
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pyoko

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Isn't it simply because AC's disk (orbit) doesnt point directly at Earth, and so we cannot see the wobble as the planet passes the sun? <div class="Discussion_UserSignature"> <p> </p><p> </p><p><span style="color:#ff9900" class="Apple-style-span">-pyoko</span> <span style="color:#333333" class="Apple-style-span">the</span> <span style="color:#339966" class="Apple-style-span">duck </span></p><p><span style="color:#339966" class="Apple-style-span"><span style="color:#808080;font-style:italic" class="Apple-style-span">It is by will alone I set my mind in motion.</span></span></p> </div>
 
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robnissen

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<font color="yellow">Isn't it simply because AC's disk (orbit) doesnt point directly at Earth.</font><br />No, that would be true for the transit method, but not for the radial wobble method, unless we were directly above the north pole of the plane. The simple fact is, that we could not currently detect our solar system from the AC system if were to look back on earth from 4.5 light years away. Personally, I'm kindof glad we have not yet found planets in the AC system, that means it is still possible that it might have planets similar to ours. Stable orbits in a trinary system can be tricky, however.
 
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mvp347

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H2Ouniverse,<br /><br />from http://en.wikipedia.org/wiki/HD_188753_Ab<br /><br />"HD 188753 Ab was claimed to be the first known exoplanet in a triple star system in 2005 by Dr. Maciej Konacki, a Polish astronomer working in the United States. An attempt to confirm the discovery, however, failed. In 2007 a team at the Geneva Observatory concluded that the claimed planet does not, in fact, exist. Konacki issued a response to this, claiming that the precision of the follow-up measurements was not in fact sufficient to confirm or deny the planet's existence. Further observations of this system are planned over the course of 2007."
 
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h2ouniverse

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Yes, as Rob points out with spectrography you measure radial velocity so all you need is a radial component twoards Earth. Even with a 60° tilt of the AC ecliptic versus AC-Earth direction you can detect something. (cos 60°=0.5). <br />But you will have no measurable radial velocity if:<br />* planet mass is too low<br />* or the tilt is too close to 90°<br />* or the planet has a long period and is currently at 90° on its orbit (wobble currently purely tangential).<br /><br />There are however two other methods for detecting planets:<br />* grav lensing: not adapted for AC (too close!!) but for systems thousands of LY away with a dense stellar background<br />* astrometry: you measure the wobble not by spectro but directly, optically! This one would be well adapted to very close systems.You observe an ondulated trajectory of the star instead of a quasi-linear one.<br />But as AC is ternary, discriminating trajectory perturbation is more difficult. <br />Worth trying however since astrometry enables to detect TANGENTIAL wobbles. GAIA should be able to do that with enough precision I hope.
 
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h2ouniverse

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Mvp,<br />I was not aware. This being said I think that even if it is more difficult for planets to emerge from accretion disks in multiple systems, on the other hand they have a greater capability to capture objects traveling from far thanks to the double-dip gravity well. So...<br />In addition planets orbits are very unstable on long term anyway, even in our Solar System. Not sure therfore that the complex gravity pattern of a multiple star system makes a real difference in terms of having planets. Then having stable circular orbits is another kettle of fish....
 
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