How is the existence of unseen planets, orbiting very distance stars determined?
I thought I read somewhere that they exert a gravitational "wobble" on the movements of the star. But are our instruments that precise that miniscule gravitational tugs can be measured from such great distances?
There are two ways in which exoplanets have been detected in other stars. You have mentioned the first one where they use the gravitational wobble caused by the planet or planets into their parent star. In fact this "miniscule gravitational tugs" you say are not at all that small. Most planets discovered in this method are Jupiter-sized planets. If you will look in the Solar System, the gravitational tug exerted by Jupiter onto the Sun displaces the Sun by half it’s own diameter from the center of the Solar System. That means, the sun revolves around center between it and Jupiter which is above its surface, or around 690,000 km. This is the barycenter, the true center of the Solar System. Most of the planets discovered in this method even exceed Jupiter’s mass and sometimes more than one is found to orbit the star. This may have displaced the parent star pretty far away from their common centers.
The next method is by the amount of dimming in the star’s light. The subtle differences in the star’s light often tell of a planet that passes in front or on the disc of the star. Since Jupiter-size planets are more than 10% the size of the sun, they can cause a considerable dip in the star’s light output. Though, sometimes debunked by a star’s true variability, this is still an effetive method in planet-hunting.
And if you have noticed, most of the hundreds of planets discovered so far through these methods are fairly in the league of Jupiter’s mass and size. That is because these planets are the only ones enough to cause a considerable dip in light and gravitational tug into their parent star. For your reference, the Earth is 330,000 times less massive than the sun.
November 30th, 2009 at 4:04 pm
Yes they are, in fact there are two ways of detecting planets that astronomers use. You described the first one. But the second one involves seeing whether a star dims when a planet moves in front of it. If you think about it that’s like seeing whether or not a light bulb dims when a spec of dust moves in front of it.
References :
November 30th, 2009 at 4:17 pm
You’re right; we *usually* detect extra-solar planets by their gravitational tug on the parent star which causes the star to "wobble" as it moves through space. We *do* have instruments that can detect that wobble, but only out to relatively short distances. Most of the extra-solar planets we know about our virtually in our own backyard.
In some cases we can find a new extra-solar planet when it transits its parent star (..moves across the face of that star..) thereby very slightly dimming the stars brightness.
Finally there’s a technique known as microlensing. Microlensing occurs when the gravitational field of a star acts like a lens, magnifying the light of a distant background star. Possible planets orbiting the foreground star can cause detectable anomalies in the lensing event light curve.
References :
November 30th, 2009 at 4:46 pm
what is actually being measured is the frequency of an absorption or emission line in the light spectrum (spectroscopy). these lines occur at particular, known frequencies for particular elements, and change ever so slightly as the "wobble" occurs. and yes, the measurements are that precise, and getting better all the time.
sometimes other methods of detecting extrasolar planets can be used, and the results generally agree with this method, so it’s considered quite reliable.
References :
http://en.wikipedia.org/wiki/Radial_velocity_method
November 30th, 2009 at 5:36 pm
There are two ways in which exoplanets have been detected in other stars. You have mentioned the first one where they use the gravitational wobble caused by the planet or planets into their parent star. In fact this "miniscule gravitational tugs" you say are not at all that small. Most planets discovered in this method are Jupiter-sized planets. If you will look in the Solar System, the gravitational tug exerted by Jupiter onto the Sun displaces the Sun by half it’s own diameter from the center of the Solar System. That means, the sun revolves around center between it and Jupiter which is above its surface, or around 690,000 km. This is the barycenter, the true center of the Solar System. Most of the planets discovered in this method even exceed Jupiter’s mass and sometimes more than one is found to orbit the star. This may have displaced the parent star pretty far away from their common centers.
The next method is by the amount of dimming in the star’s light. The subtle differences in the star’s light often tell of a planet that passes in front or on the disc of the star. Since Jupiter-size planets are more than 10% the size of the sun, they can cause a considerable dip in the star’s light output. Though, sometimes debunked by a star’s true variability, this is still an effetive method in planet-hunting.
And if you have noticed, most of the hundreds of planets discovered so far through these methods are fairly in the league of Jupiter’s mass and size. That is because these planets are the only ones enough to cause a considerable dip in light and gravitational tug into their parent star. For your reference, the Earth is 330,000 times less massive than the sun.
References :