# Detecting a less massive exoplanet -> Smaller gravitational force?Watch

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Thread starter 4 weeks ago
#1
I don't understand the physics behind this
What does gravitational force have anything to do with measuring the changes in wavelength?
Can someone pls explain it?

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4 weeks ago
#2
This is about the Doppler shift method of detecting exoplanets.

The Doppler shift comes from the movement of the star relative to the observer (us)
The star and the exoplanet are both orbiting their combined centre of mass...
If the planet is very low mass in comparison to the star, the combined centre of mass is very close to the centre of mass of the star and the amplitude of the movement of the star is very small... and the variation of speed of the star relative to the observer is small.

If the planet is closer in mass to the mass of the star, the combined centre of mass is far from the centre of mass of the star. The amplitude of the movement of the star is greater and the variation of the speed of the star relative to the observer is great.
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Thread starter 4 weeks ago
#3
(Original post by Joinedup)
This is about the Doppler shift method of detecting exoplanets.

The Doppler shift comes from the movement of the star relative to the observer (us)
The star and the exoplanet are both orbiting their combined centre of mass...
If the planet is very low mass in comparison to the star, the combined centre of mass is very close to the centre of mass of the star and the amplitude of the movement of the star is very small... and the variation of speed of the star relative to the observer is small.

If the planet is closer in mass to the mass of the star, the combined centre of mass is far from the centre of mass of the star. The amplitude of the movement of the star is greater and the variation of the speed of the star relative to the observer is great.
What do you mean by 'orbiting their combined centre of mass'? I know that centre of mass is the point where an object's entire weight seems to act or where it's mass is most concentrated but I don't understand how you can just combine two objects centre of masses like that and I can't visualise this :/
And I also don't understand how you deduced the amplitude would be small/large depending on the combined COM and I don't get what amplitude has to do with variation of speed either .-.

Can you please elaborate? Is there an equation which links amplitude and variation in speed? Or combined m and amplitude?
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4 weeks ago
#4
There's a video on this esa page http://sci.esa.int/exoplanets/60655-detection-methods/

star and planet orbit is mathematically the same as binary star orbits described on a page at nasa
https://imagine.gsfc.nasa.gov/featur...on_derive.html
(Original post by Presto)
What do you mean by 'orbiting their combined centre of mass'? I know that centre of mass is the point where an object's entire weight seems to act or where it's mass is most concentrated but I don't understand how you can just combine two objects centre of masses like that and I can't visualise this :/
And I also don't understand how you deduced the amplitude would be small/large depending on the combined COM and I don't get what amplitude has to do with variation of speed either .-.

Can you please elaborate? Is there an equation which links amplitude and variation in speed? Or combined m and amplitude?
0
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