We now have all the pieces required to detect a planet! By observing a star’s spectrum over a period of time, we can see if its spectral lines move back and forth because of the effect of a planet orbiting the star. When the star moves towards us, its light looks a little bluer, and when it moves away from us, it looks a little redder. The same effect happens with light, which is just another kind of wave. The colour of an object producing light becomes redder as it moves away from the observer and bluer as it moves closer. As it moves away from you, it will sound lower pitched, since its sound waves are now being stretched out.Īn animation demonstrating the Doppler effect. As an ambulance moves towards you, it will sound higher pitched because the sound waves it is creating will become compressed, or squished. You have probably experienced the Doppler effect in your own life. There is one more puzzle piece needed to detect the wobble of a star, and this is called the Doppler effect. Every star has a special pattern of these lines that tell us what the star is made of. These dark lines are caused by certain elements in the star’s atmosphere absorbing those very specific colours. A star’s rainbow spectrum is usually has a few missing colours or gaps. We obtain the spectrum of a star’s light by breaking apart this light into all its constituent colours, like a rainbow. The spectrum of the Sun, including several dark absorption lines associated with certain elements in its atmosphere (Credit: N.A.Sharp/NOAO/NSO/Kitt Peak FTS/AURA/NSF) A planet like Jupiter has a bigger effect on the Sun than a planet like the Earth.Ī star is much brighter than its planet, but if we could somehow detect if that star is wobbling, we could detect its planet despite not being able to see it directly! So, how do we detect a wobbling star? A star’s spectrum The bigger the planet, the more it makes its star dance. Consequently, it usually looks like the star is not moving very much. Because the star is much more massive than the planet, the centre of mass is usually very close to the centre of the star. In reality, both the star and the planets in a system orbit around a point called their centre of mass. Because planets also have gravity, they pull on the star just like the star pulls on the planets. It is often said that planets orbit around their stars. At the bottom, we see the change of position of the spectral lines of the star caused by the dance of the star, which is caused by the gravitational effect of its planet (Credit: ESO/L. On top, we see a star and a planet orbiting around their common centre of mass.
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