In my recent post, I discussed the transit method and how it is used to detect extrasolar planets. The transit method is one of the most popular methods, but so is the radial velocity method. The radial velocity method is similar to the transit method, but is used to find different characteristics of the extrasolar planet. By using the radial velocity method, astronomers can determine the mass of the extrasolar planet.
We know that the planet exerts a gravitational “tug” on the star; this tug causes the star to move. Because we know the stars so well, we can look at its spectra. This spectra is like a map of the colors of light the star emits. Sometimes colors are missing in the spectra because atoms in the star use the color to gain energy. In figure 1, if you look at the top panel of the spectra you can see the missing colors. By watching the spectra you can see the star’s motion, towards or away from you, caused by the extrasolar planet.
The spectra is then used to see the Doppler shift. Everyone has heard the Doppler shift when a police car flies past you with its siren. When the police car is coming towards you, its siren is higher pitched. When it is going away from you the pitch lowers. The Doppler shift in light is similar, when the object emitting light is coming towards you it looks as if it is full of higher pitched light, which is bluer. When the object emitting light is going away from you it is full of lower pitched light, which is redder. These shifts are called the “blue-shift” and the “red-shift”. In figure 1, if you refer to the second panel you can see the “red-shift”. It is red-shifted because relative to the top panel, or the unshifted spectra, it is shifted towards the red part of the spectra. The third panel shows the “blue shift” because the spectra is shifted towards the blue part of the spectra. The cartoon below shows how the red-shift and blue-shift is seen when the object emitting light is moving towards and away from you.
If you use the blue-shift and red-shift, you can determine which way a star is moving. If the shift is in the blue, the star is moving towards you and if it the shift is red, it is moving away from you. This movement, or Doppler shift, can also determine the mass of the extrasolar planet. You can determine the mass due to the size of the shift. If you have a bigger doppler shift, the star has a bigger movement. This bigger movement means that the star was “tugged” harder. It was “tugged” harder because the heavier planets exert more gravity. So, if you have a bigger doppler shift, you have a heavier planet.
If you combine these two methods (the transit and radial velocity method) of discovering extrasolar planets, we can determine the planet’s distance of orbit, size, and mass. Both of these methods can be used to isolate certain characteristics of the extrasolar planet. By isolating characteristics from the mass of the planet found in the radial velocity method and the size of the planet from the transit method, you can figure out what the planet is made of! If it is small and heavy, it is most likely made out of rock and water, like Earth! If it is very big but light for its size, it is most likely made out of gas, like Jupiter!
1) If a star has a blue-shift, what does this mean about the star?
2) If my hypothesis was “all doppler shifts seen in the light spectra are caused by extrasolar planets”, can you think of an example of when this hypothesis would be incorrect?
3) In figure 1, explain why the second panel is red-shifted.