# How are planets detected from the pull their gravity exerts on their parent stars?

"The planets we have so far discovered orbiting other stars were found from the pull their gravity exerts on their parent stars, or from the light they block from their stars when they pass in front of them"

I did not comprehend how they detected those planets by gravity

I found it in openstax textbook and I searched a lot and I did not comprehend exactly what it means.

• Hello yh. I've edited to add a link, please check that it is correct. Commented Sep 10, 2022 at 19:25
• Your question will need more details. What don't you understand about this sentence? It is about "exoplanets" (that is another word for "planets we have discovered orbiting other stars") The rest of the sentence seems very clear and unambiguous. I don't think you will get a good answer, unless you edit to explain what you did not comprehend. Commented Sep 10, 2022 at 19:28
• @JamesK the thing that I did not comprehend is The planets we have so far discovered orbiting other stars were found from the pull their gravity exerts on their parent stars, I did not comprehend how did the detect those planets by gravity Commented Sep 10, 2022 at 19:31
• I will try to answer, but if it is wrong I'll delete it. Commented Sep 10, 2022 at 19:33
• For an in depth explanation, search for the word "barycenter". Commented Sep 11, 2022 at 2:34

When an exoplanet orbits a star, the star pulls the exoplanet by gravity.

But Newton's third law says that if the star pulls the exoplanet, then the exoplanet must pull on the star by an equal but opposite amount.

The gravitational pull on the star by the exoplanet causes the star to move in space. The star moves in a small circle.

We can measure very accurately the speed that a star is moving in space by the doppler effect. When a star moves towards us, its light becomes slightly bluer, when it moves away it becomes redder. We can measure these changes very accurately and so we can tell when a star is moving in a small circle.

If we see a star moving in a small circle, we know that there must be an exoplanet orbiting the star. This means that we can "discover" an exoplanet even though we can't see it directly.

• Do these words apply to the sun?@james k Commented Sep 10, 2022 at 19:44
• The Earth and the other planets do cause the sun to move. Commented Sep 10, 2022 at 19:46
• @yh_2004 if this answer helped you, make sure to accept it by clicking the checkmark. Commented Sep 11, 2022 at 1:51
• @yh_2004 See here Commented Sep 11, 2022 at 14:38
• I understand that the barycenter of the Sun-Jupiter system lies just outside the Sun. Commented Sep 11, 2022 at 21:20