<p>When an exoplanet orbits a star, the star pulls the exoplanet by gravity.</p>
<p>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.</p>
<p>The gravitational pull on the star by the exoplanet causes the star to move in space.  The star moves in a small circle.</p>
<p>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.</p>
<p>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 &quot;discover&quot; an exoplanet even though we can't see it directly.</p>

CC BY-SA 4.0
2022-09-10T19:44:31.203Z
Do these words apply to the sun?@james k
yh_2004
CC BY-SA 4.0
2022-09-10T19:46:01.740Z
The Earth and the other planets do cause the sun to move.
James K
CC BY-SA 4.0
2022-09-11T01:51:53.287Z
@yh_2004 if this answer helped you, make sure to accept it by clicking the checkmark.
Nilay Ghosh
CC BY-SA 4.0
2022-09-11T14:38:01.240Z
@yh_2004 See [here](https://www.researchgate.net/figure/The-wobbling-of-the-Sun-relative-to-the-center-of-mass-of-the-solar-system-A-Monthly_fig3_259705165#:~:text=In%20fact%2C%20the%20Sun%20is,gravitational%20attraction%20of%20all%20planets.)
Barmar
CC BY-SA 4.0
2022-09-11T21:20:18.320Z
I understand that the barycenter of the Sun-Jupiter system lies just outside the Sun.
EvilSnack
CC BY-SA 4.0
2022-09-10T19:38:06.103Z
2022-09-10T19:38:06.103Z
James K
<blockquote>
<p>&quot;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&quot;</p>
</blockquote>
<p>I did not comprehend how they detected those planets by gravity</p>
<p>I found it in <a href="https://phys.libretexts.org/Bookshelves/Astronomy__Cosmology/Book%3A_Astronomy_(OpenStax)/01%3A_Science_and_the_Universe_-_A_Brief_Tour/1.06%3A_A_Tour_of_the_Universe#:%7E:text=The%20planets%20we%20have%20so%20far%20discovered%20orbiting%20other%20stars%20were%20found%20from%20the%20pull%20their%20gravity%20exerts%20on%20their%20parent%20stars%2C%20or%20from%20the%20light%20they%20block%20from%20their%20stars%20when%20they%20pass%20in%20front%20of%20them.%20We%20can%E2%80%99t%20see%20most%20of%20these%20planets%20directly%2C%20although%20a%20few%20are%20now%20being%20imaged%20directly." rel="nofollow noreferrer">openstax textbook</a> and I searched a lot and I did not comprehend exactly what it means.</p>

CC BY-SA 4.0
2022-09-10T19:25:51.257Z
Hello yh.  I've edited to add a link, please check that it is correct.
James K
CC BY-SA 4.0
2022-09-10T19:28:46.833Z
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.
James K
CC BY-SA 4.0
2022-09-10T19:31:04.067Z
@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
yh_2004
CC BY-SA 4.0
2022-09-10T19:33:16.137Z
I will try to answer, but if it is wrong I'll delete it.
James K
CC BY-SA 4.0
2022-09-11T02:34:25.440Z
For an in depth explanation, search for the word "barycenter".
Greg Miller
<exoplanet>
How are planets detected from the pull their gravity exerts on their parent stars?
yh_2004
# How are planets detected from the pull their gravity exerts on their parent stars?

(Asked by: username_0 on 2022-09-10)

> "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](https://phys.libretexts.org/Bookshelves/Astronomy__Cosmology/Book%3A_Astronomy_(OpenStax)/01%3A_Science_and_the_Universe_-_A_Brief_Tour/1.06%3A_A_Tour_of_the_Universe#:%7E:text=The%20planets%20we%20have%20so%20far%20discovered%20orbiting%20other%20stars%20were%20found%20from%20the%20pull%20their%20gravity%20exerts%20on%20their%20parent%20stars%2C%20or%20from%20the%20light%20they%20block%20from%20their%20stars%20when%20they%20pass%20in%20front%20of%20them.%20We%20can%E2%80%99t%20see%20most%20of%20these%20planets%20directly%2C%20although%20a%20few%20are%20now%20being%20imaged%20directly.) and I searched a lot and I did not comprehend exactly what it means.

**Tags:** exoplanet

> **Comments:**
> **username_1**: Hello yh.  I've edited to add a link, please check that it is correct.  
> **username_1**: 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.  
> **username_0**: @username_1 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  
> **username_1**: I will try to answer, but if it is wrong I'll delete it.  
> **username_2**: For an in depth explanation, search for the word "barycenter".  

---

## 1 answers

### Answer by username_1 on 2022-09-10. Score: 10 | Accepted answer

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.

> **Comments:**
> **username_0**: Do these words apply to the sun?@username_1 k  
> **username_1**: The Earth and the other planets do cause the sun to move.  
> **username_3**: @username_0 if this answer helped you, make sure to accept it by clicking the checkmark.  
> **username_4**: @username_0 See [here](https://www.researchgate.net/figure/The-wobbling-of-the-Sun-relative-to-the-center-of-mass-of-the-solar-system-A-Monthly_fig3_259705165#:~:text=In%20fact%2C%20the%20Sun%20is,gravitational%20attraction%20of%20all%20planets.)  
> **username_5**: I understand that the barycenter of the Sun-Jupiter system lies just outside the Sun.  

---

