<p>Knowing:</p>
<ul>
<li><p>Star's radius <span class="math-container">$r_s$</span> , luminosity <span class="math-container">$L_s$</span>, and absolute magnitude <span class="math-container">$V$</span></p>
</li>
<li><p>Planet's radius <span class="math-container">$r_p$</span> , albedo <span class="math-container">$a$</span> , and distance <span class="math-container">$d_s$</span></p>
</li>
</ul>
<p>How can you calculate the absolute magnitude of a planet in a hypothetical star system?</p>
<p>When plugging Earth value into the answer given by Micheal B. <a href="https://astronomy.stackexchange.com/questions/5957/compute-planets-apparent-visual-magnitude">here</a>, it gives me <span class="math-container">$V_p$</span> = 19.83, but it should be around -3.99, although this could be an error on my part.</p>

CC BY-SA 4.0
2022-08-07T06:32:18.790Z
Welcome to Astronomy Stack Exchange! Can you click "Edit" and add to you post what you've tried so far, and which equations you think might apply? Pure "please work this problem for me" questions are generally not well-received, and usually require some evidence of effort.  *Thanks!*
uhoh
CC BY-SA 4.0
2022-08-07T07:36:29.063Z
I don’t think absolute magnitude is well defined for planets(ie distance). For stars it’s 10 parsec, tho.
Max0815
CC BY-SA 4.0
2022-08-07T08:39:42.583Z
https://en.wikipedia.org/wiki/Absolute_magnitude
planetmaker
CC BY-SA 4.0
2022-08-07T12:52:49.483Z
So this would be the magnitude of the planet at 10 parsecs as viewed from the star (so the phase would be full). You can use HORIZONS https://ssd.jpl.nasa.gov/horizons/app.html or similar to get this for our solar system's planets and curve-fit, assuming your hypothetical planets are similar enough to ours. In particular, that they're Lambertian reflectors: https://en.wikipedia.org/wiki/Lambertian_reflectance
Barry Carter
CC BY-SA 4.0
2022-08-07T13:45:29.780Z
This question seems like it might be similar and has some good answers.
https://astronomy.stackexchange.com/questions/5957/compute-planets-apparent-visual-magnitude
Greg Miller
CC BY-SA 4.0
2022-08-07T20:48:19.923Z
Does this answer your question? [Compute Planet's Apparent Visual Magnitude](https://astronomy.stackexchange.com/questions/5957/compute-planets-apparent-visual-magnitude)
WarpPrime
CC BY-SA 4.0
2022-08-07T21:38:35.240Z
Whenever I plug Earth values into Micheal's answer to test it out, I get $V_p$ = -8.16 when it should be around -3.99; although, I could be doing something wrong..
E.UCIT
CC BY-SA 4.0
2022-08-07T23:36:17.030Z
Are you using the right units? (Parsecs for distance in Michael’s answer, even though it says light-years in the OP.)
Pierre Paquette
CC BY-SA 4.0
2022-08-08T05:15:29.940Z
@PierrePaquette I think you may be correct about using the wrong units, but when using parsecs $V_p$ = 19.76 ($r_p$ and $d_s$ in parsecs) or $V_p$ = -62.68 ($d_s$ in parsecs). I think the parsecs unit is exclusive to $d_e-p$.
E.UCIT
<planet><solar-system><apparent-magnitude><magnitude><absolute-magnitude>
How to calculate absolute magnitude of planets?
E.UCIT
# How to calculate absolute magnitude of planets?

(Asked by: username_0 on 2022-08-07)

Knowing:

* Star's radius $r\_s$ , luminosity $L\_s$, and absolute magnitude $V$
* Planet's radius $r\_p$ , albedo $a$ , and distance $d\_s$

How can you calculate the absolute magnitude of a planet in a hypothetical star system?

When plugging Earth value into the answer given by Micheal B. [here](https://astronomy.stackexchange.com/questions/5957/compute-planets-apparent-visual-magnitude), it gives me $V\_p$ = 19.83, but it should be around -3.99, although this could be an error on my part.

**Tags:** planet, solar-system, apparent-magnitude, magnitude, absolute-magnitude

> **Comments:**
> **username_1**: Welcome to Astronomy Stack Exchange! Can you click "Edit" and add to you post what you've tried so far, and which equations you think might apply? Pure "please work this problem for me" questions are generally not well-received, and usually require some evidence of effort.  *Thanks!*  
> **username_2**: I don’t think absolute magnitude is well defined for planets(ie distance). For stars it’s 10 parsec, tho.  
> **username_3**: https://en.wikipedia.org/wiki/Absolute_magnitude  
> **username_4**: So this would be the magnitude of the planet at 10 parsecs as viewed from the star (so the phase would be full). You can use HORIZONS https://ssd.jpl.nasa.gov/horizons/app.html or similar to get this for our solar system's planets and curve-fit, assuming your hypothetical planets are similar enough to ours. In particular, that they're Lambertian reflectors: https://en.wikipedia.org/wiki/Lambertian_reflectance  
> **username_5**: This question seems like it might be similar and has some good answers. https://astronomy.stackexchange.com/questions/5957/compute-planets-apparent-visual-magnitude  
> **username_6**: Does this answer your question? [Compute Planet's Apparent Visual Magnitude](https://astronomy.stackexchange.com/questions/5957/compute-planets-apparent-visual-magnitude)  
> **username_0**: Whenever I plug Earth values into Micheal's answer to test it out, I get $V_p$ = -8.16 when it should be around -3.99; although, I could be doing something wrong..  
> **username_7**: Are you using the right units? (Parsecs for distance in Michael’s answer, even though it says light-years in the OP.)  
> **username_0**: @PierrePaquette I think you may be correct about using the wrong units, but when using parsecs $V_p$ = 19.76 ($r_p$ and $d_s$ in parsecs) or $V_p$ = -62.68 ($d_s$ in parsecs). I think the parsecs unit is exclusive to $d_e-p$.  

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