# How to calculate absolute magnitude of planets?

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, it gives me $$V_p$$ = 19.83, but it should be around -3.99, although this could be an error on my part.

• 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
Aug 7, 2022 at 6:32
• This question seems like it might be similar and has some good answers. astronomy.stackexchange.com/questions/5957/… Aug 7, 2022 at 13:45
• Does this answer your question? Compute Planet's Apparent Visual Magnitude Aug 7, 2022 at 20:48
• 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.. Aug 7, 2022 at 21:38
• @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 $d_e-p$. Aug 8, 2022 at 5:15