I am currently in the process of compiling a list of stars and their planets, given the information in the European Star Catalogue found here, but I'm struggling to find out how to calculate a planets solar day. To be exact; How long, in earth hours, is the solar day of a planet found in this catalogue?

I know the catalogue is incomplete and that it's likely impossible to calculate the solar day for some of these planets due to this, but that's okay. I could also accept the sidereal day rather than the solar day.

I tried to use the formulas found here but I have a hard time finding out what numbers to plot into the equations, given the information available in the exoplanet catalogue. I've been doing a lot of googling and I'm not an astronomer, so it has been a bit of challenge!

I hope some of you can help me here.

  • 1
    $\begingroup$ You will need the planet's rotation speed and the diameter or circumference of the planet. Also if the planet is tidally locked, then you can just take the orbital period. $\endgroup$
    – WarpPrime
    Jan 5, 2021 at 14:42
  • $\begingroup$ @fasterthanlight Could you elaborate what "tidally locked" means? $\endgroup$
    – OmniOwl
    Jan 5, 2021 at 14:43
  • $\begingroup$ "Tidally locked" means an object that is always facing another object. For example, the Moon is tidally locked to the Earth as its rotational period is the same as the orbital period. Check this link: en.wikipedia.org/wiki/Tidal_locking $\endgroup$
    – WarpPrime
    Jan 5, 2021 at 14:48
  • $\begingroup$ @fasterthanlight Alright, thanks! I have a planets radius in Jupiter Radius so getting a diameter is easy enough, but I'm not entirely sure how to derive the rotational speed given the information in that catalogue. $\endgroup$
    – OmniOwl
    Jan 5, 2021 at 15:00
  • $\begingroup$ What information are you given? $\endgroup$
    – WarpPrime
    Jan 5, 2021 at 15:04

1 Answer 1


This source doesn't give sufficient information to get the length of either a stellar or sidereal day. The rotation rate of a body is not governed by the orbital parameters. The Earth, for example, has a slowing spin rate as the Moon retreats due to tidal interactions. This doesn't affect the Earth's orbital parameters.

The fields given are:

Planet Status           
Discovered in       
Semi-Major Axis
Orbital Period
Detection Method
Mass Detection Method       
Radius Detection Method     
Primary transit
Secondary transit
Impact Parameter b
Time Vr=0       
Velocity Semiamplitude K
Calculated temperature
Measured temperature    
Hottest point longitude         
Geometric albedo        
Surface gravity log(g/gH)       
Alternate Names
  • $\begingroup$ Yeah that seems to be the conclusion. Thanks though! $\endgroup$
    – OmniOwl
    Jan 5, 2021 at 16:26

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