# What are reasonable rotation rates of an Earth-like world?

I have the radius, volume, mass and density figured out. It does not have a moon. I do wonder how that will affect the rotation. The Earth's rotation was considerably faster before the creation of the moon, and I do wonder how long of a rotation period it would have and what is a reasonable planetary rotation period in general. How does this affect changes in obliquity?

The star is an F-Type Star (Beta Virginis), and I haven't figured out the exact age either. I am currently using the star's age as an approximation of the planet's.

"2.8 to 3.2 billion years" or even up to 4.1 Billion years old is what I am finding for the star.

• Just about anything between a few hours and a year is reasonable. A tidally locked planet (like Mercury) will have a 1 year rotation, while a fast planet could have a faster rotation than Earth. Commented Sep 14, 2018 at 1:22
• You're asking two different questions ("how that will effect the rotation", which is the length of the day) and ("how long of an orbit it would have", which is the length of the year). Did you inadvertently use "orbit" when you really meant "rotation"? Commented Sep 14, 2018 at 6:01
• I mean't orbit around the axis which is rotation. Clearly I should be calling it rotation and never orbit due to that being synonymous with orbit around a star. Commented Sep 14, 2018 at 16:12
• I've edited to incorporate that clarification. Remember you can edit your own posts, which is better than leaving the clarification in a comment. Commented Sep 22, 2018 at 15:07

There's a few factors that need to be addressed. For one, how big is the planet's sun? Also, how old is the planet?

Assuming that these are the same as for Earth, the rotation rate would be slowed down slightly by the Sun, but not as much as by the Moon; I think anywhere between 8 and 12 hours would make sense given Earth (which was about 6 hours IIRC before the moon formed). This is assuming the planet is Earth-sized; it will be shorter if the planet is bigger, longer if the planet is smaller.

Also keep in mind that this will affect the climate of the planet as well. If it has a 12-hour rotation, then it will likely have three Hadley cells per hemisphere like Earth, while if it's 8-hours, then it will be five likely (and at somewhat irregular latitudes for some reason). These will affect where deserts and wet areas will be (however, the equator will be always in general wet, while the poles will generally be dry). Also keep in mind the size and average temperature of the planet heavily affect it; if it is larger or colder it is more likely to have more.

• Ah thank you, it's slightly smaller in Radius and volume but it does have a higher density. Thank you for this information I've added more to the question to be a better help. So faster rotations cause more hadley cells? Commented Sep 14, 2018 at 1:26
• In general, yes. If it's slightly smaller but with a higher density, and with a F-type star but like 3 billion years old then those effects will probably mostly cancel and be left with the 8-12 hours. I mean we don't really know much about this sorta stuff esp with the Hadley cells which is actually kinda nice since it gives you more legroom Commented Sep 14, 2018 at 2:20
• Ok thanks for the quick reply, since I am here and asked this question and all what kind of effects on the native life would this have? different circadian rythmns more of a need for eyes to handle different levels of light easier? Anything else to climate besides changes to hadley cells? Commented Sep 14, 2018 at 3:20
• @BlindingLight don't be so sure that complex life would form on such a planet. (The winds would be very high.) Commented Sep 14, 2018 at 6:03
• I was more worried about it having time to develop than the winds. What is this about it being high enough to prevent complex life? How fast would these winds be? Commented Sep 14, 2018 at 16:14

The orbit depends on the mass of the central star and the distance from it. A small sun with a low mass can enable a habitable planet to have a short orbital period, larger suns can enable longer periods. Can also change the energy output if you want your planet to be habitable, for example our sun with a higher energy output might make a planet with the distance of Mars habitable for humans, which has an orbital period just short of 700 days.

Since you have a specific star in mind and want and earth-like planet, you can probably calculate/look up the goldilocks zone (centered at around 1.87AU according to quick googling, but there might be something more detailed out there), where life as we know it is possible, and pick any distance in that zone to calculate the orbital period with the formula T = 2 x pi x sqrt(a^3/µ), T being the period, a the semimajor axis (radius in your case), µ = GxM = gravitational constant*mass of the star.

The rotational period can be affected by a moon, but there really isn't any restriction to it. You can literally take your pick - tidally locked to the central star, short day/long year like Earth and Mars, more than a year and reversed rotational direction like Venus, huge planet with an extremely fast rotation like Jupiter (~10 hours, through a telescope you can see that it has a roughly ellipsoid profile due to the centrifugal forces).

Don't know what you mean by changes in "obliquity" in this case. There wouldn't be any issues with irregular orbits since it is't a dual (or more) star system, but an elliptic orbit is possible. That's your decision, though.

• I ended up making things confusing, I truly only wanted the rotation but referred to an orbit I thought I already had. Thank you for adding it though, I can always check and make sure it adds up if I suspect I've done something wrong or something was innaccurate. I did mean obliquity and not Eccentricity. I was wondering if a faster rotation would mean the tilt changing faster. Commented Sep 14, 2018 at 16:20