I would like to have a world on which humans can live unaided -- they can breathe, the climate is workable, they can eat the local vegetation, etc -- but on which a complete day is substantially shorter than it is here on earth. I think this means I need a faster rotation or a smaller planet or both, but I don't know what negative effects would come from either of those. In case it matters, the humans in question came to this planet; they weren't born here.

Without resorting to magic or advanced technology -- I want a planet that's like this "in the wild" -- can such a world exist? If so, what are its key properties?

I'd like to achieve a day that's about half that of earth's. (Even were it possible, I'm not aiming for a 43-minute asteroidal day.)

I'm not asking about the effects on the humans of the shortened day itself (disrupted circadian rhythm, psychological effects, etc).1 However, if the conditions that allow this planet to exist -- rotation, gravity, atmospheric change, tides, etc -- affect humans, I care about that.

1 That question is here.

  • $\begingroup$ Did they travel there from Earth, or evolve on the asteroid? There's some well known physiological issues that arise when a H. sapiens does not get lighting that matches a circadian rhythm of 12 hours. However, this is not a magic number... it's simply what evolution chose because there was no reason to be flexible when Earth's rotation was so reliable. $\endgroup$
    – Cort Ammon
    Commented Jun 26, 2015 at 2:35
  • $\begingroup$ They came from Earth and end up stranded here. Adjusting to the new circadian rhythm is part of the plot, but I don't want them to first have to worry about breathing, freezing to death, etc. $\endgroup$ Commented Jun 26, 2015 at 2:39
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    $\begingroup$ Don't go to short $\endgroup$ Commented Jun 26, 2015 at 4:13

5 Answers 5


There is no need to go for a different planet size. Just change the rotation rate of the planet. Since the current models suggest that, just after the moon was formed, the length of a day was about 2 hours, a 12-hour day certainly seems reasonable. Plants and animals will need to have evolved the appropriate circadian rhythms, but that's no problem.

There is no obvious gross effect of a shorter day that I can think of, but someone is sure to come up with a suggestion or two.

EDIT http://www.artificial-gravity.com/Dissertation/2_2.htm reviews various research efforts on the subject, and establishes 1 rpm as a reasonable lower limit on negative effects. Since a 12-hour day has a rotation rate 720 times slower, there seems no reason to worry.

It's worth pointing out that there are hundreds of rotating restaurants. For instance, the Space Needle restaurant rotates at 1 revolution per hour, and nobody seems to have problems with it. Of course, exposure is relatively brief, an hour or two, so there might conceivably be long-term effects, but this seems unlikely.

  • $\begingroup$ 2 hour rotations seem to be the limit for a wide range of planet sizes. Also the Earth was only spun up to this rate by the impact of Theia. About 2/3 of the planet was molten and the surface was bombarded with material that had ejected from the planet. You'd need to wait at least 80 - 100 MY before anything could live on it. $\endgroup$
    – Jim2B
    Commented Jun 26, 2015 at 2:35
  • $\begingroup$ Sorry - edit error. $\endgroup$ Commented Jun 26, 2015 at 14:52
  • $\begingroup$ Cuk and Stewart, "Making the Moon From a Fast-Spinning Earth" mygeologypage.ucdavis.edu/stewart/reprints/papers/… suggest an initial Earth rotation rate of less than 3 hours. If the impact had not occurred, there would have been little to slow the earth down, so the Earth would presumably be spinning much faster today. $\endgroup$ Commented Jun 26, 2015 at 14:55

I don't see how any negative effects could result from having a shortened day. From my understanding it would make the daily temperature fluctuate much less. They have huge planets that don't rotate at all and are perpetually freezing and burning on the respective side of the planet. This planet would not have that problem.

One consideration is that the planet probably shouldn't have a moon unless it has been formed very recently. The Moon slowly but surely lengthens the days for the planet Earth by dragging on its oceans. This will also be caused by the planet's sun, but the sun's effects should be negligible because of the huge distances.

  • $\begingroup$ I don't see how any negative effects could result from having a shortened day. => the pendant, however, is that the conditions that create this shortened day may have a negative impact; as a result, it is necessary to fully answer the question to also establish a set of conditions that (1) would create the shortened day and (2) not have any negative effects. If no such set of conditions can be found, then it is an answer as well (but proving absence is always rather difficult). $\endgroup$ Commented Jun 26, 2015 at 11:56

If this is pretty much Earth in every other respect, as long as the planet rotates regularly, it should be okay. Too little and it heats unevenly, burning sunside and freezing nightside. I'd imagine seasons are a little more mild on a planet with 12 hour days because temperatures don't get to rise or fall for as long. (I'm considering how rotating planets are a little like rotisserie.) That would drastically effect what sort of weather we'd expect. Temperature plays a huge part in weather. Less extreme temperatures means less extreme weather.

Now onto other matters:

I don't recommend the size change. Changing the size of the planet has several effects, rotation not being one of them. Rotation is partially due to distance from the star (too close and it will lock face, no rotation possible). The rotation of Earth was likely caused by a massive impact early in its formation.

If it has similar density to Earth but is smaller, it will have less mass, creating less gravitational force. This will change humans for the worse since we're evolved for Earth level gravity. Astronauts suffer from bone loss, for instance.

If the planet is larger it may have more mass (depending on what it's made of), creating stronger than Earth gravity. I suppose it could have less volume and therefore be larger and equivalent mass. It might not be as stable though.

Atmosphere is extremely crucial to get right as well, along with the magnetic field, which protects life from projectiles.

  • $\begingroup$ Earth's rotation wasn't caused by a large impact event, as far as we know. It was caused by the angular momentum of the cloud of dust and gas that preceded it and the solar system. $\endgroup$
    – John
    Commented Jun 26, 2015 at 14:46

Firstly, with regard to possibility and the planet size and rotational period, note that Jupiter and a radius more than 10x that of the Earth, but a day of only 9.8 hours. Note also that the Earth's day was originally much shorter and has increased. This is mainly due to the tidal effect of the moon. The moon moves slowly away from the Earth, carrying angular momentum.

So, to keep a shorter day, all you have to do is remove the moon.

This in itself has an effect on the Earth. Tides are much reduced - although there are still some effects from the Sun.

If you believe tides helped life evolve from the sea to the land, then maybe that's important. It's hard to argue that this is really a show stopper.

Possibly maintaining rotation rate helps maintain the magnetic field, but I am far from certain on that one. Note that the magnetic field protects from solar storms, and hence protects life on Earth.

The weather is partly driven by the rotation of the Earth. For example, the Coriolis effect will be stronger. So you might see slightly stronger storms. We might note that storms on Jupiter are extreme.

There's a general idea that life is driven by energy flows, mainly sunlight but also for example dark smoker volcanic vents. In this view, a faster spinning planet to provide stronger energy flows and help life evolve.

Overall, I see no reason why such a planet would not support a complex ecosystem and highly evolved lifeforms.

  • $\begingroup$ Jupiter spins faster and has a shorter day, but it also can't support human life. I don't know enough to know if there's any causality there. Good point about the effect of the moon; I hadn't realized it was that pronounced before asking this question. $\endgroup$ Commented Jun 26, 2015 at 12:56
  • $\begingroup$ My point was merely that you don't need a smaller planet to go faster. See also worldbuilding.stackexchange.com/questions/575/… . Jupiter cannot support human style life for many other reasons than the length of day. $\endgroup$
    – Keith
    Commented Jun 29, 2015 at 1:11

Don't remove the moon. A large moon relative to the planet (Say 1:6 mass ratio as our is) stabilizes the axis of rotation, making for a stable set of repeating seasons that the animal and plant life can grow with. Will also want moon to revolve around the planet more slowly than the planet spins.

A very short distance from its star or moon will also affect rotation due to the tidal lock forcing one face to always be pointed toward which ever one has the greatest effect of gravity on it. (The moon always has the same part facing the earth for this reason, and eventually the moon will slow the earth's rotation and the earth will speed the moon's revolution enough that they'll synchronize so that the same part of earth faces the moon at all times - around 45 current earth days.)

Rotation rate about its axis determines length of day as does planet diameter and thickness of atmosphere. A planet with a large diameter or thick atmosphere will appear to have longer days since the star's rays will continue to pass through and bend in the atmosphere to keep it light even as the star dips below the horizon. This can be seen on earth every day.

Angular momentum and gravity are of concern when you change the rotation speed. Our acceleration due to gravity is 9.8m/s/s or 1g. Increase either or both the rotation rate and diameter but keep the mass the same and the inhabitants will feel less gravity. Decrease either and they'll feel more gravity. it's a balance between the three. If the mass is increased as well as the spin rate, but the diameter is kept the same, the force of gravity can remain the same if you want that.

These all affect tides of any body of water on the planet as well as weather. The faster storms caused by the rotation would likely help equalize the temperatures but most likely will cause bands of pressure and temperature like those on Jupiter.

Keep in mind that during the time of the dinosaurs, the earth had only 23 hour days, not 24 and probably had 2-3 hour days when the moon was formed. Life arose early but complex life only arose in the past half billion years or so, so rotation rate would have been probably longer than 18 hours through most of it.

Lots of numbers to crunch, but ask any engineer friend and they can run them for you pretty quickly so you get what you need for your story. We all use these equations in our college studies, so none of it should be hard for them.

Good luck, and if your story gets published, my name is J-P for your contributions page! :)

  • $\begingroup$ The earth's rotation is slowing down, the moon is pulling away from the earth thus it's orbit is also slowing down, not speeding up. $\endgroup$
    – bowlturner
    Commented Jun 26, 2015 at 18:16

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