Ok, my head hurts after thinking about this and writing this question at least six times. Finally I have something that might pose as a question.

Can a planet have a 6 Earth-month rotation around its star at the same time it has a 12 Earth-hour day?

How does this work and is it possible? How does this affect the climate and can it be habitable or does it require assistance?

If not how much can I chip off the aforementioned times to have it maintaining habitability and being feasible?

  • $\begingroup$ Seems a little broad. Firstly, are you speeding up the planet or moving its orbit onto a shorter path? $\endgroup$ – Bellerophon May 26 '17 at 18:14
  • $\begingroup$ @Bellerophon This exactly what I am asking how do I half it? $\endgroup$ – Mendeleev May 26 '17 at 18:15
  • $\begingroup$ Sorry, I thought you were asking what would happen if we sped up a planet. Can we assume the planet in question is similar to Earth (as in size, composition, position)? $\endgroup$ – Bellerophon May 26 '17 at 18:17
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    $\begingroup$ What are you asking? Of course, you can have a planet closer to the sun and spinning faster $\endgroup$ – Joe Kissling May 26 '17 at 18:18
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    $\begingroup$ "How does this work?" for a half-Earth-year orbital period and 12 Earth-hour rotation period is trivially answerable (and I think we've even had a few very similar questions recently). When you start asking about the climate of the planet, that's where it starts getting broad. If you delete everything after the first question mark of the third paragraph (leaving only "How does this work and is it possible?" in that paragraph), I suspect this question will be fine, though I also suspect that it'll be a duplicate of something somewhere around here... (said while turning over virtual papers) $\endgroup$ – a CVn May 26 '17 at 20:52

Six-month year

Yes, it's perfectly possible for a planet to have a year of approximately six Earth months (since our months are of varying lengths, it's hard to be precise). For clarity's sake, let's say that the orbit is 180 Earth days. If it were orbiting the sun, that would be roughly halfway between Venus (224 day orbit) and Mercury (88 day orbit). Given that both of those planets exist, yes, it's perfectly possible to have a planet with a 180-day year.

Orbiting other stars, the exact distance from the star for that kind of year would vary by the mass of the star (the lower the mass of the star, the slower the orbit). We've seen planets with an orbital period of as little as 2.2 hours. It's all a balancing act between the planet and its star.

12-hour day

Also perfectly possible. The length of a planet's day is largely unrelated to its orbital period. A planet that's very close to its star will quite quickly become tidally locked, with a day the same length as its year, but a six-month orbit probably isn't close enough to do that with most stars.

A planet's rotation can be affected by things like massive impacts, orbiting moons, even other nearby planets; certainly you can handwave having a six-month year and a 12-hour day.

How does this affect climate

It depends...

And would it be habitable?

...on the star.

If you're orbiting a red dwarf star in a six-month orbit, you're probably outside the star's habitable zone. Red dwarfs don't put out anything like as much energy as our sun, so you need a much closer orbit for a planet to be habitable. At one extreme, look at a system like TRAPPIST-1, which has three planets in its habitable zone. The closest one has an orbital period of a little under 19 days.

With this kind of variation, it's really difficult to estimate what the effect might be on the climate. We can guess that you may have less temperature variation across the planet; there's less time to warm up during the day, and less time to cool down at night. Since that kind of temperature difference is what mainly drives weather systems on earth, you might see a reduction in weather fronts. On the other hand, faster rotation could make up the difference. It's difficult to say for sure.

There are a few tools you can use to roughly calculate a good planetary orbit for your world. This one lets you calculate what the orbital period would be at a given distance from the star, based on the mass of the planet and the star. This one lets you calculate the approximate habitable zone for a star of a given temperature and luminosity.

  • $\begingroup$ Note that since Saturn's day is 10 hours 42 minutes, it's also perfectly possible to have a planet with a 12 hour day. $\endgroup$ – jamesqf May 27 '17 at 4:57

What are the requirements for a planet to orbit around the sun at twice the Earth's rate?

The closer something is to the sun, the faster it will need to revolve around the sun (meaning shorter years) to maintain its orbit without falling into the sun. Conversely, the farther away from the sun something is, the more slowly it'll need to revolve to maintain its orbit without being flung away.

You can see this in a quick and dirty orbital simulation here, showing that objects closer to the star orbit more rapidly.

In fact, in 1609, based on new measurements of our own solar system, Johannes Kepler figured out some general rules. Kepler's third law of planetary motion states that the cube of a planet's semi-major axis (that is, the distance from the sun) is proportional to the square of its orbital period (year). So to get a body to orbit twice as quickly requires it to be about a third closer: I calculate that for a planet to revolve every 180 days you'd want it to be about 0.62 AU from the sun.

Would it rotate at twice the rate?

While the rate of a planet's revolution is determined by its proximity to the star, its rate of rotation is almost entirely arbitrary. Tidal force between two bodies acting over a sufficiently long period of time can cause tidal locking, although no known planets are synchronously tidally locked with their star.

Mercury is “locked” with a 3:2 spin-orbit state. So planets very close to the sun will not have arbitrary choices for day length. The distance in question is more like that of Venus, so this won’t be a problem here.

  • Jupiter has the fastest rotation in the solar system, with a day on Jupiter lasting less than ten hours Earth hours.
  • In contrast, a day on Mercury lasts 176 Earth days.
  • Venus actually rotates in retrograde (backwards, relative to its orbital revolution) exceptionally slowly, with a sidereal day lasting 243 Earth days, and a solar day lasting over 116.75 Earth days.
  • Neptune's magnetic field rotates every 16 hours

Would a planet that close to the sun be habitable?

The Circumstellar habitable zone defines the range in which a planet can sustain liquid water on its surface. Any nearer the sun from the inner edge of the habitable zone, and water is vaporized by a runaway greenhouse effect, then gets broken up by sunlight and the hydrogen is blown off into space.

An estimate from 2013 suggests that the inner edge of the habitable zone around our star can be closer to the sun than previously estimated if you take into account reduced humidity and a greater surface albedo. That paper puts the inner edge of the HZ at 0.38 AU.

This gives you plenty of room for your planet, which I'd earlier calculated would need to be at 0.624194245 AU from the sun to revolve in 180 days.

Do note that other estimates suggest that you can't get much closer than 0.99 AU before leaving the habitable zone, so the science there is still fuzzy and openly debated.

Would a planet rotating that quickly be habitable?

You can expect less temperature variation between day and night, since there'd be less time for the sun to heat things up before they rotate out of range. Winds and currents that are ordinarily caused by daily temperature differences might be weaker or more erratic. There's a chance that the increased rotation could skew the cycle of winds generated by the Coriolis Effect, but the result of such skewing seems difficult to predict.

I imagine the proximity to the sun, and the changes to the planet you'd need to implement to ensure that it can support surface water at that proximity, would have a much greater impact on the climate than the planet's rotation.


Not exactly sure what you are asking so here goes...

Moving a planet

You have a few options for moving a planet, but I'm going to suggest gravity assist. Simply put, have a large amount of mass pass by the planet to change its orbit. To move a planet closer to the sun you will need to increase its speed and probably alter its trajectory to prevent from flinging it out of the solar system. This process will take time and a whole bunch of mass. Simply cannibalize a moon (or several) from an outer planet and launch it towards the inner system in pieces on a close pass to the Earth. Done properly, the launched mass will loose velocity and the earth will gain it. Over time this process will move the Earth towards the sun.

Spinning up a planet

Warning may have geological implications

This is a little more challenging because planets have a whole bunch of inertia and halving the length of a day requires adding a bunch of angular momentum. We are also going to need to add a whole bunch of mass too. The mass will need to be launched on a trajectory that passes very close to the target planet and is moving at a speed that would be equivalent to the velocity of a geosynchronous orbit around the planet. As the mass passes by it needs to be caught by an orbital tether. Catching the mass in this manner increases the overall angular momentum of the system without decreasing the rotational velocity of the earth. With mass captured, reel it in and thanks to conservation of angular momentum, the rotation of the earth will increase. Like so:

enter image description here


Depends on several things, if we are sticking with the Earth-Sun system then the Earth will get much hotter since it will receive more the solar energy than it gets now. Probably pretty bad for the majority of life on earth but Antarctica could be a nice place to live at least.

Now, this is assuming nothing is done to alter the Earth. If we are talking moving planets then altering the atmosphere is easy. Removing the greenhouse gasses is one step that could preserve the earth because without them the earth would be very cold.

With a faster rotation expect winds to be very different, again hard to say directly but expect them to be moving faster. The Coriolis Effect will play a much larger part in the weather than it does here on earth. A combination of more winds and higher temp extremes probably will cause superstorms.

Probably still habitable, just much hotter. For example the solar system inner limit may be as low as .5 AU.


Moving the Earth would be extremely difficult, but there's no reason that such a planet couldn't form. I can't think of any reason it wouldn't work.

Such a planet orbiting the sun would probably not be habitable because of its proximity to the sun (it would be a tad closer than Venus). Around a smaller, dimmer star, a planet at this distance would be habitable, but due to the smaller gravity it would have to be closer, which would make it hotter, so the star has to be smaller... But with each iteration the change gets smaller, so you can definitely find a combination of distance and star size that allows a habitable planet to have a 6-month period. If you want to, you can mess around with this calculator to find periods given distance and star mass.

The issue is that the closer you get to a star, the more likely the planet is to be tidally locked, or at least resonant, meaning it's unlikely to spin very fast. Unless the balancing distance happens to be Mercury-ish or closer, though, I think it's completely possible, if unlikely, for such a planet to form naturally.


The two items you are asking about are not linked. The length of a day on a planet is not tied to the length of a year on a planet. For an often used example, look at Venus. A "day" on Venus is longer than a "year".

The rotation of the Earth around its axis determines the length of its day. Doubling its speed of rotation will reduce the length of the day by half. This would give the equatorial bulge quite an increase in size.

The orbital period of the Earth around the sun gives the length of its year. Cutting the length of the year in half is not as trivial as cutting the length of the day.

Doubling the orbital speed in half might intuitively seem to do it, but a stable orbit at that speed would actually be much nearer the sun. Being closer to the sun, the distance around the sun would also be much less (is it a quarter of the distance? Someone do the math). The speed of the orbit would probably need to be around 15-25% faster than "normal" to settle into a 50% year length.

And again, these two speeds are not linked. Changing one will not require that you change the other.

The Earth would be considerably warmer. Much of its current life would have a tough time. However, the planet is still within the "Goldilocks" zone, so life would likely still thrive. It would just be a different looking kind of life.


Can a planet have a 6 Earth-month rotation around its star at the same time it has a 12 Earth-hour day?

Yes. You'd need to significantly increase Earth's orbital speed around the Sun, while simultaneously reducing its angular momentum.

How does this work

Not too easily, since putting retro-rockets on rotating masses the size of Earth is really difficult.

is it possible?

I doubt whether even a Dyson-sphere civilization could do it.

How does this affect the climate

Being much closer to the Sun, with less time each day (1) exposed to the Sun and (2) cooling off would naturally have a severe impact on the planet. Probably would boil off all the water.

No, it wouldn't be habitable.

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    $\begingroup$ Orbiting closer to the sun actually means a faster orbit. Moving a planet take a long time, but it could be done using gravity assists which would be child's play for a civilization that could construct a Dyson sphere. Climate is hard to call; but where are you getting all the water on earth being boiled off? $\endgroup$ – Joe Kissling May 26 '17 at 18:44
  • $\begingroup$ If you're that much closer to the Sun, the planet is so much hotter. Thus, the water would boil off. $\endgroup$ – RonJohn May 26 '17 at 20:00
  • $\begingroup$ Venus' orbit is 224 Earth days, and this planet's would be 182 Earth days. That's well within Sol's habitable zone. $\endgroup$ – RonJohn May 26 '17 at 20:04
  • $\begingroup$ I don't quite think that you're understanding OP's question. I believe that he's asking if any planet could have these traits, rather than how shifting a planet inwards towards its sun would work. Taking out the bit about the retro-rockets might help it be more applicable to the question at hand. $\endgroup$ – user19838 May 26 '17 at 20:05
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    $\begingroup$ The Goldilocks zone is not a hard line. $\endgroup$ – Joe Kissling May 26 '17 at 20:07

It is certainly possible for a planet to have a year about 182.625 Earth days long and a day 12 Earth hours long, thus having the same number of planetary days per planetary year.

1) Year of 182.625 Earth days.

According to Wickipedia exoplanet PSR J1719-1438 b has a day 0.092 Earth days or 2.2 Earth hours long. Thus it has a year much shorter than the day you desire.


Of exoplanets that orbit in the habitable zones of their stars, TRAPPIST-1e, TRAPPIST-1f, and TRAPPIST-1g have orbital periods or years that are 6.1, 9.2, and 12.4 Earth days long, while Proxima Centauri b has a year 11.186 Earth days long.


But they orbit dim red M class stars and thus are so close that they would probably have tidally locked rotation with years and days of equal length and one side always facing the star and the other side always in darkness.

One way to avoid that is to have the habitable world be a giant moon of a giant planet. Thus the habitable world would be tidally locked to the giant planet. Furthermore, a moon's obit will not be stable unless the orbital period of the planet around their star is at least nine times a long as the moon's orbital period around its planet. So having the planet's year be 365.25 times as long as the orbital period of the habitable moon around it will be no problem.


But I think that I read somewhere that a habitable moon should orbit at a distance at least about 5 times the radius of the planet it orbits.


In our Solar system Proteus, the Neptunian moon closet to 5 radii from Neptune, has a period 1.112 Earth days long; Miranda, the Uranian moon with an orbit closest to 5 radii from Uranus, has a period 1.413 Earth days long; Tethys, the Saturnian moon with an orbit closest to 5 radii from Saturn, has a period of 1.887 Earth days, and Io, the Jovian moon orbiting closest to 5 radii from Jupiter, has a period of 1.769 Earth days.

Thus it seems unlikely that a tidally locked habitable moon of giant planet orbiting a class M star could have a day equal to 0.5 Earth days.

The list of potentially habitable exoplanets also includes Kepler-62f that orbits Kepler-62 a K2V type star, with an orbital period of 267.291 Earth days, and Kepler-442b that orbits K?V star Kepler-442 with a period of 112.3053 Earth days. Thus it is possible that your planet could orbit in the habitable zone of a K class star and have a day about 182.625 Earth days long without being tidally locked to its star or needing to orbit a giant planet in order to have a day shorter than its year.

2) Day of 0.5 earth days.

The four rocky terrestrial planets in the solar system have days at least one Earth day long.

The giant planets in the solar system have days much shorter. The 2 biggest giants, Jupiter and Saturn have days 0.41354 and 0.44401 Earth days long.

Thus one might deduce that the largest planets have the shortest days and a planet a bit smaller than Saturn is necessary to have a day 0.5 Earth days long. That would be bad.

But Earth has about 9.34 times the mass of Mars and the Martian day is slightly longer than the Earth day. And the dwarf planets Ceres and Haumea have days shorter than Earth's - 0.3781 and 0.167 Earth days.

Thus it seems possible for a planet of Earth like size to have a day much longer or shorter than Earth's day.

But Earth didn't gain an oxygen-rich atmosphere or become habitable for humans for billions of years after forming. And during those billions of years the rotation of the Earth gradually slowed due to tidal interactions with the Moon and forced the Moon farther away from Earth. So Earth's days got longer and longer.

Back when the Moon was formed the length of an Earth day was a very brief two to three hours, and a much closer Moon was orbiting the Earth every five hours.


Thus a planet that never had a large moon might have a day only 12 Earth hours long.

But some planetary scientists believe that having a large moon is necessary for a planet to remain habitable, that a large moon stabilizes the axial tilt of the planet.

So you might not be able to get away with a moon less planet. You might need a planet that originally spun very fast and gained a large moon that stabilized its axial tilt and slowed down its spin but less than Earth's Moon did, so that the planet's day is still only half an Earth day long.

So if you are writing soft science fiction don't worry about it, a planet with both a day and a year half those of Earth's is perfectly plausible for your audience. But if you are writing any type of hard science fiction you might want to get someone to calculate all the parameters of your imaginary world to make sure it is possible and everything is consistent with other factors.

You wouldn't want to write that a month on that planet was 25 Earth days long and have a fan calculate that the months must be less than 15 Earth days long for the situation to be plausible, for example.


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