# What is the fastest an earth-like planet can spin and still support life, and would it be possible to launch space-craft from it?

I was wondering, if a planet in similar size and mass to earth would have, from its creation, been spinning at this hypothetical planet speed limit (with it somehow staying in once piece) could life still evolve with the conditions that seem catastrophic to us? (huge disk shaped equator, from what I gather, and extremely intense winds)

And more importantly, could they even begin to launch space-ships, or would the terrible conditions keep them bound to the ground?

The context I am using is, Imagine that a probe travels to the planet, would it just be a big blur? And could the probe accelerate to the speed of the planet and orbit it?

• How fast is too fast is a very good question. Launching spaceships, however, would be only easier. Commented Sep 3, 2019 at 20:56
• This is why rockets launch from Florida in the US, and why some have suggested moving rocket launches to places on the equator like Kenya. Commented Jun 9, 2020 at 21:31

It is an interesting question. Firstly, because of the science-based tag, it is unlikely that any planetary body could rotate at anywhere near the limiting rotational velocity. Most planetoids in the solar system have a period more than about 2 hours - anything faster is generally very small (sub 1 km) although there are a few exceptions (one object 400 km in diameter with a 30 minute period is listed in a wiki article). It is not obvious how normal planetary creation or even a violent collision could give an earth-like body anywhere near that rotation rate.

But allowing for some wierd natural phenomenon (or artificial means of spinning up), a rapidly spinning body can form into a stable oblate spheroid or a rod-shape or a multi-lobed structure (or even a torus).

Assuming the oblate spheroid option, the maximum stable spin is when the surface gravity at the equator is equal to the centrifugal force on an object at the equator. That means that a body at the equator is (almost) in geo-stationary orbit. Any faster and the planet will fly apart.

I don't have the maths tools here to do a detailed calculation but for the earth, I believe the limiting obaticity is around 3:1 so the earth would have a radius of ~11000 km at the equator but only about 4000 km at the poles. It would be rotating approximately once every four hours.

Effective gravity at the equator (that is the sum of the effects of gravity plus centrifugal acceleration) would be almost zero - i.e you would be nearly weightless. At the poles gravity would be about 2/3 of what it is on earth. Interestingly though, on the oblate spheroidal surface, the local 'down' would still be perpendicular to the surface at every point (assuming the planet was in hydrostatic equilibrium) - although the horizon would appear much closer at the equator and much further away at the poles compared with Earth.

With regard to intense winds etc - the rate of rotation and the oblacity wouldn't in themselves cause excessive winds - but they would likely result from a combination of solar radiance (heating the atmosphere) and the Coreolis effect (which would be much greater than that which we experience on Earth). But whether this would cause mega hurricanes, or rapidly circulating latitudinal air-flows like you see on the gas giants is in the realms of exoplanet meteorology so I cant really comment. In principle there is no reason why the atmosphere might not be relatively stable.

On other effect of this oblacity is that if there was a significant axial tilt like earth's then the seasons would likely be a lot more extreme - as the poles would have a surface far more oblique to the sun and the portion of the planet s surface that could be considered 'polar' would be much larger.

No obvious reason why a hardy intelligent species couldn't evolve in these conditions. And they have the bonus that if they can, their rockets would have a much easier job taking off - so long as they built their space-ports at the equator.

• How do you rate the risk of the atmosphere escaping at the equator due to being at nearly geostationary height? And if it is not escaping, will it vary in density between poles and equator? If so how much, will it be to thin in either of the extreems? Commented Sep 4, 2019 at 7:01
• If you were close to the limit, I imagine that the atmosphere would escape very rapidly - in fact catastrophically. At 50% of the limiting rotation rate the worlds atmosphere would still be escaping much faster than Earth's, but probably only a significant issue over geological times. Regarding pressure/density. Assuming the same atmospheric volume as Earth, the pressure would be less everywhere (due to the lower local force of gravity everywhere). Commented Sep 15, 2019 at 21:06
• At limiting velocity, pressure would drop to zero, but at more reasonable rotation rates (say 50% of maximum) there would still b a significant pressure differential between poles and equator. One possible 'bonus', although atmospheric pressure would be lower at sea-level, and hence harder to breathe, the rate at which pressure would drop with altitude would also be reduced. So you might have a chance to be able to climb the enormous mountains at the equator. Commented Sep 15, 2019 at 21:06

It is probably not possible for a planet to be rotating at near the structural limit for long enough for intelligent life to evolve. The earth had a day of around ten hours right after the moon was formed, but the tidal effects of the moon and the sun slowed it down to its current 24 hours, and it is still very gradually slowing. Even without a moon, the parent star would have noticeable tidal effects (for the sun, it's 46% as large as the moon's effect).

The tidal effect would be weaker if the planet were denser and hence smaller.