# How flat may be the planet?

We know that the Earth is not flat, for hundreds of years. There are many ways to demonstrate this, starting with the masts of ships disappearing over the horizon, the ability to see farther, climbing higher, and ending with the long shadows that the sun casts at higher latitudes. In the end, you can just go into space and see the round shape of our planet firsthand.

But if the Ground got lucky with a form, this does not mean that every planet should be. Moreover, some observations suggest that we could get a flat round planet. How flat may be the planet?

To test this, you can take a solid slab of stone, steel, diamond or graphene — and to build so large flat disk as possible. If you were using traditional materials like these, you could create a thin, stable, flat disk with a radius of many kilometers. Simply put, it would be possible to create a flat world that was bigger than any object in our asteroid belt and maybe the size of the moon itself.

But it could hardly be called a planet. Back in 2006, we established three criteria that define a planet. (Since this definition was transferred to ekzoplaneta!). To be a planet, the world:

– must be in orbit around the sun (but not another body like a planet),
– must have enough mass that its self-gravity to overcome rigid forces of a body and the body has reached hydrostatic equilibrium (a round, flattened or elongated in the case of rapid rotation,
– should clear the surrounding area in the direction of motion in orbit (so that there weren’t any other large bodies of comparable size),
The second paragraph of this definition is not entirely suitable for our special, flat, thin world. If it is massive enough to achieve hydrostatic equilibrium, it cannot be considered a planet.

However, the way to create a relatively flat planet exists: to make it spin. Our planet Earth spins relatively slowly: it takes 24 hours before it will make a turnover of 360 degrees. This means that people living at the equator, as far as possible from the axis of rotation of the Earth, experiencing the extra speed at 464 meters per second (1,800 km/h), when compared with people at the poles. This extra speed influences the shape of the whole Earth and leads to the fact that it takes the form of a flattened spheroid: an almost perfect sphere which is flattened at the poles and it bulges at the equator.

The diameter of the Earth at the equator is 12 756 kilometres, while at the poles — only 12 714 kilometers. You 21 kilometers closer to the center of the Earth when standing at the North pole, not the equator. It’s not much, but there are worlds that revolve much faster. Gas giants all rotate pretty quickly, and pole of Saturn is compressed by 10% relative to the equator.

But this is not the limit. The laws of physics, the world may be much more flat. We’ve never seen, because all we have is eight planets, but opening up massive asteroids and worlds in the Kuiper belt, we met a rather strange objects. For example, a massive object Haumea in the Kuiper belt, the Equatorial diameter of which its long axis is two times larger than the short axis. The ratio of 2 to 1 is most remarkable in the context of hydrostatic equilibrium, which we met.

Scientists believe that the rapid rotation of Haumea, it was determined the collision and its two known moons: Namaka and Hiiaka. The larger Hiiaka, exerts a powerful gravitational influence on haumea, further complicating the system. Haumea is not just a world with a long the equator and flattened poles: it has three separate axes of different lengths, which make it a triaxial ellipsoid.

In other words, Haumea is just one of the extreme examples that we know, but in theory the world could be even more flat. The denser and faster than the planet rotates, the flatter it will be. In principle, the limit of flatness sets how quickly you can rotate the world before its Equatorial particles begin to jump off the planet into space, overcoming the gravitational pull. On a planet like Earth, we could achieve the maximum flattening of about 3 to 1, before our equator will begin to escape into space, a planet consisting entirely of uranium, could achieve a ratio of 5 to 1.

Flatter than the planet, the harder it will be to remain tight because of the internal forces will create friction and differential rotation in the outer layers. Just as the outer part of Saturn’s rings rotate slower than the inner particles of the rings, the tapered planet have to contend with the same forces. Theoretically, the world can be very flat, but the planet which is completely flat, as in medieval myths, and to obey the laws of physics — this will not happen.