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Saturn is the flattest planet in our solar system, with an equatorial diameter 10.7% thicker than its polar diameter. Outside our solar system, the dwarf planet Haumea is flatter still, with an equatorial diameter along its long axis double the size of its shortest axis. While all planets are roughly spherical by definition, relative flatness varies with density, spin and mass.
If you want to learn more about the shape of planets and how flat they can be, then this article can help. Continue reading to find out why Saturn is so flat, how some planets become flatter than others, and the limits on how flat a planet’s shape can be.
Why Is Saturn The Flattest Planet?
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Saturn’s polar diameter is only around 90% of its equatorial diameter, making it the flattest planet in our solar system. This flatness is partly due to the relatively low density of Saturn (0.7 gm/cm3) and partly due to its fast rotation compared to neighboring planets. Saturn has the second shortest day in our solar system, spinning on its axis every 10 hours and 34 minutes.
Jupiter is the second flattest planet after Saturn, being 6.7% thicker in diameter at its equator than at its poles. Like Saturn, it has a relative low density (1.3 gm/cm3) and a fast spin which gives a Jovian day of just under 10 hours.
In comparison, terrestrial planets in our solar system are smaller, higher density worlds which don’t spin on their axes as fast as Jupiter or Saturn. Earth, for example, has a 24 hour day and a density of 5.5 gm/cm3 . While not perfectly spherical, Earth and Mars are rounder than Saturn. Earth is only 0.3% thicker at its equator than at its poles and Mars 0.6% thicker.
What Makes A Planet Flat?
As already mentioned, the flatness of a planet is influenced by its size, density and spin, with lower density and faster spin causing a planetary bulge at the equator. But why?
Planets are roughly round in shape because if a celestial body is sufficiently massive, its own gravitational field pulls all its matter towards its center of gravity equally from all sides, forming it into a spherical shape. (A sphere is the shape in which all mass is drawn as close to a body’s center of gravity as possible.)
Now think about what is happening when a rounded object spins. The area at the outer edge has to move faster and further than areas closer to the center in order to keep up with them. You can visualize this by thinking of common spinning objects such as a wheel or a DVD.
As a planet spins, this motion creates a centrifugal force which pushes matter outwards from the center of the planet. This will be countered by the gravitational force of the planet which will be proportional to its size.
Even with a spin as fast as Saturn’s, centrifugal force remains weaker than the gravitational force exerted by a planet of that size, and matter is not thrown into space.
Still, the centrifugal force can be strong enough to distort a planet’s spherical shape. This force is experienced most strongly at the equator and falls to zero at the poles which are the centre of rotation. This uneven distribution of centrifugal force creates the equatorial bulge and corresponding flattening at the poles.
Density contributes to a planet’s more rounded or flattened shape because less dense materials are more malleable than more dense materials. The gases which make up so much of Saturn’s mass are more easily distorted by centrifugal force than the rocks, water and soil of Earth or Mars.
How Is The Flatness Of A Planet Observed?
Measuring the size and shape of a planet, including its diameter, allow us to observe its relative flatness or roundness. There are several ways to obtain these measurements.
For our own solar system, we have several decades of data obtained directly from space probes orbiting or flying past the planets. This includes photographs and electromagnetic measurement data (e.g. radar measurements of planetary diameters).
Before the days of space probes Earth-based means were used to deduce planetary size and shape.
One way to calculate size is by using very precise values of a planet’s apparent angular diameter (how big it appears against the sky) obtained with a telescope, and then scaling up this value using a measure of the planet’s distance from Earth.
In our solar system these values have long been deduced from knowledge of planetary orbits around the Sun.
An alternative approach to calculating planetary size involves using a telescope to measure the movement of moons as they eclipse the planet. This method was used to determine the equatorial and polar diameter of Mars (and therefore its slightly oblate shape) more than a century ago.
How Flat Can A Planet Be?
By definition, a planet must have sufficient mass for its own gravity to create a rounded hydrostatic equilibrium shape of spherical or oblate form. Celestial bodies which are too small to establish hydrostatic equilibrium cannot be classified as planets.
While this places a limit on how flat planets can be, Saturn isn’t close to this limit and the laws of physics allow for much flatter worlds. The flattest dwarf planet currently known is the Kuiper belt body ‘Haumea’, which has an equatorial axis diameter double the size of its shortest axis.
In theory, an even flatter planet than Haumea could exist. The theoretical limits to flatness are set by the point where a planet spins fast enough to overcome its gravitational field and start throwing equatorial matter into space.
With an Earth-like planet, a maximum flattening ratio of 3:1 (equatorial to polar diameter) may be possible before matter from the equator begins to be lost into space.
Planets with different compositions (e.g. made of uranium) might be able to flatten up to a 5:1 ratio before beginning to disintegrate under high centrifugal forces.
A Final Word…
Saturn is the flattest planet in our solar system. Haumea is the flattest known (dwarf) planet. Physical laws allow for the existence of an even flatter planet. Depending on composition, the flattest possible planet could have an oblate shape of ratio of 3:1 or even 5:1. A completely flat planet (Earth or otherwise) cannot exist.
https://solarsystem.nasa.gov/basics/chapter1-2/#:~:text=The planets Mercury, Venus, Earth,planets in the solar system.
https://www.scientificamerican.com/article/why-are-planets-round/#:~:text=Planets are round because their,and pulls everything toward it.&text=The only way to get,process is “isostatic adjustment.”