Why are planets round?

Most planets and other bodies in the Solar System are nearly perfectly spherical, but why?

Jupiter, Saturn, Uranus and Neptune, as seen by the Hubble Space Telescope. Credit: NASA, ESA, A. Simon (Goddard Space Flight Center), and M.H. Wong (University of California, Berkeley) and the OPAL team
Published: March 18, 2022 at 4:00 pm
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From the major planets in our Solar System to the very Sun itself, the sphere is the most common shape of celestial bodies, but why is this?

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Gravity is the main force that sculpts the Universe, causing the formation of planets, stars and galaxies.

It works over great distances and has no preferred orientation.

It draws matter in from all directions and when matter clumps together it forms a shape that looks the same from all directions. The shape that meets all these criteria is a sphere.

A remarkable view of Earth from Apollo 16, taken on 16 April 1972, reveals a world of blue and white with a hint of brown; (inset) the image before it was digitally restored. Credit: NASA / Toby Ord
Credit: NASA / Toby Ord

But the force of gravity also distorts the sphere that we would expect to see, meaning most of the bodies that we think of as being round are really 'almost spherical'.

Take our planet for instance. If I was to travel all around the world from pole to pole, I would have covered a distance of around 39,941km.

If I were to do the same journey, but this time around the equator, I would have travelled an additional 130km.

The Earth bulges at the equator to form a shape close to an oblate spheroid – a slightly squashed sphere.

Why are planets slightly squashed?

The centrifugal force of a chairoplane is a simple way of understanding how a planet's spin causes it to bulge. Credit: Andrew Williams / EyeEm / Getty
The centrifugal force of a chairoplane is a simple way of understanding how a planet's spin causes it to bulge. Credit: Andrew Williams / EyeEm / Getty

The reason why planets get a bit fat around the middle is due to their rotation about their axes.

Imagine the chairoplane: that fairground ride with lots of chairs hanging down from a central disc.

As the disc starts to rotates, the chairs lift and move outward from the centre or outward from the centre of rotation.

The only thing stopping the chairs from flying off is the chains.

The same thing happens to a planet when it spins. Gravity acts like the chains, pulling everything inwards, but the speed of the rotation pushes everything outwards.

Just like the chairs on the ride moving outward, as a plane rotates on its axis it grows wider around the middle.

How fast a planet rotates seems to be one of the largest influences on the size of the bulge.

  • Earth, with its 24-hour rotation, has a bulge of about 0.3%
  • One rotation of Jupiter takes 10 hours and it has a bulge of 0.7 per cent
  • Venus, which takes 243 Earth days to complete a single rotation, has no discernible bulge at all

But what of something much larger? Do stars bulge as well?

Sunspots Arturo Buenrostro, Dallas, Texas, USA, 30 October 2021 Equipment: ZWO ASI178MM camera, Lunt 60mm H-Alpha telescope with double-stack 50mm filter and B1200 blocking filter, Sky-Watcher AZ-EQ6 Pro mount
Credit: Arturo Buenrostro

Recent data taken by the Solar Dynamics Observatory (SDO) has revealed that even though the Sun rotates at a rapid 7,000km/h at its equator, the observed measurements of its bulge have found it to be much smaller than expected – virtually negligible in fact.

Is this normal for stars? The SDO only recently made the measurements of the Sun.

Our star’s brightness and turbulent surface make defining its edge hard work.

The stars of the Summer Triangle asterism. Star Vega (top left), is the 5th brightest star in the night sky. Altair can be seen below in the middle and Deneb is far left. Credit: A. Fujii
The stars of the Summer Triangle asterism: Vega (top left), Altair (below middle) and Deneb (far left). Credit: A. Fujii

Doing this for another star has proved to be more challenging still, but Altair in Aquila, which rotates at an even faster 17,000km/h, has been measured and found to have a bulge.

So why does our Sun not bulge as it spin? Well, this currently remains a mystery.

A few theories are being postulated but there is little evidence to back them as yet.

I think the fact that the Sun is lacking a bulge will be the source of much research in the future, and it shows that we can still have a number of surprises on our astronomical doorstep.

This guide originally appeared in the June 2014 issue of BBC Sky at Night Magazine.

From the major planets in our Solar System to the very Sun itself, the sphere is the most common shape of celestial bodies, but why is this?

Gravity is the main force that sculpts the Universe, causing the formation of planets, stars and galaxies.

It works over great distances and has no preferred orientation.

It draws matter in from all directions and when matter clumps together it forms a shape that looks the same from all directions. The shape that meets all these criteria is a sphere.

A remarkable view of Earth from Apollo 16, taken on 16 April 1972, reveals a world of blue and white with a hint of brown; (inset) the image before it was digitally restored. Credit: NASA / Toby Ord
Credit: NASA / Toby Ord

But the force of gravity also distorts the sphere that we would expect to see, meaning most of the bodies that we think of as being round are really 'almost spherical'.

Take our planet for instance. If I was to travel all around the world from pole to pole, I would have covered a distance of around 39,941km.

If I were to do the same journey, but this time around the equator, I would have travelled an additional 130km.

The Earth bulges at the equator to form a shape close to an oblate spheroid – a slightly squashed sphere.

Why are planets slightly squashed?

The centrifugal force of a chairoplane is a simple way of understanding how a planet's spin causes it to bulge. Credit: Andrew Williams / EyeEm / Getty
The centrifugal force of a chairoplane is a simple way of understanding how a planet's spin causes it to bulge. Credit: Andrew Williams / EyeEm / Getty

The reason why planets get a bit fat around the middle is due to their rotation about their axes.

Imagine the chairoplane: that fairground ride with lots of chairs hanging down from a central disc.

As the disc starts to rotates, the chairs lift and move outward from the centre or outward from the centre of rotation.

The only thing stopping the chairs from flying off is the chains.

The same thing happens to a planet when it spins. Gravity acts like the chains, pulling everything inwards, but the speed of the rotation pushes everything outwards.

Just like the chairs on the ride moving outward, as a plane rotates on its axis it grows wider around the middle.

How fast a planet rotates seems to be one of the largest influences on the size of the bulge.

  • Earth, with its 24-hour rotation, has a bulge of about 0.3%
  • One rotation of Jupiter takes 10 hours and it has a bulge of 0.7 per cent
  • Venus, which takes 243 Earth days to complete a single rotation, has no discernible bulge at all

But what of something much larger? Do stars bulge as well?

Sunspots Arturo Buenrostro, Dallas, Texas, USA, 30 October 2021 Equipment: ZWO ASI178MM camera, Lunt 60mm H-Alpha telescope with double-stack 50mm filter and B1200 blocking filter, Sky-Watcher AZ-EQ6 Pro mount
Credit: Arturo Buenrostro

Recent data taken by the Solar Dynamics Observatory (SDO) has revealed that even though the Sun rotates at a rapid 7,000km/h at its equator, the observed measurements of its bulge have found it to be much smaller than expected – virtually negligible in fact.

Is this normal for stars? The SDO only recently made the measurements of the Sun.

Our star’s brightness and turbulent surface make defining its edge hard work.

The stars of the Summer Triangle asterism. Star Vega (top left), is the 5th brightest star in the night sky. Altair can be seen below in the middle and Deneb is far left. Credit: A. Fujii
The stars of the Summer Triangle asterism: Vega (top left), Altair (below middle) and Deneb (far left). Credit: A. Fujii

Doing this for another star has proved to be more challenging still, but Altair in Aquila, which rotates at an even faster 17,000km/h, has been measured and found to have a bulge.

So why does our Sun not bulge as it spin? Well, this currently remains a mystery.

A few theories are being postulated but there is little evidence to back them as yet.

I think the fact that the Sun is lacking a bulge will be the source of much research in the future, and it shows that we can still have a number of surprises on our astronomical doorstep.

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This guide originally appeared in the June 2014 issue of BBC Sky at Night Magazine.

Authors

maggie aderin pocock
Maggie Aderin-PocockSpace engineer

Maggie Aderin-Pocock is a space engineer and a co-presenter on the BBC's The Sky at Night.

maggie aderin pocock
Maggie Aderin-PocockSpace engineer

Maggie Aderin-Pocock is a space engineer and a co-presenter on the BBC's The Sky at Night.

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