Quirks & Quarks

If the sun is round, why are the planets in elliptical orbits?

This burning question of the week concerns why planets have elliptical orbits around the sun given its round shape.

The burning question of the week asks why planets have elliptical orbits around the sun given its round shape

A composite image from space shows the eclipse of the sun by Earth. (NASA SDO/NASA GSFC/Michael Benson, Kinetikon Picture)

This week's question comes from Wilma David in Ottawa, Ontario. David asks: 

Given that the sun is almost perfectly spherical (not oblate), why do planets orbit the sun in an ellipse? I understand that gravity becomes weaker, the farther away a planet is from the sun, but why would the distance not be almost constant, given the round shape of the mass (the sun) that the planets orbit?

It's true the sun is almost perfectly spherical. We could also say it's a lot "rounder" than the Earth, said Dr. Ruobing Dong, assistant professor in the department of physics and astronomy at the University of Victoria.

Dr. Dong said the diameter of the Sun at the equator is only about 12 kilometres larger than the pole-to-pole diameter. By contrast the Earth has a much larger equatorial bulge — it's 47 km larger — even though the sun is roughly 100 times bigger than the Earth. 

But the fact that the sun is spherical doesn't prevent planets from having eccentric orbits. It exerts gravitational pull on planets, but as far as the planets are concerned it behaves as though all the mass was concentrated at the centre.

In 1687, Isaac Newton showed that as a consequence of this, planetary motion would obey Kepler's laws, the first of which states orbits of planets are ellipses with one focus on the sun.

So, why are planet orbits ellipses instead of circles? It turns out it doesn't have much to do with the shape of the sun or Earth. It's much more related to the initial conditions of planetary formation, said Dong — specifically, what momentum planets picked up as they formed. That is largely responsible for the shape of orbits.

An image taken by NASA and ESA's Solar and Heliospheric Observatory. (ESA/NASA)

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