Institute of Astronomy


Ask an Astronomer - Planets

Spinning Planets

Published on 15/02/2012 

How does rotation affect a planet?

Gravity affects all objects in the universe no matter how big or small they are making any two things with mass be attracted to each other. The way this is typically described is through Newton's Laws of Gravitation where the 'Gravitational Force' is increases with mass but decreases the further you move away from it. Under extreme conditions, Newton's Law fails to match what we observe out in space but then Einstein's Laws of General Relativity comes to the rescue to explain what we see!

Now on to spinning objects - if an object is spinning, it experiences an 'outward' force - you'll have experienced this when you've gone around a roundabout in a car and been pushed outwards. This centrifugal force depends how fast you're travelling around the point at the centre of the rotation and decreases the further you go away. All planets do rotate and as such have the effects of both gravity keeping them together but this centrifugal force pulling them apart. Fortunately for us, the gravitational force is much stronger - if we do the calculations for Jupiter, the centripetal force is about 8% of that created by Gravity while for Earth it's about 0.4%. As such, if Jupiter wasn't spinning, you would have feel a force 10% bigger keeping you on the surface while for Earth, the difference would be pretty much completely unnoticeable! The condition about this however - the speed with which a planet rotates doesn't have any relation to how massive it is! It depends on how it was formed and if it has experienced anything like asteroids collisions etc.

The only thing we notice about rotating planets is that they tend to bulge out at the minute (or the technical term being oblate). This is because as they spin, they want to flatten into a disk but again, the rotation speed of the planet limits the effects of this.

Twinkling stars

Published on 28/02/2011 

Why do stars twinkle and planets do not?

In fact, both stars and planets twinkle! The twinkling is due to the turbulent air in the Earth's atmosphere, blurring and distorting the image of the star. The twinkling therefore has more of an effect nearer to the horizon, where the light must travel through more of the densest parts of the Earth's atmosphere. You can see this for yourself! Compare the twinkling of a star near the horizon (such as Sirius), and one close to zenith (straight up). Objects such as the Sun, Moon and the planets are called extended sources, because the light is emitted from a disc. Objects such as distant stars are called point sources, because they appear to be a point as they are very far away. In fact, the light from extended sources can be thought of as many point sources spread over an area. The turbulent air in the atmosphere causes a point source to appear to move around on the sky ever so slightly. However if we spread many point sources over the face of the planet, all point sources move around, but we do not notice a change in the total light from the object.

Supporting life on other planets

Published on 28/02/2011 

What is the Habitable Zone?

The habitable zone is a region around a star where an orbiting planet could host liquid water. It is sometimes also known as the Goldilocks Zone, because the planet must not be too close to the star (where any water will vapourise) and not too far from the star (where water will freeze).

An exoplanet...?

Published on 28/02/2011 

What is an Exoplanet?

An exoplanet (short for extra-solar planet) refers to planets that are orbiting stars other than our Sun. So far more than 500 have been found, through a variety of different methods, but this number is always growing! (for an up to date count see

The first exoplanets were found in 1992, orbiting a pulsar, a star that has reached the end of its lifespan and collapsed to form a very dense object rotating much faster than our Sun. These exoplanets would not be a very good place to visit, due to the large amounts of radiation given off by the pulsar. The first planet discovered around a more Sunlike star, 51 Peg b, would also not be a nice place to visit - although its star is more like our Sun, this exoplanet is what we call a 'Hot Jupiter', as it's a large gaseous world (like the planet Jupiter in our Solar System) and orbits very close to its host star (and so is very hot) - in fact 51 Peg b completes one orbit of its host in only 4 (Earth)days!


Published on 23/02/2011 

Why is Pluto no longer classified as a planet?

In recent years, better observations have lead to the discovery of a number of small bodies of similar size to Pluto in the Solar System. These include Ceres, found in the Asteroid Belt between Mars and Jupiter, and Eris, found beyond Pluto.

In light of these discoveries, if Pluto were still to be classified a planet, then these other bodies should also be called planets, meaning there would be far more than the nine (or now eight) with which we are familiar. The International Astronomoical Union therefore decided to define the term 'planet' to include the eight in our solar system and to define Pluto and other similar sized objects 'dwarf planets.'