Institute of Astronomy

 

Ask an Astronomer - Planets

How many solar systems are there?

Published on 24/09/2012 
Question: 

How many solar systems are present in the Universe and what is the possibility of finding a solar system similar to ours?

Those are very good questions, and ones to which we don't quite know the answer.  There are a number of projects running at the moment that hope to try and provide some answers, such as NASA's Kepler mission and the European CoRoT, as well as various ground-based efforts.  So far they are suggesting that a reasonable fraction of stars (around 10 per cent or even more) have planets, but so far we have not really able to find systems quite like our own.

In terms of the number of solar systems present in the universe, there are something like 300 billion stars in the Milky Way, so if 10 per cent of them have planets there are around 30 billion planets in our galaxy alone, and there are over 100 billion galaxies in the observable Universe for a total of something in the order of 10^21 (that's 1 then 21 zeros) planets in the observable Universe.  There is still quite a bit of uncertainty in that number however, and we don't yet know how many of them would look like our solar system.

What happens when asteroids and planets collide?

Published on 10/06/2012 
Question: 

Is asteroid 2003/Q0104 going to hit Earth in May 2031, and if not, then by how much will it miss us and what effect could the near miss have on us; could it hit our moon, and if so, what would the effects be for us?

 

This asteroid is no longer considered to be on a collision course with Earth (or the Moon)! It was removed from this risk category back in 2003 shortly after it was discovered (http://neo.jpl.nasa.gov/risk/removed.html).

If two large objects collide in space, there are a lot of different conditions which need to be accounted for when saying exactly what the effects would be e.g. the size of the objects, their speeds etc.. If the Earth and another object e.g. an asteroid did collide, there are a range of possibilities in terms of the kind of damage it would cause. If the asteroid hit an ocean it would create a mega tsunami while if it collided with land it would cause a large crater, examples of which can be found all over the Earth e.g. Meteor Crater in Arizona, USA. Material thrown up when the crater was formed would also be thrown out of the Earth's atmosphere and would be spread around the neighboring solar system. Depending if the asteroid hit directly or with a glancing blow, this would have a significant effect on the amount of material ejected.

So in summary - there is no reason to panic in 2031! And there are a range of possible outcomes when it comes to collisions of large bodies in the solar system.

Spinning Planets

Published on 15/02/2012 
Question: 

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 
Question: 

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 
Question: 

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).