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Asked by 0mnomcookie to Ailsa, Evan, James, Ryan on 22 Jun 2011.
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Ailsa Powell answered on 22 Jun 2011:
I think planets around different stars have been discovered by looking at the wobbles in the light coming from that star. When astronomers are looking at stars I think the light coming from them should be a constant intensity, but sometimes they have noticed with some stars the intensity wobbles and the light isn’t constant. They think that this is caused by a planet coming in front of the star and altering the amount of light that reaches the observer on Earth.
Evan will know for sure though. Hopefully, I’m at least vaguely right though 😉
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Evan Keane answered on 22 Jun 2011:
Ailsa is touching on one of the ways to detect planets called “the Doppler method”. I will tell you a few ways of how you can find planets around other stars.
1. Look for transits. What is that? Well if you watch a star for a while you can measure how bright it is. If it has a planet orbiting it, when the planet passes in front of the star it will block some of the light. You will notice the star get a bit dimmer, and this will repeat every time the planet orbits. From the amount of dimming you can work out the size of the planet, and from the length of the orbit you can work out how close it is to the star, which in turn would let you work out the temperature of the planet.
2. “The Doppler Method”. The transit method would only work if the planet and star line up, so you can’t find all planets this way. This is the wobble method Ailsa mentioned. But why does a star wobble? Well you should know that a planet does not really orbit a star with the star in the middle standing still. They both orbit around the “centre of mass”. Of course the star is so much more heavy than the planet that the centre of mass is very close to the star (basically this is because the gravity force on the star from the planet is tiny compared to the gravity force on the planet from he star) so from outside it looks like the planet orbits the star and the star just wobbles, but the star is really doing a really tiny orbit around the centre of mass. A full wobble happens once every orbit. Observing the wobbles gives you duration of the orbit (and so the distance from the star, and so the temperature on the planet) and the size of the wobbles tell you about the gravity of the planet (and so its mass).
3. “The pulsar method”. The pulsar method is really cool! Remember I told you that the pulses from a pulsar could be used like the ticks of a clock – so a pulsar is like a clock in space? I also said in one of my answers that time slows down when there is strong gravity. Ok, what happens if there is a planet orbiting a pulsar? Well the gravity of the planet will slow down the ticks coming from the pulsar and we will see the ticks coming faster and slower and faster and slower periodically. This period is the time of the planet’s orbit! Although you would not want to live on a planet orbiting a pulsar – it would be VERY strange!
There are several other cool ways to detect planets around other stars (we call them “exoplanets”) but I think those 3 are enough for you to get your head around for now! 😀
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