Exploring Exoplanets with Year 7

At school this week I ran Science Club (a 20 minute lunch time session) on “How to find alien worlds…. “. For this we explored the developments within the hunt for extrasolar planets. We started off by recapping the number of planets in our own star system and then looked at the number of planets currently know – quite a few, as I write this  853, (latest numbers can be found on exoplanet.eu). This was the hook and it seemingly worked well.

To further explore this I then got a volunteer to hold a pretty bright torch and pretend to be a star. Another student was a planet who could move in front of the star. The rest were observers with telescopes on the Earth. A discussion of what they could see was led culminating in the conclusion of how this was similar to an eclipse.

We then went about looking at a graph of the light curve from HD209458 – the first discovered transiting extrasolar planet. This is a stripped down version of my 2011 “Extrasolar planets in the classroom paper”  . Firstly I explained how the graph could represent what we just did with the torch and then we went on to make some measurements.


All you have todo now is measure the time from the start to the end of the transit (i.e. were the light has dipped from the higher to the lower level) and you can determine the planetary radius!

Now I was doing this in 20 minutes with Year 7 so the next bit we kind of glossed over the Newtonian physics and the orbits – if I had longer and doing this with an older group I would have spent time here.  So to make this short (full details in the George 2011 paper):

Planetary radius = Transit Time x pi  x a / P


a = the separation between the star and the planet = 0.047 AU = 7031099.937 km

P =  time taken to orbit the star for the planet = 3.52 days = 304128 seconds

pi = 3.14159….

Its useful to compare to the radius of Jupiter: 69 911 km

And there you go planetary radius, should be something like 1.27 x the radius of Jupiter.

Not bad for about 20 minutes. The students quite enjoyed this and it worked really quite well allowing me to show off an area of science I’m truly passionate about.

(If you would like to try this with your class then it might be handy to have my excel workbook.)



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