An introduction to exoplanets
An introduction to exoplanets

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An introduction to exoplanets

Week 6: Planets galore: the contents of the Milky Way


Last week, you discovered how to measure the properties of exoplanets. You were introduced to some of the various surveys that have detected transiting exoplanets, in particular the Kepler spacecraft that has found around two-thirds of all the planets discovered to date. This week, you’ll take a look at some of the important things we’ve learned from Kepler’s mammoth haul of planets. This will lead on to the search for planets which may be habitable, the likelihood of finding another Earth-like planet, and ultimately, life out there.

Watch the following video to find out more about what you will be learning this week.

Download this video clip.Video player: boc_exo_1_video_week6_carole_upload.mp4
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Carole Haswell:
The main message this week is an exciting one, especially if like me you grew up watching science fiction on TV. As assumed in Star Trek and Doctor Who, it turns out our Galaxy has lots of planets. It wasn’t only science fiction writers who predicted it. Otto Struve, the astronomer I mentioned last week, wrote about the possibility of there being billions of planets in the Galaxy. We now have measurements which show that these predictions were correct as Lisa Kaltenegger explains.
Lisa Keltenegger:
We live in an exciting time because for the first time in human history, we actually found planets that don’t orbit our own Sun, that they orbit other suns, other stars that you see in the night sky. We found more than 3000 already, and that number allows us to estimate how many are out there.
And so we know that around every second star out there is at least a planet because smaller planets are incredibly hard to find so there could be more. And what’s fascinating to me is that around every fifth star you go out and count to five in the night sky, there is one that has the right size, so it should be a rock, and that is at the right distance so it’s not too hot and not too cold. If this is the sun, like around a bonfire where it’s just nice and warm, so you can have liquid water on the surface of the planet.
In our Galaxy, there are billions of stars. And so if every fifth of those has a planet that could potentially be like ours, we’re talking billions of worlds that just maybe could be another Earth just waiting to be discovered.
Carole Haswell:
So we’ve got billions of planets to cover. Let’s get on with it.
End transcript
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By the end of this week, you will be able to:

  • know that there are more planets than stars in the Milky Way
  • explain in words how we know this
  • describe the relative abundance of terrestrial and giant exoplanets
  • know why some exoplanets are much easier to detect than others, using the radial velocity (RV) and transit methods for planet discovery
  • understand the use of light years for measuring distance
  • understand and explain in words the concept of a habitable zone
  • explain why, despite their abundance, we know of relatively few terrestrial exoplanets.

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