• Question: Whilst we may be able to determine the distance of a close star, planet or moon, how can the diameter and size of a planet be worked out?

    Asked by Matthew S to Heather, Helen, Hugh, Julian on 19 Nov 2014.
    • Photo: Julian Onions

      Julian Onions answered on 19 Nov 2014:

      This is a great question!

      For close planets and moons in our own solar system, we can measure them at a telescope – and in most cases we’ve now visited most of them anyway.

      Stars are much harder, as they are a long way away. So mostly this is done from theory. You know how hot the star is from its pattern of light, and you know what it has to do to reach that temperature, which gives you a mass, and for the star to be stable under gravity, there is a particular radius it has to be. So really its just maths.
      In fact this is the maths usually used
      L = 4 * pi * R^2 * sigma * T^4
      which says the luminosity of a star (how bright it is) is equal to
      4 times pi times the radius of the star squared, times sigma (a constant called the stefan-oltzman constant – just a number) times the stars temperature to the 4th power.
      With a bit of algebra, and knowing the temperature and luminosity you can solve for the radius.

      Exoplants, planets around other systems, can be worked out if they are found using the transit method, where the light from the star dips a little as the planet blocks some of the light. As the planet edges into the path of the star it begins to block light, and eventually its inside the disk of the star, but while its edging in you can see the light levels drop, and that lets you know how big it is.

    • Photo: Heather Campbell

      Heather Campbell answered on 19 Nov 2014:

      Great answer form Julian.
      Just to add, to get accurate distances to stars (which we have very few at the moment) you can use a method called parallax. This is the method of looking at an object, in this case a star, from two different view points, and it appears to move compared to objects much more distant. You can measure the shift (as an angle) and you know the distance you have moved (the baseline), so you can use trigonometry to get a very accurate distance to the star. This is what the Gaia satellite is currently doing for the billion brightest stars in our galaxy, to make a 3D map of what our Milky way looks like. The baseline is the satellite orbit around the sun (out side the earth another 1.5million km further out than the sun), and it actually looks at each star~70 times over 5 years to get these v accurate distances.

      You also need the distance to get the luminosity, which Julian describes how you can get from there to the radius.

    • Photo: Helen Johnson

      Helen Johnson answered on 19 Nov 2014:

      Great answers guys!
      I don’t think I need to add much to this, but just to say that scientists often use this method of measuring the dip in light as a planet passes in front of a star, to detect planets in other solar systems. It’s usually the big Jupiter sized planets which are easily detected this way, because the dip in light is more noticeable. To date we’ve discovered about 1800 planets in other solar systems! I wonder if any have life…

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