• Question: is the sun pulling all the planets closer to it

    Asked by 439susa26 to Ahmed, Francesca, George, James, Nitheen on 13 Nov 2014.
    • Photo: James Sullivan

      James Sullivan answered on 13 Nov 2014:

      Hi –

      interesting question – The sun is pulling planets closer to itself because of gravity but the fact that the planets are moving means that they all stay (roughly) the same distance from the sun (oftentimes the orbits are more like ellipses than circles).
      If the earth stopped moving around the sun then gravity would pull the planet into the sun – but because it’s moving around all the time this doesn’t happen.

    • Photo: Francesca Paradisi

      Francesca Paradisi answered on 14 Nov 2014:

      James answered it already, the way I explain it is that you have two forces going in the opposite directions, one the sun pulling planets closer and the other the centrifugal force (like in a spinning washing machine) that is due to the planets moving around in orbits. The two are in equilibrium and so the distance of the planets from the sun stays the same (almost) even though the sun tends to pull them closer.

    • Photo: George Dowson

      George Dowson answered on 14 Nov 2014:

      James and Francesca have answered this very well I’ll just add that because it is burning hydrogen to produce light and heat, to make helium, the sun is actually getting lighter (ie less massive) by one million tonnes a second. So while the sun’s gravity is pulling the planets in, its gravity is getting fractionally weaker all the time so the planets are very very slowly moving away from the sun!

      (Don’t worry it’s only about 15cm a year!)

    • Photo: Ahmed Osman

      Ahmed Osman answered on 17 Nov 2014:

      There are two forces against each others. The Sun and the planets are staying approximately the same distance apart and have been in roughly the same places for the last several billion years. Planetary orbits are a fine balancing act between momentum and gravity. As the planets fly through space they want to move in a straight line but the gravity from the Sun attempts to pull the planets towards it, which curves their paths. The planets currently lie in a perfect balance that results in each planet moving fast enough to not be pulled closer to the Sun, but not too fast that it moves away from the Sun and out of the Solar System.