How close to the edge of a galaxy can planets exist?
Is there such a thing as a ‘galactic habitable zone’?
The same sort of habitable zones that apply to planetary systems might also apply to galaxies, which would mean that there is a specific region within a galaxy where planets could exist. With missions such as the Kepler Space Telescope finding thousands of planets outside our solar system, the number and location of these foreign worlds is grabbing people’s attention. One question that always follows discussions of exoplanets is whether or not life could exist on them and a way of figuring this out relates to the ‘habitable zone’. This region of space around a star represents the location that is ‘just right’, not too hot or too cold.
It has been suggested that this idea may also apply to planets in galaxies. This often contested idea was put forward by Guillermo Gonzalez. He suggested that a planet would need to be far enough away from the galactic centre to avoid harmful radiation but far enough away from the edge of the galaxy to ensure enough heavy elements to form the planet. This gives the Milky Way’s habitable zone as a 6,000 light-year band 25,000 light-years from the centre. This sits roughly halfway between the centre and the edge.
Currently not enough planets have been detected to put solid limits on where planets can form in the galaxy but more and more missions are collecting data and adding to our knowledge.
Image courtesy of NASA/JPL/Caltech/ESA/Harvard-Smithsonian CfA
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