Econintersect: Nearly 2,000 exoplanets (planets outside our own solar system) have been identified to date, out of a possible 50 billion in our galaxy. Of these approximately 50 million may have orbits in the habitable zone (orbits properly distanced from their star/sun to have temperatures compatible with life).
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But many of these may have to be ruled out because all such star systems much closer to the center of our Milky Way galaxy would be exposed to too much intense gamma radiation which is created at the galaxy center. Others much farther from the center of the galaxy may also not be suitable because of lack of heavy metals arriving early in their existence due to bombardment originating from the reactions inside the inner stars of the galaxy.
This report suggests that perhaps only 2 million or so (about 0.005% of all planets in the Milk Way) could satisfy all the “location” requirements for possible habitation.
And then comes the hard part: How many of them could actually have the hydrogen-oxygen-water-carbon based chemistry upon which earth life is based? In spite of the very large numbers of planets involved, finding one with life could be an extremely improbable event. This is the basis of the scientific question: Is the earth an extremely rare combination of factors or are earthlike conditions replicated elsewhere in the solar system and among the estimated 1021 planets in the known universe.
- Giant Impacts, Planet Formation and the Search for Life Elsewhere (GEI News, 12 December 2014)
- ‘Super-Earth’ Alien Planet Spotted by Ground-Based Telescope, a First (Space.com, 12 December 2014)
- For Life to Form on a Planet It Needs to Orbit the Right Kind of Star (GEI News, 05 December 2014)
- 5 Exoplanets Most Likely to Host Alien Life (Space.com, 17 April 2014)
- Exoplanets: Looking for a Potential New Home (GEI News, 24 December 2013)
- 50 Million More Earths in the Milky Way? (GEI News, 18 January 2012)
- Observable Universe (Wikipedia)
- Kepler, A Search for Habitable Planets (NASA, Ames Research Center)
- Infographic of the Day: Four Giant Exoplanets of Star HR 8799 (GEI News, 23 March 2013)