Econintersect: The solar system containing earth lies within a galaxy known as the Milky Way, about 2/3 of the distance from the center to the outer edge. 
Our sun is a relatively young star within the galaxy; most of the stars are smaller, older and cooler red dwarfs. A new research paper to be published in a forthcoming issue of Astronomy & Astrophysics reports on new measurements that give the first direct estimate of the number of “light” planets around red dwarf stars. The “light” planets of interest for possible conditions to support life are actually rocky super-earths with masses from near earth size up to approximately 10x earth. The bulk of planets surrounding red dwarfs in the Milky Way are these super-earths; giant gaseous planets like Jupiter and Saturn are relatively rare. The process that has been used for direct detection of super-earths associated with stars in the Milky Way is known as HARPES (High Accuracy Radial velocity Planet Searcher) that looks at spectral shifts resulting from gravitational forces between stars and planets. Click on caption graphic for larger picture with description.
In January GEI News reported on then current estimates that up to 50 million earth-like planets with habitable conditions could exist in the Milky Way. The key factor in habitability is the surface temperature of a planet which must fall in the range at which water is liquid. The latest paper states there are about 160 billion red dwarfs in the Milky Way galaxy and from 28% to 95% of them (most probable number 41%) have super-earth planets in the habitable zone. This puts the number of potential earth-like planets in our galaxy in the tens of billions, much larger than the 50 million suggested in the January 2012 GEI News article.
There are factors that could prevent potential planets from being habitable. From Scientific Computing:
“The habitable zone around a red dwarf, where the temperature is suitable for liquid water to exist on the surface, is much closer to the star than the Earth is to the Sun,” says Stephane Udry (Geneva Observatory and member of the team). “But, red dwarfs are known to be subject to stellar eruptions or flares, which may bathe the planet in X-rays or ultraviolet radiation, and which may make life there less likely.”
Future studies may be able to determine the nature of planetary atmospheres and even detect signs of life. Again from Scientific Computing:
“Now that we know that there are many super-Earths around nearby red dwarfs, we need to identify more of them using both HARPS and future instruments. Some of these planets are expected to pass in front of their parent star as they orbit — this will open up the exciting possibility of studying the planet’s atmosphere and searching for signs of life,” concludes Xavier Delfosse, another member of the team.
For planets that might actually be probed with equipment sent from earth, the number will be very small. Even if the “proximity window” is opened up to 30 light years distance there may be only about 100 potential earth-like planets. And 30 light years is far beyond the range of current earth space travel technology.
The following picture of the Milky Way Viewed from the top) is from Windows2universe.org showing the location of our sun. The second picture is of the Milky Way viewed from the side. The Milky Way is one of about 200 billion galaxies.
Scientific Computing lists the members of the HARPES research team who are authors of the forthcoming paper:
The team is composed of X. Bonfils (UJF-Grenoble 1 / CNRS-INSU, Institut de Planetologie et d’Astrophysique de Grenoble, France [IPAG]; Geneva Observatory, Switzerland), X. Delfosse (IPAG), S. Udry (Geneva Observatory), T. Forveille (IPAG), M. Mayor (Geneva Observatory), C. Perrier (IPAG), F. Bouchy (Institut d’Astrophysique de Paris, CNRS, France; Observatoire de Haute-Provence, France), M. Gillon (Universite de Liege, Belgium; Geneva Observatory), C. Lovis (Geneva Observatory), F. Pepe (Geneva Observatory), D. Queloz (Geneva Observatory), N. C. Santos (Centro de Astrofisica da Universidade do Porto, Portugal), D. Segransan (Geneva Observatory), J.-L. Bertaux (Service d’Aeronomie du CNRS, Verrieres-le-Buisson, France), and Vasco Neves (Centro de Astrofisica da Universidade do Porto, Portugal and UJF-Grenoble 1 / CNRS-INSU, Institut de Planetologie et d’Astrophysique de Grenoble, France [IPAG]).
Sources:
- Many Billions of Rocky Planets in Habitable Milky Way Zones Scientific Computing, 29 March 2012
- 50 new worlds join the exoplanet list Bad Astronomy Blog, Discover Magazine, 12 September 2011
- 50 Million More Earths in the Milky Way GEI News, 18 January 2012
