(Hat tip Bad Astronomy), About 1% of the stars in the Milky Way could have earth like planets (smaller rock like planets) Another study shows the Milky Way is twice as big as previously thought. So instead 100 billion stars there could be 200 billion and of those 1% or two billion could have earth like worlds around sun like stars. Centauri Dreams also has coverage of the earth like worlds study
A Spitzer Space Telescope study found that 10-20% of young stars had these disks of dusty debris around them. As it happens, about 10% of the stars in the Milky Way can be categorized as sun-like, which is about 10 billion stars. If 10% of them have rocky planets, as this study indicates, then there may be a billion Earths orbiting stars in our galaxy alone! And that’s only for stars like the Sun; lower mass stars also can form planetary systems, and there are far more of them then stars like the Sun. It is entirely possible that there are many billions of terrestrial planets in the galaxy… and there are hundreds of billions of galaxies in the Universe.
Up to 62 percent of the surveyed stars have formed, or may be forming, planets. The correct answer probably lies somewhere between the pessimistic case of less than 20 percent and optimistic case of more than 60 percent.
In separate but related news, the Milky way galaxy is twice as big as previously thought
Astrophysicist Professor Bryan Gaensler led a team that has found that our galaxy - a flattened spiral about 100,000 light years across - is 12,000 light years thick, not the 6,000 light years that had been previously thought.
The University of Sydney team's analysis differs from previous calculations because they were more discerning with their data selection. "We used data from pulsars: stars that flash with a regular pulse," Professor Gaensler explains. "As light from these pulsars travels to us, it interacts with electrons scattered between the stars (the Warm Ionised Medium, or WIM), which slows the light down.
"If you know the distance to the pulsar accurately, then you can work out how dense the WIM is and where it stops - in other words where the Galaxy's edge is.
"Of the thousands of pulsars known in and around our Galaxy, only about 60 have really well known distances. But to measure the thickness of the Milky Way we need to focus only on those that are sitting above or below the main part of the Galaxy; it turns out that pulsars embedded in the main disk of the Milky Way don't give us useful information."
Choosing only the pulsars well above or below us cuts the number of measurements by a factor of three, but it is precisely this rejection of data points that makes The University of Sydney's analysis different from previous work.