Assuming a 1,000 ton space vehicle and constant acceleration at 0.722 m/s2, which an exhaust velocity of 10,000 km/s with a mass-flow rate equal to “Daedalus’s” second-stage would produce, we get the following figures for travel times to various Solar System destinations.
45.5 billion terrestrial planets in the habitable zones of host stars in our galaxy. Another estimate was 1000 times lower.
Kipping’s figures are truly mind-blowing when he turns to the larger universe. A figure of roughly 100 million habitable environments per galaxy can now be turned around for an estimate of habitable worlds in the visible universe. The number works out to 10^18, or 10 million trillion.
The Guo paper is “Probability Distribution of Terrestrial Planets in Habitable Zones around Host Stars,” Astrophysics & Space Science Vol. 323, No. 4 (October, 2009). Preprint available. David Kipping’s Web pages are packed with good information. Start with these articles.