Billions of planets similar to Earth

Photo: The Canadian Press. All rights reserved.

This artist rendering released Monday Jan. 7,2013 by Harvard-Smithsonian Center for Astrophysics shows the different types of planets in our Milky Way galaxy detected by NASA's Kepler spacecraft. A new analysis of Kepler data found there are at least 17 billion planets the size of Earth. (AP Photo/Harvard-Smithsonian Center for Astrophysics)

A new estimate released Monday suggests that the Milky Way galaxy is home to at least 17 billion planets similar in size to Earth.

It doesn't mean all are potentially habitable, but the sheer number is a welcome starting point in the search for worlds like our own.

Scientists have yet to find a twin Earth -- one that's not only the right size but also located in a zone that's not too hot and not too cold where water might exist in liquid form.

Two independent groups came up with the new estimate after a fresh analysis of data gathered by NASA's Kepler spacecraft, launched in 2009 to track down other Earths. The craft spots a planet when it passes in front of the planet's star.

One team led by Francois Fressin of the Harvard-Smithsonian Center for Astrophysics estimated at least one in six stars has an Earth-size planet orbiting it. Using a different method, researchers at the University of California, Berkeley and University of Hawaii determined 17 per cent of stars host planets that are one to two times the diameter of Earth.

The findings were presented at the American Astronomical Society.