Is our Milky Way galaxy home to other planets the size of Earth? Are Earth-sized planets common or rare? NASA scientists seeking answers to those questions recently revealed their discovery.
"We went from zero to 68 Earth-sized planet candidates and zero to 54 candidates in the habitable zone - a region where liquid water could exist on a planet’s surface. Some candidates could even have moons with liquid water," said William Borucki of NASA’s Ames Research Center, Moffett Field, Calif., and the Kepler Mission’s science principal investigator. "Five of the planetary candidates are both near Earth-size and orbit in the habitable zone of their parent stars."
Planet candidates require follow-up observations to verify they are actual planets.
"We have found over twelve hundred candidate planets - that’s more than all the people have found so far in history," said Borucki. "Now, these are candidates, but most of them, I’m convinced, will be confirmed as planets in the coming months and years."
The findings increase the number of planet candidates identified by Kepler to-date to 1,235. Of these, 68 are approximately Earth-size; 288 are super-Earth-size; 662 are Neptune-size; 165 are the size of Jupiter and 19 are larger than Jupiter. Of the 54 new planet candidates found in the habitable zone, five are near Earth-sized. The remaining 49 habitable zone candidates range from super-Earth size -- up to twice the size of Earth -- to larger than Jupiter. The findings are based on the results of observations conducted May 12 to Sept. 17, 2009 of more than 156,000 stars in Kepler’s field of view, which covers approximately 1/400 of the sky.
"The fact that we’ve found so many planet candidates in such a tiny fraction of the sky suggests there are countless planets orbiting stars like our sun in our galaxy," said Borucki. "Kepler can find only a small fraction of the planets around the stars it looks at because the orbits aren’t aligned properly. If you account for those two factors, our results indicate there must be millions of planets orbiting the stars that surround our sun."
“We’re about half-way through Kepler’s scheduled mission," said Roger Hunter, the Kepler project manager. "Today’s announcement is very exciting and portends many discoveries to come. It’s looking like the galaxy may be littered with many planets.”
Among the stars with planetary candidates, 170 show evidence of multiple planetary candidates, including one, Kepler-11, that scientists have been able to confirm that has no fewer than six planets.
"Another exciting discovery has been the tremendous variations in the structure of the confirmed planets – some have the density of Styrofoam and others are denser than iron. The Earth's density is in between."
"The historic milestones Kepler makes with each new discovery will determine the course of every exoplanet mission to follow," said Douglas Hudgins, Kepler program scientist at NASA Headquarters in Washington.
Kepler, a space telescope, looks for planet signatures by measuring tiny decreases in the brightness of stars caused by planets crossing in front of them - this is known as a transit.