The group of planets,ranging in size from twice to four times the radius of Earth,is “the most compact system of planets ever discovered,” Jack Lissauer,a Kepler co-investigator,said at a press conference Wednesday.
The five inner planets orbit the star,Kepler-11,closer than any planet orbits our sun. The sixth planet orbits Kepler-11 at a distance smaller than that between the sun and Venus.
NASA also announced the discovery of 68 planet candidates that are approximately the size of Earth. Five of the candidates orbit in the habitable zone of stars,a region where liquid water could exist on a planet’s surface. Candidates require further observation to confirm they are actual planets.
“The holy grail of the Kepler mission is the discovery of an Earth-sized planet orbiting in the habitable zone of a star like our own sun,” said Douglas Hudgins,Kepler program scientist at NASA Headquarters. “However,that’s going to take time.”
The Kepler satellite measures small decreases in the brightness of stars caused by planets crossing in front of them,which are known as transits. Because it takes three transits to confirm the existence of planets,the satellite has yet to find a planet with an orbit similar to Earth’s.
The satellite was launched two years ago,so it will be at least a year before a third transit could be detected.
“We are,in some sense,the first generation,” said William Borucki,Kepler science principal investigator at NASA’s Ames Research Center. He said the Kepler mission will build the foundations in the quest for life on other planets. “The next generation will build the walls,the third generation will put the ceiling in and the fourth generation is going to enjoy it.”
Hudgins said the milestones reached by Kepler will determine the course of the quest for extra-solar planets – those outside our solar system – in the future. But the discoveries also open the mission to further pursuits.
“When Kepler launched,everyone was talking about finding planets,” said John Troeltzsch,Kepler program manager at Ball Aerospace,the company that built and operates the satellite. “Now it’s opening up all kinds of new questions. We’re talking about moons,we’re thinking about the atmospheres of these planets and what they’re made of.”
Yale astronomy Professor Debra Fischer said the Kepler mission has added significantly to the knowledge of extra-solar planets. She said the data shows that low-mass planets and planetary systems are more common that previously thought.
“I feel this is an incredible historic moment,” Fischer said.