NASA announced some preliminary results of studies by its Kepler probe on June 15: the probe's camera chips had pointed out 706 potential planets in its first 43 days of operation. Though cautionary comments pepper the announcement, it's thrilling.
These are in addition to five planets that were announced earlier by the project, places where life seems extremely unlikely.
[An artist's rendering of Kepler keeping watch on exoplanets. Image credit, NASA/JPL-Caltech]
First, a heap of those ifs, ands and buts: Kepler can find planets by the dimming of a star when a planet crosses in front, but the announced results are only targets that seemed promising. Some could be nothing more than binary stars eclipsing the target stars. Of those that really are planets, many could be outside the star's "habitable zone, the region that's neither too hot nor too cold for liquid water.
"These stars represent a full range of temperatures, sizes and ages," says the media release. "Many of them are stable, while others pulsate." Some have starspots, equivalent to sunspots. A few "produce flares that would sterilize their nearest planets."
The space agency released detailed readings on about 300 targets but withheld data on 400, giving Kepler project researchers the first crack at determining more about them. Assuming they kept the best for themselves, that's encouraging. NASA pledged to release information on the 400 next February.
As the Kepler Science Council announced on June 15, "The 28-member Kepler science team also is using ground-based telescopes and NASA's Hubble Space Telescope and Spitzer Space Telescope to perform follow-up observations on a specific set of 400 objects of interest."
Kepler launched into an orbit around the sun on March 6, 2009. It has been examining a region in the constellations Cygnus and Lyra, recording tiny blips in the light streaming from 156,000 stars. It can detect a transit, when a planet passes between the probe and a star, even if the planet is as small as Earth.
Isn't it discouraging to find only 700 possible planets out of 156,000 stars? No, that statistic doesn't give a full picture. The 700 potential planets were found in only a little more than six weeks, and Kepler has been staring at those stars for almost a year and a half. Scads more must have been picked up by now.
Presumably, the vast majority of planets wouldn't eclipse their stars from our vantage point and Kepler could never detect them. If a planet that did eclipse were the same distance from its star as Earth is from our sun, the dimming would occur only once a year. Odds are, that one time would not have happened after the first 43 days.
If an eclipsing planet were twice as far from its star, the dip in starlight might not occur until next week.
Some commentators jumped the gun and claimed Kepler had detected many Earthlike planets. On Aug. 2 this error prompted S. Pete Worden, director of NASA Ames Research Center, to pass along a statement from the Kepler Science Council:
"Recently there have been reports to the effect that Kepler has discovered many Earth-like planets. This is not the case. Analysis of the current Kepler data does not support the assertion that Kepler has found any Earth-like planets."
That means no target has been confirmed – so far – as hosting an Earthlike planet. If astronomers need to wait through two or three eclipses to feel comfortable in concluding the dips in starlight intensity result from a planet of the right size and distance crossing in front of its star, the confirmation period could extend to two or three years or longer.
The council's statement continued, "Kepler is producing excellent results and is on a path to achieving all its mission requirements and actually determining the frequency of Earth-size planets, especially in habitable zones. We will announce our results when they become available and are confirmed."
Further details came July 27 from Dimitar Sasselov, a co-investigator on the Kepler Science Team. Based at the Harvard Smithsonian Center for Astrophysics, Sasselov said the majority of the 306 planets about which data were released were the size of Neptune and smaller. "This is the good news," he said in a written statement.
"As of today none of the candidates smaller than 2 Earth radii is in the habitable zones; their orbits are too small, which is why it was easier to spot them after just 43 days. Habitable planets will take a lot more time, as Kepler needs to observe more than one transit."
Eventually the analysts will winnow out which, if any, planet candidates are the right size and distance from their stars. Meanwhile, we should be proud of the job the probe and its scientists are doing. Soon we will know much more about the likelihood that life exists in other star systems.