Discovery of smallest planet yet a 'milestone' in search for another Earth
The Kepler space telescope has found a planet smaller than Mercury orbiting a distant star. The discovery suggests Kepler has the precision to find a planet more like Earth.
For nearly 20 years, astronomers have filled the pantheon of planets beyond the solar system with objects ranging from behemoths several times Jupiter's mass to small orbs somewhat less hefty than Earth.Skip to next paragraph
In Pictures Looking into the skies: Telescopes
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Now, a team reports finding the smallest planet yet – a true pipsqueak orbiting a sun-like star some 215 light-years away near the constellation Cygnus. The planet has a mass at least 1 percent of Earth's mass, probably more. And it's about 30 percent Earth's size. This is slightly larger than the moon, which is 27 percent the size of Earth.
The planet, one of three in the system, orbits its host star once every 13.4 days.
"This is a milestone" on the path to finding Earth-like planets, says Dr. Barclay, who is the lead author of a report of the find, which appears in Thursday's issue of the journal Nature.
The observations come courtesy of NASA's Kepler mission, a sensitive space telescope that observes more than 150,000 stars simultaneously in and near the head of Cygnus around the clock. Kepler's ultimate goal is to find Earth-like planets orbiting sun-like stars at Earth-like distances.
Kepler 37b, as the planet is known, is more like a mini-Mercury but on a much tighter orbit. But "the sort of precision we need from the instrument" to achieve the mission's goal "is demonstrated through this discovery."
In essence, the planet's very weak signature at some 9.3 million miles from its star is comparable to the signature researchers expect to see from an Earth-mass planet 93 million miles from its star, Earth's average distance from the sun. A planet betrays itself to Kepler by dimming its star's light repeatedly as it passes in front of it, a process astronomers call the transit method for detecting extrasolar planets.
The detection of Kepler 37b was extremely difficult, Barclay says. Many transits were needed to build up enough data to spot the dimming the planet imparts to the star. And Kepler's ability to take very precise measurements of the star's own light helped the team develop a highly accurate estimate of the star's size and mass.