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data from NASA's Kepler and Spitzer Space Telescopes, scientists have
made the most precise measurement ever of the size of a world outside
our solar system, as illustrated in this artist's conception.Image Credit: NASA/JPL-Caltech.Full image and caption
to NASA's Kepler and Spitzer Space Telescopes, scientists have made the
most precise measurement ever of the radius of a planet outside our
solar system. The size of the exoplanet, dubbed Kepler-93b, is now known
to an uncertainty of just 74 miles (119 kilometers) on either side of
the planetary body.
The findings confirm Kepler-93b as a "super-Earth" that is about
one-and-a-half times the size of our planet. Although super-Earths are
common in the galaxy, none exist in our solar system. Exoplanets like
Kepler-93b are therefore our only laboratories to study this major class
With good limits on the sizes and masses of super-Earths, scientists
can finally start to theorize about what makes up these weird worlds.
Previous measurements, by the Keck Observatory in Hawaii, had put
Kepler-93b's mass at about 3.8 times that of Earth. The density of
Kepler-93b, derived from its mass and newly obtained radius, indicates
the planet is in fact very likely made of iron and rock, like Earth.
"With Kepler and Spitzer, we've captured the most precise measurement
to date of an alien planet's size, which is critical for understanding
these far-off worlds," said Sarah Ballard, a NASA Carl Sagan Fellow at
the University of Washington in Seattle and lead author of a paper on
the findings published in the Astrophysical Journal.
"The measurement is so precise that it's literally like being able to
measure the height of a six-foot tall person to within three quarters
of an inch -- if that person were standing on Jupiter," said Ballard.
Kepler-93b orbits a star located about 300 light-years away, with
approximately 90 percent of the sun's mass and radius. The exoplanet's
orbital distance -- only about one-sixth that of Mercury's from the sun
-- implies a scorching surface temperature around 1,400 degrees
Fahrenheit (760 degrees Celsius). Despite its newfound similarities in
composition to Earth, Kepler-93b is far too hot for life.
To make the key measurement about this toasty exoplanet's radius, the
Kepler and Spitzer telescopes each watched Kepler-93b cross, or
transit, the face of its star, eclipsing a tiny portion of starlight.
Kepler's unflinching gaze also simultaneously tracked the dimming of the
star caused by seismic waves moving within its interior. These readings
encode precise information about the star's interior. The team
leveraged them to narrowly gauge the star's radius, which is crucial for
measuring the planetary radius.
Spitzer, meanwhile, confirmed that the exoplanet's transit looked the
same in infrared light as in Kepler's visible-light observations. These
corroborating data from Spitzer -- some of which were gathered in a
new, precision observing mode -- ruled out the possibility that Kepler's
detection of the exoplanet was bogus, or a so-called false positive.
Taken together, the data boast an error bar of just one percent of
the radius of Kepler-93b. The measurements mean that the planet,
estimated at about 11,700 miles (18,800 kilometers) in diameter, could
be bigger or smaller by about 150 miles (240 kilometers), the
approximate distance between Washington, D.C., and Philadelphia.
Spitzer racked up a total of seven transits of Kepler-93b between
2010 and 2011. Three of the transits were snapped using a "peak-up"
observational technique. In 2011, Spitzer engineers repurposed the
spacecraft's peak-up camera, originally used to point the telescope
precisely, to control where light lands on individual pixels within
Spitzer's infrared camera.
The upshot of this rejiggering: Ballard and her colleagues were able
to cut in half the range of uncertainty of the Spitzer measurements of
the exoplanet radius, improving the agreement between the Spitzer and
"Ballard and her team have made a major scientific advance while
demonstrating the power of Spitzer's new approach to exoplanet
observations," said Michael Werner, project scientist for the Spitzer
Space Telescope at NASA's Jet Propulsion Laboratory, Pasadena,
JPL manages the Spitzer Space Telescope mission for NASA's Science
Mission Directorate, Washington. Science operations are conducted at the
Spitzer Science Center at the California Institute of Technology in
Pasadena. Spacecraft operations are based at Lockheed Martin Space
Systems Company, Littleton, Colorado. Data are archived at the Infrared
Science Archive housed at the Infrared Processing and Analysis Center at
Caltech. Caltech manages JPL for NASA.
NASA's Ames Research Center in Moffett Field, California, is
responsible for Kepler's ground system development, mission operations
and science data analysis. JPL managed Kepler mission development. Ball
Aerospace & Technologies Corp. in Boulder, Colorado, developed the
Kepler flight system and supports mission operations with the Laboratory
for Atmospheric and Space Physics at the University of Colorado in
Boulder. The Space Telescope Science Institute in Baltimore archives,
hosts and distributes Kepler science data. Kepler is NASA's 10th
Discovery Mission and was funded by the agency's Science Mission
For more information about the Kepler mission, visit: