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NASA : The Most Precise Measurement of an Alien World's Size
Using 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.
Thanks 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
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 of planet.
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
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 Kepler measurements.
"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, California.
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'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
For more information about the Kepler mission, visit: