The problem with measuring the physical size of extrasolar planets is that they orbit stars many light-years distant. But now, through a joint effort between NASA’s Kepler and Spitzer space telescopes, the vital statistics of one alien planet has been gauged through the most precise interstellar measurement made to date.
The “Super-Earth,” called Kepler-93b, orbits a star 300 light-years from Earth and was already known — from observations of the star’s ‘wobble’ by the Keck observatory in Hawaii — to have a mass 3.8 times that of Earth. Although it may be called a “super-Earth”, there is very little similarity between Kepler-93b and our planet, however; it orbits its star at one-sixth the orbital distance at which Mercury orbits the sun — it is therefore a roasted world. But with the help of this most recent measurement, astronomers have deduced that it is likely of a similar composition as our planet, albeit a little more molten.
“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. A paper detailing Ballard’s team’s findings has been published in the Astrophysical Journal.
Over seven orbits that Kepler-93b completed between 2010 and 2011, the researchers were able to use transit data from Kepler and corroborate those measurements with Spitzer’s infrared data. By combining both instruments’ data, a very precise measurement of the star’s girth could be made. From this, a precise measurement of the planet’s diameter could be made.
And the precision acquired is jaw-dropping. Kepler-93b is 11,700 miles (18,800 kilometers) wide with an error margin of only plus or minus 150 miles (240 kilometers). In other words, we now know Kepler-93b’s diameter to a precision of more than 98.7 percent.
“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 in a NASA/JPL news release.
With information about the exoplanet’s mass and diameter in hand, it was a relatively simple matter to gain a hint as to its composition. Kepler-93b is likely a rocky world with an iron core, much like Earth’s interior.