The Measurement of Kepler-93b’s Size Is the Best One for Exoplanet Yet

July 25, 2014

By combining data from two space telescopes and improving measurement techniques, astronomers have made the most precise measurement of an exoplanet’s radius so far.

Artist's illustration of Kepler-93b, with the diameter a bit less than 1.5 that of the Earth.

Artist's illustration of Kepler-93b, with the diameter a bit less than 1.5 that of the Earth. Credit: NASA/JPL-Caltech

Kepler-93b is an exoplanet some 300 light-years away from us, categorized as a super-Earth - a class of planets with higher mass than Earth, but smaller than Uranus' and Neptune's. Exoplanets are interesting because while there are plenty of them in the galaxy, they are of course absent in our system.

By combining data from NASA’s Kepler and Spitzer Space Telescopes, astronomers have been able to measure Kepler-93b’s radius so precisely, the margin of error is only 1.2%. This is incredible considering the distance of the system the planet is in.

"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, USA. "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."

Kepler-93b is about 18,800 kilometers in diameter, give or take 240 kilometres, making it about 1.5 times larger than Earth in diameter. Planet’s mass was previously measured at 3.8 times the mass of the Earth, which implies it is dense and most likely made of iron and rock.

However, like many other super-Earths discovered so far, Kepler-93b is hardly habitable, considering it is six times closer to its star than Mercury is to the Sun. Kepler-93b’s surface temperature is estimated at 760 degrees Celsius.

Kepler Space Telescope observed the planet transit its star, blocking a small portion of its light while doing so. The telescope also watched the dimming of the star caused by seismic waves within star’s interior, caused by Kepler-93b. With that information, Ballard’s team was able to measure star’s radius more precisely, which was prerequisite for measuring radius of the Kepler-93b itself.

At the same time, Spitzer Space Telescope observed the planet in infrared part of the spectrum, partially using a “peak-up” observation method, in order to confirm Kepler Space Telescope’s data as accurate. Spitzer’s peak-up camera was originally used to point telescope more precisely, but was repurposed in 2011 to control where light lands on individual pixels within the telescope’s camera. This resulted in better measurements, cutting the margin of error in half, when measuring exoplanet’s radius with Spitzer.

"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.

By improving measurements of exoplanets’ diameters and masses, we can better understand what they are made of. This is especially interesting in the case of super-Earths, since we have no opportunity to examine them up-close.

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