Astronomers have made the most precise measurement so far of an exoplanet's size — for Kepler-93b, which orbits a star around 100 parsecs away.

Sarah Ballard at the University of Washington in Seattle and her colleagues estimated the planet's diameter at about 18,800 kilometres (1.48 times that of Earth), plus or minus 240 kilometres.

They used NASA's Kepler space telescope to monitor seismic activity inside the planet's host star. They also used the Spitzer Space Telescope to observe Kepler-93b as it transited the star, applying a technique that ensured that for each measurement, light from the star fell on the centre of the same pixel in Spitzer's camera.

This allows for precision measurements of exoplanets' radii and masses, and even of the structure of their parent stars, the authors write.

Astrophys. J. 790, 12 (2014)