There are now about 1750 confirmed exoplanets, and several thousand candidates awaiting followup measurements. Most of them have been discovered by the Kepler satellite which looks for planetary transits: the slight dimming of starlight when an exoplanet crosses the face of the star as seen from Earth. The orbital period of the exoplanet is determined from multiple transits and its mass from the star's mass and stellar wobble. The duration and details of the transit dip provide a measure of the planet’s size, and together with the mass yields an average density -- and hence a clue to the planet’s composition (with low density around 1 gram/cm^3 implying a watery composition and high density like the Earth's of 5.5 grams/cm^3 implying a rocky planet). Measuring the planet’s density precisely is a difficult task, however, especially for smaller, Earth-sized planets, and so far just fifty-eight exoplanets have their masses known with a high degree of confidence.
CfA astronomers David Charbonneau, Francois Fressin, Li Zeng, and their colleagues teamed up with experts on the Spitzer Space Telescope to improve by a factor of two the diameter measurement of the Earth-sized exoplanet known as Kepler-93b. They used the Spitzer spacecraft’s pointing stability to position the star steadily on just one small region of one camera pixel, and the camera's precise infrared calibration to detect exceptionally small intensity variations. From seven transits, and complementary other data, they determined that the planet has a period of 4.72673978 +- 0.00000097 days and a radius of 1.481 +- 0.019 Earth-radii (an uncertainly of slightly over 1% corresponding to a mere 120 kilometers). Its mass is 3.8+- 1.5 Earth-masses, and so its density is 6.3 +- 2.6 grams-cm^-3: rocky and very similar to Earth’s average density.
The astronomers compare Kepler 93b to the seven other exoplanets with radii smaller than 1.5 Earth-radii whose masses are well known, and find that all of them are rocky, with about half of them indistinguishable from one another in composition. They also conclude from exoplanet models that the Kepler93b surface temperature is about 760 Celsius (recall it has an annual orbit of only about 5 days), and that the probability that it has an atmosphere is only about 3%. The results not only extend our knowledge of Earth-sized exoplanets, they illustrate the remarkable power of new technology (in this case Spitzer) to keep improving our ability to characterize the family of exoplanets.
"Kepler-93b: A Terrestrial World Measured to Within 120 km, and a Test Case for a New Spitzer Observing Mode," Sarah Ballard,William J. Chaplin, David Charbonneau, Jean-Michel Desert, Francois Fressin, Li Zeng,Michael W. Werner, Guy R. Davies, Victor Silva Aguirre, Sarbani Basu, Jørgen Christensen-Dalsgaard,Travis S. Metcalfe, Dennis Stello, Timothy R. Bedding, Tiago L. Campante, Rasmus Handberg,Christoffer Karoff, Yvonne Elsworth, Ronald L. Gilliland, Saskia Hekker, Daniel Huber,Steven D. Kawaler, Hans Kjeldsen, Mikkel N. Lund, and Mia Lundkvist, ApJ 790, 12, 2014.