Mass and radius of the exoplanet HD 97658b

A planet more massive than Earth orbits a star less massive than the Sun. Image: NASA Art

HD 97658 is a nearby (21 pc) K1V dwarf whose planet was detected in 2011 by radial velocity measurements. The minimum mass msini is eight Earth masses. Early transit confirmation measurements suggested a low density of 1.4 grams per cubic centimeter, making the planet likely similar in composition to Neptune, with a hydrogen and helium envelope. The new observations give a higher density and suggest a rocky core with a gaseous envelope of lighter elements:

“Measuring an exoplanet’s size and mass leads to a determination of its density, which in turn allows astronomers to say something about its composition,” Dragomir said. “Measuring the properties of super-Earths in particular tells us whether they are mainly rocky, water-rich, mini gas giants, or something entirely different.”

The average density of HD 97658b is about four grams per cubic centimeter, a third of the density of lead but denser than most rocks. Astronomers see great significance in that value – about 70 percent of the average density of Earth – since the surface gravity of HD 97658b could hold onto a thick atmosphere. But there’s unlikely to be alien life breathing those gases. The planet orbits its sun every 9.5 days, at a distance a dozen times closer than we are from our Sun.

The new data are from the Canadian MOST micro-satellite telescope, launched exactly 10 years ago. The paper is Diana Dragomir, et al., “MOST Detects Transits of HD 97658b, a Warm, Likely Volatile-rich Super-Earth,” 2013, ApJ, 772, L2; doi:10.1088/2041-8205/772/1/L2.


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