HD 97658b, a SuperEarth ExoPlanet, 8x Earth Mass and 2x Diameter

Hovering about 70 light-years from Earth –– that's "next door" by astronomical standards –– is a star astronomers call HD 97658, which is almost bright enough to see with the naked eye. But the real "star" is the planet HD 97658b, not much more than twice the Earth's diameter and a little less than eight times its mass. HD 97658b is a super-Earth, a class of planet for which there is no example in our home solar system.

While the discovery of this particular exoplanet is not new, determining its true size and mass is, thanks to Diana Dragomir, a postdoctoral astronomer with UC Santa Barbara's Las Cumbres Observatory Global Telescope (LCOGT). As part of her research, Dragomir looked for transits of this exoplanet with Canada's Microvariability & Oscillations of Stars (MOST) space telescope. The telescope was launched in 2003 to a pole-over-pole orbit about 510 miles high. Dragomir analyzed the data using code written by LCOGT postdoctoral fellow Jason Eastman. The results were published online today in the Astrophysical Journal Letters.

A super-Earth is an exoplanet with a mass and radius between those of the Earth and Neptune. Don't be fooled by the moniker though. Super-Earth refers to the planet's mass and does not imply similar temperature, composition, or environment to Earth. The brightness of HD 97658 means astronomers can study this star and planet in ways not possible for most of the exoplanet systems that have been discovered around fainter stars.

HD 97658b was discovered in 2011 by a team of astronomers using the Keck Observatory and a technique sometimes called Doppler wobble. But only a lower limit could be set on the planet's mass, and nothing was known about its size.

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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, which is too close to be in the Habitable Zone, nicknamed The Goldilocks Zone. The Goldilocks nickname is apropos: If a planet is too close to its star, it's too hot; if it's too far away, it's too cold, but if it's in the zone, it's "just right" for liquid water oceans, one condition that was necessary for life here on Earth.