HD 97658b is a super-Earth located just 70 light years from Earth - almost next door by the standards of astronomy. For the first time ever, astronomers have been able to accurately measure the size and mass of that world.
This exoplanet was first discovered in 2011, when it revealed itself through the gravitational influence that it has on its local star, HD 97658. This "Doppler wobble" of the star allowed astronomers to assign a lowest possible mass to the planet, and calculate its orbital period. That data did not provide any information regarding the diameter of the planet, however.
As the planet revolves around its star, it passes between the Earth and HD 97658. By measuring the amount of dimming which occurs, astronomers were able to make the first good measurement of the size of the world. These transits were recorded by the Microvariability & Oscillations of Stars (MOST) space telescope, a Canadian orbiting observatory.
"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. Measuring the properties of super-Earths in particular tells us whether they are mainly rocky, water-rich, mini gas giants, or something entirely different," Diana Dragomir, an astronomer with the Las Cumbres Observatory Global Telescope, said.
The planet was found to have a diameter about twice that of the Earth and a mass roughly eight times that of our home planet. That makes the density of the nearby super-Earth about 250 pounds per cubic foot. That is denser than most rocks, although it is only about one-third as dense as lead, or 70 percent of the average density of the Earth. This would suggest that HD 97658b could hold on to a substantial atmosphere.
The star HD 97658 is almost bright enough to be seen with the naked eye, and is easily seen with a pair of binoculars in the constellation Leo. This is one of the few super-Earths which have been seen transiting their stars. Astronomers are seeking out such worlds with orbits at the right distance from their stars for water to stay liquid.
"[S]tudying HD 97658b's properties is part of the progression toward understanding what exoplanets in the habitable zone might be like," Dragomir said.
Super-Earths are considered to be those planets found around other stars that are between the sizes of Earth and Neptune. They can have compositions and temperatures far different from our home world, however, many inhospitable. Life on this super-earth is considered unlikely by most astronomers, since the planet orbits so close to its star that the planet is likely too hot for life to have ever risen.
