Newly Identified Planet Bucks Planetary Trends

Located a mere 70 light-years from Earth, astronomers have uncovered the identity of a planet roughly twice the diameter of our own planet and a little less than eight time its mass. Orbiting a star named HD 97658, the neighbor falls in a class of planets known as super-Earths.

First discovered in 2011, the study to determine its true size and mass was led by Diana Dragomir, a postdoctoral astronomer with the University of California, Santa Barbara's Las Cumbres Observatory Global Telescope (LCOGT) network.

As part of her research, Dragomir looked for transits of this exoplanet with Canada's Microvariability and Oscillations of Stars (MOST) space telescope, analyzing the resulting data using code written by LCOGT postdoctoral fellow Jason Eastman.

Transits occur when a planet's orbit carries it in front of its parent star, slightly decreasing its level of brightness as seen from Earth. These dips in light occur every orbit, however, only if it is aligned with the Earth's line of sight.

The first report of transits in the HD 97658 system in 2011 turned out to be a false alarm, and could have been the end of the story had Dragomir not known that the ephemeris of the planet's orbit (a timetable to predict when the planet might pass in front of the star) was not exact.

Convincing the MOST team to widen the search parameters, the researchers recorded tantalizing evidence of a transit - tantalizing, but not indisputable.

A year later, the scientists revisited HD 97658 and, sure enough, found clear evidence of the planet's transits, allowing them to estimate the planet's true size and mass for the first time.

"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 explained in a statement. "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 the nearby planet is about four grams per cubic centimeter, which is about one-third the density of lead and denser than most rocks.

Astronomers see great significance in this value: at about 70 percent of the average density of Earth, the surface gravity of the neighboring planet could hold onto a thick atmosphere. 

At this point, more than 600 exoplanets have been confirmed, the vast majority of which have been discovered by radial velocity surveys. However, these are severely biased toward the detection of systems with massive planets in small orbits. The newly identified planet bucks this trend, orbiting its star at a distance farther than many of the currently known exoplanets.

To add to its exotic nature, the planet is only the second super-Earth known to transit a very bright star.

To Dragomir, however, the study isn't simply about the planet under examination, but instead represents one more piece to a massive puzzle scientists around the world are currently working to solve in regards to discovering alien life.

"This discovery adds to the still small sample of transiting super-Earths around bright stars," Dragomir said. "In addition, it has a longer period than many known transiting exoplanets around bright stars, including 55 Cnc e, the only other super-Earth in this category. The longer period means it is cooler than many closer-in exoplanets, so studying HD 97658b's properties is part of the progression toward understanding what exoplanets in the habitable zone might be like."