Throwing light on new super-Earth

• Nasa's astronomers confirm the existence of the nearest rocky planet outside our solar system

Nasa did not take a vacation in July, closing out a busy month by confirming the discovery of the nearest rocky planet outside our solar system.

Larger than Earth and at "a mere" 21 light years away, the exoplanet - so-called because it orbits a star other than the Sun - offers a unique opportunity for research, according to Nasa scientists, because it is so close. Most known planets are hundreds of light years away.

Named HD 219134b, the exoplanet orbits too close to its star to sustain life, and cannot be seen directly, even by telescopes, but the star it orbits is visible to the naked eye in dark skies in the Cassiopeia constellation, near the North Star. 

The rocky planet was first detected by a telescope instrument on the Canary Islands called HARPS-North, which measured the planet's mass and orbit by the gravitational "tug" it exerts on the star it orbits. The planet was determined to have a mass 4.5 times that of Earth, and a "speedy" three-day orbit around its star, according to Nasa.

Spitzer, a space telescope that detects heat radiation, followed up on the initial data, and discovered the planet transits its star. The planet's size is about 1.6 times that of Earth, according to Spitzer's measurements. Combining the size and the mass picked up by HARPS-North, scientists determined a density of 3.5 ounces per cubic inch, which makes HD 219134b a rocky planet.

Rocky planets that are bigger than Earth, like the one just discovered, belong to a growing class of planets termed super-Earths, according to Nasa.

"Thanks to Nasa's Kepler mission, we know super-Earths are ubiquitous in our galaxy, but we still know very little about them," said co-author Michael Gillon of the University of Liege in Belgium, lead scientist for the Spitzer detection of the transit, in a statement. "Now we have a local specimen to study in greater detail. It can be considered a kind of Rosetta Stone for the study of super-Earths."

The conclusion that our closest rocky neighbor crosses its star has Nasa anticipating a scramble to find out as much as possible from the ground and space. Only a small fraction of exoplanets can be detected transiting their stars due to their relative orientation to Earth. When the orientation is just right, the planet's orbit places it between its star and Earth, dimming the detectable light of its star. As the planet passes before its star, researchers can tease chemical data out of the dimming starlight that results. If the planet is determined to have an atmosphere, chemicals in it can imprint patterns in the observed starlight, according to Nasa.

"Transiting exoplanets are worth their weight in gold because they can be extensively characterised," said Michael Werner, the project scientist for the Spitzer mission at NASA's Jet Propulsion Laboratory, in a statement. "This exoplanet will be one of the most studied for decades to come."

/KT