Discovey of the closest rocky exoplanet

Astronomers have detected the nearest rocky planet known beyond our solar system, using the HARPS-North ground-based instrument and the NASA Spitzer Space Telescope. The planet, called HD 219134b, is a mere 21 light-years away, within a stone's throw at the scale of the Galaxy. It is a bit larger than Earth, and of similar composition. This planet is also the closest exoplanet, or a planet beyond our solar system, that can be seen "transiting," crossing in front of its star, making it the perfect planetary specimen for future studies. The paper presenting the discovery has been accepted for publication in the journal Astronomy & Astrophysics.

"This exoplanet will be one of the most studied for decades," said Ati Motalebi of the Geneva Observatory (Switzerland), the first author of the scientific publication presenting the discovery. "Indeed, transiting exoplanets can be extensively characterized, especially if they orbit around nearby bright stars."

Only a small fraction of exoplanets have their orbit oriented relative to Earth in such a way that they can be seen crossing periodically their stars. Astronomers are very interested about these "transiting exoplanets", because when they pass in front of their stars, the starlight dims. This dimming can reveal not only the size of the planet but also valuable clues about its atmospheric composition and its surface conditions.

HD 219134b is located in the familiar Cassiopeia constellation near the North Star. While the planet can't be seen directly, even by space telescopes, its star is visible to the naked eye in dark skies next to one leg of Cassiopeia's "W" shape. At 21 light-years away, it is one of the closest known exoplanets. Only 12 confirmed exoplanets are closer to Earth than HD219134b, the closest one, named GJ674b, being 15 light-years away. Still, none of these 12 closer planets transit, so astronomers have no information about their size and composition.

The planet was first detected by the HARPS-North instrument on the Italian 3.6-meter Galileo National Telescope in the Canary Islands, using the "radial velocity" technique in which an exoplanet's mass and orbit can be measured by the tugs that it exerts on its host star. The NASA/Spitzer space telescope followed up on the finding, discovering that the planet transits its star. Combining the HARPS-North and Spitzer observations allowed the team to find the planet to have a mass and radius of respectively 4.5 and 1.6 times those of the Earth. Combining these size and mass gives a density of 6 grams per cubic centimeter -- indicating a rocky composition similar to Earth's, making it enter the "telluric" planetary class. Still, the planet orbits much too close to its star to be habitable, its "year" lastingonly 3 days.

Determine its chemical composition"Most of the known transiting exoplanets similar in size to Earth are hundreds of light-years away, way too far to be amenable for characterization with existing or even near-future telescopes. This one sits just next-door, which opens up many exciting opportunities for its detailed study!" said Michaël Gillon, astronomer at the University of Liege, Belgium, and leader scientist for the transit detection with the Spitzer space telescope.
 
Now that astronomers know that HD 219134b is out there and transits its star, scientists will be scrambling to observe it from the ground and space. The goal will be to tease chemical information out of the dimming starlight that occurs as the planet passes. Chemicals in the atmosphere of the planet can absorb certain wavelengths of the starlight, imprinting patterns in the light telescopes collect.  

This planet can expect years of attention. Future telescopes will likely observe it intensively, gathering ever-exquisite detail.  Furthermore, the  HARPS-North observations revealed three more planets in the star system, farther out from HD 219134b. Two of these planets are relatively small and not too far from the star. Michaël Gillon said "We are now preparing new observations with Spitzer to determine if any of these other planets transits the star too, which would make this nearby system even more interesting...