How common are small planets?

Built and operated by a Consortium of three European countries (Switzerland, Italy, and UK) and the US, the new high-precision spectrograph called HARPS-N helps the planet-scouting Kepler spacecraft to confirm potential Earth analogues. HARPS-N stands for High Accuracy Radial velocity Planet Searcher – North (to distinguish it from its southern-hemisphere twin HARPS). This spectrograph was designed to detect tiny radial velocity signals induced by planets as small as Earth.

Astronomers, with the financial support of the ETAEARTH project, trained it to pick out tiny fluctuations caused by short-period planets with radii as small as Earth's. Next, objects of longer periods were monitored. Most recently, they started searching for Earth-like planets around

Around Kepler-78, a bright solar-type star approximately 400 light years away, in the Cygnus constellation, ETAEARTH scientists confirmed the existence of an Earth-sized planet outside our solar system that has a composition similar to our planet. Kepler-78b was first spotted in 2013 by the Kepler Space Telescope, which continuously scans the night sky in search of planets transiting in between distant stars and us.

Kepler observations allowed ETAEARTH scientists to determine the radius of this exoplanet by the amount of starlight blocked as it passes in front of its host star. To determine its mass, they employed the radial velocity method to calculate how much its gravitational pull makes Kepler-78 wobble, and thus determine the mass of the orbiting planet.

With these two quantities known, scientists could calculate Kepler-78b’s density and determine what it is made of. The exoplanet is 1.2 times the size of Earth and 1.7 times more massive, suggesting that it is primarily made of rock and iron. It is the exoplanet most similar to Earth in composition known to-date. Details of the breakthrough discovery were published in Nature.

Kepler, nonetheless, has collected observations from a large number of exoplanets. The ultimate aim of the ETAEARTH project is to obtain statistical information that will help astronomers understand the broad trends of exoplanet distribution in the habitable zone of their stars.

ETAEARTH project findings are expected to bring us closer to answering one of the most fundamental questions of modern astronomy: Is Earth the unique habitat for the complex biology called life?