An Earth-like planet characterized by HARPS-N at the TNG

Artist’s impression of the Kepler-78b

Kepler-78b is an extra solar planet orbiting a Sun-like star located inthe constellation of Cygnus, some 400 light-years away from Earth. So far, the astronomers have characterized more than 1000 exoplanets, but Kepler-78b is a special one. According to the study published today in the Nature magazine, it has a mass and density very similar to those of the Earth. The authors of the study, an international team of astronomers led by Francesco Pepe of the Geneva University and the team members coming from various institutions of Switzerland, Italy, UK and USA, show that Kepler-78b is made of rock and iron, just like our Earth. Thus, Kepler- 78b is now the smallest exoplanet for which the mass and radius are known accurately.

Kepler-78b was first spotted by the NASA’s Kepler satellite. Kepler detected the tiny variation of the host star’s light caused by the passage of the planet in front of it. Shortly after the discovery, the Italian Telescopio Nazionale Galileo (TNG) in La Palma, Canary Islands, pointed the planet’s star. Indeed, the science team of HARPS-N, one of the world’s most precise spectrographs, mounted on the TNG, decided to focus on the observations of the Kepler-78b. A very intense observational campaign during the spring-summer months gave its fruits. Exploiting the unique characteristics and accuracy of the HARPS-N, the team measured the slight wobble of the host star caused by the exoplanet orbiting it. It was showed that the planet has Earth-like mass and density. An independent team observed Kepler-78b with the Keck telescope, Hawaii. The measurements of the two teams agreed with each other, thus increasing the confidence of the result.

Emilio Molinari, the director of the TNG and a coauthor of the article stresses the importance of the use of HARPS-N: “we are especially proud of such TNG and HARPS-N performance, as it shows that the discovery of the Earth’s twin planet is becoming more and more feasible. A special thanks is due to the staff people who made it possible.”

Kepler-78b has a radius of only 1.17 times that of the Earth, while the mass is 1.86 Earth masses. These numbers yield a density of 5.57 grams per cubic centimeter, and imply a composition of rock and iron, thus making Kepler-78b the most Earth-like exoplanet known so far. However, Kepler-78b has a very short orbital period of only 8.5 hours and therefore it orbits at a close distance from its host star. This means that the temperature on the surface of the planet should be somewhere between 3000 and 5000 degrees, effectively ruling out any possibility of life as we imagine it.

The Kepler-78b is destined to vanish, since the tidal forces will continuously draw it closer to its star. Eventually it will move so close that the star’s gravity will rip the planet apart. This may happen within three billion years, the theorists predict. “Interestingly, our solar system could have had a planet like Kepler-78b. If it had, the planet would have been destroyed early during the system evolution, leaving no signs today”, says Emilio Molinari.

Even if without any possibility of hosting life, the discovery and characterization of Kepler-78b, a planet of Earth-like dimensions, density and composition, is a big step forward in our search of extraterrestrial life. HARPS-N will produce more amazing results, allowing us to obtain an increasingly complete picture of the exoplanets formation and evolution, and one day we may possibly find a habitable planet with Earth-like characteristics.

TNG contact person: Gloria Andreuzzi, tel. (+34)-922-433645