The European Southern Observatory (ESO) has announced its exoplanet-hunting HARPS (High Accuracy Radial velocity Planet Searcher) has discovered 50 new exoplanets, making it the largest amount of exoplanets that has been announced at the one time. Bringing the number of planets discovered outside our solar system to 645, the 50-planet haul includes 16 super-Earths (planets with a mass between one and ten times that of Earth), including one that orbits at the edge of the habitable zone of its star.

Whereas NASA's Kepler spacecraft looks for fluctuations in the brightness of stars to detect planets passing in front of it, HARPS is a high precision echelle spectrograph that observes Doppler shifts in the spectrum of the star around which a planet orbits. In contrast to the majority of planets discovered by the transit method employed by Kepler, which are very distant from us, the planets found by HARPS are around stars that are much closer, making them better targets for many kinds of additional follow-up observations.

HARPS discovered its first super-Earth in the habitable zone, (Gliese 581 d), in 2007. More recently, it was also used to demonstrate that the other candidate super-Earth in the habitable zone around star Gliese 581 (Gliese 581 g) doesn't exist.

In the eight years since HARPS achieved first light, its observations have allowed astronomers to improve the estimate of how likely it is that a star like the Sun is host to low-mass planets as opposed to gaseous giants. By studying the properties of all the HARPS planets discovered so far, the team has found that about 40 percent of stars similar to the Sun have at least one planet lighter than Saturn. Additionally, the majority of exoplanets of Neptune mass or less appear to be in systems with multiple planets.

With upgrades to both hardware and software, the team is increasing the sensitivity of HARPS, to search for rocky planets that could support life. One potential candidate is the newly discovered HD 85512 b, which is estimated to be just 3.6 times the mass of the Earth and is located at the edge of the habitable zone where water may be present in liquid form if conditions are right.

"This is the lowest-mass confirmed planet discovered by the radial velocity method that potentially lies in the habitable zone of its star, and the second low-mass planet discovered by HARPS inside the habitable zone," says Lisa Kaltenegger of the Max Planck Institute for Astronomy and Harvard Smithsonian Center for Astrophysics.

The team says that HARPS is now so sensitive that it can detect radial velocity amplitudes of significantly less than 4 km/h (2.5 mph), allowing it to detect planets under two Earth masses. Earth induces a 0.32 km/h (0.2 mph) radial velocity on the Sun.

HARPS is currently installed on ESO's 3.6 m Telescope at La Silla Observatory in Chile but a copy of HARPS is to be installed on the Telescopio Nazionale Galileo in the Canary Islands, to survey stars in the northern sky. Additionally, a new and more powerful planet-finder, called ESPRESSO, (Echelle SPectrograph for Rocky Exoplanet and Stable Spectroscopic Observations), will be installed on ESO's Very Large Telescope in Chile in 2016. It will boast radial velocity precision of 0.35 km/h (0.22 mph) or less, giving it the ability to discover Earth-mass planets in the habitable zone of low-mass stars.

"In the coming ten to twenty years we should have the first list of potentially habitable planets in the Sun's neighborhood. Making such a list is essential before future experiments can search for possible spectroscopic signatures of life in the exoplanet atmospheres," concludes Michel Mayor, who leads the ESO's HARPS team.