The HARPS spectrograph on the 3.6-meter telescope at ESO’s La Silla Observatory in Chile is the world’s most successful planet finder. The HARPS team, led by Michel Mayor from the University of Geneva, Switzerland, announced September 12 the discovery of more than 50 new exoplanets orbiting nearby stars, including 16 super-Earths. This is the largest number of such planets ever announced at one time.
“The harvest of discoveries from HARPS has exceeded all expectations and includes an exceptionally rich population of super-Earths and Neptune-type planets hosted by stars very similar to our Sun. And even better — the new results show that the pace of discovery is accelerating,” said Mayor.
In the 8 years since it started surveying stars like the Sun using the radial velocity technique, HARPS has been used to discover more than 150 new planets. About two-thirds of all the known exoplanets with masses less than that of Neptune were discovered by HARPS. These exceptional results are the fruit of several hundred nights of HARPS observations.
Working with HARPS observations of 376 Sun-like stars, astronomers have now also improved the estimate of how likely it is that a star like the Sun is host to low-mass planets — as opposed to gaseous giants. They find that about 40 percent of such stars have at least one planet less massive than Saturn. The majority of exoplanets of Neptune’s mass or less appear to be in systems with multiple planets.
With upgrades to both hardware and software systems in progress, HARPS is being pushed to the next level of stability and sensitivity to search for rocky planets that could support life. Scientists selected 10 nearby stars similar to the Sun for a new survey. HARPS has already observed these stars which are known to be suitable for extremely precise radial velocity measurements. After 2 years of work, the team of astronomers has discovered five new planets with masses less than 5 times that of Earth.
“These planets will be among the best targets for future space telescopes to look for signs of life in the planet’s atmosphere by looking for chemical signatures such as evidence of oxygen,” said Francesco Pepe from the Geneva Observatory in Switzerland.
One of the recently announced newly discovered planets, HD 85512 b, is estimated to be only 3.6 times the mass of the Earth and is located at the edge of the habitable zone — a narrow zone around a star in which 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,” said Lisa Kaltenegger from the Max Planck Institute for Astronomy in Heidelberg, Germany, and the Harvard Smithsonian Center for Astrophysics in Cambridge, Massachusetts.
The increasing precision of the new HARPS survey now allows the detection of planets under 2 Earth masses. HARPS is now so sensitive that it can detect radial velocity amplitudes of significantly less than 2 mph (4 km/h).
“The detection of HD 85512 b is far from the limit of HARPS and demonstrates the possibility of discovering other super-Earths in the habitable zones around stars similar to the Sun,” said Mayor.
These results make astronomers confident that they are close to discovering other small rocky habitable planets around stars similar to our Sun. New instruments are planned to further this search. These include a copy of HARPS to be installed on the Telescopio Nazionale Galileo in the Canary Islands to survey stars in the northern sky, as well as a new and more powerful planet-finder called ESPRESSO to be installed on ESO’s Very Large Telescope (VLT) in 2016. Looking further into the future, the CODEX instrument on the European Extremely Large Telescope (E-ELT) will also push this technique to a higher level.
“In the coming 10 to 20 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,” said Mayor.