The HARPS can attain a precision of 0.97 m/s (3.5 km/h),[2] making it one of only two instruments worldwide with such accuracy.[citation needed] This is due to a design in which the target star and a reference spectrum from a thorium lamp are observed simultaneously using two identical optic fibre feeds, and to careful attention to mechanical stability: the instrument sits in a vacuum vessel which is temperature-controlled to within 0.01 kelvins.[3] The precision and sensitivity of the instrument is such that it incidentally produced the best available measurement of the thorium spectrum.[citation needed] Planet-detection is in some cases limited by the seismic pulsations of the star observed rather than by limitations of the instrument.[4]
It was initially used for a survey of one-thousand stars.[citation needed]
Since October 2012 the HARPS spectrograph has the precision to detect a new category of planets: habitable super-Earths. This sensitivity was expected from simulations of stellar intrinsic signals, and actual observations of planetary systems. Currently, the HARPS can detect habitable super-Earth only around low-mass stars as these are more affected by gravitational tug from planets and have habitable zones close to the host star.[6]
Discoveries
This is an incomplete list of exoplanets discovered by the HARPS. The list is sorted by the date of the discovery's announcement. As of December 2017, the list contains 134 exoplanets.
CAFE (Calar Alto Fibre-fed Echelle spectrograph) installed on the Calar Alto Observatory's 2.2-metre telescope in 2014, and the CARMENES mounted on the 3.5-metre telescope in 2016.