Free Astronomy Magazine September-October 2015

PLANETOLOGY take into account and that involve working at the limits of current in- strumental capabilities and data analysis. The most restrictive factor implies the need to discover a plan- et with an orbital period of 10-11 years, and hence collect within a comparable timeframe or (better) longer, uniform data, possibly with the same instrument and appro- priate temporal coverage. The fact that extrasolar planets have been monitored as a whole for two decades does not automatically mean discovering a Jupiter twin at the right distance from a Sun twin. As mentioned, it is also essential a certain level of uniformity of the data collected and it is indispens- able an extraordinary spectroscopy precision; requirements that only the HARPS (High Accuracy Radial velocity Planet Searcher) instrument on the ESO 3.6-metre Cassegrain telescope has in the past met and still continue to meet today in its hunt for extrasolar planets. HARPS has been operating since 2003 and, consequently, for some planetary systems there are record- ings and documentation that – at least from the temporal viewpoint – are suffi- cient for revealing systems analogous to that of Sun-Jupiter. HARPS’ particularity is to be able to highlight even tiny shifts toward the blue and red of certain spectral lines (compared to laboratory-measured standard positions), that are interpreted as light source move- ments at speeds typ- ically between a few km/sec and a few hun- dred km/sec. For a star with a uni- form and stable pho- tosphere, such shifts are a true reflection of the stellar motion with respect to the observer, and hence of its radial velocity. But stars, especially those of the solar A bove, the his- toric 3.6-metre Cassegrain tele- scope, installed at ESO in 1977. This instrument has over the years benefited from sig- nificant upgrades, including the HARPS spectro- graph, thanks to which it has been possible to discover and confirm nu- merous exoplanets. Left, at the focal plane of the tele- scope we can see the Fibre Adapter (in green), which sends to HARPS the light collected. [ESO/H.H. Heyer]