ESPRESSO - Searching for other Worlds The Echelle SPectrograph for Rocky Exoplanet- and Stable Spectroscopic Observations ESPRESSO is a new-generation spectrograph for ESO's VLT. The amazing 'spectroscopic' precision of this instrument will provide the community with new scientific capabilities which are unique world- wide. In particular, the ESPRESSO Consortium will invest the Guaranteed-Time Observation (GTO), awarded in return to the major investment, into two major scientific programmes:
  • Search for rocky extra-solar planets in the habitable zone of their host stars;
  • Determination of the possible variability of physical constants.

The capabilities of ESPRESSO will allow to do much more, such as extra- galactic spectroscopy, stellar abundances in neighboring galaxies, QSOs observation and cosmology. In particular, the foreseen 4-UT mode, in which all the four 8-m telescopes are connected incoherently to form a 16-m equivalent telescope, will allow to reach extremely faint objects.

Project Context Since the discovery of 51 Peg b by Michel Mayor and Didier Queloz (1995) huge progress has been made in the detection and characterization of extra-solar planets, the understanding of their formation and their internal structures and atmospheres. The variety of known planets and planetary systems increases impressively. Today, one very topical and exciting branch of the research on extra-solar planets is the search and characterization for very low-mass planets.
HARPS on ESO’s 3.6-m telescope has played a fundamental role in this domain. Its superb detection efficiency and its unique radial-velocity precision have actually allowed us, starting in 2004 with the discovery of the first ‘Neptune-mass planet’ around the star µ Ara, to explore a mass range which suffered from a strong detection bias before. Out of the 19 extra-solar planets with masses below 25 MEarth, 15 have been discovered using the HARPS instrument. A quick look into data of the on-going high-precision program on HARPS shows furthermore that this kind of planets is very common and may be found around as much as about 30% of solar-type stars. It looks like we opened the exciting ‘Pandora Box’ of very low-mass planets.
It is widely recognized today that HARPS delivered and is still delivering fantastic and important results. The discussion about Super- Earths would probably not exist at all today without HARPS. In contrast to ten years ago, it is now commonly agreed that very low- mass planets exist, that they can be detected by means of very precise radial-velocity measurements and that therefore, it is worth improving this technique even further. On the other hand, HARPS has been designed to reach ‘only’ the 1 ms-1 precision level. Although, with an effective precision of the order of 30 cms-1, it is doing better than specified, it will hardly be able to detect ‘real’ Earths in the habitable zone of their host star (our Earth induces a radial-velocity variation of 9 cms-1 on our Sun).

The astronomical community and ESO have recognized that a new HARPS for the VLT is necessary to push the detection limits down to Earth- like planets. ESO’s Scientific Technical Committee recommended to ESO, in October 2007, to initiate as soon as possible the Call for Proposal for a High-Resolution Ultra-Stable Spectrograph for the VLT called ESPRESSO. The ESPRESSO Consortium, composed of institutes from various European countries, has answered to ESO’s call for a Phase A study and was successfully selected for this task in Autumn 2008. Phase A started in January 2009 and will last until end 2009. If approved by ESO, the project will continue with Phase B starting in 2010 and last for about 4 years. Installation and commissioning of ESPRESSO at the VLT is foreseen in 2014.

The instrument ESPRESSO is an high-resolution, fiber-fed and cross-dispersed echelle spectrograph (R = 140’000) for the visible wavelength range (350 nm – 720 nm). Its main feature is the spectroscopic stability and the radial-velocity precision. The requirement is to reach 10 cms-1, but the aimed goal is to obtain a precision level of few cms-1. The combination of the extreme precision with a large telescope makes it to the most powerful spectrograph in the world. Besides being optimized for the search of extra-solar planets and thus for the highest radial-velocity performance, it remains a conventional spectrograph. In addition, ESPRESSO can be used at reduced spectral resolution with up to four Unit Telescopes of the VLT and will become therefore the first spectrograph worldwide operated on a 16-m equivalent telescope.