What great news that ESPRESSO, the Echelle SPectrograph for Rocky Exoplanet and Stable Spectroscopic Observations, has just achieved ‘first light.’ The spectrograph is installed on the European Southern Observatory’s Very Large Telescope at the Paranal Observatory in northern Chile and its powers are prodigious. For ESPRESSO makes it possible, for the first time, to combine the light of all four telescopes at the VLT. This creates an instrument with the light collecting power of a 16-meter telescope, a major enhancement to the exoplanet hunt.
Image: The room where the light beams coming from the four VLT Unit Telescopes are brought together and fed into fibres, which in turn deliver the light to the spectrograph itself in another room. One of the points where the light enters the room appears at the back of this picture. Credit: ESO/P. Horálek.
Thus the enthusiasm of lead scientist Francesco Pepe (University of Geneva):
ESPRESSO isn’t just the evolution of our previous instruments like HARPS, but it will be transformational, with its higher resolution and higher precision. And unlike earlier instruments it can exploit the VLT’s full collecting power — it can be used with all four of the VLT Unit Telescopes at the same time to simulate a 16-metre telescope. ESPRESSO will be unsurpassed for at least a decade — now I am just impatient to find our first rocky planet!”
Image: This colorful image shows spectral data from the First Light of the ESPRESSO instrument on ESO’s Very Large Telescope in Chile. The light from a star has been dispersed into its component colours. This view has been colourised to indicate how the wavelengths change across the image, but these are not exactly the colours that would be seen visually. Close inspection shows many dark spectral lines in the stellar spectra and also the regular double spots from a calibration light source. The dark gaps are features of how the data is taken, and are not real. Credit: ESO/ESPRESSO team.
We’re going to be hearing a lot from ESPRESSO because it will greatly improve our powers of radial velocity observation. Remember what we are doing when we use these techniques. Radial velocity involves extracting the tiny Doppler signature of star motion as the star is pulled first one way, then another, by the planets around it. Compared to the size of the star, the movements are small but we can trace them in the star’s light spectrum.
Repeating changes to the spectrum as it shifts toward red, toward blue, back toward red, give us the data we need to identify a planet, and until the Kepler mission came along, radial velocity was the primary means we used to find such worlds. 51 Pegasi’s planet, the first found to orbit a main-sequence star, was found using radial velocity methods in 1995. Now we use a mixture of methods including transit studies, direct imaging and gravitational microlensing.
Image: The Echelle SPectrograph for Rocky Exoplanet and Stable Spectroscopic Observations (ESPRESSO) successfully made its first observations in November 2017. Installed on ESO’s Very Large Telescope (VLT) in Chile, ESPRESSO will search for exoplanets with unprecedented precision by looking at the minuscule changes in the properties of light coming from their host stars. This view shows the inside of one of the ESPRESSO front-ends where all the active components of the spectrograph are located. Credit: Giorgio Calderone, INAF Trieste.
Combining methods can be hugely useful, for while radial velocity lets us measure the mass and orbit of the planet (I won’t, for the purposes of this post, get into the problem that RV mass measurements can produce only minimum mass estimates), transits can help us deduce its density. But for those planets that do not transit, we’d like to move further and further down the scale, making detections of ever smaller worlds, terrestrial-class planets perhaps like our own.
Image: the moment of first light, with the team jubilant in the VLT control room. Credit: Giorgio Calderone, INAF Trieste.
From an ESO fact sheet on ESPRESSO:
The radial velocity technique has been so far the most productive in terms of extra-solar planet detections. Low mass planets (one to few Earth masses) are especially interesting because according to formation models they could represent the bulk of the planet population. However they are more elusive and require extremely stable instruments. The HARPS instrument, with a precision better than 1m/s, has discovered up to now the vast majority of planets with masses smaller than Neptune, giving an invaluable experience in view of the realization of more precise instruments. With a radial velocity precision better than 10cm/s, an Earth mass planet in the habitable zone of a low mass star can be detected.
Exactly so. ESPRESSO picks up where HARPS left off. While HARPS could achieve a precision of one meter per second, ESPRESSO gets us down below 10 centimeters per second. The upscaling is the result of ESPRESSO’s placement to tap the four VLT telescopes as well as advances in spectroscopic technology. The benefit will be in characterizing much less massive planets unavailable for our scrutiny through transits or direct imaging, further bulking up the exoplanet catalog and deepening our statistical analysis of planets near us in the galaxy.