The Very Large Telescope turns 25
A look at its contribution to the study of exoplanets
On 26 May the European Southern Observatory (ESO) had an international celebration to mark 25 years since the first light of the Very Large Telescope (VLT), which occurred overnight between 25 and 26 May 1998. The VLT represents the pinnacle of scientific and engineering achievement that was possible in the 20th century. It is the flagship instrument for ESO, built in the Atacama desert in Chile, at the Paranal Observatory, where sky conditions are ideal for astronomy. At the time it was the largest optical telescope ever built and it is still one of the most advanced optical telescopes in the world. Over 10,000 scientific papers have been published which use observations from the telescope, in a wide range of different areas within astronomy.
On the occasion of the anniversary, ESO commented on its continuing status in astronomy:
“Still very much at the forefront of science and engineering, the VLT is expected to continue making key discoveries for years to come.”
Within the field of exoplanets, the VLT has contributed to a number of milestones, including the first direct image of an exoplanet. 2M1207 b is a giant planet, around 5 times the mass of Jupiter, which was observed by the VLT in 2004. This amazing achievement was made possible thanks to an instrument known as NACO, which uses so-called ‘adaptive optics’ techniques to reduce the impact of the Earth’s own atmosphere on observations of stars, allowing for much sharper images than otherwise possible. Since then, a newer instrument, the Spectro-Polarimetric High-contrast Exoplanet Research instrument, known as SPHERE, has been introduced at the VLT with the primary aim of directly observing exoplanet systems. Commenting on that successes of NACO and SPHERE, Dr. Matthew Kenworthy, Associate Professor at Leiden University said to the Extrasolar Times:
“NACO was one of the early pioneering cameras on the Very Large Telescopes, discovering low mass substellar companions and imaging the brighter disks around young stars. The lessons learned had an impact on the SPHERE high contrast imaging camera, which has produced many spectacular discoveries over the past decade.”
In 2020, SPHERE took the first direct image of a multi-planet system around a Sun-like star, allowing scientists to further investigate the environment of these planet systems. Kenworthy said:
“A wonderful surprise for me has been the ability of SPHERE to image circumstellar disks, the material from which planets and moons form. SPHERE has shown a wide diversity of disks, rings, and spiral arms, rich with information that tell us about forming planets, interactions with other passing stars, and the differences that binary stars can make in the evolution of planetary systems.”
Another VLT instrument, known as CRIRES, resulted in the first direct detection of an exoplanet's orbital velocity and the detection of wind on the planet, through the detection of carbon monoxide in its atmosphere. Professor Ignas Snellen, Professor of Observational Astrophysics at Leiden University — who later also led a pioneering study using CRIRES to measure the day-night length of a (different) planet — commented on the exciting discovery when it was made in 2010:
“HD209458b is definitely not a place for the faint-hearted. By studying the poisonous carbon monoxide gas with great accuracy we found evidence for a super wind, blowing at a speed of 5000 to 10 000 km per hour.”
Despite the growing importance of direct imaging techniques, the overwhelming majority of over 5000 known exoplanets have never been directly imaged. Instead, they are observed indirectly, with techniques that measure how planets affect the stars they orbit. One such technique is known as radial velocity (or RV) observations. Sometimes also referred to as the Doppler method, RV observations are used to detect exoplanets by measuring the tiny shifts in a star's spectrum of light, caused by its gravitational interaction with an orbiting planet. As the planet orbits, its gravitational pull causes the star to wobble, resulting in a periodic variation in the star's radial velocity toward and away from Earth. By analysing these velocity changes, scientists can infer the presence, mass, and orbital characteristics of the exoplanet. RV observations were used in 1995 to detect 51 Peg b, the first exoplanet around a Sun-like star — a discovery awarded the 2019 Nobel Prize in Physics.
In this context, the VLT is home to an instrument called the Echelle SPectrograph for Rocky Exoplanet and Stable Spectroscopic Observations — most astronomers know it simply as ESPRESSO — a highly-stabilised spectrograph capable of extremely high-precision RV observations. So far, exoplanet searches have been unable to detect an Earth-sized planet in orbit around a Sun-like star, mainly due to the extreme technical difficulty this poses. The RV signal caused by the Earth causes the Sun to wobble with a peak velocity of 9 cm per second. Until recently, the best instruments were capable of detecting wobbles of stars to a level of about a meter per second. An impressive feat, measuring the movement of stars light years away to about human walking pace, but still insufficient to detect an Earth analogue.
Since ESPRESSO saw first light in 2017, it has been widely considered the world’s state-of-the-art RV instrument. It shows the enticing potential of reaching down to the 10 cm per second level, meaning this is one of the best chances we have of finding an Earth-like exoplanet with RV observations. Additionally, ESPRESSO has the opportunity to investigate the atmospheres of exoplanets through measuring their spectra, allowing researchers to look at what gases are present at these distant worlds, and whether any of them have the potential to support life.
Looking to the future, it is clear that the VLT will continue to play a vital role in the characterisation of exoplanets. Undoubtedly, we will be celebrating even more scientific milestones in the next 25 years. Happy birthday!
Good article that is both interesting and accessible (well, in part) to a layman like me. Tim N.