ESO’s PoET Telescope Begins Operations to Detect Earth-like Exoplanets
Radial-velocity spectroscopy has reached a level where stellar surface activity now limits the detection of Earth-mass planets around Sun-like stars. Modern spectrographs can measure extremely small Doppler shifts in stellar spectra. However, signals produced by granulation, magnetic regions, and starspots often mask or imitate planetary signatures. The European Southern Observatory’s Paranal Solar ESPRESSO Telescope (PoET) was developed to address this problem. The instrument achieved first light on April 9, 2026, at the Paranal Observatory in Chile.
Although PoET has a modest aperture of 60 centimetres, its scientific importance lies in the role it plays within the radial-velocity detection chain. Rather than imaging solar structure at extreme spatial resolution, the telescope measures how local solar activity alters the integrated solar spectrum. These measurements provide a calibration framework for interpreting stellar spectra observed with the Very Large Telescope.
A solar telescope designed for spectroscopic calibration
Most solar telescopes concentrate on imaging magnetic fields, chromospheric dynamics, or fine photospheric structure. PoET, however, follows a different strategy. It studies how solar surface features influence precision spectroscopic measurements that resemble those obtained from distant stars.
The radial-velocity technique depends on detecting extremely small spectral shifts caused by orbiting planets. In the case of Earth-mass planets around Sun-like stars, these shifts approach the current instrumental precision limit. At this level, variations produced by stellar activity dominate the signal. As a result, astronomers must separate planetary signatures from activity-driven distortions.
PoET addresses this challenge by using the Sun as a resolved stellar laboratory. Because the solar surface can be observed directly, it becomes possible to measure how individual structures contribute to the integrated spectral signal. This approach allows researchers to identify activity-related velocity variations with far greater confidence than is possible for distant stars.
Supporting the ESPRESSO spectrograph at the Very Large Telescope
PoET operates in close coordination with the ESPRESSO spectrograph at ESO’s Very Large Telescope. ESPRESSO represents one of the most stable radial-velocity instruments currently available. It measures spectral line shifts with extremely high precision and has already contributed to the detection of several low-mass exoplanets.
During nighttime observations, ESPRESSO receives light from the VLT Unit Telescopes. During the daytime, PoET sends sunlight into the same spectrograph through optical fibres. This configuration allows astronomers to use ESPRESSO almost continuously throughout the observing cycle. At the same time, it provides a steady stream of high-precision solar reference data.
ESPRESSO operates at spectral resolving powers near 140,000 in high-resolution mode and about 190,000 in ultra-high-resolution mode. These capabilities make it well-suited for analysing the subtle spectral variations produced by solar activity. PoET therefore extends the usefulness of ESPRESSO beyond nighttime stellar observations and strengthens its calibration performance.
Simultaneous measurements of local and disc-integrated solar light
A key feature of PoET is its ability to observe both the full solar disc and selected solar regions at the same time. This capability distinguishes it from most solar reference instruments used in spectroscopy.
When astronomers observe distant stars, they receive only disc-integrated light. Individual surface structures remain unresolved. Consequently, activity-driven velocity variations appear mixed in the recorded spectrum. This situation makes it difficult to determine which physical processes produce the observed signal.
PoET solves this problem by recording two complementary data streams simultaneously. One observing channel measures the integrated light from the entire solar disc. A second channel targets smaller regions that may contain sunspots, faculae, or granulation patterns. Astronomers can then compare the local measurements with the global spectrum and determine how specific surface features modify radial-velocity signals.
Improving the reliability of radial-velocity planet searches
Surface activity remains one of the dominant sources of uncertainty in radial-velocity surveys. Magnetic structures modify spectral line shapes. Convective flows shift line positions. Oscillations introduce additional variability. Together, these effects can imitate planetary signals or hide them inside noise.
PoET provides a controlled environment for studying these processes in detail. Because the Sun can be observed with spatial resolution, astronomers can identify exactly which structures produce measurable velocity shifts. They can then test correction techniques using real observations rather than theoretical models alone.
This strategy improves the reliability of planetary detections obtained with ESPRESSO. It also prepares the way for future spectrographs that aim to reach even higher precision levels. As radial-velocity measurements approach the centimetre-per-second regime, accurate correction of stellar activity will become increasingly important.
Installation at Paranal Observatory and international development
PoET operates at Paranal Observatory in northern Chile at an altitude of about 2635 metres above sea level. The site offers stable atmospheric conditions and a large number of clear observing days each year. These conditions support consistent solar observations throughout the annual cycle.
The telescope observes solar regions at spatial scales ranging from about 1 arcsecond to roughly 55 arcseconds. This range allows researchers to study both compact magnetic features and larger surface structures while maintaining compatibility with precision spectroscopic measurements obtained by ESPRESSO.
The instrument was developed through an international collaboration led by researchers in Portugal. Additional components were manufactured in Italy, while the telescope enclosure was constructed in Chile. Scientific operations are coordinated from the Centre for Astrophysics of the University of Porto in cooperation with ESO staff at Paranal Observatory.
ESO will distribute PoET observations through its Science Archive Facility. This policy ensures that the broader astronomical community can use the data in future research programs.
Clear skies!