In 2025, the European Space Agency dark universe detective spacecraft Euclid turned its attention to the heart of the Milky Way for just 26 hours. In just over one day, Euclid was able to create the largest and most detailed photo of this region of our galaxy ever made.
The image, packed with 60 million stars, could help scientists hunt for extrasolar planets, exoplanets, in this region known as the galactic bulge.
Euclid is designed to study dark energy, the mysterious force that drives the accelerating expansion of the universe, by studying distant galaxies. That means the space telescope is powerful enough to distinguish individual stars in the central bulge of the Milky Way. Other telescopes fail to do this because they are too blinded by the densely packed stars in this region.
Euclid was requested to monitor the central bulge of the Milky Way to assist astronomers in the hunt for exoplanets because this is the perfect region for so-called "microlensing" events to occur.
"To catch microlensing, you need to observe parts of the sky that are crowded with stars, such as close to the center of our galaxy," team leader Jean-Philippe Beaulieu of the Institut d'Astrophysique de Paris in France said in a statement.
Microlensing is a weak form of gravitational lensing that occurs when objects with mass cause the very fabric of space to warp. When light from a background source passes through this warping of space, its path is curved. This can be used to study the background source; for example, scientists have used it to great effect with the James Webb Space Telescope (JWST) to study some of the most distant and early galaxies. However, the curvature of light from background sources can also be used to detect faint objects like planets.
Spotting planets using microlensing requires one star to pass in front of another and act as a gravitational lens. The presence of a planet causes a tiny perturbation in the lensing of light from the background star. It's a small effect, but one that has been used very effectively in the detection of exoplanets.
"During the last twenty years, almost 300 exoplanets have been discovered using this technique, all with ground-based telescopes and all towards the center of our galaxy," Beaulieu said. "This image from Euclid includes 51 known planetary systems – and it will assist in studying many more that will be found."
Despite Euclid's study of the central bulge pointing the way forward in observing new microlensing events, there are no such events in the data from the ESA spacecraft. That is because detecting such events takes around 20 days.
Instead, it will be up to telescopes like the forthcoming Nancy Grace Roman Space Telescope to observe this region for longer periods and compare with this day's worth of Euclid data to find microlensing events.
"In 24 hours, Euclid has already captured the stars involved in all the future microlensing events that the Roman space telescope will detect, but before the stars and planets involved have aligned," team member Natalia Rektsini of the Institut d'Astrophysique de Paris said.
"This means that anyone who detects a microlensing event in the same region, for example, with Roman, will be able from now on to use Euclid data as a time reference in the past and see how the stars looked before they overlapped. Since Euclid can clearly separate individual stars, one can then measure how fast they move over time and use that information to confirm the existence of a planet and determine its mass. This would not be possible with data from one point in time."
One reason this is so exciting to exoplanet hunters is that other techniques used to spot these distant worlds tend to excel at detecting hot and massive planets close to their host stars.
Microlensing, however, can be used to detect more diminutive planets much farther from their host stars and with chillier temperatures. That means it could be used to detect ice giants like Uranus and Neptune in wide orbits around central bulge stars.
"This result shows what a relatively small, dedicated team can achieve within a large international mission," says Valeria Pettorino, Euclid Project Scientist at ESA.
"That's why this Euclid data will be a time reference for past and future missions and enable studies of exoplanets and their masses. This data can also be used for other scientific applications, from brown dwarfs and binary stars to stellar motions and dust across our galaxy."
