Euclid Galactic Bulge Survey

On March 23rd 2025 the ESA Euclid telescope was pointed towards a location close to the Galactic Centre (Figure 1), in order to obtain an unprecedented deep, wide-field, high-resolution view of the inner bulge region of our Milky Way galaxy.

Nine adjoining fields (Table 1), spanning a total area of 4.8 deg2, were imaged by Euclid, using its optical VIS camera, over a period of about 24 hours. Each field comprises 16 dithered exposures of 400 sec each, giving a total exposure of 1.8 hours per field. Euclid’s optical sensitivity and resolution (0.16 arcsec) is comparable to the Wide Field Camera 3 (WFC3) instrument on the Hubble Space Telescope. However, each Euclid field covers an area that is 270 times larger than a WFC3 field, underlining the true nature of Euclid as a sky surveyor.

As well as offering an unprecedented crisp view of stars over a wide area of the inner Galaxy, the principal science driver of the Euclid Galactic Bulge Survey is to assist in the search for exoplanets. To date, over 200 distant exoplanets have been discovered towards the Galactic bulge using the gravitational microlensing method. Microlensing involves the transient magnification of a background star caused by a close (milli-arcsecond) temporary alignment on the sky of a passing foreground star or planetary system. The foreground host star can induce a magnification signal lasting for weeks or months, whilst its planets may cause signal perturbations on timescales of hours to a few days.

The Euclid Galactic Bulge Survey covers an area that contains many historical planetary microlensing signals observed from ground-based surveys over the last 20 years. Over the passage of time the foreground planetary systems of many of these historic events will have separated from the background star at a level that can be detected by Euclid’s high resolution VIS camera. Being able to detect directly the foreground host star light, and its separation over time from the background magnified star, allows confirmation of the planetary nature of the event, and a more direct and precise measurement to be made of the planetary masses.

The survey also covers the region that the NASA Nancy Grace Roman Space Telescope (Roman) will monitor over a 5-year period from 2027 for its Galactic Bulge Time Domain Survey. The Euclid Consortium team will use Euclid images to identify the foreground planetary host lenses that are seen by Roman several years later to microlens background stars. In this way, Euclid will help to reveal the nature of the host system and its motion over time, allowing more accurate measurements of the planetary masses than would otherwise be possible with Roman data alone.

The Euclid Exoplanet Science Working Group is responsible for the design and science analysis of the Euclid Galactic Bulge Survey. As the data are composed of very dense fields close to the centre of the Galaxy, they required special processing by the Euclid Science Ground Segment, and by the team expert in calibrating images obtained via the visible instrument. The Euclid Exoplanet Science Working Group will provide catalogues of stars (astrometry and photometry) in this unique field as part of the second Euclid Quick Data Release (Q2).

Processed data products from the Euclid Galactic Bulge Survey will be made public under the Euclid Quick Data Release (Q2) scheduled for 2026.

Figure 1: Outline of the mosaic of nine Euclid Galactic Bulge Survey pointings (in red) shown on a Gaia image of the Milky Way.

Image adaptation for Euclid, credit: J.-C. Cuillandre/ESA/Euclid/Euclid Consortium/NASA; ESA/Gaia/DPAC.

License: CC BY-SA 3.0 IGO. The original Gaia image is available here.

Table 1: Euclid Galactic Bulge Survey field centres