From instrumental design
to scientific exploitation

Gaia EDR3 Day

» Monday 30 November 2020

A day of presentations on Gaia will be held, online, on 3 December 2020, on the occasion of the publication of Gaia EDR3.

On the occasion of the publication of the first part of the third Gaia catalogue, Gaia EDR3 (Gaia Early Data Release 3), a day is jointly organised by the CNES, the Bordeaux Astrophysics Laboratory (LAB), the Observatoire de la Côte d’Azur (OCA), the Paris Observatory, and the UTINAM/OSU Theta Institute.

The day will be divided into three parts :

from 11 to 12 am : a press conference on the Gaia EDR3 catalogue and its first results;

from 14 to 17h : a series of webinars on some of the very numerous scientific results obtained with Gaia DR2 (for enlightened scientists and amateurs). These presentations will be given in French, but with slides in English. They will be recorded and later available on the YouTube channel of the Paris Observatory;

from 21 to 22h: a conference for the general public.

Zoom login link:

Programme of the Press Conference.
Welcome by Fabienne Casoli, President of the Paris Observatory - PSL

11h00-11h15 : Gaia and its 3rd catalogue. Its content and validation (Catherine Turon and Frédéric Arenou, GEPI, Observatoire de Paris)

11h15-11h22 : The Gaia catalogue of nearby stars (Céline Reylé, UTINAM Institute, OSU THETA)

11h22-11h29 : The Galactic Anticentre as seen by Gaia (Georges Kordopatis, Lagrange, OCA)

11h29-11h36 : Exploration of the Magellanic Clouds (Laurent Chemin, Universidad de Antofagasta, Chile, ex GEPI and LAB)

11h36-11h41 : Stroll in the Milky Way (Clément Hottier, GEPI, Observatoire de Paris)

11h41-11h48 : The next Gaia catalogues (Chantal Panem, CNES)
Questions and Answers

12h Opening of the Gaia EDR3 database and visualization of queries to the archive in real time from the ESAC (European Space Astronomy Centre) in Madrid.

Afternoon programme
Organised by the Paris Observatory.

14:00 Fabienne Casoli, President of the Paris Observatory - PSL: introduction

14:05 Paola Di Matteo (GEPI): The disc of the Milky Way unveiled by Gaia: ridges, undulations and spiral structure
Since the publication of its second catalogue, the Gaia mission has made new discoveries. It has become clear that the processes of non-equilibrium in the Milky Way, such as interactions with satellites, the effect of the bar and the spiral arms, have considerably shaped the disc’s current structure and kinematics. I will present a summary of these discoveries, as well as the modelling work carried out by our team to interpret them. Finally, I will show how it is possible to combine the parallaxes, proper motions and radial velocities of millions of stars on the disc to map our Galaxy and reveal its spiral structure.

14:25 Pierre Kervella et al (LESIA): stellar and sub-stellar companions using Gaia’s own movements
The census of the stellar and substellar companions of nearby stars is largely incomplete, especially for low-mass brown dwarfs and long-period exoplanets. The companions influence the proper motion of their host by their gravitational reflex motion. Using the Hipparcos and Gaia DR2 catalogues, we have determined the long-term proper motion of stars common to these two catalogues. We then searched for a proper motion anomaly (PMa) between the long-term proper motion vector and the DR2 (or Hipparcos) measurements, indicating the presence of a disturbing secondary object. We focused our analysis on classical Cepheids, RR Lyrae stars and nearby stars located at less than 50 pc. I will focus on the specific case of the radial velocity exoplanet Proxima c, whose mass could be estimated from Gaia astrometry. We have also recovered the masses of the planetary companions of epsilon Eri, epsilon Ind and beta Pic, and detected the signature of a possible planet with a few Jovian masses orbiting tau Ceti. This type of analysis opens up the possibility of identifying otherwise inaccessible long-period orbital companions. Gaia’s complementarity with velocimetry and transit techniques (more sensitive to short orbital periods) already seems remarkably powerful.

14:45 Misha Haywood (GEPI): History of a galactic encounter
The history of a galaxy is also the history of its encounters with other galaxies. Gaia’s second Data Release highlighted that the Milky Way merged with one of its satellite galaxies more than 9 billion years ago. This was a major encounter that shaped the evolution of our Galaxy. I will discuss the circumstances of this discovery, the characteristics and implications of this galactic event.

15:05 Bruno Sicardy (LESIA): stellar occultations with Gaia
Stellar occultations by objects in the solar system have led to many very fruitful campaigns (discovery of rings around Chariklo and Haumea, seasonal effects of Pluto...). These occultations have greatly benefited from Gaia DR2, and today allow us to tackle new themes.

15:15 Jean Schneider (LUTh): Searching for interstellar asteroids with Gaia
Gaia provides regular alerts of objects in the solar system: At the same time, Zwicky Transit Facility (ZTF) also produces alerts of objects from the solar system. In liaison with the FINK group at IN2P3 (, which has privileged access to ZTF alerts, I will describe the project to correlate Gaia’s alerts with those of ZTF to search for interstellar asteroids. Once LSST is operational, the operation will continue with LSST.

15:30 Clement Hottier (GEPI): "Dusting off" the Galaxy with Gaia
Gaia’s second data release provided us with the largest catalogue of star parallaxes. This information on the third dimension not only helps us understand the distribution of stars in the Galaxy, but also the extinction that dims their lights. Indeed, by combining the Gaia DR2 catalogue with infrared photometry, we have mapped interstellar extinction in three dimensions using two methods. The first allows us to probe the first kiloparsecs around the sun, while the second gives us access to more distant areas. These two complementary maps reveal the dust components of the large structures of the galactic disc such as the spiral arms.

- Pause -

16:00 Louise Breuval (LESIA) : Leavitt’s Law of the Milky Way Cepheids thanks to Gaia DR2 parallaxes and open host clusters
Cepheid stars play a considerable role as astronomical distance standards, thanks to the simple relationship between their period of pulsation and their intrinsic luminosity, the PL relationship (Leavitt 1912). Uncertainty about this relationship is the main factor contributing to the error budget of the Hubble constant, a parameter that describes the expansion of the Universe. The value of the Hubble constant is currently at the centre of a major controversy: while it is estimated at 67.4 +/- 0.5 km/s/Mpc by the Planck satellite (Planck Collaboration 2018), the local measurement based on Cepheids is 4 sigma larger, with a value of 74.0 +/- 1.4 km/s/Mpc (Riess et al. 2019). This difference may provide evidence of physics beyond the standard model. Therefore, accurate and precise measurements of the Cepheid distance are of paramount importance to improve the calibration of the PL relationship and to try to solve this crisis. We propose an original approach to avoid the direct use of potentially unreliable Cepheid parallaxes. We obtain 36 indirect, unbiased and accurate distances for galactic Cepheids. Using this sample, we calibrate the PL relationship in the near infrared and optical bands. We revise the Hubble constant of 76.18 +/- 2.37 km/s/Mpc from Riess et al (2016) based on the Milky Way Cepheids: using our sample, we obtain 72.8 ± 2.7 km/s/Mpc.

16:20 Francois Hammer (GEPI) : Gaia DR2: consequences on the nature of spheroidal dwarf galaxies and the mass of the Milky Way
The spheroidal dwarfs around the Milky Way are the smallest galaxies observed and are supposed to contain a large fraction of dark matter, given their excessive speed dispersion. However, most of them are located close to their pericentre due to their orbit and the mass distribution of the Milky Way, both derived from Gaia DR2. This leads to a very low probability, P = 2 10-7, that they are long-lived satellites such as the subhalos predicted by cosmological simulations. Rather, their proximity to their pericentre suggests that they are affected by tidal shocks which provide sufficient kinematic energy to explain their velocity dispersions. The dependency of the properties of dark matter on their distance from the Milky Way seems to favour dSphs subject to tidal and out-of-equilibrium effects rather than self-balancing systems dominated by dark matter.

16:40 Frédéric Arenou (GEPI): Gaia, from DR2 to EDR3
Slowly but surely, the astrometry and photometry of Gaia is improving, and Gaia EDR3, more accurate and precise, will replace Gaia DR2, and increase the limiting distance of the survey for a greater number of sources. We will present the main properties of this new data publication, highlighting the useful results, the caveat of the scientific validation and the expected improvements compared to DR2.

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Programme of the General Public Conference
Organised by OCA and CNES.

21h00-21h05 : Introduction on the Gaia mission by Philippe Laudet (Head of the Astrophysics and Astronomy programmes at CNES)
21h05-21h22 : The solar system as seen by Gaia by Paolo Tanga (astronomer at the Observatoire de la Côte d’Azur)
21h22-21h39 : Studying the galaxy with 1 billion stars by Orlagh Creevey (assistant astronomer at the Observatoire de la Côte d’Azur)
21h39-21h56 : Gravitational lenses with Gaia with Christine Ducourant (assistant astronomer at the Bordeaux Observatory)

Leaders: Frédérique Auffret and Frédéric Arenou at the Paris Observatory and Chantal Panem at CNES Toulouse.

Press contacts: Frédérique Auffret ( at the Paris Observatory, Raphael Sart ( at CNES Toulouse and Marc Fulconis ( at the OCA.

Local managers on behalf of DPAC, the Gaia Data Processing and Analysis Consortium: Frederic Arenou and Catherine Turon at the Paris Observatory, François Mignard and Paolo Tanga at the OCA, Céline Reylé at the Besançon Observatory (UTINAM) and Clara Nicolas at CNES Toulouse.