Gaia: science essays


If the rate of expansion one second after the Big Bang had been smaller by even one part in a hundred thousand million million, [the Universe] would have recollapsed before it reached its present size. On the other hand, if it had been greater by a part in a million, the Universe would have expanded too rapidly for stars and planets to form.

Stephen Hawking

Gaia is a satellite mission of the European Space Agency, launched in 2013. It is operating today, and it should remain operational until about 2023–24.  Because of the enormous amount of data processing involved, improved catalogues will continue to be released well after the end of satellite operations, probably until around 2030.

Gaia's goal is to measure the distances and motions of more than two billion stars in our Galaxy and beyond, all with unprecedented accuracy, barely imaginable even 25 years ago. These observations are enabling an enormous advance in our understanding of the Universe. As of mid-2022, several thousand scientific papers have been written on its findings.  The ESA Gaia www pages include many topical news highlights.

Gaia builds on the success of ESA's pioneering Hipparcos mission, which was operated in orbit between 1989–1993.  The Hipparcos Catalogue of nearly 120,000 star positions, distances, and space motions was published in 1997.


In these short weekly 'essays', I have picked out some of the scientific highlights of the Gaia mission as they are emerging, or as they caught my attention. They are not necessarily the most important. They do not follow any specific sequence. They are not a complete review of a given topic. Many will be quickly superseded by new results. But they offer a snapshot of some of the exciting discoveries that Gaia is making across all of astronomy.

I've also included some essays on related topics, including the history of astrometry, and some more technical, managerial, or developmental aspects of both the Hipparcos and Gaia missions. In each, I have included a footnote DR1, DR2, EDR3, DR3, etc to indicate which of the (latest) data releases the essay refers to (see essay #10), with DR0 signifying technical or historical material not connected with any specific data release.

Who are they written for?  Anyone who might have a general interest in science and astronomy, including amateur astronomers, young scientists starting out on their careers, mid-career scientists looking in on Gaia for the first time to get a feeling of what is possible, and even specialists looking in from different areas of astronomy, or physics more generally.

My thanks go to many people: to all those I worked with on the Hipparcos and Gaia projects over almost 30 years, to those now dedicating huge reserves of their time, energy, and skill to the ongoing data processing, and to those who have entered into the Gaia catalogue and published the results described here.

Click on the access PDF icon to access the file. Only a few references are included, and these are 'discreetly' hyperlinked for those who want to read more... where references appear in the form (Einstein 1908) or, clicking on the text (even though not highlighted!) should lead to the relevant online article.


In a few cases, I've recorded an interview on the subject (see science interview page).


78. Gaia's first exoplanets

The first of thousands?

Pre-Gaia DR3, the NASA exoplanet archive tabulated more than 5000 exoplanets, with just one discovered from astrometry. With just over 34 months of data, Gaia DR3 is accompanied by 130,000 astrometric orbit solutions, including 1843 brown dwarf companion candidates, and 72 exoplanet candidates.

26 June 2022


77. The Galactic escape velocity

Constraints on the halo mass

What is the total mass of our Galaxy? How far out does our Galaxy halo extend? The distribution of stellar velocities, and in particular the ‘escape’ velocity from the solar neighbourhood, holds a number of clues. Estimates from Gaia are converging on a Milky Way mass of about 10^(12) times the mass of our Sun.

19 June 2022


76. Data Release 3

Gaia's latest stunning data release

Gaia is now almost seven years into a possible 10-year data collection phase. Today marks the latest data release, Gaia DR3. For the same stretch of time and the same set of observations as EDR3 (Early Data Release 3), DR3 presents a stunning wealth of new data products derived from this first 3 years of mission data.

12 June 2022


75. The local mass density

Gaia as a dark matter detector

Stars in the Galactic disk 'bounce' slowly up and down around its mid-plane as a result of the force exerted by the matter comprising the disk itself. The detailed stellar motions depend on the total disk mass, both visible and dark matter. The Gaia data are throwing new light on the disk structure and its dark matter content.

5 June 2022


74. Open clusters with Gaia

More sites of recent star formation

Gaia is revolutionising the study of Galactic open clusters. High-quality distances allow cluster membership to be refined, space motions convey details of their dynamics and dispersion, and its unprecedented multi-epoch multi-colour photometry further contributes to classifying membership and chemistry.

29 May 2022


73. White dwarf pollution and exoplanets

Remarkable clues about ancient planets

Heavy element pollution in white dwarf atmospheres is attributed to the accretion of rocky planetesimals which have been scattered and torn apart to form a dusty debris disk that can be accreted by the white dwarf. Deep insights into the nature of the associated planetary systems are now being assembled.

22 May 2022


72. The warp of our Galaxy

The mystery of its origin continues

Various explanations have been proposed for our Galaxy's warped structure, including infall of intergalactic material, a close encounter with a companion galaxy, or misalignment of the disk and its dark matter halo. Gaia is contributing to an improved picture of its structural complexity, but the underlying driving mechanism remains uncertain.

15 May 2022


71. More halo streams from Gaia

The origin of the Galaxy's stellar halo

Ancient signatures of tidal infall, responsible for our Galaxy's stellar halo, remain evident because orbital time-scales in the outer parts of the Milky Way extend to billions of years. As a result, the halo retains kinematic evidence of the surviving remnants of accretion. A number of these are being found and characterised by Gaia.

8 May 2022


70. The Local Bubble

Do supernovae pose an existential threat?

Our Sun lies within a low-density region of the interstellar medium known as the local cavity. This is partially filled with hot, low-density gas, about 100 pc in size, and referred to as the Local Bubble. Its detailed morphology can be probed through our knowledge of stellar distances, and these are being transformed by Gaia.

1 May 2022


69. HD 140283: as old as Methuselah?

Our Galaxy's oldest stars

Today, most astronomers would probably place their bets on the microwave background radiation providing the most secure estimate of the age of the Universe. But there are a number of nearby stars whose estimated ages push at the limits of this inferred upper bound. Accurate distances are crucial to a better understanding.

24 April 2022