Gaia: science essays
In these short (mostly 2-page) 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 offer a snapshot of some of the 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 (described in essays #10 and #76), 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 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 www.gaia.com, clicking on the text (even though generally 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).
New: As of early July 2025, have converted Essays 1–130 into audio "discussion-type" podcasts, entirely using generative AI. They are available at my Gaia Essay YouTube channel, and I describe their construction in Essay 227.​​
As of July 2025, my essays will be monthly (on the first Monday of the month) until further notice.​​
This table page lists all essays, updated to the end of June 2025 (1–235 inclusive), in tabular form. It includes a simple search on the title field.​​​​
New: This Gaia Science Tree (v3.0, July 2025) presents essays 1–235 (Jan 2021–Jun 2025) as a hyperlinked "mind map"
* all end-nodes are hyperlinked to the given essay number (links are to "legacy" copies at the CERN-Zenodo site)
* catalogue content topics are at top right, background material at bottom left, otherwise moving "outwards" clockwise in the diagram
* I have prepared this as a didactic tool. Please feel free to make use of it as you wish
Please make use of this subscribe page to receive an email (usually Monday) when each new essay is published
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Essays through to the end of 2023 (1–156 inclusive) also appear in a hyperlinked indexed form in the Bulletin of the American Astronomical Society (BAAS Vol. 56, Issue 1, 15 March 2024): ADS 2024BAAS...56a.008P
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16. Quasars, as seen by Gaia
Distant objects characterised by Gaia
More than 500,000 quasars are being observed by Gaia. In addition to their role in defining a highly accurate inertial reference system, many quasars are also of individual interest. They are being used as probes of cosmological evolution and structure formation, and several hundred show multiple images as a result of gravitational lensing.
19 April 2021
15. The Enceladus stream
Remnant streams of an ancient merger
Present theories of galaxy formation argue that large galaxies, such as our own Milky Way, have been built up from a series of mergers with smaller galaxies over the past several billions of years. Remnant streams of these mergers are being revealed by Gaia, with the Enceladus stream being particularly prominent.
12 April 2021
14. Testing modified gravity
Gaia probes the weak-gravity regime
Dark matter has been invoked to explain various perplexing observations. Yet despite much experimental effort, there has been no detection to date. Could some modified theory of gravity provide an alternative explanation? Gaia is providing a probe in the weak gravity regime using wide-separation binary stars.
5 April 2021
13. The distance to the Pleiades
Gaia sets the record straight
The Pleiades star cluster, in the constellation Taurus, has been recognised as a group of stars since antiquity, and it has long played an important role in establishing the cosmic distance scale. The Pleiades distance provided a puzzling controversy for Hipparcos. Gaia has at last set the record straight.
29 March 2021
12. Planet mandalas
Beautiful fingerprints of planetary systems
Each planet around a star pulls on the star as it orbits, affecting the motion of the star itself. Multiple planets pull by different amounts, and with different periods. The result is that the star moves along a complex path through space, each planetary system with its own distinct 'fingerprint'. Gaia is measuring these remarkable movements.
22 March 2021
11. Astrometric microlensing
A manifestation of General Relativity
Gravitational microlensing is the bending of light due to a massive body. It occurs through the chance alignment of stars along the line-of-sight. The changing brightness of the background star gives a powerful technique for discovering exoplanets. For the first time, tiny shifts in the positions of the distorted images can also be detected by Gaia.
15 March 2021
10. Catalogue data releases
The data releases so far (to EDR3)
Gaia is more than six years into a possible 10-year data collection phase. Three data releases have been issued to date. Each successive catalogue supersedes the previous. Each comprises more observations, and an increased temporal baseline, and an improving accuracy of the astrometric parameters as a result.
8 March 2021
9. Gaia and GDP
Does it have any impact on economic growth?
Scientists are keen to emphasise that their contributions are essential for the advancement of human knowledge, and that they often have unexpected benefits much further down the line. Some economists are actually trying to quantify the returns on a country's investment made in the areas of fundamental or basic science.
1 March 2021
8. Why radial velocities?
And why they aren't easy to acquire
Classical astrometry measures star positions in two angular coordinates. But star motions along the line-of-sight can't be determined in this way. Great efforts were made to measure radial velocities from the ground for Hipparcos. Gaia is in the process of tracking more than a million of these radial motions directly from space.
22 February 2021
7. On-board detection for Gaia
How stars are detected on-board
Gaia uses a combination of advanced optical imaging and powerful onboard processing to detect every star that enters its field of view, down to a million times fainter than observable with the human eye. Each of these more than two billion stars are then tracked with its high-precision measurement system as the satellite scans the sky.
15 February 2021
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