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
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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30. The motion of globular clusters
Measuring their space motions
The space motions of globular clusters and dwarf spheroidal galaxies depend on our Galaxy's gravitational potential, and therefore its mass distribution. Together with knowledge of their chemistry and ages, these provide strong constraints on theories of formation of our Galaxy, including when and how the halo and disk actually formed.
26 July 2021
29. White dwarf surveys
Important objects in huge numbers
White dwarfs are of great importance for theories of star formation and evolution, of degenerate matter at extremely high density, for distance scale determination, and for understanding planet survival beyond a star's main-sequence lifetime. Pre-Gaia, known numbers were around 10,000. Gaia is in the process of characterising some 100,000.
19 July 2021
28. Solar activity - and dark matter?
Some speculation on dark matter
The rotation of the Sun is central to the two main hypotheses which try to explain the 11-year solar activity cycle: attributable either to a turbulent dynamo operating in or below the Sun's convection envelope, or to a large-scale oscillations of a fossil magnetic field in its radiative core. Some particularly speculative ideas could be tested with Gaia.
12 July 2021
27. The celestial reference frame
Defining the reference system
An absolute reference system is as central to Gaia as it was for Hipparcos, and the accuracy for the link correspondingly more demanding. But Gaia's limiting magnitude, at 20–21 mag, allows very large numbers of quasars to be observed by the instrument itself. And this means that the problem can be tackled much more directly.
5 July 2021
26. Polar motion
The wobbling Earth
The Earth's rotation axis moves, on timescales ranging from years to millennia, as a result of various external and internal forces. The wobbling motion of the Earth has been characterised, back to about 1900, by re-examining star measurements using the Hipparcos star positions. Further insight could be possible with Gaia.
28 June 2021
25. The origin of Oumuamua
Origin of an interstellar visitor
The astronomy world was captivated by the announcement of an interstellar traveller in October 2017. Named Oumuamua, from the Hawaaian for 'scout', it was discovered crossing the solar system some 40 days after its closest approach to the Sun. Gaia is in the process of pin-pointing its progenitor star system.
21 June 2021
24. Occcultations of Europa and Triton
Applications to bodies in our solar system
A stellar occultation occurs when a solar system object passes in front of a star. The brightness drop can be used to determine the occulting object's size and shape, the presence of an atmosphere around it, and structures such as rings or moons. Gaia is allowing occultation events to be accurately predicted in much larger numbers.
14 June 2021
23. The Maunder minimum
Probing Earth's climate
A particularly cold period in Europe, which caused great hardship, was a 70-year interval from around 1645–1715, today known as the 'Maunder Minimum'. Hipparcos made some first steps in establishing whether this sort of behaviour is common in solar-type stars. Further progress can be expected with Gaia.
7 June 2021
22. Hypervelocity stars
These strange objects get more mysterious
Hypervelocity stars are a rare and exotic type of star, racing through our Galaxy with velocities of 500–1000 km per second or more. How did they acquire these enormous speeds, how and where were they formed, where are they going, and what can they tell us about the structure and origin of our Galaxy? Gaia has all the answers!
31 May 2021
21. Measuring exoplanet radii
Gaia pins down the sizes of vast numbers
An accurate exoplanet radius, together with its mass, yields the planet's mean density, and a broad categorisation of its nature. Combined with periods and host star properties, this leads to great insights into both the physics of planetary atmospheres and interiors, and the physics of planet formation and evolution. Gaia is transforming the field.
24 May 2021
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