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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100. The Fermi paradox
Do alien civilisations exist?
Following my last three essays, which looked at the historical debate about the existence of life on other worlds, and the search for anomalous stars which may be the first steps along the path for discovering alien civilisations, I look in more details at the Fermi paradox: if such civilisations exist, where are they?
28 November 2022
99. Searching for Dyson spheres
Searches for alien civilisations
I look at one specific search for alien civilisations which is being assisted by Gaia: the search for so-called Dyson spheres. These are hypothetical megastructures that might encompass a star, capturing a significant fraction of its emitted energy, and satisfying that civilisation's continuously growing energy needs.
21 November 2022
98. Boyajian's star(s)
More ideas for SETI searches
Amongst Kepler's exoplanet discoveries was the curious KIC 8462852 (Boyajian's star), which displays an unusual lightcurve whose nature is still under debate, even in the context of being an interesting SETI target. I describe a search for other similar stars, and the surprising spatial clustering of some of the new candidates.
14 November 2022
97. Life on other worlds?
A historical perspective
This week, I will review the philosophical debates that have raged since antiquity on whether ‘populated’ worlds exist beyond our own. While deviating from the topic of Gaia directly, it nonetheless provides an interesting historical background to the search for exoplanets, and life on other worlds, which are being assisted by Gaia.
7 November 2022
96. Is the Earth flat?
A message for scientists and educators?
Why do some people, in the face of overwhelming evidence to the contrary, cling vigorously to the idea that the Earth is flat? If it is because of the way today's big science is conducted and communicated, the question may hold an important message for scientists and educators.
31 October 2022
95. Our Galaxy's tumbling motion
Big movements of our Galaxy's disk
I look at two large-scale dynamical phenomena which are, today, believed to affect the bulk motion of our Galaxy's disk with respect to its dark matter halo. The first is a tumbling motion tied to their primordial origin, while the other is related to the orbit of our neighbouring Large Magellanic Cloud.
24 October 2022
94. The mass of the Local Group
The sum of all our neighbours
I look at the latest estimates of the total mass of our Local Group of galaxies. These take into account increasingly subtle effects related to their detailed dynamics, many being clarified by Gaia. And they must also be consistent with the increasingly detailed predictions of numerical models of the formation and evolution of structure in the Universe.
17 October 2022
93. The mass of the Milky Way
Why it's so difficult to measure
What is the mass of our Milky Way galaxy? Why is it important to know? Why is it so difficult to measure? And what is Gaia contributing to our knowledge? Here, I take a look at the Gaia-related papers that have been trying to tackle this problem, and see why the Galaxy halo still resists our attempts to fully characterise it.
10 October 2022
92. Diffuse interstellar bands
A slowly advancing mystery
Diffuse interstellar bands, or DIBs, comprise some 600 known absorption features widely observed in stellar spectra. Known for more than a century, only one has been securely identified. Nearly half a million sources show the 862 nm DIB in their Gaia spectra, providing new insight into interstellar absorption.
3 October 2022
91. Cerium and the Galaxy infall history
Its use in Galactic archaeology
As one of the developments in Galactic archaeology being enabled by Gaia, I will explain what cerium is, why it is relevant to astronomy, how it is measured by Gaia, and what its occurrence tells us about the formation, and in particular the complex infall history, of our Galaxy.
26 September 2022
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