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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120. Solar analogues – and our Sun
What these stars tell us about our Sun
Solar analogues are loosely defined as stars that are `very similar' to the Sun. Their identification provides the basis for a number of important applications across astronomy, including their value in determining the colours of our own Sun. Compared with some 100 candidates previously known, more than 5000 have already been identified in Gaia DR3.
17 April 2023
119. Brown dwarfs with Gaia
What can Gaia say about these faint objects?
Brown dwarfs are objects with masses below the hydrogen-burning limit. They are faint in the optical, with most isolated objects, even the nearest, close to Gaia's survey limit. Those in binary systems can be detected through the orbital motion of the brighter component. Many new brown dwarfs have already been discovered in Data Release 3.
10 April 2023
118. The `plane of satellites' problem
Gaia resolves one of the conflicts with the Lambda CDM cosmology
Since the emergence of the Lambda CDM parameterisation of the standard `big bang' cosmological model in the late 1990s, various difficulties emerged in reconciling numerical simulations with observations at galaxy scales. Here I describe the `plane of satellites' problem, and how Gaia EDR3 data has been used to resolve it.
3 April 2023
117. The Gaia phase-space spiral
Who wants to know about the phase-space spiral?
I have previously looked at some of the kinematic features that have been revealed, using the Gaia data, in the local stellar population. An important Gaia discovery has been rich structure particular prominent in plots of the vertical location in the solar neighbourhood versus the vertical velocity. I explain the nature of this `phase-space spiral', and theories as to its origin.
27 March 2023
116. The Arcturus and HR 1614 streams
Probing two more of the local kinematic groups
Following my previous essay, I look at two more of the two dozen or so kinematic groups known to exist in the solar neighbourhood: the Arcturus group and the HR 1614 group. Data in Gaia DR2 allow an unprecedented view of the local kinematic structure, and detailed studies suggest that both are `phase-space waves' that could have been cause by an ancient merger.
20 March 2023
115. The Hercules stream
Gaia provides convincing evidence for its origin
Stars in the local solar neighbourhood do not move at random, but instead include groups or streams of stars sharing a common bulk motion around the Galaxy. Their reality long debated, Gaia spectacularly confirms their existence. For the Hercules stream, accurate distances and motions confirm that it results from resonant motions driven by our Galaxy's rotating bar.
13 March 2023
114. Our Galaxy's spiral arms
New insights into long-standing questions
While it is well established that our Milky Way is a barred spiral galaxy, the detailed structure of its spiral arms is difficult to establish since our vantage point lies deep within the disk. Despite the ubiquity of spiral galaxies, even the driving mechanism of spiral arms remains unclear. The millions of stars with accurate astrometry in Gaia DR3 is casting new light on this long-standing question.
6 March 2023
113. Gravitational redshift
Tests of another remarkable prediction from General Relativity
General relativity predicts that light escaping from a gravitational potential well, such as the surface of a star, will be redshifted, by some 636 km/s in the case of our Sun. The effect is most pronounced for white dwarfs. Gaia astrometry is furthering these studies, and allowing the effects to be characterised for main sequence stars in open clusters and wide binaries.
27 February 2023
112. Our Galaxy's central bar
What is it, why is there?
While it has long been suspected that our own spiral Milky Way galaxy has a central bar, its structure, its orientation, and its pattern speed, are still subject to much debate. Gaia is helping to pin down all of these properties. Remarkably, Gaia's vast compilation of accurate stellar orbits provides a measure of the bar's deceleration!
20 February 2023
111. The distance to the Galactic centre
A new method to measure this important distance
The distance to the Galactic centre is arguably the most fundamental distance entering treatments of our Galaxy's structure and dynamics, but it has been notoriously difficult to measure. A novel method, based on the kinematics of the bar region at the centre of the Milky Way, brings a new and fundamental approach to determining this important quantity.
13 February 2023
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