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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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.
2 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.
25 April 2022
68. Gaia photometry
Background to the why and how
Multi-colour photometry of stars can be carried out from the ground. But there were compelling reasons to make these observations onboard Gaia, at the multiple epochs coinciding with the astrometric measurements. I explain the reasoning, and the challenges involved in the technical implementation.
18 April 2022
67. Where is Gaia?
Choice of the L2 orbit
Gaia observes from the Sun-Earth Lagrange point L2, about 1.5 million km from Earth, in the direction away from the Sun. Various interesting properties of the L2 orbit have made it a popular destination for science missions observing deep space. But a special observation programme is need to continuously monitor Gaia's position.
11 April 2022
66. Exoplanet habitability: TESS and Gaia
Advancing the search for life
The number of known terrestrial-type planets in the `habitable zone' are increasing, based on transit measurements with the space missions Kepler and TESS. Gaia is fixing their distances, and characterising their properties, allowing the list of potentially habitable planets to be sharpened, and adding key targets to SETI searches.
4 April 2022
65. Hot Jupiters and star clustering
New evidence for their formation
Amongst exoplanets known today are the perplexing class of `hot Jupiters', gas giants orbiting close to their host star. What brought them to such bizarre orbits? Gaia shows that their existence correlates with ancient star clusters, and with other waves and ripples of star densities in space and space motion in our Galaxy.
28 March 2022
64. Solar system objects in DR2
New insights in dynamics and taxonomy
Hipparcos observed around 100,000 stars, but just 48 asteroids. Gaia is expected to determine highly accurate orbits and reflectance spectra for 350,000 solar system objects, permitting profound studies of their dynamics, structure, and taxonomy. Their properties will shed new light on the formation and evolution of our solar system.
21 March 2022
63. Catalogue validation
How confident should users be?
When making the successive Gaia data releases available to the world-wide scientific community, an important question is to what extent the positions, distances, proper motions, photometry, and radial velocities can be ‘trusted’? What sort of independent validation can be made before publication? Many tests are made, and I describe a few.
14 March 2022
62. Variability across the HR diagram
Which evolutionary states show variability?
Variable stars have long been recognised as offering deep insights into stellar structure and evolution. Similarly, the Hertzsprung–Russell diagram provides a quantitative visualisation of all stages of stellar evolution. Together with its empirical analogue, the colour–magnitude diagram, it has enabled many advances in stellar astrophysics.
7 March 2022
61. Discovering variability with Gaia
How is Gaia identifying variable stars
There are enormous numbers of known variable stars, of dozens of different types. Some variability is 'intrinsic', being due to the luminosity of the star itself changing with time. Others are 'extrinsic', meaning that something affects the stellar light on the way from the star to us. Gaia is, again, transforming the field.
28 February 2022
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