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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73. White dwarf pollution and exoplanets
Remarkable clues about ancient planets
Heavy element pollution in white dwarf atmospheres is attributed to the accretion of rocky planetesimals which have been scattered and torn apart to form a dusty debris disk that can be accreted by the white dwarf. Deep insights into the nature of the associated planetary systems are now being assembled.
23 May 2022
72. The warp of our Galaxy
The mystery of its origin continues
Various explanations have been proposed for our Galaxy's warped structure, including infall of intergalactic material, a close encounter with a companion galaxy, or misalignment of the disk and its dark matter halo. Gaia is contributing to an improved picture of its structural complexity, but the underlying driving mechanism remains uncertain.
16 May 2022
71. More halo streams from Gaia
The origin of the Galaxy's stellar halo
Ancient signatures of tidal infall, responsible for our Galaxy's stellar halo, remain evident because orbital time-scales in the outer parts of the Milky Way extend to billions of years. As a result, the halo retains kinematic evidence of the surviving remnants of accretion. A number of these are being found and characterised by Gaia.
9 May 2022
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
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