Credit course-original figures and separate external redraws

Every script-backed figure in lectures 1 to 6 now names its generating script in an HTML comment above the figure directive and ends its caption with the course-original credit. Figures that redraw, digitize, or annotate published material keep only the external attribution: the Kepler-laws redraw, the annotated LROC nearside mosaic, the Robinson tropopause profiles, the Blum aggregate-growth data, and the Fulton radius-valley histogram. The caption convention in scripts/figures/README.md now states the rule for these derivative figures.

Author: timlichtenberg

book/01_introduction/introduction.md

@@ -150,22 +150,24 @@ Trajectory of NASA's Voyager 2 spacecraft during its grand tour of the outer sol
 In 1992, Aleksander Wolszczan and Dale Frail announced the discovery of planets orbiting a pulsar, the first confirmed exoplanets {cite:p}`Wolszczan1992`. Three years later, Michel Mayor and Didier Queloz detected 51 Pegasi b, the first planet orbiting a Sun-like star: a "hot Jupiter" with a 4.2-day orbit that challenged all existing formation theories {cite:p}`MayorQueloz1995`. The French-led CoRoT space mission (Convection, Rotation and planetary Transits) then announced the first transiting rocky exoplanet, CoRoT-7b, in 2009, a super-Earth with a measured radius of $1.58\,\Rearth$ and a 0.85-day orbit {cite:p}`Leger2009`. NASA's Kepler mission {cite:p}`Borucki2010` operated from 2009 to 2018 and discovered thousands of transiting exoplanets, while the ongoing TESS mission (Transiting Exoplanet Survey Satellite, launched 2018) surveys the brightest nearby stars. JWST, launched in December 2021, is now characterising exoplanet atmospheres through transmission and emission spectroscopy ({ref}`Lecture 13 <lecture13>`).
 The cumulative tally of confirmed detections is shown in {numref}`fig:exoplanet-cumulative`.
 
+<!-- Generated by scripts/figures/L01_introduction/fig_exoplanet_cumulative.py -->
 ```{figure} figures/exoplanet_cumulative.avif
 :name: fig:exoplanet-cumulative
 :width: 700px
 :align: center
 
-Cumulative number of confirmed exoplanets per discovery year, colour-coded by detection method. The first detection (1992, pulsar timing) was followed by the radial-velocity revolution of the late 1990s and 2000s, and then the explosion of transit discoveries during and after NASA's Kepler mission (2009 to 2018). The visible step-jumps in 2014 and 2016 are the Kepler team's statistical-validation batch releases, in which hundreds (2014) and over a thousand (2016) Kepler candidates were promoted to confirmed planets simultaneously rather than one-by-one. Credit: NASA Exoplanet Archive (Caltech), accessed 2026-05-08 {cite:p}`NASAExoplanetArchive2026`.
+Cumulative number of confirmed exoplanets per discovery year, colour-coded by detection method. The first detection (1992, pulsar timing) was followed by the radial-velocity revolution of the late 1990s and 2000s, and then the explosion of transit discoveries during and after NASA's Kepler mission (2009 to 2018). The visible step-jumps in 2014 and 2016 are the Kepler team's statistical-validation batch releases, in which hundreds (2014) and over a thousand (2016) Kepler candidates were promoted to confirmed planets simultaneously rather than one-by-one. Credit: NASA Exoplanet Archive (Caltech), accessed 2026-05-08 {cite:p}`NASAExoplanetArchive2026`. Course-original figure.
 ```
 
 The same set of detections plotted in mass-period space ({numref}`fig:exoplanet-mass-period`) reveals strong selection biases: large short-period planets dominate the upper-left of the diagram, while Earth analogues in the lower-right remain sparsely populated.
 
+<!-- Generated by scripts/figures/L01_introduction/fig_exoplanet_mass_period.py -->
 ```{figure} figures/exoplanet_mass_period.avif
 :name: fig:exoplanet-mass-period
 :width: 700px
 :align: center
 
-Planet mass (or $M\sin i$) versus orbital period for all confirmed exoplanets, colour-coded by detection method, with the eight solar-system planets shown as gold stars for reference. Selection effects favour large, short-period planets in the upper-left of the diagram. The Earth-analogue regime, near mass $\sim 1\,\Mearth$ and orbital period $\sim 1$ year, sits in the lower-middle of the plot and remains sparsely populated because radial-velocity and transit surveys with multi-year baselines have only recently begun to access it; future direct-imaging missions (in particular the Habitable Worlds Observatory and the LIFE interferometer concept) are explicitly designed to fill this region by spatially separating reflected or thermal planet light from the host star. Credit: NASA Exoplanet Archive (Caltech), accessed 2026-08-21 {cite:p}`NASAExoplanetArchive2026`.
+Planet mass (or $M\sin i$) versus orbital period for all confirmed exoplanets, colour-coded by detection method, with the eight solar-system planets shown as gold stars for reference. Selection effects favour large, short-period planets in the upper-left of the diagram. The Earth-analogue regime, near mass $\sim 1\,\Mearth$ and orbital period $\sim 1$ year, sits in the lower-middle of the plot and remains sparsely populated because radial-velocity and transit surveys with multi-year baselines have only recently begun to access it; future direct-imaging missions (in particular the Habitable Worlds Observatory and the LIFE interferometer concept) are explicitly designed to fill this region by spatially separating reflected or thermal planet light from the host star. Credit: NASA Exoplanet Archive (Caltech), accessed 2026-08-21 {cite:p}`NASAExoplanetArchive2026`. Course-original figure.
 ```
 
 Within the small-planet regime, the Kepler sample reveals a bimodal radius distribution ({numref}`fig:radius-gap`) with peaks near $1.3\,\Rearth$ and $2.4\,\Rearth$ separated by a "radius valley", interpreted as the boundary between rocky planets and those retaining a thin H/He envelope.
@@ -241,20 +243,22 @@ The table below summarises the key physical and orbital properties of the eight
 
 Two patterns stand out immediately. First, **density decreases with distance** ({numref}`fig:density-vs-distance`): the inner planets have $\rho > 3900$ kg m$^{-3}$ (rock and metal), while the outer planets have $\rho < 1700$ kg m$^{-3}$ (gas and ice). Saturn is famously less dense than water. The **canonical** interpretation is that this gradient reflects the temperature structure of the protoplanetary disk: rock condenses everywhere, ices only beyond the snow line at $\sim 3$ to $5$ AU, and hydrogen and helium are accreted as gas by the cores that form quickly enough to capture it before the disk dispersed. This picture is the starting point developed in {ref}`Lecture 2 <lecture02>`; we will see there that modern formation models continue to refine, and in some places challenge, several parts of it. Second, **mass is concentrated in Jupiter** ({numref}`fig:mass-vs-distance`): it contains more than twice the mass of all other planets combined. We will quantify this in the blackboard derivation below.
 
+<!-- Generated by scripts/figures/L01_introduction/fig_density_vs_distance.py -->
 ```{figure} figures/density_vs_distance.avif
 :name: fig:density-vs-distance
 :width: 600px
 :align: center
 
-Bulk density of the eight solar-system planets versus orbital semi-major axis (log $x$). The terrestrial planets (red) cluster at $\rho > 3900$ kg m$^{-3}$; the giant planets (blue) sit below 1700 kg m$^{-3}$, with Saturn the lowest at 687 kg m$^{-3}$, below the density of liquid water (grey dashed line). The tan dashed line marks the **uncompressed density of typical silicate mantle rock** ($\sim 3300$ kg m$^{-3}$, the 1-bar density of olivine and peridotite): Mars sits close to this value, while Earth, Venus, and Mercury exceed it because (a) iron-rich cores raise the bulk density and (b) self-gravity compresses their interiors. The formation context of this density gradient is developed in {ref}`Lecture 2 <lecture02>`. Data from {cite:p}`NASAFactSheet`.
+Bulk density of the eight solar-system planets versus orbital semi-major axis (log $x$). The terrestrial planets (red) cluster at $\rho > 3900$ kg m$^{-3}$; the giant planets (blue) sit below 1700 kg m$^{-3}$, with Saturn the lowest at 687 kg m$^{-3}$, below the density of liquid water (grey dashed line). The tan dashed line marks the **uncompressed density of typical silicate mantle rock** ($\sim 3300$ kg m$^{-3}$, the 1-bar density of olivine and peridotite): Mars sits close to this value, while Earth, Venus, and Mercury exceed it because (a) iron-rich cores raise the bulk density and (b) self-gravity compresses their interiors. The formation context of this density gradient is developed in {ref}`Lecture 2 <lecture02>`. Data from {cite:p}`NASAFactSheet`. Course-original figure.
 ```
 
+<!-- Generated by scripts/figures/L01_introduction/fig_mass_vs_distance.py -->
 ```{figure} figures/mass_vs_distance.avif
 :name: fig:mass-vs-distance
 :width: 600px
 :align: center
 
-Planetary mass versus orbital semi-major axis on a log-log scale, on the same x-axis as {numref}`fig:density-vs-distance`. The grey dashed line marks the **sum of all planetary masses except Jupiter** ($\approx 129\,\Mearth$); Jupiter alone, at $317.83\,\Mearth$, exceeds this sum by a factor of $\approx 2.5$ and sits well above the line. Jupiter and Saturn together account for more than $90\%$ of the total planetary mass in the solar system. The formation context of this mass concentration is developed in {ref}`Lecture 2 <lecture02>`. Data from {cite:p}`NASAFactSheet`.
+Planetary mass versus orbital semi-major axis on a log-log scale, on the same x-axis as {numref}`fig:density-vs-distance`. The grey dashed line marks the **sum of all planetary masses except Jupiter** ($\approx 129\,\Mearth$); Jupiter alone, at $317.83\,\Mearth$, exceeds this sum by a factor of $\approx 2.5$ and sits well above the line. Jupiter and Saturn together account for more than $90\%$ of the total planetary mass in the solar system. The formation context of this mass concentration is developed in {ref}`Lecture 2 <lecture02>`. Data from {cite:p}`NASAFactSheet`. Course-original figure.
 ```
 
 ### Classification
@@ -326,12 +330,13 @@ The more precise form of Kepler's third law is $P^2 = 4\pi^2 a^3 / [G(M_* + M_p)
 The Sun contains **99.87%** of the solar system's total mass. Jupiter alone accounts for 71% of the planetary mass ({numref}`fig:ss-mass-budget`). This extreme concentration of mass in the central star is a fundamental property of planetary systems, and one that planet formation theory must explain ({ref}`Lecture 2 <lecture02>`).
 ```
 
+<!-- Generated by scripts/figures/L01_introduction/fig_ss_mass_budget.py -->
 ```{figure} figures/ss_mass_budget.avif
 :name: fig:ss-mass-budget
 :width: 700px
 :align: center
 
-Mass budget of the solar system. *Left:* the Sun contains 99.87% of the total mass; the eight planets together contribute the remaining 0.13%. *Right:* among the planets, Jupiter accounts for $\sim 71\%$ and Saturn $\sim 21\%$; the ice giants Neptune ($3.8\%$) and Uranus ($3.3\%$) make up most of the rest; the four terrestrial planets (Mercury, Venus, Earth, Mars) together contribute only $0.44\%$ of the planetary mass and are grouped into a single wedge for legibility. Data from {cite:p}`NASAFactSheet`.
+Mass budget of the solar system. *Left:* the Sun contains 99.87% of the total mass; the eight planets together contribute the remaining 0.13%. *Right:* among the planets, Jupiter accounts for $\sim 71\%$ and Saturn $\sim 21\%$; the ice giants Neptune ($3.8\%$) and Uranus ($3.3\%$) make up most of the rest; the four terrestrial planets (Mercury, Venus, Earth, Mars) together contribute only $0.44\%$ of the planetary mass and are grouped into a single wedge for legibility. Data from {cite:p}`NASAFactSheet`. Course-original figure.
 ```
 
 
@@ -359,12 +364,13 @@ The four terrestrial planets at approximate relative scale. From left to right:
 
 The atmospheric divergence is striking: {numref}`fig:vem-atmospheres` shows that the three planets span four orders of magnitude in surface pressure and several hundred kelvin in surface temperature.
 
+<!-- Generated by scripts/figures/L01_introduction/fig_vem_atmospheres.py -->
 ```{figure} figures/venus_earth_mars_atmospheres.avif
 :name: fig:vem-atmospheres
 :width: 700px
 :align: center
 
-Surface temperature (red bars, left axis) and surface pressure (blue bars, right axis, log scale) for Venus, Earth, and Mars, with the dominant atmospheric species labelled below. Despite similar bulk compositions and shared formation environment, the three terrestrial planets span a factor of $\sim 10^4$ in surface pressure and $\sim 500$ K in surface temperature. Data from {cite:p}`NASAFactSheet`.
+Surface temperature (red bars, left axis) and surface pressure (blue bars, right axis, log scale) for Venus, Earth, and Mars, with the dominant atmospheric species labelled below. Despite similar bulk compositions and shared formation environment, the three terrestrial planets span a factor of $\sim 10^4$ in surface pressure and $\sim 500$ K in surface temperature. Data from {cite:p}`NASAFactSheet`. Course-original figure.
 ```
 
 By comparing these three cases, we can isolate which differences arise from distance to the Sun, planetary mass, internal activity, or historical contingency. The same logic applies to moons: comparing Io, Europa, Ganymede, and Callisto ({numref}`fig:galilean-moons`), all orbiting Jupiter but differing in composition and tidal heating, reveals how a single variable can drive vastly different geological outcomes.