Sources

1. The Blue Marble (AS17-148-22727)

   NASA / Apollo 17. Dated 1972-12-07. Cover photograph. Earth as a living world with a thin bright band of air. Public domain.

2. Mars: Facts

   NASA Science. Dated 2026-07-10. Mars size, sol, year, temperature, air, ice, magnetic past.

3. First radiation measurements from the surface of Mars

   Hassler et al. / SwRI / NASA Curiosity RAD. Dated 2013-12-09. About 0.64 to 0.67 mSv per day on the surface from galactic cosmic rays.

4. MOXIE

   NASA / JPL / MIT. Dated 2023. Making oxygen from Mars carbon dioxide has been demonstrated at small scale.

5. Moon Facts

   NASA Science. Dated 2026-08. Moon gravity, temperature, airless surface, polar ice.

6. Facts About Earth

   NASA Science. Dated 2025-12-05. Earth air mix, magnetosphere.

7. Titan: Facts

   NASA Science. Dated 2025-04-25. Titan pressure, temperature, nitrogen and methane air.

8. Cassini-Huygens at Titan

   NASA / ESA / ASI. Dated 2005-2017. Huygens landing, methane cycle, surface pebbles.

9. Europa

   NASA Science. Dated 2026. Ice shell, inferred ocean, radiation environment.

10. Galileo magnetometer inference of a Europan ocean

    Kivelson et al. / Galileo. Dated 2000. Induced magnetic field as evidence for a salty ocean.

11. Europa Clipper

    NASA / JPL. Dated 2024-10. A mission on the way to measure ice and ocean. Not a claim of life.

12. Europa's Lyman-alpha emissions from HST/STIS observations

    Roth, Retherford, et al. / Astronomy and Astrophysics. Dated 2026-05. Hubble hints of Europa water plumes from 2014 no longer hold. Not a photograph of a geyser. Clipper can look again.

13. Exoplanets

    NASA Science. Dated 2026. TRAPPIST-1 system, habitable zone as a starlight idea.

14. JWST observations of TRAPPIST-1 planets

    NASA / STScI / independent teams. Dated 2026-04. Atmosphere of TRAPPIST-1e still unconfirmed. Inner worlds show no thick air.

Habitable is not used here to mean comfortable for humans. People and life stay two questions. Artist concepts are labeled.