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Reading time ~5 min
Taking notes? Power 1 seeds for today
Space is big in a way that breaks the words we usually use for big. The distance from Chennai to Delhi is about 2,200 kilometers. The distance to the Moon is 175 times that. And the Moon is, in astronomical terms, glued to us. To talk about anything beyond it, you need new units, and this course uses three.
The astronomical unit. One astronomical unit (AU) is the average distance from Earth to the Sun: about 150 million kilometers. It exists because kilometers stop being useful embarrassingly fast. Saying "Mars is 78 million kilometers away at closest approach" tells most people nothing. Saying "Earth is 1 AU from the Sun and Mars is 1.52 AU" tells you something real: Mars is about one and a half times farther out. The AU turns the solar system into a map with a scale you can hold.
Some anchors worth memorizing. Earth: 1 AU. Mars: 1.52. Jupiter: 5.2. Neptune: 30. Voyager 1, the farthest machine humans have ever sent, has been flying for almost fifty years and has reached about 165 AU.
Now here is the problem. The nearest star apart from the Sun, Proxima Centauri, sits at roughly 270,000 AU. The unit we invented to make the solar system manageable becomes useless one star later. So we switch units again.
The light-year. Light is the fastest thing there is. It goes from the Moon to Earth in a little over one second, and from the Sun to Earth in about eight minutes. A light-year is the distance light covers in one year: about 9.5 trillion kilometers. Proxima Centauri is 4.24 light-years away. The exoplanets on your Goldilocks candidate list run from Proxima b at 4.24 light-years to Kepler-452 b at about 1,800.
That range matters practically. At 40 light-years (TRAPPIST-1), telescopes like JWST can study a planet's star in detail and sometimes probe the planet itself. At 1,800 light-years, we can barely study the star. Distance is not just a number on your planet's data card. It decides what evidence can exist.
The part nobody tells you: light-years are also time. Light from Kepler-452 left its star around the year 226. Everything we know about that star is news from eighteen centuries ago. Every "now" in astronomy is really a "then," and the farther out you look, the older the light. When your data card gives a distance in light-years, it is also quietly telling you the age of your evidence.
The habitable zone, and its trap. The habitable zone (HZ) is the band of orbits around a star where a planet could, in principle, keep liquid water on its surface: close enough that water doesn't freeze permanently, far enough that it doesn't boil away. Around the Sun, that band runs from about 0.95 to 1.37 AU. Earth orbits inside it. So far so good.
But the zone moves with the star. TRAPPIST-1 is a dim red dwarf, so its habitable zone huddles between 0.022 and 0.032 AU, closer to its star than Mercury is to the Sun. Planets that close tend to get locked with one face permanently toward the star, and they sit in the blast radius of stellar flares. "In the habitable zone" turns out to be the beginning of a habitability argument, not the end of one. A planet can be in the zone and still be a terrible place to be alive. Keep that sentence; you will use it in your Report.
Sources
Every number in this reading is checkable. Notice the source types; they're not all the same kind of evidence.
- AU and light-year definitions, distance figures: NASA Exoplanet Exploration Program, "What is a light-year?", exoplanets.nasa.gov/faq [government science agency]
- Voyager 1 distance: NASA/JPL Voyager mission status, voyager.jpl.nasa.gov [government science agency: live-updating; the number grows ~3.5 AU per year]
- Habitable zone boundaries (Sun, TRAPPIST-1): the course's Habitability Zone Calculator lookup tables, computed from stellar luminosities in the NASA Exoplanet Archive, exoplanetarchive.ipac.caltech.edu [primary data archive, via a course tool; OPVL the tool too: click "Show assumptions"]
- Kepler-452 b distance: NASA Kepler-452 b discovery announcement (July 2015), nasa.gov [agency press release: a release, not a paper; the distance carries real uncertainty]
Go deeper: the same NASA Exoplanet FAQ has the animations this reading can't show you.