Block 5: Exoplanet Detection: Transits and What They Tell Us

What today is about

How do we know what we know about planets we will never visit?

Every exoplanet in your Goldilocks Report candidate list was discovered the same way: someone watched a distant star very carefully and noticed it got a tiny bit dimmer at regular intervals. That's the transit method: periodic dimming of a host star as a planet passes between it and our telescope. The Kepler space telescope used this method to confirm more than 2,600 exoplanets between 2009 and 2018. JWST uses it today to probe atmospheres.

What a transit observation tells you is precise but limited. From the dip's depth, you get the planet's size relative to the star (smaller star + smaller dip = smaller planet). From the dip's timing, you get the orbital period. From multiple dips over a long observation, you get orbital stability. What a single transit does NOT tell you: the planet's composition, its mass, its atmospheric composition (you need follow-up spectroscopy for that), or anything about life. Pop-sci coverage often blurs these lines: a transit detection becomes "scientists discover habitable planet." Your job in the Goldilocks Report is to keep these lines sharp.

This matters for OPVL. Every claim about your chosen exoplanet (its mass, atmosphere, surface conditions, habitability) has a different source, a different observation technique, and a different level of certainty. A transit-confirmed radius is one of the most reliable measurements in exoplanet science. An atmospheric biosignature claim, by contrast, often rests on a single observation that subsequent re-analysis may overturn (see Gliese 581 d, K2-18 b's DMS controversy). Knowing where each claim comes from is the move.

This is exactly where the Lovelock → Thompson lineage from U0 Block 2 lands. Thompson et al. (2022) calculate that biological methane on an Earth-like TRAPPIST-1e could be detectable in roughly 5–10 JWST transits: a concrete example of how a transit observation gets turned into a habitability claim, and a peer-reviewed source you can cite directly if TRAPPIST-1e is your planet. Lovelock proposed the idea in 1965; Thompson made it a number you could test with a real telescope.