A flicker can expose a planet. When a world crosses the face of its star, the resulting transit dims the light by an amount that reveals the planet's size relative to the star. Given the star's size, astronomers can estimate the planet's radius.
The subtler prize is an atmosphere. During a transit, a small fraction of starlight passes through any gas surrounding the planet before reaching a telescope. Different molecules absorb particular wavelengths, leaving patterns in the light. By comparing spectra taken during and outside the transit, astronomers use transmission spectroscopy to search for gases such as water vapor. The signal is faint. Clouds and hazes can obscure absorption features, while activity on the star can produce misleading signals; identifying a molecule often requires repeated observations and careful modeling. A detection alone does not establish whether a planet supports life.
Size, too, is only a beginning. A radius cannot by itself distinguish a rocky world from one wrapped in a deep envelope of gas; a mass measurement, obtained independently, can provide an estimate of average density. Even then, different internal compositions can fit the same data. Artists may furnish such worlds with rings and neighboring planets, but those details belong to imagination unless observations support them. What the transit offers instead is less extravagant and more exacting: a small interruption in a star's light, from which an unseen world slowly takes shape.
