AAS Nova
Transit method
Finding planets by watching a star dim slightly as one passes in front.
When a planet crosses the face of its star from our point of view, the star dims — by about one percent for a Jupiter, and one hundredth of a percent for an Earth. Measure that dip precisely enough, repeatedly, and you have the planet's size and its orbital period. This is how Kepler found most of the thousands of planets we know about, and how Webb now studies their atmospheres: during a transit, a sliver of starlight passes through the planet's air on the way to us, and the molecules there leave their fingerprints in the spectrum. The catch is geometry — only planets whose orbits happen to line up edge-on to us ever transit at all.
Where it has come up lately
Related terms
Radial velocity
Finding planets by the wobble they induce in their star's motion.
Habitable zone
The orbital band where a planet could hold liquid water — no more than that.
Sub-Neptune
The most common kind of planet in the galaxy, and there is not one in our solar system.
Hot Jupiter
A gas giant orbiting closer to its star than Mercury does to the Sun.
Questions
What is Transit method?
Finding planets by watching a star dim slightly as one passes in front.
Why does it matter?
This is how Kepler found most of the thousands of planets we know about, and how Webb now studies their atmospheres: during a transit, a sliver of starlight passes through the planet's air on the way to us, and the molecules there leave their fingerprints in the spectrum. The catch is geometry — only planets whose orbits happen to line up edge-on to us ever transit at all.