An exoplanet is a planet that orbits a star other than the Sun. We usually cannot take a resolved photograph of the planet: the star is far brighter, and the system is far away. Instead, exoplanet science often starts by measuring a small, repeatable effect of the planet on light or motion from the star.
What can we observe?¶
The planet changes what an observer receives from the star. Different methods turn different changes into evidence:
| Method | What changes | What the change can constrain |
|---|---|---|
| Transit | received starlight gets slightly dimmer | planet-to-star radius ratio and orbital timing |
| Radial velocity | stellar spectral lines shift as the star moves | a mass-related orbital signal |
| Direct imaging | light from the planet is separated from stellar light | brightness and atmosphere in favorable systems |
COMP 521 begins with transits because they connect a visible measurement to a geometric model that we can calculate, test, and gradually improve.
The key scientific habit¶
The observation is not “we saw a planet.” The observation is a pattern in the data—for example, a repeatable decrease in received starlight. A model links that pattern to a possible physical explanation. Good scientific computing keeps those two pieces distinct:
observable: the received light as a function of time;
model: a prediction for how an orbiting planet blocks some of the star;
inference: what properties are consistent with the observation and the model;
limits: other phenomena or model choices can sometimes make similar signals.
Next connection¶
Continue with What is a transit?, then see how the measurement is recorded in a light curve.