Starspots distort exoplanet transits because the planet blocks one strip of a star while the telescope measures light from the entire visible disk. If that strip differs from the disk average, a transit can give a biased radius or spectrum even when its light curve looks smooth.
Crossed and uncrossed spots do different things
When a planet crosses a dark spot, it blocks material that was already dimmer than the surrounding photosphere. Relative to a spotless transit model, the light curve shows an upward bump. Crossing a bright facular region can produce the opposite anomaly.
Uncrossed dark spots reduce the out-of-transit baseline. If the planet crosses a brighter, quiet region, the same blocked light becomes a larger fraction of the total, making the normalized transit deeper. No visible bump is required. The NASA Exoplanet Exploration Program SAG21 report explains why both situations matter for transmission spectroscopy.
A simple radius-bias calculation
Ignore limb darkening and suppose a fraction f of the apparent stellar disk has spot intensity c relative to the quiet photosphere. If the transit path is quiet, δobs ≈ δquiet/[1 − f(1 − c)]. The contrast is wavelength-dependent, so this is a single-passband approximation.
With illustrative values f = 0.10, c = 0.50 and δquiet = 1%, the baseline is 95% of its quiet value. The observed depth becomes about 1.053%. Inferring radius from the square root of depth would overestimate the radius ratio by about 2.6%.
This is not a correction to apply to every active star. Unknown spot coverage, faculae, geometry and limb darkening require a more complete model. It shows why a small depth bias can matter physically.
Why the spectrum can imitate an atmosphere
Cool spots and hot bright regions have different spectra from the quiet photosphere. Their contributions change with wavelength, which can introduce slopes or molecular-looking structure into the inferred transmission spectrum. A feature observed during transit therefore needs testing against both stellar and planetary models.
A smooth rotational light curve does not prove a spotless star. Symmetric or persistent spot distributions can change the average spectrum without producing a large rotational modulation.
Timing and duration can also be biased
A spot-crossing anomaly on one side of a transit can pull a symmetric fitted model toward it, shifting the inferred midpoint or contact parameters. The simulation study of stellar spots in precision transit curves explores these effects. Freeing more fit parameters does not necessarily recover the true orbit when the stellar model is wrong.
How observers reduce the ambiguity
Use repeated transits, simultaneous multiwavelength data, out-of-transit monitoring and independent activity diagnostics. Fit or explicitly flag resolved anomalies. Report results across plausible stellar-surface models rather than assuming one spot solution is uniquely measured.
Compare other causes of changing depth and asymmetric transit shapes. Stellar activity can affect both, but it is only one of the explanations to test.
Revised October 26, 2026. Spot coverage and the resulting radius bias are illustrative.