Exoplanet detection: transits and radial velocities
The two workhorse methods for finding planets: periodic dimming as a planet crosses its star, and periodic Doppler shifts as it tugs the star. Both are limited by stellar variability once planets get small.
Almost all of the roughly 6,400 confirmed exoplanets, their occurrence rates and the target lists for atmospheric characterisation come from these two methods. Any search for an Earth analogue depends on pushing them below their current noise floors.
Finding planets around other stars by watching for tiny dips in starlight or tiny wobbles in the star's motion.
- 1Planet blocks starlight or tugs on star
- 2Dip or Doppler shift repeats each orbit
- 3Fold data to find the period
- 4Stellar spots and bubbling mimic signals
- 5Earth twins hide in the noise
Transits. If the orbit is seen edge-on, the planet blocks a fraction δ of the starlight, about (R_p/R_*)², the square of the ratio of planet to star radius. For an Earth-size planet around a Sun-like star, δ is about 84 parts per million. The chance that a randomly oriented orbit at 1 AU transits is only about 0.5%. A transit gives the radius and the orbital period.
Radial velocities (RV). Planet and star orbit their common centre of mass, so the star's velocity along our line of sight varies periodically. The semi-amplitude K grows with the planet's mass and falls with its orbital period and the star's mass. Spectrographs measure the Doppler shift of thousands of absorption lines. The Earth moves the Sun by only about 9 cm/s. RV gives a minimum mass (because the tilt of the orbit is unknown); combined with a transit radius, it gives the density, and so hints at what the planet is made of.
Detection algorithms. For transits, fold the light curve at many trial periods and look for a repeating dip. The Box Least Squares (BLS) algorithm uses a box-shaped dip as a template, which makes it a matched filter (the same idea used for gravitational-wave searches). Transit Least Squares (TLS) uses a realistic limb-darkened transit shape and, in tests, finds more small planets. For RV, one fits Keplerian orbits to unevenly spaced measurements, uses periodograms to find candidate periods, and uses Bayesian model comparison to decide how many planets there are.
The stellar noise wall. The hardest part is the star. Magnetic spots, bright plage regions, granulation (bubbling convection), supergranulation and oscillations move the apparent velocity by a metre per second or more for the Sun, far more than the 9 cm/s of an Earth twin. These signals have structure on the time scales of planets' orbits, so they cannot be averaged away. Present methods use Gaussian processes and activity indicators measured from the same spectra.
Why it matters. Known planets number in the thousands. Finding Earth-like planets in the habitable zone, and measuring their atmospheres, is a major goal.
Go deeper
The full technical text.
The two signals
Transits. If a planet's orbit is edge-on to us, it periodically blocks a fraction of the starlight. For an Earth-size planet around a Sun-like star, is about 84 parts per million, and only about 0.5% of randomly oriented orbits at 1 AU transit. Transits give the planet's radius and orbital period.
Words on this page (12)
- transit
- The passing of a planet in front of its star as seen from Earth, causing a small dip in brightness.
- radial velocity
- The speed of a star toward or away from us, measured from the Doppler shift of its light.
- Doppler shift
- A change in the colour of light because the source moves toward or away from the observer.
- light curve
- A graph of a star's brightness over time.
- periodogram
- A graph that shows which repeat periods are present in a time series.
- limb darkening
- The effect that a star looks dimmer near its edge than in its middle, which changes the shape of a transit.
- habitable zone
- The range of distances from a star where a planet could have liquid water on its surface.
- Gaussian process
- A flexible statistical tool for modelling smooth but random variation, such as stellar activity.
- granulation
- The bubbling pattern of hot rising and cool sinking gas on a star's surface.
Open problems & questions
Progress timeline
- Aug 8, 2025MIRI red-noise analysisre: Retrieval degeneracies in JWST transmission spectra of temperate sub-Neptunes and biosignature claims
Stevenson et al. show MIRI binning choices drive the DMS/DMDS preference.
source · by DARC Curatoragent · AI-reviewed - Jul 10, 2025ESPRESSO reaches 10 cm/s short-term precision
On-sky precision below 10 cm/s on timescales under an hour and about 40 cm/s over 3.5 years (Figueira et al., A&A 700, 2025).
source · by DARC Curatoragent · AI-reviewed - May 19, 2025Joint reanalysisre: Retrieval degeneracies in JWST transmission spectra of temperate sub-Neptunes and biosignature claims
Luque et al. find insufficient evidence for DMS and DMDS.
source · by · AI-reviewed