What Is the Transit Method? How Scientists Detect Exoplanets

The transit method is the most successful technique ever developed for finding planets beyond our solar system. Here is how a tiny dip in starlight can reveal an entire world — and how S.O.L.A.R.I.S. uses it to discover new exoplanets every week.

Since the first confirmed exoplanet detection in 1995, astronomers have discovered more than 5,700 worlds orbiting other stars. The vast majority of those discoveries — over 75% — were made using a single technique: the transit method. It is elegant in its simplicity, powerful in its results, and accessible enough that citizen science projects like S.O.L.A.R.I.S. can apply it to real satellite data from home computers.

How Does the Transit Method Work?

Imagine watching a distant streetlight. If a moth flies in front of it, the light dims slightly for a brief moment. The transit method works on exactly the same principle, except the "streetlight" is a star and the "moth" is a planet.

When a planet's orbit carries it directly between its host star and our telescopes, the planet blocks a small fraction of the star's light. This event is called a transit. By continuously monitoring a star's brightness over time and recording this data as a light curve, astronomers can detect the characteristic dip that signals a planet is crossing the stellar disk.

The critical requirement is geometric alignment. The planet's orbital plane must be oriented so that it passes between the star and our line of sight. For randomly oriented orbits, the probability of this alignment is small — typically 0.5% for an Earth-like orbit around a Sun-like star. This is why transit surveys must monitor hundreds of thousands of stars simultaneously to find a meaningful number of transiting planets.

What Can You Learn from a Transit?

A transit is far more than a simple detection event. The shape, depth, and timing of the light curve dip encode a wealth of physical information about the planet:

• Planet radius — The depth of the dip is directly proportional to the ratio of the planet's area to the star's area. A deeper dip means a larger planet.
• Orbital period — The time between successive transits reveals how long the planet takes to complete one orbit.
• Distance from the star — The orbital period, combined with the star's mass (known from spectral type), gives the planet-star distance via Kepler's third law.
• Equilibrium temperature — Knowing the star's luminosity and the planet's distance, scientists calculate the temperature the planet would have if it absorbed and re-emitted starlight in equilibrium.
• Transit duration — How long the dip lasts constrains the planet's orbital inclination and the star's size.

When combined with radial velocity measurements (which measure the star's wobble due to the planet's gravitational pull), the transit method also yields the planet's mass and therefore its density — telling us whether a planet is rocky like Earth or gaseous like Neptune.

A Brief History: Kepler and TESS

The transit method was proposed theoretically in the 1950s, but it took decades of technological progress before it could be applied in practice. Two NASA missions transformed it into the dominant planet-finding technique.

Kepler (2009–2018)

NASA's Kepler Space Telescope stared at a single patch of sky containing about 150,000 stars for four years, watching for transit signals. It was a revelation. Kepler discovered over 2,600 confirmed exoplanets and showed that planets are more common than stars in our galaxy. Its extended mission, K2, continued observations of different sky fields until the spacecraft ran out of fuel in 2018.

TESS (2018–present)

The Transiting Exoplanet Survey Satellite (TESS) picked up where Kepler left off, but with a different strategy. Instead of staring at one patch, TESS surveys nearly the entire sky, monitoring each sector for 27 days (or longer for overlapping sectors near the ecliptic poles). TESS focuses on bright, nearby stars, making follow-up observations much easier. It collects brightness measurements every 2 minutes for targeted stars and every 10 minutes for full-frame images.

TESS data is publicly available through NASA's Mikulski Archive for Space Telescopes (MAST) — and this is exactly the data that S.O.L.A.R.I.S. processes.

How S.O.L.A.R.I.S. Applies the Transit Method

S.O.L.A.R.I.S. takes the transit method from the professional observatory into the hands of volunteers worldwide. The project's automated pipeline downloads TESS 2-minute cadence light curves and processes them through two stages:

1. BLS Detection — The Box Least Squares (BLS) algorithm scans each light curve for periodic box-shaped dips. It tests thousands of possible orbital periods and identifies the best-fitting transit signal, reporting its depth, duration, and significance.
2. MCMC Fitting — Once a candidate signal is found, Markov Chain Monte Carlo (MCMC) fitting refines the planet's parameters — radius, period, inclination, and temperature — by exploring the full probability landscape of the model. This yields not just best-fit values but also uncertainty estimates.

Since its launch on March 5, 2026, S.O.L.A.R.I.S. has searched over 35,000 stars and discovered 54 exoplanet candidates — 35 of which orbit within their star's habitable zone. The project's most remarkable find, SOLARIS-002 (TIC 103245015), has an Earth Similarity Index of 98.3%, a radius of 1.02 Earth radii, and an equilibrium temperature of -26°C. Created by Cassius Mehlhopt, S.O.L.A.R.I.S. is not affiliated with NASA but uses publicly available NASA TESS data.

Why the Transit Method Remains the Gold Standard

Other exoplanet detection techniques exist — radial velocity, direct imaging, gravitational microlensing, astrometry — but the transit method offers a unique combination of advantages. It works on large numbers of stars simultaneously, provides the planet's radius directly, and yields enough orbital information to assess whether a planet might be habitable. It is also the only method that enables atmospheric characterization through transmission spectroscopy, where starlight filtered through a planet's atmosphere during transit reveals its chemical composition.

As TESS continues its extended mission and projects like S.O.L.A.R.I.S. scale up the analysis, the transit method will remain at the frontier of exoplanet science — turning pinpoints of starlight into discoveries of new worlds.

Join the Search

S.O.L.A.R.I.S. discoveries are exoplanet candidates based on statistical analysis of TESS photometry. Professional follow-up observations are needed for confirmation. The project is not affiliated with NASA.