{ "id": "epa4zgbbtr", "slug": "transit-photometry", "title": "Transit photometry", "updated": "2026-09-30", "topic_path": [ { "id": "physical", "label": "Physical world and mathematics", "api_url": "https://www.edgepedia.org/api/v1/topics/physical" }, { "id": "physical.astronomy", "label": "Astronomy", "api_url": "https://www.edgepedia.org/api/v1/topics/physical.astronomy" }, { "id": "physical.astronomy.cosmos", "label": "Cosmology and observation", "api_url": "https://www.edgepedia.org/api/v1/topics/physical.astronomy.cosmos" }, { "id": "physical.astronomy.cosmos.cosmos.techniques", "label": "Observational techniques: astrometry, photometry, spectroscopy", "api_url": "https://www.edgepedia.org/api/v1/topics/physical.astronomy.cosmos.cosmos.techniques" } ], "geo": [ { "id": "geo.nongeo.t1946.physical.astronomy.cosmos", "label": "Non-geographic · 1946 to 2000: Cosmology and observation", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1946.physical.astronomy.cosmos", "path": [ { "id": "geo.nongeo", "label": "Non-geographic", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo" }, { "id": "geo.nongeo.t1946", "label": "Non-geographic · 1946 to 2000", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1946" }, { "id": "geo.nongeo.t1946.physical", "label": "Physical world and mathematics", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1946.physical" }, { "id": "geo.nongeo.t1946.physical.astronomy", "label": "Astronomy", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1946.physical.astronomy" }, { "id": "geo.nongeo.t1946.physical.astronomy.cosmos", "label": "Cosmology and observation", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1946.physical.astronomy.cosmos" } ] } ], "excerpt": "Transit photometry detects exoplanets by measuring the periodic dip in a star's brightness when a planet crosses its disk, and has found over 4000 known exoplanets.", "snippet": "Transit photometry detects exoplanets by measuring the periodic dip in a star's brightness when a planet crosses its disk, and has found over 4000 known exoplanets.", "node": "physical.astronomy.cosmos.cosmos.techniques", "markdown": "# Transit photometry\n\nTransit photometry detects exoplanets by measuring the periodic dip in a star's brightness when a planet crosses the stellar disk as seen from Earth. It has produced more than 4000 of the several thousand known exoplanets, making it the most productive discovery method to date.[1](https://www.scitepress.org/Papers/2025/138158/138158.pdf) • [2](https://arxiv.org/pdf/1803.06896) The observed transit of [HD 209458](https://www.edgepedia.org/hd-209458) b was recorded and published.[3](https://iopscience.iop.org/article/10.1086/312458) • [4](https://beta.iopscience.iop.org/article/10.1086/312457) Beyond discovery, a transit light curve yields the planet's radius, the stellar density, and, in favorable systems, the planet's mass and atmospheric spectrum.\n\n| Key fact | Value |\n|---|---|\n| Signal measured | Fractional flux drop with depth \\( \\delta = (R_{p}/R_{\\star})^{2} \\)[5](https://ar5iv.labs.arxiv.org/html/0912.0887) |\n| First observed transit | HD 209458 b, November 1999; reported in 2000 in The Astrophysical Journal[3](https://iopscience.iop.org/article/10.1086/312458) • [4](https://beta.iopscience.iop.org/article/10.1086/312457) |\n| Typical hot-Jupiter signal | Depth ~1%, duration 1.5–4 hr, transit probability ~10%[5](https://ar5iv.labs.arxiv.org/html/0912.0887) • [6](https://ar5iv.labs.arxiv.org/html/1210.2471) |\n| Geometric transit probability | ~0.45% for a 1 AU orbit around a solar-type star[5](https://ar5iv.labs.arxiv.org/html/0912.0887) |\n| Precision required | 3–5 mmag from the ground; 20 ppm from space (Kepler design goal)[5](https://ar5iv.labs.arxiv.org/html/0912.0887) • [7](https://archive.stsci.edu/kepler/manuals/KSCI-19033-002.pdf) |\n| False positives | Over 95% of transit-like signals in wide-field surveys[5](https://ar5iv.labs.arxiv.org/html/0912.0887) |\n| Leading instruments | Kepler (0.95 m, space), TESS (all-sky, space), SuperWASP, HATNet/HATSouth, NGTS (ground)[7](https://archive.stsci.edu/kepler/manuals/KSCI-19033-002.pdf) • [8](https://heasarc.gsfc.nasa.gov/docs/tess/docs/TESS_observatory_guide_v1.1.pdf) |\n\n## How it works\n\nThe dip is simple geometric blocking: the planet covers a fraction of the stellar disk proportional to its area, so the relative flux loss is \\( \\delta = (R_{p}/R_{\\star})^{2} \\), where \\( R_{p} \\) and \\( R_{\\star} \\) are the planet and star radii.[5](https://ar5iv.labs.arxiv.org/html/0912.0887) The total duration \\( T \\) (first to fourth contact) and the ingress/egress time \\( \\tau \\) (first to second contact, or third to fourth contact) measure how tilted the orbit is and how fast the planet travels; for circular orbits the impact parameter follows \\( b^{2} = 1 - \\delta^{1/2} \\cdot T/\\tau \\), and the stellar density follows \\[ \\rho_{\\star} = \\frac{3P}{G \\cdot \\pi^{2}}\\left(\\frac{\\delta^{1/4}}{\\sqrt{T \\cdot \\tau}}\\right)^{3} \\].[9](https://juliaastro.org/Transits/stable/introduction/)\n\nSeager and Mallén-Ornelas showed that three geometric equations (depth, shape, duration) plus Kepler's third law and a stellar mass–radius relation uniquely determine \\( M_{\\star} \\), \\( R_{\\star} \\), \\( R_{p} \\), the semi-major axis, and the inclination from a light curve with two or more transits, assuming a circular orbit, a dark companion, and negligible limb darkening.[10](https://home.strw.leidenuniv.nl/~radhakrishnan/AFS/lectures/Seager_Ormelas_2003.pdf) Stellar surfaces are dimmer toward the limb, so real transits are deeper at the center and shallower near the edges; Mandel and Agol (2002) published exact analytic formulae for light curves with quadratic or nonlinear limb darkening that make fast model fitting possible.[11](https://doi.org/10.1086/345520)\n\nThe chance of catching a transit is small. For random orientation the probability is roughly \\( R_{\\star}/a \\) for circular orbits: about 0.45% at 1 AU for a solar-type star, 0.47% for Earth as seen by a distant observer, and 0.099% for Jupiter.[5](https://ar5iv.labs.arxiv.org/html/0912.0887) • [12](https://warwick.ac.uk/fac/sci/physics/research/astro/theses/edwarybryant.phd.pdf)\n\n## How it is done\n\nA survey monitors large star fields repeatedly, then processes each light curve in a standard sequence. The practitioner first normalizes and flattens the photometry to remove trends, then runs a period search over a trial grid, takes the period, epoch, and duration at maximum power, phase-folds the data to confirm the signal visually, and masks it to search for additional planets.[13](https://docs.lightkurve.org/tutorials/3-science-examples/exoplanets-identifying-transiting-planet-signals.html)\n\nThe standard search tool is the box least squares (BLS) algorithm of Kovács, Zucker, and Mazeh (2002, [Astronomy](https://www.edgepedia.org/astronomy) and [Astrophysics](https://www.edgepedia.org/astrophysics)), which models a transit as an upside-down top hat with four parameters (period, duration, depth, and reference time) and optimizes them by minimizing the squared difference from the observations.[14](https://doi.org/10.1051/0004-6361:20020802) • [13](https://docs.lightkurve.org/tutorials/3-science-examples/exoplanets-identifying-transiting-planet-signals.html)\n\nCompleteness is measured by injection and recovery: the Kepler DR25 pipeline injected synthetic transits into real light curves and measured detection efficiency across fourteen standard transit durations from 1.5 to 15 hr.[15](https://kvmexoweb.ipac.caltech.edu/docs/KSCI-19111-002.pdf) Candidates surviving vetting are then validated statistically; modern validation pipelines typically require a probability of at least 0.99 that a candidate is a true transiting planet.[16](https://arxiv.org/abs/2603.22597v1)\n\n## Origin\n\n[Otto Struve](https://www.edgepedia.org/otto-struve)'s 1952 Observatory paper, \"Proposal for a project of high-precision stellar radial velocity work,\" predicted the detection method that the field later followed.[2](https://arxiv.org/pdf/1803.06896) The first systematic detection program was the TEP network on [CM Draconis](https://www.edgepedia.org/cm-draconis), also the first systematic search for circumbinary planets; the large-sample implementation was the FRESIP proposal, which became the Kepler mission.[2](https://arxiv.org/pdf/1803.06896) The first observed exoplanet transit was of HD 209458 b, a planet already known from radial velocities: Gregory W. Henry and colleagues measured a 0.017 mag drop at the predicted transit time and reported the first extrasolar planet observed to transit its star in 2000 in The Astrophysical Journal[17](https://doi.org/10.1086/312458), while [David Charbonneau](https://www.edgepedia.org/david-charbonneau) and colleagues independently detected two transits in work published the same day in The Astrophysical Journal.[18](https://doi.org/10.1086/312457) In work posted to arXiv in 2001, David Charbonneau and colleagues reported the detection of an exoplanet atmosphere, the sodium doublet, using transit observations.[19](https://doi.org/10.48550/arxiv.astro-ph/0111544)\n\n## Variants\n\nGround-based wide-field surveys trade precision for star count, typically reaching 3–5 mmag (best cases 1–2 mmag at 1–2 m class telescopes).[5](https://ar5iv.labs.arxiv.org/html/0912.0887) WASP and HATNet/HATSouth together have discovered over 300 transiting exoplanets, and the bulk of early JWST targets came from these surveys.[20](https://discovery.ucl.ac.uk/id/eprint/10191993/1/121911A.pdf) NGTS at Paranal uses twelve 20-cm f/2.8 astrographs over ~100 square degrees, reached first light in January 2015, and is most sensitive to planets with orbits under 20 days.[21](https://eso.org/sci/publications/messenger/archive/no.165-sep16/messenger-no165-10-12.pdf) NGTS-1b, a hot Jupiter transiting an M-dwarf, was reported by Daniel Bayliss and colleagues in 2017 in [Monthly Notices of the Royal Astronomical Society](https://www.edgepedia.org/monthly-notices-of-the-royal-astronomical-society).[22](https://doi.org/10.1093/mnras/stx2778)\n\nSpace missions reach the ppm regime. Kepler, launched March 7, 2009 into a trailing heliocentric orbit, uses a 0.95 m Schmidt telescope feeding a 94.6 million pixel CCD array with a 16.1 degree field of view, and was designed for 20 ppm precision on 12th magnitude G2V stars over 6.5 hr; it must stare at one field for at least 3.5 years to see Earthlike planets complete three orbits.[7](https://archive.stsci.edu/kepler/manuals/KSCI-19033-002.pdf) TESS surveys the whole sky over two years, covering ~400 times Kepler's sky area and targeting stars 10–100 times brighter, with >200,000 pre-selected stars at 2-minute cadence plus 30-minute full-frame images of 24° × 96° fields; its 600–1000 nm bandpass is redder than Kepler's, and it targets planets with periods under 10 days and radii under 2.5 Earth radii around bright stars.[8](https://heasarc.gsfc.nasa.gov/docs/tess/docs/TESS_observatory_guide_v1.1.pdf) PLATO, described by H. Rauer and colleagues in 2014 in Experimental Astronomy, extends this line with 26 cameras.[23](https://doi.org/10.1007/s10686-014-9383-4)\n\n## Applications\n\nTransits alone give the planet's radius relative to the star and the stellar density, which helps rule out giant-star blends.[10](https://home.strw.leidenuniv.nl/~radhakrishnan/AFS/lectures/Seager_Ormelas_2003.pdf) Combined with radial velocities, they give true masses and bulk densities: for HD 209458 b, Charbonneau's team derived a radius of 1.27 ± 0.02 \\( R_{\\mathrm{Jup}} \\) and an inclination of 87.1° ± 0.2°[4](https://beta.iopscience.iop.org/article/10.1086/312457), and Henry's team a mean density of 0.27 g cm⁻³, identifying the companion as a gas giant.[3](https://iopscience.iop.org/article/10.1086/312458)\n\nIn multi-planet systems near first-order mean-motion resonances, transit timing variations (TTVs) reveal planet masses from photometry alone; this method was most successfully employed by Kepler, with systems such as [K2-19](https://www.edgepedia.org/k2-19) and [K2-24](https://www.edgepedia.org/k2-24) measured this way.[6](https://ar5iv.labs.arxiv.org/html/1210.2471) • [12](https://warwick.ac.uk/fac/sci/physics/research/astro/theses/edwarybryant.phd.pdf) Because the planet's atmosphere is backlit during a transit, wavelength-dependent transit depths give transmission spectra; the sodium doublet of HD 209458 b was detected in 2001.[19](https://doi.org/10.48550/arxiv.astro-ph/0111544)\n\n## Limitations and alternatives\n\nThe method's core drawbacks are the low probability of properly aligned systems and astrophysical phenomena that mimic transits.[24](https://link.springer.com/rwe/10.1007/978-3-319-30648-3_117-2) In wide-field survey data, impostors such as grazing eclipsing binaries and blends constitute over 95% of detected transit-like signals.[5](https://ar5iv.labs.arxiv.org/html/0912.0887) Some blends are subtle: the OGLE-TR-33 candidate, with a 2% dip and seemingly confirming radial-velocity variations, turned out to be a hierarchical triple containing an eclipsing binary.[25](https://google.iopscience.iop.org/article/10.1088/0004-637X/761/1/6) Statistical validation addresses this: Kepler candidates passing photometric vetting typically have false-positive probabilities around 5% or less.[25](https://google.iopscience.iop.org/article/10.1088/0004-637X/761/1/6)\n\nRadial-velocity confirmation of small planets is hard: Earth induces a semi-amplitude of ~9 cm s⁻¹ on the Sun, below the best HARPS precision of 50–100 cm s⁻¹, and the [CoRoT-7](https://www.edgepedia.org/corot-7) confirmation campaign required over 100 spectra and more than 70 hours over 4 months.[5](https://ar5iv.labs.arxiv.org/html/0912.0887) Compared with radial velocity, astrometry, microlensing, and direct imaging, transit photometry's advantage is yield and the rich parameter set obtainable, especially combined with radial velocities; the published literature gives only qualitative statements on sensitivity comparisons with microlensing, astrometry, and direct imaging, and no quantitative head-to-head benchmark has been published.[24](https://link.springer.com/rwe/10.1007/978-3-319-30648-3_117-2) • [1](https://www.scitepress.org/Papers/2025/138158/138158.pdf)\n\n## References\n\n1. [Analysis of Extra-Planets Searching and Detection Approaches: Radial Velocity, Transition and Gravitational Microlensing (SciTePress 2025)](https://www.scitepress.org/Papers/2025/138158/138158.pdf)\n2. [Planet transits and their history (historical review, arXiv:1803.06896)](https://arxiv.org/pdf/1803.06896)\n3. [A Transiting \"51 Peg-like\" Planet (Henry et al. 2000, ApJ 529, L41)](https://iopscience.iop.org/article/10.1086/312458)\n4. [Detection of Planetary Transits Across a Sun-like Star (Charbonneau et al. 2000, ApJ 529, L45)](https://beta.iopscience.iop.org/article/10.1086/312457)\n5. [Blue Dots Team Transits Working Group Review (arXiv:0912.0887)](https://ar5iv.labs.arxiv.org/html/0912.0887)\n6. [Exoplanet Detection Methods (Winn, arXiv:1210.2471)](https://ar5iv.labs.arxiv.org/html/1210.2471)\n7. [Kepler: A Search for Terrestrial Planets (Kepler Instrument Handbook, KSCI-19033-002)](https://archive.stsci.edu/kepler/manuals/KSCI-19033-002.pdf)\n8. [TESS Observatory Guide (NASA HEASARC)](https://heasarc.gsfc.nasa.gov/docs/tess/docs/TESS_observatory_guide_v1.1.pdf)\n9. [Transits.jl documentation, Introduction](https://juliaastro.org/Transits/stable/introduction/)\n10. [A Unique Solution of Planet and Star Parameters from an Extrasolar Planet Transit Light Curve (Seager & Mallén-Ornelas 2003, ApJ 585:1038)](https://home.strw.leidenuniv.nl/~radhakrishnan/AFS/lectures/Seager_Ormelas_2003.pdf)\n11. [Kaisey Mandel, Eric Agol (2002). Analytic Light Curves for Planetary Transit Searches. The Astrophysical Journal.](https://doi.org/10.1086/345520)\n12. [PhD thesis on NGTS/TESS transiting exoplanets (University of Warwick)](https://warwick.ac.uk/fac/sci/physics/research/astro/theses/edwarybryant.phd.pdf)\n13. [Identifying transiting exoplanet signals in a light curve (Lightkurve documentation)](https://docs.lightkurve.org/tutorials/3-science-examples/exoplanets-identifying-transiting-planet-signals.html)\n14. [G. Kovács, S. Zucker, T. Mazeh (2002). A box-fitting algorithm in the search for periodic transits. Astronomy and Astrophysics.](https://doi.org/10.1051/0004-6361:20020802)\n15. [Planet Detection Metrics: Per-Target Detection Contours (KeplerPORTs, KSCI-19111-002)](https://kvmexoweb.ipac.caltech.edu/docs/KSCI-19111-002.pdf)\n16. [Automatic search for transiting planets in TESS-SPOC FFIs with RAVEN: over 100 newly validated planets and over 2000 vetted candidates](https://arxiv.org/abs/2603.22597v1)\n17. [Gregory W. Henry and colleagues (2000). A Transiting “51 Peg–like” Planet. The Astrophysical Journal.](https://doi.org/10.1086/312458)\n18. [David Charbonneau and colleagues (2000). Detection of Planetary Transits Across a Sun-like Star. The Astrophysical Journal.](https://doi.org/10.1086/312457)\n19. [Charbonneau, David and colleagues (2001). Detection of an Extrasolar Planet Atmosphere. arXiv (Cornell University).](https://doi.org/10.48550/arxiv.astro-ph/0111544)\n20. [High precision ground-based CCD photometry from the Next Generation Transit Survey (SPIE)](https://discovery.ucl.ac.uk/id/eprint/10191993/1/121911A.pdf)\n21. [The Next Generation Transit Survey Becomes Operational (ESO Messenger 165, 2016)](https://eso.org/sci/publications/messenger/archive/no.165-sep16/messenger-no165-10-12.pdf)\n22. [Daniel Bayliss and colleagues (2017). NGTS-1b: a hot Jupiter transiting an M-dwarf. Monthly Notices of the Royal Astronomical Society.](https://doi.org/10.1093/mnras/stx2778)\n23. [H. Rauer and colleagues (2014). The PLATO 2.0 mission. Experimental Astronomy.](https://doi.org/10.1007/s10686-014-9383-4)\n24. [Transit Photometry as an Exoplanet Discovery Method (Deeg & Alonso, Handbook of Exoplanets, Springer)](https://link.springer.com/rwe/10.1007/978-3-319-30648-3_117-2)\n25. [An Efficient Automated Validation Procedure for Exoplanet Transit Candidates (Morton 2012, ApJ 761:6)](https://google.iopscience.iop.org/article/10.1088/0004-637X/761/1/6)\n\n---\n*Topic: Encyclopedia › Physical world and mathematics › Astronomy › Cosmology and observation › Observational techniques: astrometry, photometry, spectroscopy*\n\n*Initially written Sep 29, 2026 · Reviewed: Sep 30, 2026 · Edited: Sep 30, 2026 · Last review: Sep 30, 2026*\n\n*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*\n\nLicense: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license\n", "same_as": [], "url": "https://www.edgepedia.org/transit-photometry", "markdown_url": "https://www.edgepedia.org/transit-photometry.md", "license": { "name": "Edgepedia Community License 1.0", "url": "https://www.edgepedia.org/edgepedia/license", "summary": "Free with credit, commercial use included. AI training is open to everyone. For other uses, organizations over USD 100M in revenue or 100M monthly users license separately.", "spdx": "LicenseRef-Edgepedia-Community-1.0" }, "credit": "\"Transit photometry\", Edgepedia (EdgeChat), https://www.edgepedia.org/transit-photometry. Edgepedia Community License 1.0.", "credit_md": "\"[Transit photometry](https://www.edgepedia.org/transit-photometry)\", Edgepedia (EdgeChat), [https://www.edgepedia.org/transit-photometry](https://www.edgepedia.org/transit-photometry). [Edgepedia Community License 1.0](https://www.edgepedia.org/edgepedia/license).", "credit_html": "\"Transit photometry\", Edgepedia (EdgeChat), https://www.edgepedia.org/transit-photometry. Edgepedia Community License 1.0.", "speakable": "Transit photometry detects exoplanets by measuring the periodic dip in a star's brightness when a planet crosses its disk, and has found over 4000 known exoplanets." }