🔭 astrophysics

TEPCat: The Transiting Extrasolar Planet Catalogue

This paper introduces TEPCat, a critical online database that compiles and maintains the high-precision physical and orbital properties of known transiting extrasolar planets to support scientific research.

Original authors: John Southworth

Published 2026-02-27

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CC BY 4.0

Original authors: John Southworth

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the universe as a massive, chaotic library. For decades, astronomers have been finding new "books" (planets) that orbit other stars, but they were scattered across different shelves, written in different languages, and often missing key pages. Some books had great summaries, others had just a title, and many were just rumors.

Enter TEPCat (The Transiting Extrasolar Planet Catalogue). Think of TEPCat not just as a library, but as the ultimate, living "Wikipedia" or "Master Index" for planets that we can actually see passing in front of their stars.

Here is a simple breakdown of what this paper is about, using some everyday analogies:

1. What is TEPCat?

Imagine you are a chef trying to cook a perfect meal, but you need to know the exact weight, temperature, and origin of every single ingredient. If you had to ask 50 different farmers for this info, and they all gave you answers in different units (some in pounds, some in kilograms, some in "handfuls"), you'd be in trouble.

TEPCat is the central kitchen scale. It takes all the messy, scattered data about thousands of exoplanets (planets outside our solar system) and standardizes it. It tells scientists exactly:

Where the planet is (coordinates).
When it will pass in front of its star again (the schedule).
How big and how heavy the planet and its star are.
How hot the planet is.

It's a "one-stop shop" so scientists don't have to hunt through hundreds of old papers to find the facts they need.

2. Why do we need it? (The "Why")

The paper explains that the number of known planets has exploded.

The Past: In the 1990s, finding a new planet was like finding a needle in a haystack. We only knew a handful.
The Present: Thanks to space telescopes like Kepler and TESS, we are finding planets like raindrops. We now have thousands.

Without a central database, this information would be a mess. If a scientist wants to study "Hot Jupiters" (giant, scorching planets close to their stars), they need to grab a specific list of them instantly. TEPCat provides that list, updated monthly, so researchers can focus on the science rather than the search.

3. How does it work? (The "How")

The author, John Southworth, built this system like a highly organized filing cabinet that updates itself.

The Input: Every day, the author (and his team) scans new scientific papers and pre-prints.
The Sorting: They separate the planets into categories:
- "Well-studied": Planets we know a lot about (mass, size, temperature).
- "Little-studied": Planets we've spotted but don't know much about yet (mostly from the massive Kepler mission).
The Output: They turn this raw data into easy-to-read web pages and downloadable files that computers can read. They also generate cool charts and graphs automatically.

4. The "Rules of the Game" (Classification)

The paper also discusses how we name and categorize these worlds. It's like sorting a bag of mixed candy:

The Size Limit: If an object is too heavy (over 13 times the mass of Jupiter), it's not a planet; it's a "Brown Dwarf" (a failed star). If it's heavier than 75 Jupiters, it's a star. TEPCat draws the line at 13 Jupiters to keep things simple.
The Temperature: Planets are sorted by how close they are to their sun:
- Hot: Very close (like a sunbather on a beach).
- Warm: Middle distance.
- Cold: Far away (like an iceberg).
The "Transit" vs. "Eclipse": The paper clarifies some confusing terms. When a planet crosses in front of a star, it's a Transit (like a bug flying across a car headlight). When it goes behind the star, it's an Occultation (like the bug hiding behind the headlight). TEPCat makes sure everyone uses the right words.

5. The "Time Machine" Feature

One of the coolest parts of TEPCat is that it keeps snapshots of itself every month.
Imagine if Wikipedia deleted its history every time it updated. You wouldn't know what the article looked like last year. TEPCat saves a "time capsule" of the database every month. This allows scientists to say, "In 2023, we thought this planet was this big, but in 2024, new data showed it was actually smaller." It ensures scientific honesty and reproducibility.

Summary

In short, this paper is the user manual and mission statement for TEPCat. It tells the scientific community:

"We have built a massive, constantly updating, free-to-use database that organizes the chaos of exoplanet discoveries. It helps us measure, compare, and understand the thousands of new worlds we are finding, ensuring that no one has to reinvent the wheel every time they want to study a new planet."

It turns a chaotic galaxy of data into a structured, usable map for the next generation of space explorers.

1. Problem Statement

The field of exoplanet research has experienced exponential growth over the last 25 years, with thousands of transiting extrasolar planets (TEPs) discovered. While individual studies provide high-precision physical and orbital properties for specific systems, the scientific community lacks a single, critically compiled, and easily accessible database that aggregates this heterogeneous data.

Data Fragmentation: Properties are scattered across thousands of discovery papers, often using different analysis methods, units, and error conventions.
Reproducibility Issues: Without a centralized, version-controlled repository, it is difficult for researchers to reproduce results or track the evolution of parameter measurements over time.
Target Selection Needs: Upcoming space missions (e.g., Ariel, PLATO, CHEOPS) require curated lists of high-quality targets with known physical properties to optimize observation strategies.
Inconsistency: Previous attempts to compile data often included low-precision "validated" candidates without distinguishing them from well-characterized systems, cluttering the dataset.

2. Methodology

The paper outlines the design, implementation, and maintenance strategy of TEPCat (the Transiting Extrasolar Planet Catalogue).

Data Curation Strategy:
- Scope: TEPCat includes Transiting Extrasolar Planets (TEPs) and Transiting Brown Dwarfs (BDs). It excludes systems with masses $>75 M_{Jup}$ (classified as eclipsing binaries) and objects with radii $>0.5 R_{Jup}$ lacking mass constraints (to avoid ambiguity between planets, BDs, and low-mass stars).
- Classification: Objects are split into "well-studied" (homogeneously re-analyzed or high-precision data) and "little-studied" (large surveys like Kepler/TESS with lower precision or unknown masses).
- Data Sources: Data is manually extracted from peer-reviewed literature, arXiv preprints, and major journals (A&A, ApJ, MNRAS, AJ, PASP). The author checks for new papers daily.
Data Structure:
The catalogue is organized into three primary tables, plus a consolidated view:
1. Observables Table: Sky position (RA/Dec), apparent magnitudes ( $V, K$ ), orbital ephemerides ( $T_0, P_{orb}$ ), transit duration, and depth.
2. Properties Table: Physical parameters of the host star (mass, radius, $T_{eff}$ $T_{e f f}$ , metallicity, density) and the planet (mass, radius, surface gravity, density, equilibrium temperature).
  - Error Handling: All values include separate upper and lower error bars. Upper limits are handled by setting the value to zero with the limit as the upper error.
  - Calculations: Derived quantities (e.g., semi-major axis $a$ , surface gravity $\log g$ , density $\rho$ ) are calculated internally using IAU 2012/2015 constants and displayed on the web but excluded from raw data files to ensure transparency.
3. Obliquities Table: Sky-projected ( $\lambda$ ) and true ( $\psi$ ) orbital obliquities.
4. Consolidated Table: A single file combining all 57 quantities per system for ease of use.
Implementation:
- Backend: Data is stored in fixed-width ASCII files for speed and clarity.
- Processing: A monolithic IDL (Integrated Data Language) script processes the input files to resolve bibliographic codes (ADS bibcodes), validate data ranges, calculate derived parameters, and generate HTML webpages and machine-readable files (ASCII/CSV).
- Version Control: To ensure scientific reproducibility, monthly snapshots of the entire database are archived (e.g., 2023mar). A "News" section tracks all modifications, ranging from single parameter updates to the addition of dozens of new systems.

3. Key Contributions

Centralized Repository: TEPCat serves as the primary, freely accessible source for physical and observable properties of transiting systems, distinct from the NASA Exoplanet Archive or the Extrasolar Planets Encyclopaedia.
Standardization: It enforces consistent units (e.g., $M_{Jup}$ , $R_{Jup}$ defined as 71,492 km) and error reporting conventions across thousands of disparate studies.
Dynamic Classification: The paper proposes and discusses a classification scheme for planets based on mass and orbital period (Hot/Warm/Cold Jupiters, Neptunes, Super-Earths) and distinguishes between transits, occultations, and partial eclipses.
Historical Context: The paper provides a comprehensive history of TEP discovery, from Struve's 1952 proposal to the current era of space-based surveys, analyzing the impact of different missions (SuperWASP, Kepler, TESS, CoRoT).

4. Results and Analysis

The paper presents several visualizations and statistical analyses derived from the TEPCat database (as of February 2026):

Discovery Statistics:
- Total Count: As of the snapshot, there are 3,764 TEPs and 51 transiting Brown Dwarfs.
- Mission Dominance: The Kepler mission remains the largest contributor (2,237 TEPs), followed by TESS (708 TEPs). Ground-based surveys like SuperWASP (185) and HATNet/HATSouth (145 combined) also contribute significantly.
Sky Distribution:
- Ground-based discoveries are roughly isotropic but avoid the galactic plane due to stellar crowding.
- Space-based discoveries are clustered: Kepler in a single field, K2 near the ecliptic, CoRoT toward the galactic center/anti-center, and TESS covering almost the entire sky.
Physical Properties:
- Mass-Radius Diagram: Shows a clear main-sequence locus for stars, while planets and brown dwarfs show significant scatter. The "Brown Dwarf Desert" (a paucity of objects between 13–75 $M_{Jup}$ ) is visible.
- Mass-Density Relation: Confirms trends identified by Hatzes & Rauer (2015), showing distinct regimes for low-mass planets, giant planets, and stars, with revised boundary masses (0.5 $M_{Jup}$ and 70 $M_{Jup}$ ).
- Obliquity: The catalogue compiles Rossiter-McLaughlin measurements, showing a mix of aligned and misaligned systems, with ongoing debate regarding the prevalence of polar orbits.

5. Significance

Scientific Utility: TEPCat is critical for population studies, atmospheric characterization (transmission spectroscopy), and orbital decay research. It allows researchers to define unbiased samples for statistical analysis.
Mission Support: It directly aids in target selection for major space missions (Ariel, Twinkle, CHEOPS, PLATO), ensuring that observation time is allocated to systems with the most reliable physical parameters.
Reproducibility: By archiving monthly snapshots and providing machine-readable data, TEPCat addresses the "moving target" problem in exoplanet science, where parameters are frequently revised as new data becomes available.
Community Resource: The author explicitly welcomes corrections and updates, fostering a collaborative environment to maintain the highest data quality. The paper serves as both a technical manual for the database and a review of the current state of transiting exoplanet science.