---
source: https://iinventedlight.com/library/notes/exoplanets-basics.md
title: "Exoplanets: what they are and how they are found"
license: CC-BY-4.0
license_url: https://creativecommons.org/licenses/by/4.0/
source_license: "US government work, public domain; restated in our own words"
attribution: "Summary by iinventedlight (AI-assisted). Sources: NASA, In Depth, US government work, public domain; NASA, How We Find and Characterize, US government work, public domain."
author: "iinventedlight (synthesis of cited sources); text drafted with AI assistance"
date: 2026-11-04
status: published
checked_on: 2026-11-04
as_of: 2026-11-04
confidence: medium
untested: "No observation or experiment was done; only the fetched government pages were read."
category: coverage-gaps
topic_area: "science-explain: astronomy"
tags: science, astronomy, exoplanets, kepler
sources:
  - id: S1
    title: "In Depth"
    publisher: NASA
    url: https://science.nasa.gov/exoplanets/facts/
    accessed: 2026-11-04
    license: "US government work, public domain"
  - id: S2
    title: "How We Find and Characterize"
    publisher: NASA
    url: https://science.nasa.gov/exoplanets/how-we-find-and-characterize/
    accessed: 2026-11-04
    license: "US government work, public domain"
---
# Exoplanets: what they are and how they are found

## Short answer
An exoplanet is a planet outside our solar system, usually orbiting another star. Most are found by two methods: watching a star dim slightly when a planet crosses in front (transit), or measuring the star's wobble as the planet tugs it (radial velocity). [S1][S2]

## Key points
- The first exoplanet around a Sun-like star was confirmed in 1995: 51 Pegasi b, a hot gas giant. [S2]
- Kepler data suggest there are more planets than stars in our galaxy. [S2]
- The closest known exoplanet, Proxima Centauri b, is about 4 light-years away. [S1]
- Scientists commonly use five discovery methods; transit and radial velocity are the main two. [S1]

## Claims
- id: C1
  claim: An exoplanet is a planet outside our solar system.
  source: S1
  evidence: "is a planet outside of our solar system that usually orbits another star"
- id: C2
  claim: Transit and radial velocity are the two main detection techniques.
  source: S1
  evidence: "The two main techniques are the transit and radial velocity methods."
- id: C3
  claim: Kepler data indicate more planets than stars in our galaxy.
  source: S2
  evidence: "there are more planets than stars in our galaxy"
- id: C4
  claim: The wobble method measures changes in a star's radial velocity.
  source: S2
  evidence: "The wobble method measures changes in a star’s “radial velocity.”"

## Sources
- [S1] In Depth. NASA. https://science.nasa.gov/exoplanets/facts/ . License: US government work, public domain. Accessed 2026-11-04.
- [S2] How We Find and Characterize. NASA. https://science.nasa.gov/exoplanets/how-we-find-and-characterize/ . License: US government work, public domain. Accessed 2026-11-04.

License: this note is CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Summary by iinventedlight (AI-assisted).