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Exoplanets

Exoplanets are planets beyond our Solar System, most of them orbiting other stars. The first were found in the 1990s, and NASA now counts more than 6,000 confirmed. Because planets are faint next to their stars, almost all are found indirectly — by the wobble they cause in their star or the dip in starlight when they pass in front of it — and those measurements reveal their size, mass and what they are made of.

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Combining Methods

A radial-velocity measurement sees only the part of the star's motion along our line of sight, so on its own it gives M sin(i), where i is the orbit's inclination — a minimum mass. A transiting orbit must be lined up with our view, which pins down the inclination; this is why the transit-plus-wobble combination is so powerful.

ρˉ=Mp43πRp3\bar{\rho} = \dfrac{M_p}{\tfrac{4}{3}\pi R_p^{3}}

Mean density from a transit radius and a radial-velocity mass.

Transit spectroscopy adds composition: starlight filtered through a planet's atmosphere during transit carries the signatures of the gases it passed through. Hubble has detected helium and water vapour in exoplanet atmospheres this way. Direct imaging with coronagraphs is so far limited mainly to young giant planets still glowing from their formation.

Info: Kepler's survey showed that small planets are very common; NASA estimates there are more planets than stars in our galaxy.
Full explanation — the complete reference version every reading depth is based on

What it is

An exoplanet is any planet outside our Solar System. Most orbit other stars, though some 'rogue' planets drift through space with no star at all. Planets are tiny and dim compared with their stars, which is why none was confirmed until the 1990s: first planet-sized bodies around a pulsar in 1992, then in 1995 51 Pegasi b, the first planet proven to orbit a Sun-like star — a giant planet whipping round its star in about four days.

How we find them

  • Radial velocity (the wobble method): a planet's gravity makes its star move slightly towards and away from us, alternately squeezing and stretching the starlight's wavelengths. Jupiter moves the Sun by about 12 m/s; Earth by only about 0.09 m/s.
  • Transits: if a planet's orbit is lined up with our view, it crosses its star once per orbit and dims it slightly. The depth of the dip gives the planet's size; the time between dips gives its year.
  • Direct imaging: blocking the star's glare with a coronagraph can reveal the planet itself — so far mainly young giant planets still glowing from their formation.
transit depth=(DplanetDstar)2\text{transit depth} = \left(\dfrac{D_{\text{planet}}}{D_{\text{star}}}\right)^2

The fraction of starlight blocked equals the planet's disk area divided by the star's, which is the square of the ratio of their diameters.

Worked example

A Jupiter-sized planet is about 143,000 km across and a Sun-like star about 1.4 million km. (143,000 ÷ 1,400,000)² ≈ 0.0104, so the planet blocks about 1% of the starlight. Earth, about 12,700 km across, in front of a star half the Sun's size (700,000 km) would block only (12,700 ÷ 700,000)² ≈ 0.0003, about 0.03% (our own calculation).

Why it matters

Combining a planet's size (from transits) with its mass (from the wobble) gives its density, which separates gas giants from rocky worlds. HD 209458 b, for example, has about 0.73 times Jupiter's mass but 1.39 times its radius, so it is a gas giant. Astronomers are especially interested in planets in a star's habitable zone, where liquid water could exist on the surface — water being essential for life as we know it.

Common misconception: A common misconception is that a planet in the habitable zone must have life, or even water. The zone only marks the distances where surface water could be liquid; whether a planet actually has water, an atmosphere or life has to be found out separately.

Where it connects

Exoplanet systems invite comparison with our own (Solar System), orbit stars whose light we analyse (Stars), and are found and studied with large telescopes in space and on the ground (Telescopes).

Assumptions and limits

  • A transit only happens if the orbit is lined up with our view, so most planets never transit as seen from Earth.
  • The wobble method measures only the star's motion along our line of sight, so on its own it gives M sin(i) — a lower limit on the mass — rather than the true mass.
  • Both methods find big planets close to their stars most easily, so the known planets are not a fair sample of all planets.

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Sources and methodology

  • An exoplanet is any planet beyond our Solar System; most orbit other stars, but some free-floating 'rogue' planets are not bound to any star. (awaiting scientific review)
  • As of 1 October 2026, NASA reports more than 6,000 confirmed exoplanets. (awaiting scientific review)
  • In 1992 Aleksander Wolszczan and Dale Frail reported two or more planet-sized bodies orbiting the millisecond pulsar PSR1257+12, the first such objects detected outside the Solar System. (awaiting scientific review)
  • On 6 October 1995 Michel Mayor and Didier Queloz announced 51 Pegasi b, the first planet proven to orbit a solar-type star: a gas giant comparable to Jupiter that circles its star in about four days. (awaiting scientific review)
  • Mayor and Queloz published their discovery in the 1995 Nature paper 'A Jupiter-mass companion to a solar-type star'. (awaiting scientific review)
  • The radial-velocity (wobble) method detects a planet from the way its gravity makes its star move slightly towards and away from us, alternately squeezing and stretching the wavelengths of the starlight. (awaiting scientific review)
  • Jupiter's gravity makes the Sun move at about 12 m/s, while Earth's makes it move at only about 0.09 m/s. (awaiting scientific review)
  • During a transit a star dims by the ratio of the planet's disk area to the star's: a Jupiter-sized planet (143,000 km across) in front of a Sun-like star (1.4 million km across) blocks about 1/100 of the light, a 1% dip. (awaiting scientific review)
  • The time between successive transit dips gives a planet's orbital period, and with the star's mass that gives the planet's distance from its star. (awaiting scientific review)
  • When a planet is studied with both methods, transit photometry gives its size and the radial-velocity method its mass, so its density — and therefore its structure — can be calculated. (awaiting scientific review)
  • HD 209458 b, a gas giant whose discovery was announced in 1999, has about 0.73 times Jupiter's mass and 1.39 times Jupiter's radius and orbits its star every 3.5 days. (awaiting scientific review)
  • A star's habitable zone is the distance from it at which liquid water could exist on an orbiting planet's surface; habitable zones are wider for hotter stars and much tighter, and very close in, for small dim red dwarfs. (awaiting scientific review)
  • NASA's Kepler space telescope, launched on 6 March 2009, stared at about 150,000 stars in one patch of sky and detected planets by watching for transits. (awaiting scientific review)
  • The early wobble technique revealed one planet after another, many of them large 'hot Jupiters' on tight, close-in orbits. (awaiting scientific review)
  • The NASA Exoplanet Archive records a planet's best mass as either its true mass or M sin(i), where i is the inclination of the orbital plane to our line of sight. (awaiting scientific review)
  • Exoplanets imaged directly so far are mainly giant planets that are still hot enough from their formation to glow on their own. (awaiting scientific review)

Claims marked “awaiting scientific review” cite the sources listed but have not yet been signed off by a scientific reviewer.

Content status: published 1 October 2026.

  • Scientific review: this version has not yet been signed off by a scientific reviewer.
  • The Advanced explanation has not yet been reviewed for age suitability.