The study of planets outside our solar system.

Image: Planet Hunters* transit mapping tool.

An exoplanet is a planet orbiting a star other than the Sun. The first confirmed detection was only in 1992, orbiting a pulsar, and in 1995 around a normal star. Early detections were by terrestrial telescope. The large number of recent detections has been made by the NASA Kepler telescope, launched in 2009. Apart from being fascinating in itself, the significance of exoplanets is that they form in the early years of a star’s lifecycle. Observing large numbers, at different stages of a star’s life, tells us about planet formation. The light from an exoplanet can be used to infer the chemical composition of the planet and atmosphere.


  1. We have a good idea of how the planets may have formed in our solar system but it is not complete
  2. The NASA Kepler space telescope has, since 2009, found thousands of planets orbiting other stars, in just a small region of space
  3. Studying these exoplanets may give ideas about planet formation and composition.


Methods of detection of exoplanets, in order of the number of detections to date:

  1. Transit: dip in luminosity of the star
  2. Radial velocity: change of colour of the light from the star, using Doppler spectroscopy
  3. Gravitational micro-lensing: light from the star is bent and focused by the gravity of the planet
  4. Direct imaging: precise blocking of the light from the star, allowing the dark planet to be seen
  5. Astrometry: direct detection of very small movements of the star.

Kepler space telescope

The NASA telescope was launched in 2009 to observe a small section of the Milky Way galaxy continuously, from an orbit around the Sun. Kepler uses the transit method of detection, and is responsible for by far the largest number of detection. The satellite suffered a failure in 2013, and was re-purposed as K2.

Exoplanets Data Explorer

An interactive table and plotter for exploring and displaying data from the Exoplanet Orbit Database.


TRAPPIST-1 is a planetary system discovered in 2015 in the constellation of Aquarius, 39 light years away. At least seven planets have been detected, orbiting an ultra-cool red dwarf star that is slightly larger but much more massive than the planet Jupiter.

The initial discovery was made by TRAPPIST, the TRAnsiting Planets and PlanetesImals Small Telescope, at the La Silla Observatory in Chile. Additional planets were subsequently identified using other terrestrial telescopes, and the NASA Spitzer Space Telescope.

The TRAPPIST-1 system is fascinating because of the nature of the planets. The system is composed of seven temperate terrestrial planets, of which five are similar in size to Earth, and two are intermediate in size between Mars and Earth. Three of the planets orbit within the habitable zone.

Planet Hunters

Planet Hunters is a web site that publishes data from the Kepler telescope to give members of the public the opportunity to spot potential exoplanet occurrences.

This is the real data used to identify exoplanets. In the graphic at the top of the page,  “Each point on the light curve represents one measurement of a star’s brightness taken by NASA’s Kepler Space Telescope. These measurements are taken approximately every 30 minutes. The higher the dot, the brighter the star appears.”

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