Discovery of other planets
In 1995 Michel Mayor and Didier Queloz of the Geneva Observatory discovered a
planet orbiting the star 51 Pegasi, 50 ly away. Their results were confirmed by Geoff
Marcy and Paul Butler using California’s Lick Observatory. They used the radial velocity
method to detect the planet.
Methods of detection
Ways of detecting extrasolar planets:
1 Astrometric method
2 Radial velocity method
3 Transit method
As a planet orbits a star it pulls on the star making it wobble. The astrometric method
involves making very precise measurements (accuracy of at least 0.001”) of a star’s
position in the sky relative to other stars. If they have an orbiting planet their position
changes in a periodic way.
The Hubble Space Telescope used astrometry to discover a planet around star Gliese
876 in 2002.
Radial velocity method
Based on the Doppler effect; a star wobbling due to an orbiting planet will periodically
move towards and away from the Earth. Absorption lines in it’s spectrum will shift to the
blue and the red in a cyclic fashion. These shifts are very small as the star’s motion
around it’s orbit is very slow, so high resolution instruments are needed to detect this.
Velocity variations down to 1 m s-1 can be detected with spectrometers such as HARPS
(High Accuracy Radial Velocity Planet Searcher) at the ESO La Silla Observatory in Chile,
or HIRES at the Keck Observatory in Hawaii.
This method looks for a drop in stellar brightness as a planet crosses in front (transits).
The extent of the drop depends on the relative sizes of the star and the planet. For this
method to work the orientation of the planet’s orbit has to be aligned with our line-of-
sight for us to see a transit. Also results often have to be confirmed by radial velocity
The Spitzer Space Telescope was used to study variations in brightness of the star HD
209458 which dims by up to 1.7%.
Searching for extrasolar planets
Hubble survey called the Sagittarius Window Eclipsing Extrasolar Planet Search (SWEEPS)
took place in Feb 2004. Looked at 180 000 stars in the nuclear bulge 26 000 ly away.
ESA COROT launched Dec 27 2006: using transit method, able to detect planets a few
times to several times larger than the Earth.
NASA Kepler mission launched March 7 2009: using transit method to scan
100 000 stars in the constellation of Cygnus. It is sensitive enough to detect planets
smaller than Earth.
NASA Space Interferometry Mission scheduled for 2014 will use astrometry.
ESA Darwin probe scheduled for 2013 will image planets directly.
NASA New Worlds Mission will use an occulter to block the star’s light so we are able
to observe the planets.
Searching for extrasolar planets
Spectral Type: K1 - K2V (orange dwarf star)
Distance: 63 ly
An extrasolar planet was discovered orbiting HD 189733b; observers in France saw the planet transiting
across the face of the star. It has an orbital radius of ~ 0.031 AU and a period of 2.219 days. The planet
has a mass of 1.15 x mass Jupiter.
Spectral Type: G2.5IVa or G4-5Va (yellow dwarf star)
Apparent magnitude: 5.49
Distance: 50.1 ly
Temperature: 5665 K
51 Pegasi is a star similar to the Sun that has an orbiting planet (discovered in 1995). The planet has a
mass of 0.5 x Jupiter and orbits close to the star at a distance of 0.051 AU with a period of only 4.2 days.