Undreamed Shores - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 247

In this way, in 1992, the Polish astronomer Alexander Wolszczan
and his Canadian colleague Dale Frail, working at the Arecibo Obser-
vatory in Puerto Rica, discovered a planetary system around the pul-
sar PSR B1257 + 12, which lies nearly 1,000 light years from Earth in the
direction of the constellation Virgo. 153 They detected two planets
orbiting their pulsar with periods of 66.6 and 98.2 days respectively
(they speculated on a third, later confirmed, and there may be a
fourth). These 'pulsar planets' came as a surprise, for it was thought
that a supernova should blast away everything in its path.
Then, in 1995, came unequivocal evidence of a planet orbiting a
yellow main sequence star—a star more like our own. Fifty light
years away in the constellation of Pegasus, 154 51 Pegasi b was discov-
ered around its star. The scientists who found it, Michel Mayor and
Didier Queloz of the Observatoire de Haute-Provence, did not see it,
any more than Wolszczan and Frail could have seen the planets
reflected in the remaining faint light emitted by pulsar PSR B1257 + 12.
Rather, they observed that the parent star was wobbling very slightly,
as it was tugged by the gravitational effect of its orbiting planet. The
gravitational effect of the planet produces changes in the position of
the star relative to the Earth. Even across the vastness of space those
changes can be detected, using a spectroscope, by the Doppler effect.
This is the same effect that causes the siren of an oncoming ambu-
lance to sound higher-pitched as it approaches, as the sound waves
(to a stationary observer) become more closely spaced, and deeper-
toned as the ambulance recedes, the sound waves then 'stretching
out'. When the wobble of the star moves it towards the Earth its vis-
ible light spectrum is blue-shifted by the Doppler effect, while it is
red-shifted as the star moves away. It was the wobble technique
(radial velocity, it is called more formally) that Michel Mayor and
Didier Queloz used to detect Pegasi b. This technique not only shows
the presence of a planet, but also gives information on its mass, on