The Astronomy Book

291

understand that their existence
is insignificant in the context
of the universe.
The principle is named after
Nicolaus Copernicus, who changed
the way humankind saw itself by
relegating Earth from the center of
the solar system to one of several
planets that orbited the sun. By
the late 20th century, successive
discoveries had moved the solar
system from the center of the
universe to a quiet wing at
the edge of a galaxy containing
200 billion other stars. The galaxy
was not special either, simply one
of at least 100 billion arranged in
vast filaments that extended for
hundreds of millions of light-years.
Nevertheless, planet Earth and the
solar system were still regarded as
very special—since there was no
evidence that any other stars had
planets, let alone planets capable
of supporting life. Since Mayor’s
and Queloz’s discovery, however,
this idea has also succumbed to
the Copernican principle.

Wobbling light
Queloz and Mayor found 51 Pegasi b
using a system called Doppler
spectroscopy. Also known as
the radial velocity or “wobble”
method, Doppler spectroscopy
can detect an exoplanet by its
gravitational effects on its host star.
The star’s gravity is far greater than
that of the planet, and this is what
keeps the planet in orbit. However,
the planet’s gravity also has a small
effect on the star, making it wobble
back and forth as the planet moves
around it. The effect is tiny: Jupiter
changes the sun’s speed by about
12 miles/s (7.4 km/s) over a period of
11 years, while Earth’s effect is only
0.1 miles/s (0.16 km/s) each year.

In 1952, US astronomer Otto Struve

had suggested that this kind of

star wobble could be detected

as small fluctuations in a star’s

spectrum. As the star moved away

from Earth, its emissions would

be slightly redshifted from the

norm. When it wobbled back again

toward the observer, the light

would be blueshifted. The theory

was solid but detecting the wobble

required an ultrasensitive detector.

See also: The Copernican model 32–39 ■ Radio telescopes 210–11 ■ Studying distant stars 304–05 ■
Looking farther into space 326–27 ■ Kumar (Directory) 339

THE TRIUMPH OF TECHNOLOGY

We are getting much

closer to seeing solar

systems like our own.

Didier Queloz

That detector was a spectrograph

named ELODIE developed by

Mayor in 1993. ELODIE was about

30 times more sensitive than any

previous instrument. Even then,

it was only capable of measuring

velocity changes of 7 miles/s

(11 km/s), which meant it was

limited to detecting planets

about the size of Jupiter.

Improving the search

In 1998, an even more sensitive

spectrograph, named CORALIE,

was installed at La Silla Observatory

in Chile, which again was

searching for planets using the

radial velocity technique. In 2002,

Michel Mayor began overseeing

HARPS (High Accuracy Radial

velocity Planet Searcher) at the

same site, using a spectrograph

capable of detecting exoplanets

about the size of Earth. The wobble

method of detection was very slow,

so new techniques of spotting

exoplanets were developed. ❯❯

When a large Jupiter-like planet orbits its

star, it exerts a gravitational pull on the star.

Both star and planet revolve around a common

center of gravity. The “wobble” in the star’s

orbit allows the planet to be detected.

Unseen

planet

Longer wavelength

indicates the star

is retreating.

Shorter wavelength indicates

the star is advancing.

Star wobbles from one

position to another