The Science Book

327

See also: Nicolaus Copernicus 34–39 ■ William Herschel 86–87 ■

Christian Doppler 127 ■ Edwin Hubble 236–41

A

stronomers have long

pondered the possibility of

planets orbiting stars other

than our Sun, but technology has,

until recently, limited our ability

to detect them. First to be found

were planets that orbited pulsars—

rapidly spinning neutron stars

whose radio signals vary slightly

as their planets pull them this way

and that. Then, in 1995, Swiss

astronomers Michel Mayor and

Didier Queloz discovered 51 Pegasi

b—a Jupiter-sized planet orbiting

a Sunlike star about 51 light years

from Earth. Since then, more than

1,000 other extrasolar planets, or

“exoplanets,” have been confirmed.

Planet hunter

Astronomer Geoffrey Marcy at the

University of California, Berkeley,

along with his team, currently

holds the record for the most

planets found by a human observer,

including 70 out of the first 100.

Such distant planets are too

faint to be seen directly, but can

be revealed indirectly. The effect

of a planet’s gravity on its host star

produces variations in the star’s

radial velocity—the speed at which

it moves toward or away from

Earth—which can be measured

from changes in its light frequency.

Whether any exoplanets support

life remains to be seen. ■

FUNDAMENTAL BUILDING BLOCKS

WORLDS BEYOND

THE SOLAR

SYSTEM

GEOFFREY MARCY (1954–)

IN CONTEXT

BRANCH

Astronomy

BEFORE
1960s Astronomers hope to
detect new planets through
measurement of “wobbles” in
the paths of stars, but such
movements remain beyond the
range of even the strongest
telescopes today.

1992 Polish astronomer
Aleksander Wolszczan finds
the first confirmed extrasolar
planets in orbit around a pulsar
(a burned-out stellar core).

AFTER
2009–2013 NASA’s Kepler
satellite discovers more than
3,000 candidate exoplanets by
looking for minute drops in the
brightness of stars as planets
pass in front of them. Based
on Kepler data, astronomers
predict there could be as many
as 11 billion Earthlike worlds
orbiting Sunlike stars in the
Milky Way galaxy.

The radial velocity method relies on

detecting slight Doppler shifts (p.127)

in a star’s light frequency as it is pulled

back and forth in relation to Earth by

the gravity of an orbiting planet.

Exoplanet

Host star

Redshift as star recedes from Earth

Blueshift as star moves

toward Earth