The Astronomy Book

292

The most successful method
was the transit method, which
looked for periodic changes in the
brightness of a star. These changes
were very small and happened
when a planet transited the star,
passing between the star and the
observer, and causing it to dim very
slightly. The best place to look for
exoplanets by the transit method
was out in space and so, in 2009,
the Kepler observatory, named
after the man who first described
planetary orbits (pp.50–55), was
launched to do just that.

Staring at one place
Kepler was placed in a heliocentric
orbit, trailing behind Earth as it
circled the sun. The craft was
designed to keep its aperture firmly
fixed on a single patch of space,
called the Kepler field. This made
up only about 0.25 percent of the
whole sky, but the spacecraft could
see 150,000 stars in that area. To
find exoplanets, Kelper would have
to concentrate on this single field
of view for years on end. It was

unable to see individual exoplanets,

but could identify stars that were

likely to have them.

Kepler could only detect the

transits of exoplanets with orbital

paths that crossed the spacecraft’s

line of sight. Many exoplanets would

be orbiting at the wrong angle for

EXOPLANETS

that. Those that were correctly

oriented would only transit their star

once every orbital period (the planet’s

year), so Kepler’s method was better

at finding planets that orbited close

to their star, taking a few years and

months (or even weeks and days)

to complete each revolution.

Candidate stars

By the start of 2013, Kepler had

identified about 4,300 candidate

stars that might have extrasolar

planetary systems. Unfortunately,

the guidance system used to keep

Kepler locked on target then failed,

bringing its planet hunt to an end

about three years sooner than

expected. However, the data it

had collected was enough to keep

researchers busy for years to come.

Kepler’s candidate stars could only

be confirmed as planetary systems

using radial velocity measurements

from ground-based observatories,

such as HARPS in Chile and the

Keck Telescope in Hawaii. (Radial

velocity is the velocity of the star in

the direction of Earth.) So far, about

a tenth of Kepler’s candidate stars

Most stars are

orbited by planets.

Exoplanets can be

detected by their effects

on their host star.

Many exoplanets have

been found by several

different techniques.

According to the Copernican principle, if the sun has

a planetary system, it is likely that other stars do as well.

Statistical analysis of

the data reveals how

common exoplanets are.

The Kepler observatory

looked outward from the plane

of the ecliptic, so that Earth,

the moon, and the sun

did not obscure the view.

Photometer

Solar array

High gain

antenna

Solid state

recorder

Star

trackers

Radiator

Avionics

Ssun shade