WWW.ASTRONOMY.COM 55changes in starlight over time
is a fundamental skill that
amateur astronomers inter-
ested in doing science should
learn. I’ll describe how you
can do these measurements
and discuss some recent tech-
nology that makes it easy to
observe and record exoplanet
transits with a minimum
investment in equipment,
time, and skills.
Discoveries
Early exoplanet discover-
ies were made with large
telescopes using the radial-
velocity method. This tech-
nique employs the Doppler
shift to detect the “wobble”
of stars caused by large,
Jupiter-sized planets tugging
on them. With this method,
professional astronomers
measured the shift of the
absorption lines in the parent
star’s spectrum.
In the early 2000s, astrono-
mers began observing exoplan-
ets using another technique,
called the transit method.
With it, professional astrono-
mers discovered several exo-
planets by watching them pass
in front of their parent stars
(see “Catch a planet” on page
54). The transit method, virtu-
ally the same as observing a
Venus transit from Earth, is
the primary technique ama-
teurs now use to detect and
measure the telltale dip in a
star’s brightness that indicates
the presence of an exoplanet.Photometry
Amateurs have been doing
photometry of variable stars,
eclipsing binaries, and aster-
oids for several decades usingtheir backyard observato-
ries. Early pioneers, such as
Douglas Hall, Russ Genet,
and Mark Trueblood, used
personal computers and
photoelectric photometers
to obtain precise brightness
measurements of variable
stars. Since then, the standard
of precision for observing
these objects, which vary
in brightness by up to a few
magnitudes, has been 1 per-
cent, or about 0.01 magnitude.
Standard practice is to use
the differential photometry
method — measuring the dif-
ference in brightness between
a variable star and a compari-
son star to construct a light
curve of the variable. A dif-
ferential measurement is
required to remove changes
in brightness common to the
comparison and target stars.This precision is still the
standard when doing photo-
metric measurements. The
fundamental difference,
though, between variable stars
and exoplanet transits is the
amount of brightness change
of these objects. For typical
variable stars, that change can
range from 0.5 to 3 magni-
tudes, a significant amount.
In contrast, exoplanet transits
typically cause the light to dip
only 1 or 2 percent, or about
0.01 to 0.02 magnitude. As
you may suspect, this mea-
surement is difficult if the
error is the same or more
than the expected dip in
brightness.
“High-precision photom-
etry” refers to a total error of
a measurement less than 0.5
percent (0.005 magnitude).
To get this result, I first use aThis artist’s illustration shows Dimidium (51 Pegasi b), the first exoplanet discovered that orbited a star similar to the Sun. It was discovered October 6, 1995, orbiting
Helvetios (51 Peg), a magnitude 5.5 star slightly more than 50 light-years away in the constellation Pegasus the Winged Horse. NASA/JPL-CALTECH