418 PART 4^ |^ THE SOLAR SYSTEM
amount of infrared radiation from each system varies. When the
planets pass behind their parent stars, the total infrared bright-
ness of the systems noticeably decreases. Th ese measurements
confi rm the existence of the planets and indicate their tempera-
tures and sizes.
Notice how the techniques used to detect these planets
resemble techniques used to study binary stars (see Chapter 9).
Most of the planets were discovered using the same observational
methods used to study spectroscopic binaries, but some were
found by observing the stars as if they were eclipsing binaries. In
contrast, a few extrasolar planets have been found by a technique
called microlensing, in which an extrasolar planet passes pre-
cisely between Earth and a background star, briefl y magnifying
the distant star’s brightness by gravitational lensing.
Th e planets discovered so far tend to be massive and have
short orbital periods because lower-mass planets or longer-periodthe discovery. In fact, almost 400 planets have been discovered in
this way, including at least three planets orbiting the star Upsilon
Andromedae (Figure 19-16b) and fi ve orbiting 55 Cancri—true
planetary systems. Over 25 such multiple-planet systems have
been found.
Another way to search for planets is to look for changes in
the brightness of the star when the orbiting planet crosses in
front of the star, called a transit. Th e decrease in light during a
planet transit is very small, but it is detectable, and astronomers
have used this technique to fi nd many planets. From the amount
of light lost, astronomers can tell that these planets with Jovian
masses have Jovian diameters, and thus Jovian densities and
compositions.
Th e Spitzer Space Telescope has detected infrared radiation
from two large hot planets already known from star wobble
Doppler shifts. As these planets orbit their parent stars, the
51 PegasiUpsilon Andromedae5–50–100–150–100–50050100150050100101992 1994 1996 1998 200015 20
Time (days)Time (yr)Velocity (m/s)Velocity (m/s)25 30
ab■ Figure 19-16
Just as someone walking a lively dog is tugged around, the star 51 Pegasi is pulled back and forth by the gravity of the planet
that orbits it every 4.2 days. The wobble is detectable in precision observations of the star’s Doppler shift. Someone walking
three dogs is pulled about in a more complicated pattern, and you can see something similar in the Doppler shifts of Upsilon
Andromedae, which is orbited by three planets detected so far. The infl uence of its shortest-period planet has been removed in
this graph to reveal the orbital infl uences of the other two planets.