CHAPTER 19 | THE ORIGIN OF THE SOLAR SYSTEM 417
dog on a leash; the dog runs around pulling on the leash, and
even if it were an invisible dog, you could plot its path by watch-
ing how its owner was jerked back and forth. Astronomers can
detect a planet orbiting another star by watching how the star
moves as the planet tugs on it.
Th e fi rst planet detected this way was discovered in 1995
orbiting the sun-like star 51 Pegasi. As the planet circles the star,
the star wobbles slightly, and that very small motion of the star
is detectable by Doppler shifts in the star’s spectrum
(■ Figure 19-16a) (see Chapters 7 and 9). From the motion of
the star and estimates of the star’s mass, astronomers can deduce
that the planet has at least half the mass of Jupiter and orbits
only 0.05 AU from the star. Half the mass of Jupiter amounts to
160 Earth masses, so this is a large planet. Note also that it
orbits very close to its star, much closer than Mercury orbits
around our sun.
Astronomers were not surprised by the announcement that
a planet had been found orbiting a sun-like star; for years astron-
omers had assumed that many stars had planets. Nevertheless,
they greeted the details of this discovery with professional skepti-
cism (How Do We Know? 19-2). Th at skepticism led to
careful tests of the data and further observations that confi rmedObserving Extrasolar Planets
A planet orbiting another star is called an extrasolar planet.
Such a planet would be quite faint and diffi cult to see so close to
the glare of its star. But there are ways to fi nd these planets. To
understand one important way, all you have to do is imagine
walking a dog.
You will remember that Earth and the moon orbit around
their common center of mass, and two stars in a binary system
orbit around their center of mass. When a planet orbits a star, the
star moves very slightly as it orbits the center of mass of the
planet–star system. Th ink of someone walking a poorly trained
Inner Solar SystemInner Epsilon Eridani
SystemMars JupiterEpsilon Eridani bAsteroid beltKuiper beltComet beltInner asteroid beltEarthJupiterSaturnUranusNeptuneSolar SystemEpsilon Eridani System
Epsilon Eridani bProposed planetProposed planetAsteroid beltInner asteroid beltOuter asteroid belt■ Figure 19-15
Dust in debris belts around older main-sequence stars indicates ongoing col-
lisions of asteroids and comets. Such activity in our solar system is ultimately
driven by the gravitational infl uence of planets. The locations of debris belt
edges may be defi ned by orbits of adjacent planets. The inferred architecture
of the Epsilon Eridani planetary system is shown in comparison with our solar
system. (Artist’s conception based on Spitzer Space Telescope and Caltech Submillimeter
Observatory observations by D. Backman et al.; NASA/JPL-Caltech/R. Hurt)