Introduction to Planetary Science

466 chapter 24
and to have a core that is capable of generating
a magnetic field. Therefore, in our solar system
fi = 1/9 = 01 (including Pluto). This criterion
reduces the number of habitable planets in the
Milky Way galaxy to 300.
What fraction of these habitable planets is
actually inhabited by intelligent beings who
have developed a technological civilization?
The answer to this question depends partly
on the ages of the planets and partly on the
favorable outcomes of random events in the
histories of these planets. In our solar system,
it took more than 3.5 billion years for unicellular
organisms to evolve into a technological
civilization. Similar amounts of time may be
required on other habitable planets that orbit
Sun-like stars. However, even if sufficient time
was available, the process may have been terminated
prematurely by catastrophic impacts of
large asteroids that killed all organisms and may
have destroyed the planet itself. Nevertheless,
we tentatively conclude that the destruction of
inhabited planets is a rare event and therefore
set fc = 09. By doing so, we say that 10% of
the habitable planets in the Milky Way galaxy
fail to be inhabited by technological civilizations
because they are destroyed by celestial accidents.
The only civilizations with whom we can
communicate must exist on planets whose age is
similar to that of the Earth. Planets that revolve
around stars that formed more recently than
the Sun may be inhabited, but the inhabitants
have not yet achieved the technology required
to communicate with us at the present time.
Similarly, civilizations on planets that revolve
around stars that are significantly older than our
Sun may have ceased to exist for a variety of
reasons and therefore are no longer broadcasting
signals at the present time.
These considerations lead to the final parameter
(fl) of the Drake equation that expresses the
fraction of the lifetime of a planet during which a
communicative civilization exists. Although our
civilization has been “communicative’ only for
about 100 years, we assume that such civilizations
on other planets may last for as long as
the dinosaurs dominated the Earth from the Early
Jurassic to the Late Cretaceous Epochs or about
140 × 10 6 yeas. Since the age of the Earth is
46 × 10 9 y, the parameter fl = 140 × 10 6 /46 ×
10 9 = 3 × 10 −2 .
Therefore, the number of technologically
advanced civilizations in the Milky Way galaxy
that may be able to communicate by broadcasting
signals that our SETI project could intercept is:
N =300 × 10 9 × 10 −8 × 1 × 1 × 01 × 09
× 3 × 10 −2 = 8
This result probably underestimates the number
of planets that are inhabited by technologically
advanced civilizations depending primarily on
the number of Sun-like stars and Earth-like
planets that exist in the Milky Way galaxy and on
how long such civilizations survive on average.
Therefore, we conclude that planets that harbor
civilizations of intelligent beings are rare, but we
are probably not alone in the Milky Way galaxy.
24.6 Summary
Close examination of stars in the Milky Way
galaxy primarily by Doppler spectroscopy has
revealed that at least 150 of them have unseen
companions. Many of these companions are
actually brown-dwarf stars with masses that
range from 12 to 75 times the mass of Jupiter.
Brown dwarfs are composed of hydrogen and
helium and generate energy in their cores only by
fusion of deuterium (heavy isotope of hydrogen)
to helium. Consequently, brown-dwarf stars have
low luminosities which make them difficult
to see with telescopes. The orbits of many
brown dwarfs are elliptical and their perihelion
distances are very close to their central stars.
The resulting tidal interactions may cause brown
dwarfs to be ejected from their orbits and to
become “floaters” in interstellar space. Surveys
of star-forming nebulae suggest that browndwarf
stars are about as numerous as red dwarfs
and other kinds of stars. Some brown-dwarf stars
may have companions of their own including
planets.
Some stars in the Milky Way galaxy have
unseen companions whose masses are less than
12 Jupiters. These bodies are classified as extrasolar
planets although even the smallest of these
“exoplanets” are much more massive than any of
the terrestrial planets of the solar system. Some
of the exoplanets have nearly-circular orbits
located at distances of less than 0.25 AU from