CHAPTER 19 | THE ORIGIN OF THE SOLAR SYSTEM 419
planets are harder to detect. Low-mass planets don’t tug on their
stars very much, and present-day spectrographs can’t detect the
very small velocity changes that these gentle tugs produce.
Planets with longer periods are harder to detect because astrono-
mers have not been making high-precision observations for a
long enough time. Jupiter takes 11 years to circle the sun once,
so it will take years for astronomers to see the longer-period
wobbles produced by planets lying farther from their stars. You
should not be surprised that the fi rst planets discovered are mas-
sive and have short orbital periods.
Th e new planets may seem odd for another reason. As you
have learned for our solar system, the large planets formed far-
ther from the sun where the solar nebula was colder and ices
could condense. How could big planets near their stars (called
“hot Jupiters”) have formed? Th eoretical calculations indicate
that planets forming in an especially dense disk of matter could
spiral inward as they sweep up gas and planetesimals. Th at means
it is possible for a few planets to become the massive, short-pe-
riod planets that are detected most easily.
Many of the newly discovered extrasolar planets have
elliptical orbits, and that seems odd when compared with our
solar system, in which the planetary orbits are nearly circular.
Th eorists point out, however, that planets in some young
planetary systems can interact with each other and can be
thrown into elliptical orbits. Th is eff ect is probably rare in
planetary systems, but astronomers fi nd these extreme systems
more easily because they tend to produce easily detected
wobbles.
Th e preceding paragraphs should reassure you that massive
planets in small or elliptical orbits do not fundamentally
contradict the solar nebula theory. In fact, as astronomers con-
tinue to refi ne their instruments to detect smaller velocity
shifts in stars, they fi nd lower-mass planets. A planet with only
about 5 times the mass of Earth, less than^1 __ 3 the mass of Uranus
or Neptune, was found by the Doppler shift technique in- Th is small extrasolar planet orbits its red dwarf parent
star in only 13 Earth-days at a distance of 0.07 AU. With a
diameter estimated to be 1.5 times larger than Earth and a
What does it mean to be skeptical yet also
open to new ideas? “Scientists are just a bunch
of skeptics who don’t believe in anything.” That
is a Common Misconception among
people who don’t understand the methods and
goals of science. Yes, scientists are skeptical
about new ideas and discoveries, but they do
hold strong beliefs about how nature works.
Scientists are skeptical not because they want
to disprove everything but because they are
searching for the truth and want to be sure that
a new description of nature is reliable before it
is accepted.
Another Common Misconception
is that scientists automatically accept the work of
other scientists. On the contrary, scientists skep-
tically question every aspect of a new discovery.
They may wonder if another scientist’s instru-
ments were properly calibrated or whether the
scientist’s mathematical models are correct. Other
scientists will want to repeat the work themselves
using their own instruments to see if they can
obtain the same results. Every observation is
tested, every discovery is confi rmed, and only an
idea that survives many of these tests begins to
be accepted as a scientifi c truth.
Scientists are prepared for this kind of treat-
ment at the hands of other scientists. In fact,
they expect it. Among scientists it is not bad
manners to say, “Really, how do you know that?”
or “Why do you think that?” or “Show me the
evidence!” And it is not just new or surprising
claims that are subject to such scrutiny. Even
though astronomers had long expected to dis-
cover planets orbiting other stars, when a planet
was fi nally discovered circling 51 Pegasi, astron-
omers were skeptical. This was not because they
thought the observations were necessarily fl awed
but because that is how science works.
The goal of science is to tell stories about
nature. Some people use the phrase “telling a
story” to describe someone who is telling a fi b.
But the stories that scientists tell are exactly
the opposite; perhaps you could call them
“antifi bs” because they are as true as scientists
can make them. Skepticism eliminates stories
with logical errors, fl awed observations, or
misunderstood evidence and eventually leaves
only the stories that best describe nature.
Skepticism is not a refusal to hold beliefs.
Rather, it is a way for scientists to fi nd and
keep those natural principles that are worthy of
acceptance.A laboratory cell for the investigation of cold
fusion claims that were thereby found to be false.
(Photo by Steven Krivit of a device at the U.S.
Navy SPAWAR Systems Center in San Diego)Scientists: Courteous Skeptics