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Chapter 10

AY 101 Chapter Notes - Chapter 10: Orbital Period, Planetary Migration, Methods Of Detecting Exoplanets

Course Code
AY 101
Raymond White

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The Big Picture
In this chapter, we have explored one of the newest areas of astronomy-the study of planetary systems
beyond our own. As you continue your studies, please keep in mind the following important ideas:
In a period of barely two decades, we have gone from knowing of no other planets around other stars to
knowing that many or most stars have one or more planets. As a result, there is no longer any question
that planets are common in the universe.
The discovery of other planetary systems represents a striking confirmation of a key prediction of the
nebular theory of other planetary systems represents a striking confirmation of a key prediction of the
nebular theory of solar system formation. Nevertheless, the precise characteristics of other planets and
planetary systems pose challenges to details of the theory that scientists are still investigating.
While we have already identified thousands of extrasolar planets or candidate planets, nearly all of these
have been found with indirect methods. These methods allow us to determine many properties of the
planets, but we will need direct images or spectra to learn about them in much more detail.
It is too soon to know if planetary systems with layouts like ours are rare or common, but technology
already exists that could allow us to answer this question and to learn if Earth-like planets are also
common. It is only a matter of time until we know the answer to these fundamental questions.
Summary of Key Concepts
How do we detect planets around other stars?
We can look for a planet’s gravitational effect on its star through the astrometric method, which looks
for small shifts that reveal the back-and-forth motion of stars. For the small fraction of planetary
systems with orbits aligned edge-on to Earth, we can search for transits, in which a planet blocks a
little of its star light as it passes in front of it.
What properties of extrasolar planets can we measure?
All detection methods allow us to determine a planet’s orbital period and distance from its star. The
astrometric and Doppler methods can provide masses (or minimum masses), while the transit method
can provide sizes. In cases where transit and Doppler methods are used together, we can determine
average density. In some cases, transits (and eclipses) can provide other data, including limited data
about atmospheric composition and temperature.
How do extrasolar planets compare with planets in our solar system?
The known extrasolar planets have a much wider range of properties than the planets in our solar
system. Many orbit much closer to their stars and with more eccentric orbital paths: some extrasolar
jovian planets, called hot Jupiters, are also found close to their stars. We have also observed properties
indicating planetary types, such as water worlds, that do not fall neatly into the traditional terrestrial and
jovian categories.
Do we need to modify our theory of solar system formation?
Our basic theory seems sound, but we have had to modify it to allow for planetary migration and a
wider range of planetary types than we find in our solar system. Many mysteries remain, but they are
unlikely to require major change to the nebular theory of solar system formation.
Are planetary systems like ours common?
Current evidence indicates that planetary systems are very common, though we do not yet have enough
data to know for sure whether systems with layouts like ours-and possibly Earth-like planets-are also
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