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Lecture Outlines
Chapter 15
Astronomy Today
8th Edition
Chaisson/McMillan
© 2014 Pearson Education, Inc.
Chapter 15
Exoplanets
© 2014 Pearson Education, Inc.
Units of Chapter 15
15.1
Modeling Planet Formation
15.2
Solar System Regularities and Irregularities
15.3
Searching for Extrasolar Planets
15.4
Exoplanet Properties
Discovery 15-1 The Closest Exoplanet
15.5
Is Our Solar System Unusual?
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15.1 Modeling Planet
Formation
We now have many
planetary systems other
than our own to test
formation theories – 900
confirmed exoplanets and
2500 candidates as of mid2013
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15.1 Modeling Planet
Formation
Review of condensation theory:
• Large interstellar cloud of gas
and dust starts to contract,
heating as it does so
• Sun forms in center; dust
provides condensation nuclei,
around which planets form
• As planets grow, they sweep
up smaller debris near them
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15.2 Solar System Regularities
and Irregularities
Condensation theory covers the 10 points mentioned at
the beginning.
What about the exceptions?
1. Two large bodies may have merged to form Venus.
2. Earth–Moon system may have formed after a collision.
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15.2 Solar System Regularities
and Irregularities (cont.)
3. Late collision may have caused Mars’s north–south
asymmetry and stripped most of its atmosphere.
4. Uranus’s tilted axis may be the result of a glancing collision.
5. Miranda may have been almost destroyed in a collision.
6. Interactions between jovian protoplanets and planetesimals
could be responsible for irregular moons.
7. Binary Kuiper belt objects (including the Pluto-Charon
system) could have formed through collisions before ejection
by interactions with the jovian planets.
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15.2 Solar System Regularities
and Irregularities (cont.)
Many of these explanations have one thing in
common—a catastrophic, or near-catastrophic,
collision at a critical time during formation.
Normally, one does not like to explain things by
calling on one-time events, but it is clear that the
early solar system involved almost constant
collisions. Some must have been exceptionally large.
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15.3 Searching for Extrasolar
Planets
Most extrasolar planets have been discovered
indirectly, through their gravitational or optical effects,
and they cannot be seen directly due to the glare of
their star. However, a few dozen exoplanets have
indeed been detected this way.
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15.3 Searching for Extrasolar
Planets
Many planets around other stars have been detected
because they are large enough to cause the star to “wobble”
as the planet and star orbit around their common center of
mass.
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15.3 Searching for Extrasolar
Planets
If the “wobble” is transverse
to our line of sight, it can
also be detected through the
Doppler shift as the star's
motion changes.
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15.3 Searching for Extrasolar
Planets
An extrasolar planet may
also be detected if its orbit
lies in the plane of the line
of sight to us. The planet
will then eclipse the star,
and if the planet is large
enough, some decrease in
luminosity may be
observed.
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15.4 Exoplanet Properties
More than 900 extrasolar planets have been
discovered so far, with about 2700 more
candidates waiting to be confirmed:
• Most are in the “cold Jupiter” or “cold Neptune”
category due to size and distance from parent star
• Orbits are generally somewhat smaller than the
orbit of Jupiter
• Most orbits have high eccentricity
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15.4 Exoplanet Properties
The upper plot shows masses
and orbital semimajor axes for
hundreds of knows extrasolar
planets, with Jupiter, Neptune,
and Earth for comparison.
The lower shows planetary radii
and orbital semimajor axes for
thousands of exoplanet
candidates.
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15.4 Exoplanet Properties
Orbits of many of the known extrasolar planets. Note that
some of them are very close to their star:
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15.4 Exoplanet Properties
Planets orbiting within 0.15 AU of their stars are called “hot
Jupiters”; they are not included in the previous figure but are
numerous.
Stars with composition like our Sun are much more likely to
have planets, showing that the “dusty disk” theory is
plausible.
Some of these “planets” may actually be brown dwarfs, but
probably not many.
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Discovery 15-1 The Closest
Exoplanet
Recently, a planet with a mass close to the mass of the
Earth has been discovered orbiting our closest neighbor
star system, Alpha Centauri. But it is about 25 times closer
to its parent stars (Alpha Centauri A and B) than the Earth is
to our Sun. The figures below show the Alpha Centauri
system (left) and an artist’s conception of this planet (right).
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15.4 Exoplanet Properties
This figure shows two transiting super-Earths plus the nine
“habitable” exo-Earths (as of mid-2013), compared to Earth
and Neptune.
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15.5 Is Our Solar System
Unusual?
The other planetary systems discovered so far
appear to be very different from our own.
Selection effect biases sample toward massive
planets orbiting close to parent star; lower-mass
planets cannot be detected this way.
© 2014 Pearson Education, Inc.
15.5 Is Our Solar System
Unusual?
This is an example of a “cold Jupiter” in another system. Its
orbit is very similar to that of Jupiter’s (blue). Also included is
an artist’s conception of such a planet.
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15.5 Is Our Solar System
Unusual?
Current theories include the
possibility that Jupiter-like
planets could migrate
inward, through friction with
the solar nebula
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15.5 Is Our Solar System
Unusual?
A number of Earthlike planets have now been observed,
although due to detection difficulties most exoplanets still fall
into the “hot Jupiter” category, making other planetary
systems look quite different from our own.
Until we are able to observe much smaller planets at much
larger distances from their parent stars, we will not know just
how unusual our own system is – or if it is unusual at all.
© 2014 Pearson Education, Inc.
15.5 Is Our Solar System
Unusual?
This figure shows the size of
the habitable zone – where
there is a possibility of liquid
water being present – as a
function of the mass of the
parent star. Note that the
presence or absence of a
greenhouse effect (runaway or
otherwise) can affect the
surface temperature of a planet
considerably.
© 2014 Pearson Education, Inc.
Summary of Chapter 15
• Condensation theory leads us to expect that other systems
will be coplanar with planets orbiting in the same sense.
• Random collisions will lead to irregular properties.
• Most extrasolar planets have been discovered through
wobbling of parent stars, or through transits.
• There are 900 planets and 2500 candidates so far.
• 20% of systems, and 33% of candidates, have multiple
planets per system.
• 20 Earths and super-Earths are in habitable zones.
• We don’t yet have enough information to tell how unusual
our own system is.
© 2014 Pearson Education, Inc.