6.5 Other Planetary Systems

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6.5 Other Planetary Systems

• Our Goals for Learning
• How do we detect planets around other stars?
• What have other planetary systems taught us
  about our own?

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How do we detect planets around other stars?
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We detect planets around other stars by looking
for a periodic motion of the stars they
orbit. We measure the motion through the
Doppler shift of the stars spectrum
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The size of the wobble tells us the planets
mass The period of the wobble tells us the
radius of its orbit (Keplers 3rd law)
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We can also detect planets if they eclipse their
star Fraction of starlight blocked tells us
planets size
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What have other planetary systems taught us about
our own?
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Orbits of known extrasolar planets compared to
the orbit of Jupiter
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Over 300 known extrasolar planets as of early
2009 Most are more massive than Jupiter and
closer to their star than Earth is to
Sun Revisions to the nebular theory are
necessary! Planets can apparently migrate
inward from their birthplaces.
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Is Earth Unusual?

• Data arent sensitive enough yet to tell if
  planets like Earth are common or rare
• Available methods can only detect big planets (2
  or more Earth masses usually much more than 2)
• CoRoT or Kepler satellites might detect
  Earth-mass planets soon

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What have we learned?

• How do we detect planets around other stars?
• So far, we are only able to detect extrasolar
  planets indirectly by observing the planets
  effects on the star it orbits. Most discoveries
  to date have been made with the Doppler
  technique, in which Doppler shifts reveal the
  gravitational tug of a planet (or more than one
  planet) on a star.

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What have we learned?

• What have other planetary systems taught us
  about our own?
• Planetary systems exhibit a surprising range of
  layouts, suggesting that jovian planets sometimes
  migrate inward from where they are born. This
  lesson has taught us that despite the successes
  of the nebular theory, it remains incomplete.

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Suppose you found an isolated star with the same
mass as the Sun moving back and forth with a
period of 16 months. What could you conclude?

1. It has a planet orbiting at less than 1 AU
   distance.
2. It has a planet orbiting at greater than 1 AU
   distance.
3. It has a planet orbiting at exactly 1 AU
   distance.
4. It has a planet, but we dont have enough info to
   know at what orbital distance.

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Activity 24

• Pages 87-89

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1A. Which orbit belongs to the star, and which to
the planet?

1. Small orbit star, big orbit planet
2. Small orbit planet, big orbit star

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1B. At the same instant, the star and the planet

1. Are on the SAME side of the center of mass (both
   to the left, or both to the right)
2. Are on OPPOSITE sides of the center of mass (one
   on the left and one on the right)

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1C 1D

1. The planet moves faster, so it takes less time to
   complete an orbit
2. The star moves slower, but has less distance to
   go, so it takes less time to complete an orbit
3. Neither of the above

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2A. What is the peak radial velocity of the star
51 Peg (Figure 2)?

1. 66 meters/second
2. 55 meters/second
3. -55 meters/second
4. -62 meters/second

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2B. What is the largest radial velocity of the
planet?

1. 140 km/second
2. 140 meters/second
3. -140 km/second
4. -140 meters/second