21st CENTURY ASTRONOMY

THIRD EDITIONHester | Smith | Blumenthal | Kay | Voss

Chapter 6Lecture Outline

The Birth and Evolution of Planetary Systems

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Stars Form and Planets Are Born

• Our Solar System is only a tiny part of the universe• Theories of its origin must explain the contents:

planets, moons, asteroids, etc.

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Stars Form and Planets are Born

• Young stars are surrounded by rotating disks of gas and dust.

• The Sun should also have been surrounded.

• The rest of the Solar System formed from that rotating disk

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Protostellar Sun

• Protostar: large, hot ball of gas; not a star yet• Forms in a collapsing cloud of gas and dust• Star will form at the very center, where it is densest

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Protoplanetary Disk

• The rest of the mass is the protoplanetary disk.• The planets and other objects in the Solar System

will form from it.• Flattened disk is due to angular momentum.

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Angular Momentum

• The angular momentum of a system will be conserved.

• Result: a spinning sphere will become a flattened, rotating disk.

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Angular Momentum

• The collapse is slowed perpendicular to the rotation axis, but not parallel to it!

• Most of the gas lands on an accretion disk.

• Accretion = growth by infall.

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Growth of Particles

• Within the disk, small particles will collide and stick.

• Small particles are blown into larger ones by gas motions

• This leads to larger particles, about 1 km, called planetesimals.

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Growth of Particles

• Once they reach this size, planetesimals will pull more particles onto them by gravity, leading to planets.

• Today’s remaining planetesimals: asteroids, comets.

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The Inner Disk Is Hot

• The gravitational energy of the infalling material is converted into heat.

• Material that lands on the inner part of the disk has fallen farther and has more energy to convert into heat.

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The Inner Disk Is Hot

• Particles in the outer disk do not have as far to fall

• Also, the protostar at the center is contracting and heating up.

• This also heats the inner disk more.

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The Inner Disk

• The temperature difference between the inner and outer disks causes a difference in composition.

• In the inner disk, only materials that do not melt at high temperatures can form or remain.

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The Inner Disk

• Refractory = does not melt at high temperature.• The outer disk has volatile materials like ices.• Volatile = can melt or evaporate at moderate

temperatures.

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Gathering Atmospheres

• Planets can gather gasses from the disk.• This makes the primary atmosphere.• Low-mass planets cannot hold on to their primary

atmospheres• Some low-mass planets later emit gasses from their

interiors (e.g., from volcanoes), producing a secondary atmosphere.

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Loss of Atmospheres

• Low-mass planets would lose their primary atmospheres.

• Recall that temperature measures the speed of motion of gas atoms.

• Low-mass planets have low escape velocities.• Some gas atoms can go fast enough to escape

small (or hot) planets.

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Our Solar System

• The 4 inner planets are rocky.• The 4 outer planets are gaseous giants.• Asteroids and comets are leftover planetesimals, while

moons formed from the giant planets’ accretion disks.

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Our Solar System Is Not Special

• The physical processes that led to the Solar System should be commonplace.

• We can see young stars with disks.• Planet: a body that orbits a star and has a mass

less than 13 Jupiters.

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Extrasolar Planets

• We have found more than 330 extrasolar planets, or “exoplanets.”

• Four main techniques to find these planets:• Spectroscopic radial velocity method• Transit method• Microlensing method• Direct imaging

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Spectroscopic Radial Velocity

• Gravity is a mutual force, so both stars and planets orbit one another.

• Motion can be detected by Doppler shifts.• Some stars have periodic velocity changes and

therefore they are orbited by planets.

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Transit and Microlensing

• A planet passing in front of a star (transiting) can decrease the total brightness of the star.

• Microlensing makes a star temporarily brighter through a planet’s gravity focusing its light.

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Direct Imaging

• It is very difficult to directly see a faint planet in the bright glow of its star, but it can be done.

• 11 planets have been identified this way so far.

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Known Planetary Systems

• Most planetary systems we have found do not resemble ours.

• Many known planets have 1 to 10 times the mass of Jupiter.

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Known Planetary Systems

• Some of these orbit close to their stars and are called hot Jupiters.

• It is easier to find these very large planets due to the greater “wobble” they cause for their stars.

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Known Planetary Systems

• These hot Jupiters most likely formed far from their stars, and then migrated inward.

• These discoveries help us realize that we are just a part of the vast universe.

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Concept Quiz Hot Protostars

We know that stars have different temperatures. Consider a newly forming star that was much hotter than the proto-Sun. What would we expect about its planets?

A. The planets orbit at random angles around the star.

B. Rocky planets might be formed over a wider range of distances than in our Solar System.

C. The star would be “naked,” without a surrounding disk.

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Concept Quiz Other Planets

Which of these is not a reason why we can find planets around other stars?

A. The planet’s gravity causes the star to orbit.

B. We can take images and directly see the planets.

C. We can detect radio signals from life on the planets.

D. A star’s light could be affected by its planet.

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Concept Quiz Protoplanets

Which of the following statements is false?

A. Planetary systems begin as a disk of material around a protostar.

B. Planetesimals accrete material to become planets.C. All the planetesimals in our Solar System have become

planets.

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Solar System Formation

Click the above picture to launch the animation

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Visual Analogy: Traffic Circle

Click the above picture to launch the animation

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This concludes the Lecture PowerPoint presentation for Chapter 6

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For more learning resources, please visit the StudySpace website for 21st Century Astronomy athttp://wwnorton.com/studyspace

21st CENTURY ASTRONOMY

THIRD EDITIONHester | Smith | Blumenthal | Kay | Voss