Astronomy 1010-H Planetary Astronomy Fall_2015 Day-25

Course Announcements How is the sunset/sunrise observing going? SW-chapter 6 posted: due Today SW-chapter 7 posted: due Fri. Oct. 30

 Our Solar System is only a tiny part of the universe. There are many more like it.  Theories of its origin must explain its contents: planets, moons, asteroids, etc.

The Solar System

The Ecliptic April 17, 2002

The Planetary Orbits

The Inner Planets

The Outer Planets Jupiter Saturn Uranus Neptune

“Facts” that must be accounted for in any theory of solar system formation Almost all the planets orbit in the same plane All the planets orbit in the same direction Almost all the planets rotate in the same direction as they orbit The inner planets are rocky bodies while the outer planets are gaseous and/or icy bodies Almost 99% of the mass of the solar system is in the sun Most of the angular momentum of the solar system is in the planets

 Young stars are surrounded by rotating disks of gas and dust.  The infant Sun would also have been surrounded by such.  The rest of the Solar System formed from that rotating disk.

 Protostar: large, hot ball of gas; not a star yet.  Forms in a collapsing cloud of gas and dust.  Forms at the center, where it is densest.  When the right conditions are met, it becomes a star.

 The rest of the mass is the protoplanetary disk.  The planets and other objects in the Solar System will form from it.  The flattened disk is a result of angular momentum conservation.

 The cloud begins as a diffuse spherical collection of material.  Parts of it are going in the same direction.  The angular momentum of the system is conserved.  Result: a spinning sphere will become a flattened, rotating disk.

 The collapse is slowed perpendicular to the rotation axis, but not parallel to it!  It is easier for the parts along the rotation axis to fall in.  Most of the gas lands on an accretion disk, which continues the rotation.  Accretion = growth by infall.

 Angular momentum depends on the rotational speed of an object, its mass, and how its mass is distributed.  A spinning uniform sphere’s angular momentum:  The spinning angular momentum of a collapsing sphere in space stays constant.  As it collapses, it must speed up. Speed is inversely related to the rotational period. MATH TOOLS 7.1

 The idea of conservation of energy is a powerful one.  One type of energy can be converted into another.  Example: a hydroelectric plant using falling water to turn a turbine to generate electricity.  In the protoplanetary system, gravitational potential energy is turned into kinetic energy as an object falls closer to the center, and then thermal energy once it hits the other disk material. CONNECTIONS 7.1

 Within the disk, small particles will collide and stick.  Small particles are blown into larger ones by gas motions.  This leads to larger particles (~ 1 km in size) called planetesimals.

 Once they reach this size, planetesimals will pull more particles onto them by gravity, leading to planets.  Today’s remaining planetesimals: asteroids, comets.

 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.

 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 than the outer.

 The temperature difference between the inner and outer disks causes a difference in composition.  Inner disk: Only materials that do not melt at high temperatures can form or remain.

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