ASTR 121 (O'Connell) Study Guide

11. PLANETARY SYSTEMS:
OURS AND OTHERS


Comparison of the planets, based on NASA images.
Sizes are to scale, but separations are not.


The four large satellites of Jupiter discovered by Galileo in 1610 with his small telescope were the first new constituents of the solar system identified in recorded history. Since that time, astronomers have identified thousands of Solar System bodies (planets, satellites, smaller rocky and icy objects) with telescopes and spacecraft. Most remarkably, we have recently discovered planetary systems around other nearby stars.

This lecture describes the general properties of our planetary system and those around other stars and how we believe these originated.


A. INVENTORY OF THE SOLAR SYSTEM

By terrestrial standards, the density of matter in the Solar System is extremely low, and the planets are separated by enormous gaps. Other than the Sun, no solar system object is self-luminous at visible wavelengths, and all shine by reflected sunlight.

Contents of the Solar System:


Planetary Orbits


B. SYSTEMATICS OF PLANET ORBITS

Systematic characteristics of the orbits: It is important that NONE of the above is required by Newton's laws.

Instead, these systematics must be the product of special physical conditions prevailing during formation of planets. That is, they provide information on the formation process.

For a diagram of the current location of the planets, click here.


C. SEGREGATION OF PHYSICAL PROPERTIES

The four "inner planets" (Mercury, Venus, Earth Mars) show a striking dissimilarity from the four large "outer planets" (Jupiter, Saturn, Uranus, Neptune):

INNER (TERRESTRIAL) OUTER (JOVIAN)
Size & Mass** Small Large
Density Large Small
Composition Si,O,Al,Mg,Fe
Rocky		 H,He
Gas Giants
These differences constitute another major clue about the formation process of the planetary system.


D. ORIGIN OF THE SOLAR SYSTEM

Since the time of Galileo, there have been many models for the origin of the solar system. They all fall into two main categories:


E. THE INTERSTELLAR MEDIUM AND STAR FORMATION

We know that stars are forming continuously out of the interstellar medium at a rate of about 1 solar mass/year throughout our Galaxy:

Dust plays the important role of a refrigerant for interstellar gas. Parts of the ISM, if well shielded by dust grains, can become very dense and cold (only about 10o K). These are the regions which are primed to turn into nurseries for newly born stars.



A beautiful example of a likely stellar nursery is shown above. This is the "Eagle Nebula" imaged by the Hubble Space Telescope. The extended, dark, sculpted "elephant trunk" running across the image is a cold, dusty region. It is surrounded by hot gas (greenish-blue), which is evaporating the cold material away. The small globules on the end of the finger-like protuberances are the densest regions of the cloud, possibly containing protostars like the Sun. Click on the image for a full view. For more pictures and information, click here.


F. PLANET FORMATION IN THE NEBULAR THEORY

  1. A dense, cold cloud in the ISM collapses under gravity:


  2. As it collapses, it spins up & flattens because of the conservation of angular momentum (first illustrated by Kepler's Second Law).

      ===> A rotating, flattened "protoplanetary" disk is a natural consequence of star formation. The disk in the case of our solar system is called the "solar nebula."


    Note! The scale of this picture is much smaller, by several 1000x, than the scale of the previous picture.

  3. The dense concentration of material in the center of the disk is the "protosun."

      The protosun begins to heat up, first from energy released by gravitational collapse, later by nuclear reactions

  4. The protosun heats the inner protoplanetary disk to a higher temperature than the outer disk.

  5. The heating determines the kinds of solids which can survive in a given part of the disk and generates the segregation of planetary properties:

      Only "refractory" (high melting point) solids survive in the inner disk. These tend to be heavy, rocky materials. Only a small fraction of the total inner disk is in this form since heavy elements are not abundant.

      "Volatile" materials are those with low melting points. They include the ices of water, methane, and ammonia (H2O,CH4,NH3). These will not be found in solid form in the inner disk.

      On the other hand, these ices can exist in solid form in the cool outer disk. These are hydrogen-rich compounds. Because H is abundant, there is a large amount of such icy material in the outer disk.

      Solids in the outer nebula are more similar in chemical composition to the Sun than are inner nebula solids

  6. Larger bodies grow from the solids, not from gas, through collisions and sticking together (or "accretion").

      Accretion produces solid bodies with a range of sizes in the sequence grains ==> "planetesimals" ==> "protoplanets," where the distinction in size between the two latter classes is not firmly defined. A protoplanet is an object over about 500 km in diameter. For larger protoplanets (about 15x the mass of the Earth), gravitational fields begin to attract gas from the nebula.

      In the inner disk, small, rocky proto-planets form.

      In the outer disk, large, "gas-giant" proto-planets form.



    Computer simulation of protoplanetary disk


  7. Final assembly: violent infall of fragments, which heats the protoplanets. Collisions between protoplanets and large fragments can have drastic effects, producing extensive melting/resurfacing or even shattering them.

  8. Elapsed time is short by cosmic standards: probably ~10 million years (though is possible could occur much faster).

  9. This model explains the systematics in the orbital geometry, motions, and compositions listed in Sections B and C above.


Strong support for the nebular theory has emerged. We now have direct detections of what appear to be protoplanetary disks around nearby stars by the Hubble Space Telescope and infrared telescopes. An image of a young planetary disk is shown at the right (this star is in a stage where it is producing a gas jet perpendicular to the disk).


G. EXTRA-SOLAR PLANETS DETECTED!

After 500 years of speculation, extra-solar planets were first detected in October 1995. ("Extra-solar" means planetary systems other than that around the Sun.)

Methods

Properties

Earth-Like Planets?

Existing techniques are not sensitive enough to detect Earth-sized planets, but new approaches to do so are being aggressively pursued.

51 Peg b
Artist's concept of giant planet around
nearby star 51 Pegasi


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Last modified March 2008 by rwo

Drawings of stages in the nebular theory from ASTR 161, University of Tennessee. Computer simulation of protoplanetary disk by G. Bryden. Artwork of extra-solar planet from Extra Solar Visions, copyright © 1996 John Whatmough. Velocity curve of 51 Peg from G. Marcy & P. Butler. Text copyright © 1998-2008 Robert W. O'Connell. All rights reserved. These notes are intended for the private, noncommercial use of students enrolled in Astronomy 121 at the University of Virginia.