ASTR 1210 (O'Connell) Study Guide


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 members of the Solar System identified in recorded history. They instantly increased the known membership of the Solar System from 8 to 12. Since that time, astronomers have identified tens of thousands of Solar System bodies (planets, satellites, and smaller rocky or icy objects) with telescopes and spacecraft.

Most remarkably, after thousands of years of speculation about other worlds like ours in the universe, we have recently discovered planets in orbit around other stars---exoplanets.

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. Seen from the Earth, the second and third-brightest celestial objects are Earth's Moon and Venus.

Contents of the Solar System:

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

Planetary Orbits

An oblique view of the planetary orbits drawn to scale
(though the planet sizes shown are not to scale).

B. Systematics of Planet Orbits

Systematic characteristics of the orbits of "classical" planets: It is important to understand 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 the planets. That is, they provide clues to the process that forms planets.

C. Segregation of Physical Properties

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

Size & Mass** Small Large
Density Large Small
Composition Si,O,Al,Mg,Fe
Gas Giants
These differences constitute another major clue about the processes that formed 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 essential role of a refrigerant for interstellar gas. Parts of the ISM, if well shielded by dust grains against heating by surrounding stars, can become very dense and cold (only about 10o K above absolute zero temperature). 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 in the picture below:

Eagle Nebula
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 with masses 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).

    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 "protostar" ("protosun" in the illustration here).

  4. The protostar heats the inner protoplanetary disk to a higher temperature than the outer disk. Here is a typical temperature profile for the 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:

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

    Computer simulation of protoplanetary disk

  7. Final assembly: the violent infall of fragments heats the protoplanets. Collisions between protoplanets and large fragments can have drastic effects, producing extensive melting/resurfacing in a merged protoplanet or even shattering the originals into smaller pieces.

  8. The elapsed time for proto-planet formation is short by cosmic standards: probably a few million years (though under the right conditions it could occur much faster).

  9. The interiors of proto-planets that are large enough are heated and partially melted by the violent accretion and by the decay of short-lived radioactive isotopes. The melted interiors will differentiate, with heavy metallic materials settling to the center and lighter, rocky materials rising to the exterior.

  10. Gravitational interactions between planets, or between planets and the residual protoplanetary disk (as in the image above), can drastically change the orbits of the new planets, moving large planets inward, tossing small bodies outward, or pushing planets into more strongly elliptical orbits.

  11. Here is a pictorial summary of the nebular model.

The nebular model successfully 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 protoplanetary disks around nearby stars by the Hubble Space Telescope, infrared telescopes, and the ALMA millimeter-wave array, operated by the National Radio Astronomy Observatory here in Charlottesville. A high priority goal for the infrared James Webb Space Telescope, to be launched in 2018, is to probe in detail the physics of star and planet formation. An HST 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). Yet stronger evidence, based on the expectation in the nebular theory that planets will be common, is found in the next section.

G. Exoplanets

Speculation about planets around other stars extends as far back in history as the ancient Greeks. The philosophical implications of discovering other planetary systems for the context in which we should view the Earth and the human race have been widely discussed. But for hundreds of years, despite much effort, no planetary companions to other stars were found.

Finally, extra-solar planets around other Sun-like stars were detected in October 1995. ("Extra-solar" means planetary systems other than that around the Sun, now usually abbreveviated simply to "exoplanets".) We have not only detected exoplanets, but we have established that they are relatively common around Sun-like stars in the Galaxy. And, using special techniques and highly sensitive detectors, we have begun to probe the composition and structure---even the meteorology---of some exoplanets.

Detection Methods

The initial detections of exoplanets were technically very difficult (or they would have been found sooner!).

We cannot simply take a picture and see the planet. The images of the star and planet are blended together in an ordinary telescope (see the discussion of telescope resolution in Study Guide 14), and the feeble reflected light of the planet is completely overwhelmed by the bright star.

Instead, several sophisticated methods have been been developed for finding exoplanets (see this article). We discuss below only the two most widely used of those. The "Doppler method" is particularly important because it is the primary means by which we can obtain estimates of an exoplanet's mass.

Both of these techniques are biased in the sense they are much more sensitive to larger planets at small distances (say less than 1 AU) from a star.

The Kepler Mission

Exoplanet Count


How Many Planets in the Galaxy?

Exoplanet Art

The discovery of exoplanets inspired an explosion of artists' impressions of exoplanet systems and surfaces. Some nice ones are shown in the composite below. Bear in mind that no telescope is powerful enough to produce images like this, so they are all entirely speculative. Almost all contain some unphysical features (like showing the shape of other planets in the parent system).

Exoplanet Art

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Last modified June 2017 by rwo

Drawings of stages in the nebular theory from ASTR 161, University of Tennessee. Computer simulation of protoplanetary disk by G. Bryden. Velocity curve of 51 Peg from G. Marcy & P. Butler. Text copyright © 1998-2017 Robert W. O'Connell. All rights reserved. These notes are intended for the private, noncommercial use of students enrolled in Astronomy 1210 at the University of Virginia.