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 hundreds 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, astronomers 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.

Contents of the Solar System:

For a diagram of the current location of the planets in their orbits, 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 the terrestrial and Jovian 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 per year throughout our Galaxy:

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

Star formation out of the interstellar medium is an amazing process. The compression of the dilute ISM into the body of a star occurs because of the combined effects of gas cooling, which reduces the thermal pressure that keeps clouds inflated, and relentless gravitational attraction. The compression in volume is an astonishing factor of about 1024. The mass that ultimately resides in a star like the Sun originally occupied a volume about 10 light years on a side, a factor of 100 million larger across.

We now have a good, general understanding of how stars and planets form. The detailed physics and chemistry of the processes involved are still being worked out, but progress over the next decade or so should be rapid because of powerful new observational facilities like the Atacama Large Millimeter Array (ALMA, a radio telescope, which can detect molecules and dust grains in planetary systems) and the James Webb Space Telescope designed to attack these specific problems. ALMA is operated by the National Radio Astronomy Observatory here in Charlottesville. JWST, the successor to the Hubble Space Telescope, was launched in 2021.

Here is the sequence of steps in the formation of stars and their planetary systems:

  1. Gas in an interstellar cloud cools down and slowly compresses. Dust grains shield the interior and allow it to fall to very low temperatures. After a certain point, a dense, cold cloud in the ISM will start to collapse faster under its own self-gravity:

  2. As it collapses, the cloud 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 the collapse of the interstellar cloud and is expected to accompany star formation in all enviroments. In the case of our solar system the disk is called the "solar nebula." In the densest parts of the disk, dust grains grow in size and begin to accumulate into larger solids.

    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).

      The protostar heats up, first from energy released by gravitational collapse, and later, once its core reaches a critical density and pressure, by nuclear reactions. The generated heat counterbalances the self-gravity of the protostar and prevents it from collapsing further.

  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. Because material in the disk is so dense, large parts of the disk can remain very cold for some time even if the inner regions are quite hot.

  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 be vaporized in the inner disk.

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

      The innermost radius in the disk where icy materials can remain solid is called the "frost line".

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

  6. Larger bodies grow initially 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 ==> pebbles ==> "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 can rapidly attract gas from the nebula.

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

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

    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 now thought to be very short by cosmic standards, ranging from a few hundred thousand to a few million years.

  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.

Observational Support for the Nebular Model

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 after the invention of telescopes, no planetary companions to other stars were found despite much effort.

Finally, extra-solar planets around other Sun-like stars were positively 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

Properties of Important Types of Exoplanets

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 January 2024 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 © 1998-2024 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.