ASTR 121, O'CONNELL. [Spring 2009] Study Guide 11

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.

	INNER (TERRESTRIAL)	OUTER (JOVIAN)
Size & Mass**	Small	Large
Density	Large	Small
Composition	Si,O,Al,Mg,Fe Rocky	H,He Gas Giants

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.

From the Earth, the second and third-brightest Solar System objects are Earth's Moon and Venus.

Contents of the Solar System:

The Sun (99.8% of total mass)

8 "classical" planets (0.1%). (Jupiter > sum of others.)

Tens of thousands of smaller rocky or icy "minor" planets, ranging in size down to a few hundred meters. The largest of these, including Pluto, are now called "dwarf planets."

Most of these are members of the asteroid belt between Mars and Jupiter or the "Kuiper Belt" in the outer solar system, beyond Neptune. There has been much recent controversy on how to characterize the larger ones, but we will postpone discussion of this until Study Guide 20.

Over 160 satellites

Comets (small, icy bodies; perhaps a trillion of these)

Meteoroids (small, rocky bodies), dust, gas

B. SYSTEMATICS OF PLANET ORBITS
Systematic characteristics of the orbits:

Orbits for all (except Pluto) lie close to the plane of the Earth's orbit (the ecliptic plane)

The picture above shows an oblique view of the planetary orbits drawn to scale (though the planet sizes shown are not to scale). You can see the "cozy" inner solar system (Mercury through Mars) separated from the outer solar system by the asteroid belt.
For an edge-on plot of the orbits showing the near-coincidence of the orbital planes, click here.

Orbits, though technically ellipses, are nearly circular

The direction of revolution of the planets in their orbits is the SAME (counterclockwise from above Earth's N. pole); the direction of spin on their rotation axis is the same for most.

Orbits show systematic spacing (Bode's "Law"): separation between orbits increases with orbit size (see diagram above)

It is important that NONE of the above is required by Newton's laws.

For example: Newton's laws imply the orbit of a given planet will be in a fixed plane, but the orbits of other planets can be in different planes; revolution directions do not have to be the same; orbits can be highly elliptical rather than nearly circular; etc.

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

**See the image at the top of this page for a graphic comparison of planet sizes.

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:

"Catastrophic/tidal theory": a passing star pulls material from Sun, which cools to form planets

Would imply planets are rare, since close encounters are extremely rare
Physically unlikely: expelled material would diffuse, not condense

"Nebular theory": planets are byproducts of normal star formation

Planets form from the cloud ("nebula") of debris surrounding a forming star
Would imply planets are common
There is now excellent supporting evidence for the nebular theory, not least of which is item "G" below

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:

The interstellar medium (ISM) is the dilute matter between stars

It constitutes a few % of the mass of our Galaxy

Contents:

Gas (atoms, molecules)

Composed of: hydrogen 71%; helium 27%; all other elements (C,N,O,Fe, etc) only 2%. When hot, this gas is visible as "gaseous nebulae" (producing spectra containing, among other features, bright emission lines of hydrogen, as discussed in the Supplements).

"Dust"

Very small solid particles or "dust grains"; smoke-like. These absorb and redden light passing through them. Absorption by dust creates "dark clouds" seen against bright sources such as the Milky Way.

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 10^o 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.

here is an MPEG video of a zoom into the Eagle Nebula.

F. PLANET FORMATION IN THE NEBULAR THEORY

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

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. In the case of our solar system the disk is called the "solar nebula."

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

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

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

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 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.
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

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

Computer simulation of protoplanetary disk

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.

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

Interiors of young planets that are large enough are heated amd partially melted by the energy dissipated by accretion and by the decay of short-lived radioactive isotopes. The interiors differentiate, with heavy metallic materials settling to the center and lighter, rocky materials rising to exterior.

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, or pushing planets into more strongly elliptical orbits.

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 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). 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 time 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.
Finally, extra-solar planets around other Sun-like stars were first 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 Sunlike stars in the Galaxy.

Methods

Technically very difficult (or would have been done sooner)!

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

Instead, the most successful method used to date is based on detecting the VERY SMALL (~10 meters/sec) CHANGES in the STAR'S MOTION induced by the GRAVITY of an orbiting planet.

This animation shows how their mutual gravity causes both the star and its planet to move around a common "center of gravity"
The motions are detected using the Doppler effect (see the Supplements).

Shown at the right is the velocity curve for the parent star of the first identified extra-solar planet. Click for an enlargement. Note! a velocity of 10 meters/sec can be achieved by a fast sprinter; so we can now detect stars moving at human speeds!

After a number of planets were detected by this "radial velocity" technique, astronomers realized that they could also find them by searching for PLANETARY TRANSITS, that is, the very tiny change in light from a star when a planet crosses its disk from our point of view (a partial eclipse).

See this illustration for a sample "light curve". This method can catch only the small fraction of planets whose orbital plane has the right orientation. However, with stable and sensitive digital cameras, it has proven possible to detect many transiting planets even with modest-sized telescopes.

Properties

Count (as of March 2009): 340 planets in 290 planetary systems; 37 of these are multiple planet systems; 58 are transiting planets.
Masses: 0.01-13 Jupiter masses.

[Note: Objects larger than 13 Jupiters are considered to be "brown dwarfs"---not planets but stars which never ignited sustained hydrogen burning.]

Orbits: 0.05-8 AU.

Surprise: Jupiter-sized objects with such small orbits (artist's concept below). Gas giants like Jupiter and Saturn are at distances of 5 AU and greater in our solar system, and it is thought (see above) that they cannot form at small distances from the Sun. Probably, these planets form at large distances but, because of interactions with the protoplanetary disk, migrate over time nearer the star.
Because planet searchers thought massive planets would always be distant from their parent stars, with long orbital periods, most did not look for short-period oscillations of the stars (until after first detection).

Using special techniques and highly sensitive detectors, astronomers have begun to study the composition and structure (even the meteorology) of transiting exoplanets.

Earth-Like Planets?
Existing techniques are not sensitive enough to detect Earth-sized planets. The smallest well-determined mass for an exoplanet is about 3-4 times the mass of Earth. But new approaches to finding smaller planets are being aggressively pursued.

One promising method for detecting thousands of planets even at very great distances from us, even planets as small as Earth, is to search for planetary transits from spacecraft. The expected changes in brightness during an Earth-like planetary transit are only about 0.01%, which is why the atmosphere-free environment of space is needed to make possible very stable light-measurements.
The COROT and Kepler space missions will search many stars for planets with this technique.

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

Reading for this lecture:

Alert! Most lectures from now on will cover more material in the text than has been the case up to now. Try to keep up with the reading.
Study Guide 11
Bennett textbook: Ch. 7 except pp. 214-223; Ch. 8; p. 417.

Reading for next lecture:

Bennett textbook: Secs. 8.5, 9.1, 9.6
Study Guide 12

Web Links:
More information on systematics of the Solar System (SEDS)
Java Simulations of Solar System Orbits (UMd)
Multimedia Tour of the Solar System
Best color images of interstellar gas and dust nebulae (Anglo-Australian Observatory, David Malin)
Tutorial on ExtraSolar Planets (ASU)
University of California Extra Solar Planets Search Page
Extra Solar Planets Encyclopaedia
Techniques for Detection of Extra Solar Planets
Detection of Extra Solar Planets by Transit Photometry (STARE Project)
Extra Solar Visions (artwork by John Whatmough)
Java Simulations of Orbits for Selected Extra Solar Planets (UMd)

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Last modified March 2009 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 © 1998-2009 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.