ASTR 1210 (O'Connell) Study Guide
11. PLANETARY SYSTEMS:
OURS AND OTHERS
Comparison of the planets, based on
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
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
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 celestial objects are Earth's
Moon and Venus.
Contents of the Solar System:
- The Sun (99.8% of total mass)
- 8 "classical" planets (0.1% of the mass). Jupiter's mass
is greater than the masses of the other planets combined.
- 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 over what to call the larger
ones, but we will postpone discussion of this until Study Guide 20.
- Over 170 satellites of planets
- Comets (small, icy bodies; perhaps a trillion of these)
- Meteoroids (small, rocky bodies), dust, gas
An oblique view of the planetary orbits drawn to
(though the planet sizes shown are not to scale).
B. Systematics of Planet Orbits
Systematic characteristics of the orbits:
It is important to understand that none of the above is required by
- Orbits for all (except Pluto) lie close to the plane of the
Earth's orbit (the ecliptic plane)
In the picture above, you can see the "cozy" inner solar system
(Mercury through Mars) separated from the vast outer solar system by
the asteroid belt.
edge-on plot of the orbits showing the near-coincidence of the orbital
planes, click here.
- The 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.
- The orbits show systematic spacing (Bode's "Law"): the
separation between orbits increases with orbit size.
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 the planets. That is,
they provide clues to the process that forms planets.
For a diagram of the current location of the 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**
**See the image at the top of this page for a graphic comparison
of planet sizes.
These differences constitute another major clue about the
processes that formed the planetary system.
Note: we are deliberately ignoring Pluto here. It is not
representative of the "classical" planets. Instead it is a member of
a huge group of small, rocky/icy bodies that reside the outer solar
system. See Study Guide 20.
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
This would imply planets are rare, since close encounters
between stars are extremely rare
The process is also physically unlikely: expelled (hot) material would
diffuse quickly before it was able to condense
- "Nebular theory": planets form from the cloud ("nebula")
of debris surrounding a forming star
This would imply planets are common because they are byproducts
of normal star formation
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:
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). 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:
- The interstellar medium (ISM) is the dilute matter between stars.
It constitutes a few % of the mass of our Galaxy
Gas (atoms, molecules)
Composition: 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
Very small solid particles
or "dust grains". These are
smoke-like. They absorb and redden light passing through them.
Absorption by concentrations of dust creates
"dark clouds" seen
against bright sources such as the Milky Way.
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.
For Hubble Space Telescope images of another spectacular star-forming
region in the Carina
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 "protostar" ("protosun" in the illustration here).
The protostar begins to heat up, first from energy released by gravitational
collapse, later by nuclear reactions
- The protostar 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
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"
Computer simulation of protoplanetary disk
- Final assembly: violent infall of fragments heats the
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 a few
million years (though under the right conditions it could
occur much faster).
- 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 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, tossing small bodies outward, 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
Strong support for the nebular theory has emerged. We now have direct
detections of protoplanetary disks around nearby stars by
Hubble Space Telescope
telescopes. A high priority goal of several powerful new
millimeter-wave array, operated by the National Radio Astronomy
Observatory here in Charlottesville, and
the James Webb Space
Telescope, is to probe in detail the physics of star and planet
formation. 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
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. And, using special techniques and highly
sensitive detectors, we have begun to probe the composition and
structure---even the meteorology---of some exoplanets.
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
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
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 Doppler method for finding exoplanets is based on
detecting the tiny changes in the star's
motion induced by the gravity of an orbiting planet.
These amount to only a few times 10 meters/sec.
Recall that in Newtonian gravity, a planet exerts the same force on
its parent star as the star exerts on the planet. The star therefore
accelerates, though the acceleration is very small because it
is inverse proportion to its mass. The animation at right shows how
their mutual gravity causes both the star and its planet to
move around a common "center of gravity." (The stellar motion in the
animation is greatly exaggerated compared to the motion induced by
The motions can be detected by the Doppler effect on the
spectrum of the star (see
spectrographs, with high sensitivity and stability, are
- Shown at the right is the Doppler-derived velocity curve for the
parent star of the first identified extra-solar planet around a
Sun-like star. Click for an enlargement. The amplitude of the
velocity change is +/-50 m/s, but the precision of the data is better
than 10 meters/sec--- a velocity that can be achieved by a fast
sprinter. So we can now detect stars moving at human
- The shape of the velocity curve allows us to determine the
planet's orbital shape.
- By applying Kepler's Third Law (see Study
Guide 8), this method provides an estimate for the mimimum
mass of the planet. (It is a minimum because we usually do not
know the inclination of the plane of the planet's orbit to the line of
sight; a tilt of this plane will reduce the observed radial
After a number of planets were detected by the Doppler
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). In principle, astronomers could have
detected these with many of the electronic devices available
since the 1950's---but they had never configured their observing
techniques to look for effects this small.
For a diagram showing the viewing configuration for a
transit, click here.
The picture at the right shows a typical "light-curve" for a planetary
transit. Note that the amplitude of the eclipse is only 1.5% of the
star's normal brightness.
This method can catch only the small fraction of planets whose orbital
planes have the right orientation such that eclipses occur from the
Earth's point of view. However, with stable and sensitive digital
cameras, it has proven possible to detect many transiting planets even
with modest-sized telescopes. Many groups around the world, including
amateur astronomers, are now surveying the sky for planet transits.
The transit method is important because, by determining the fraction
of the parent star's surface that is occulted, it provides an
estimate of the radius of the exoplanet.
As of September 2013 we have found 986 planets in 750 planetary systems.
168 of these are multiple planet systems. Over 350 are transiting
planets. For a complete list, see the
Extra Solar Planets Encyclopaedia.
Hot Jupiters: "Hot" Jupiters are gas giant planets (~ Jupiter
mass or larger) in very small orbits---less than 0.4 AU,
the radius of the orbit of Mercury. The tightest orbits are
less than 0.01 AU in radius. Their atmospheres are heated
to high temperatures by their parent star; many are puffed-up
or losing material. Under some circumstances, the atmospheres
may evaporate altogether.
This was the first class of exoplanets to be discovered around
Sun-like stars because they produce large Doppler signals. However,
they were a great surprise because in our solar system the
large planets are all at distances great than 5 AU, 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, they migrate over
time nearer the star.
Because early 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 the first detection).
[Note: objects with masses larger than 13 x Jupiter's are considered
"brown dwarf" stars, not planets.]
Super-Earths: These are exoplanets with masses significantly
larger than Earth's but smaller than 10 Earth masses. They are
smaller than Uranus or Neptune. Several dozen of these have been
detected so far. The exoplanet system Kepler 62 has four
Super-Earths. Some Super-Earths have masses and radii that suggest
they are made of rocky materials, like Earth, or of ice/water. Some
are in or near their
star's habitable zone, where
surface temperatures allow the presence of liquid water. These would
be excellent candidates for Earth-like biospheres, but we have
no evidence to date that such exist.
Earth-Like Planets? Only one Earth-mass exoplanet has been
tentatively identified yet. It is a member of the nearest star-system
to the Sun, Alpha Centauri (a triple star). It is too close to its
parent star to be able to host Earth-like life. A number of Earth-sized
candidates have been found by the Kepler mission, but none of these
are confirmed yet; see next section.
The Kepler Mission
Kepler is a small space telescope carrying a huge electronic
camera (42 CCD detectors) which steadily surveys a large patch of sky
in the Milky way between Lyra and Cygnus searching for planetary
transits in the tens of thousands of stars observable in the
field. Planetary systems here will be very distant (typically over a
thousand light years away), but the strength of the mission is that it
can find a large number of potential planets. The Kepler survey field
is shown at the right.
Kepler was launched in 2009, and so far it has detected over 2700
planetary "candidates"---but each of these must be confirmed by
the Doppler radial velocity method before they are accepted as planet
detections (over 100 to date).
By design, Kepler is able to detect the very tiny eclipses (0.01%)
caused by Earth-sized planets. It has identified several dozen
candidates of near-Earth size, but none of these have yet been
confirmed to have Earth-like masses; only four are in the habitable
zone of their parent star.
In May 2013, Kepler lost a key component of its attitude-control
system and its ability for precision pointing. This has ended
its planetary discovery program, regretfully just as it was
productively exploring the Earth-like planetary regime in both size
and orbital period.
How Many Planets in the Galaxy?
Based on the excellent statistics from Kepler, astronomers estimate there
are 50-150 billion planets in our Galaxy, though again most of these are
not Earth-like nor in a habitable zone. There are probably at least as
many planets in the Galaxy as there are stars.
Artist's concept of an evaporating "Hot Jupiter"
in orbit 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: Chapter 7 except pp. 202-212; Chapter 8; "Summary of Key Concepts" for Chapter 13.
Reading for next lecture:
Bennett textbook: Secs. 8.5, 9.1, 9.6
Study Guide 12
October 2013 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
© 1998-2013 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.