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Chapter 9
Our Solar System and
Others
Introduction
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Astronomers have long speculated about the origin of
our Solar System.
They have noted regularities in the way the planets orbit
the Sun and in the spacing of the planetary orbits.
But until recently, astronomers have been limited to
studying one planetary system: our own.
In 1600, Giordano Bruno was burned, naked, at the
stake for wondering about the plurality of worlds, to use
the term of that day.
Finally, after hundreds of years of wondering whether
they existed, another planetary system was discovered
in 1991.
And in the last decade, planetary systems galore were
discovered around stars like the Sun.
Introduction
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While in 1990 we knew of only 9 planets,
all around our Sun, as of this writing (late2005) we know of more than 160
additional planets around other stars (see
figure).
In this chapter, we will first discuss our
own Solar System and its formation.
Then we will describe how we find other
planetary systems and how they may have
been formed.
This discussion, in turn, should give us
insights into how well we understand the
origin of our own Solar System.
9.1 The Formation of the Solar System
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Many scientists studying the Earth and the planets are
particularly interested in an ultimate question:
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We can accurately date the formation by studying the
oldest objects we can find in the Solar System and
allowing a little more time.
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How did the Earth and the rest of the Solar System form?
For example, astronauts found rocks on the Moon older than
4.4 billion years.
We have concluded that the Solar System formed about
4.6 billion years ago.
9.1a Collapse of a Cloud
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Our best current idea is that 4.6
billion years ago, a huge cloud of
gas and dust in space collapsed,
pulled together by the force of
gravity.
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What triggered the collapse isn’t
known.
It might have been gravity pulling
together a random cloud, or it
might have been a shock wave
from a nearby supernova (see
figure).
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As the gas pressure increased,
eventually the rate of collapse
decreased to a slower
contraction.
9.1a Collapse of a Cloud
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You have undoubtedly noticed that ice skaters
spin faster when they pull their arms in (see
figure).
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Similarly, the cloud that ultimately formed the
Solar System began to spin faster as it collapsed
and contracted.
The original gas and dust may have had some
spin, and this spin is magnified in speed by the
collapse, because the total amount of “angular
momentum” doesn’t change.
It spins faster because the quantity known as
angular momentum stays steady in a rotating
system, unless the system is acted on by an
outside force (assuming the force is not
directed at the center of the object).
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The amount of angular momentum depends on
the speed of the spin and on how close each bit
of spinning mass is to the axis of spin; see our
discussion at the end of Chapter 5.
9.1a Collapse of a Cloud
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Objects that are spinning around tend to fly off, and in our
case this force eventually became strong enough to
counteract the effect of gravity pulling inward.
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Thus the Solar System stopped contracting in one plane.
Perpendicular to this plane, there was no spin to stop the
contraction, so the solar nebula ended up as a disk.
The central region became hot and dense, eventually
becoming hot enough for the gas to undergo nuclear
reactions (see Chapter 12), thus forming our Sun.
9.1a Collapse of a Cloud
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In the disk of gas and dust, we calculate that the dust began to clump
(see figures).
Smaller clumps joined together to make larger ones, and eventually
planetesimals, bodies that range from about 1 kilometer up to a few
hundred kilometers across, were formed.
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Gravity subsequently pulled many planetesimals together to make
protoplanets (pre-planets) orbiting a protosun (pre-Sun).
9.1a Collapse of a Cloud
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The protoplanets then contracted and cooled to make the
planets we have today, and the protosun contracted to
form the Sun (see figure).
Some of the planetesimals may still be orbiting the Sun;
that is why we are so interested in studying small bodies
of the Solar System like comets, meteoroids, and
asteroids.
Most of the unused gas and dust, however, was blown
away by a strong solar wind.
9.1b Models of Planet Formation
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In one of the main models for the formation
of the outer planets, the solar nebula first
collapsed into several large blobs.
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The two outermost blobs lost most of their gaseous
atmospheres as a result of strong ultraviolet light from the
young Sun that also removed the gas from the nebula at
Saturn’s orbit and beyond (see figures, below).
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These blobs then became the outer planets
(see figure, right).
As a result, they became the moderately massive planets,
Uranus and Neptune.
Because of the strength of their internal gravity, Saturn
lost only a portion of its gas, while Jupiter was
unaffected, thus becoming the most massive planet.
9.1b Models of Planet Formation
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In another model, a solid core (resembling a terrestrial planet)
condensed first for each of the outer planets.
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For this second model, the relative amounts of elements in the
rocks and in the gases would also differ from planet to planet.
Spacecraft found that Jupiter, Saturn, Uranus, and Neptune
have different relative amounts of some of the elements in their
atmospheres.
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The gravity of this core then attracted the gas from its
surroundings.
Additionally, the atmospheres of Jupiter and Saturn are very much
more massive than the atmospheres of Uranus and Neptune.
In this model, the cores of Uranus and Neptune may contain 10
to 15 times the mass of the Earth.
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Also in this model, Jupiter’s core is thought to have only 0 to 3
Earth masses, much less than the cores of Uranus and Neptune,
which is an argument against this model.
9.1b Models of Planet Formation
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In the inner Solar System, the terrestrial planets are the
accumulation of planetesimals.
Our Earth and its neighbor planets were formed out of
planetesimals made of rocky material.
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These terrestrial planets never became massive enough to
accumulate a massive atmosphere the way the giant planets
did.
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The rocky material, mainly silicates, condensed at the temperatures
of these planets’ distances from the Sun and were not blown
outward by particles from the forming Sun.
And, because the inner planets are closer to the Sun and therefore
hotter, gas in their atmospheres moved relatively fast.
Thus free hydrogen and helium escaped from the Earth’s low
gravity, while Jupiter and the other giant planets have huge
atmospheres of hydrogen and helium, matching the atmosphere
of the Sun.
9.1b Models of Planet Formation
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Early on, the Sun would have been spinning very fast.
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In 2001, John Chambers and George Wetherill of the
Carnegie Institution of Washington proposed a model
extending standard ideas of Solar-System formation to
explain not only the range of planets we see but also the
relative emptiness of the asteroid belt.
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But much of the excess angular momentum was transported
upward by a “bipolar outflow”; see the discussion in Chapter
12.
Many people mistakenly think that the asteroids are the
remains of a large planet that exploded, but the asteroid belt
actually contains so little material that not even a small
moon could have been present there.
In their model, planetesimals formed everywhere
throughout the solar nebula, including the asteroid belt.
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Some of the planetesimals coalesced into planets.
9.1b Models of Planet Formation
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Jupiter grew especially rapidly, and its gravity kicked out
material in the asteroid-belt region that orbited an integer
number of times in the time that Jupiter itself made one orbit
(or some other integer number of orbits).
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Chambers and Wetherill have added the idea that objects in this
region that are not quite in the places affected so strongly by
Jupiter’s gravity were pushed into these zones by gravitational
encounters among themselves, including some planetesimals
and small planets that had formed there.
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This aspect of Jupiter’s gravity, and that it formed gaps in the
asteroid belt, has long been known.
They have checked the idea with computer simulations.
The process also made the orbits of the giant planets more
circular, matching observations.
Further, it sent some objects that would contain volatile
substances (those that evaporate easily) like water to crash into
the Earth.
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This process may explain how we got our oceans, something more
often attributed to comets.
9.1b Models of Planet Formation
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It is interesting that nothing about our current model of
planetary-system formation implies that the Solar System
is unique.
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As we will see next, we are increasingly finding systems of
planets around other stars.
To our surprise, their properties aren’t like those of our
Solar System: We can’t see if they have small, rocky
planets close in to the star, but we know that they don’t
all have massive ones farther out.
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Maybe some of our modelling for our own Solar System has
been wrong because we looked for regularity in the
distribution of planets while we really had a random
distribution.
9.2 Extra-solar Planets (Exoplanets)
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People have been looking for planets around
other stars for decades.
Many times in the last century, astronomers
reported the discovery of a planet orbiting
another star, but for a long time each of these
reports had proven false.
Finally, in the 1990s, the discovery of extrasolar planets (planets outside, “extra-,” the
Solar System) seemed valid.
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They have also become known as exoplanets.
9.2a Discovering Exoplanets
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Since exoplanets shine only by reflecting a small amount
of light from their parent stars (i.e., the stars that they
orbit), they are very faint and extremely difficult to see in
the glare of their parent stars with current technology.
So the search for planets has not concentrated on visible
sightings of these planets.
Rather, it has depended on watching for motions in the
star that would have to be caused by something orbiting
it.
9.2a(i) Astrometric Method
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The earlier reports, now rejected, were based
on tracking the motion of the nearest stars
across the sky with respect to other stars.
The precise measurement of stellar positions
and motions is called astrometry, so this
method is known as the astrometric method.
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If a star wobbled from side to side, it would reveal that a planet was
wobbling invisibly the other way, so that the star/planet system was
moving together in a straight line. (Technically, the “center of mass” of the
system has to move in a straight line, unless its motion is distorted by
some outside force. The center of mass is illustrated in the figure, and is
described more fully in Chapter 11, when we discuss binary stars. Both the
star and the planet orbit their common center of mass, though the star is
much closer to the center of mass than the planet is. Thus, the star moves
very slightly, in a kind of “reflex motion” caused by the orbiting planet.)
9.2a(i) Astrometric Method
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Astrometric measurements have been made over
the last hundred years or so, and a few of the
nearby stars whose motions in the sky were
followed seemed to show such wobbling.
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But the effects always turned out to be artifacts of
the measuring process.
Nevertheless, the astrometric method is still
being used by some astronomers, and maybe
they will eventually detect an exoplanet with it.
9.2a(ii) Timing of Radio Pulsars
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The first extra-solar planet was discovered in 1991 around
a pulsar, a weird kind of collapsed star (see our discussion
in Chapter 13) that gives off extremely regular pulses of
radio waves with a period that is a fraction of a second.
The pulses came more frequently for a time and then less
frequently in a regular pattern.
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So the planet around this pulsar must be first causing the
pulsar to move in our direction, making the pulses come
more rapidly, and then causing it to move in the opposite
direction, making the pulses come less often. (As described
above, the planet and star are actually orbiting their
common center of mass.)
9.2a(ii) Timing of Radio Pulsars
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But this planet must have formed after the catastrophic
explosion that destroyed most of the star after its inner
core collapsed; the planet could not have survived the
stellar explosion.
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Thus, it was not the kind of planet that is born at the same
time as the star, like Earth.
Even when the existence of two more planets (and
possibly a fourth planet) orbiting that pulsar was
established, all with masses comparable to those of the
terrestrial planets in our Solar System, the pulsar system
seemed too unusual to think much about, except by
specialists.
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It didn’t help that another pulsar planet, discovered slightly
earlier, turned out to be a mistaken report.
9.2a(iii) Periodic Doppler Shifts
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In the 1990s, techniques were developed using the Doppler effect.
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Recall that Chapter 2 describes how the Doppler effect is a shift in the
wavelengths of light that has been emitted, caused by motion of the source
or the receiver along the line of sight (that is, by the “radial velocity”).
The planet has a large orbit around the center of mass,
moving rapidly.
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But the parent star, being much more massive than the
planet, is much closer to the center of mass (see figure,
right) and therefore moves much more slowly in a smaller
orbit.
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With sufficiently good spectrographs and
numerous observations, this slight “wobble” can
be detected as a periodically changing Doppler
shift in the star’s spectrum (see figure, left); the
radial velocity of the star varies in a periodic
way.
9.2a(iii) Periodic Doppler Shifts
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The breakthrough came because new computer methods
were found that measure the changing Doppler shifts very
precisely.
On a computer, the star’s spectrum can be simulated
including changes in the wavelengths of light, just as
happens in the Doppler effect.
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These simulated spectra can be compared to the observed
spectrum of that star, until an excellent match is found.
This method allows very small Doppler shifts to be
detected, and the speeds of stars toward or away from us
can be measured to a precision of 1 meter/sec, a leisurely
walking speed. (This very high precision was only recently
achieved, with the Keck-I telescope; at most other sites,
the precision has typically been 3 meters/sec or worse.)
9.2a(iii) Periodic Doppler Shifts
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The first surprising report came in 1995 from a Swiss
astronomer and his student, Michel Mayor and Didier
Queloz.
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They found a planet around a nearby star, 51 Pegasi.
One strange thing about the planet is that it seemed
to be a giant planet, at least half as massive as
Jupiter, but with an orbit far inside what would be
Mercury’s orbit in our Solar System.
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The planet orbited 51 Peg in only 4.2 days, much faster
than any planet in our Solar System.
9.2a(iii) Periodic Doppler Shifts
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Two American astronomers, Geoff Marcy and Paul Butler,
then at San Francisco State University and the University
of California, Berkeley, had been collecting similar data on
dozens of other stars.
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They hadn’t run their spectra through the computer
programs they were writing to measure the Doppler shifts.
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But they had assumed, reasonably, that a planet like Jupiter
around another star would take years to orbit, so they were
collecting years of data while perfecting their analysis
techniques to measure exceedingly small speeds.
When they heard of the Mayor and Queloz results, they
quickly examined their existing data and also observed 51
Peg.
They soon verified the planet around 51 Peg and
discovered planets around several other stars (see the
drawing opening this chapter).
9.2a(iii) Periodic Doppler Shifts
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Most of the new objects
turned out to be giant planets
either in extremely elliptical
orbits or in circular orbits very
close to the parent stars (see
figure).
Most of these planets are
orbiting stars within 50 lightyears from us, not extremely
far away but not the very
closest few dozen stars either.
Stars this close are bright
enough for us to carry out the
extremely sensitive
spectroscopic measurements.
9.2a(iii) Periodic Doppler Shifts
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One limitation of the method is that
we generally don’t know the angle of
the plane in which the planets are
orbiting their parent stars.
The Doppler-shift method works only
for the part of the star’s motion that
is toward or away from us, and not
for the part that is from side to side.
So the planets we discover can be
more massive than our
measurements suggest; we are able
to find only a minimum value to their
masses (see figure).
9.2a(iii) Periodic Doppler Shifts
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In the first few years of exoplanet discovery, this problem
left the nagging question of whether the objects were
really planets or merely low-mass companion stars.
They might even be objects called “brown dwarfs,” which
have between about 10 and 75 times Jupiter’s mass, not
quite enough to make it to “star status” (see our
discussion in Chapter 12, and in Section 9.2c below).
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Nevertheless, most astronomers believed these objects are
planets, because there is a large gap in mass between them
and lowmass stars.
Very few intermediate-mass objects (10 –75 Jupiter
masses) had been found, yet they should have been easily
detected if they existed.
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This gap suggested that the new objects are much more
numerous than brown dwarfs.
9.2a(iii) Periodic Doppler Shifts
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The discovery, in 1999, of a system of three planets around the
star Upsilon Andromedae clinched the case that at least most of
the objects are planets.
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It seems most unlikely that a system would have formed with
four closely spaced stars (or brown dwarfs) in it, while a system
with one star and three planets is reasonable.
One of Upsilon Andromedae’s planets even has an orbit that
corresponds to Venus’s in our Solar System, not as elliptical as
the orbits of the planets around other stars.
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Other multiple-exoplanet systems were found thereafter.
This planet is in a zone that may be not too hot for life nor too cold
for it.
Though such a massive planet would be gaseous, and so not
have a surface for life to live on, it could have a moon with a
solid surface, just as the giant planets in our Solar System have
such moons.
9.2a(iii) Periodic Doppler Shifts
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The exciting announcements continue.
We now know of several systems that each contain
a few planets.
Our methods are still not sufficiently sensitive to
find “minor” bodies like our own Earth.
However, a considerable advance was announced in 2005: the existence
of a planet whose mass is as low as only 7 to 8 times Earth’s (see figure),
by a group of scientists including Geoff Marcy and Paul Butler.
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Astonishingly, the planet orbits its parent star with a period of only two
days, meaning that it is only 1/50 of an A.U. out, just 10 times the star’s
radius.
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This exoplanet orbits a star, known as Gliese 876 or GJ876, that is only 15
light-years from Earth.
It is so close to its star’s surface that its temperature is probably 200°C to
400°C, so it isn’t a candidate for bearing life as we know it.
Being so hot, yet having relatively low mass and thus moderately weak
gravity, it could not have retained a lot of gas.
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It is therefore apparently the first rocky, terrestrial-type planet ever found
orbiting another star.
9.2a(iii) Periodic Doppler Shifts
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Finding this planet required improving the Keck telescope’s system for
detecting small Doppler shifts (see figure, left).
The detailed observations (see figure, right) allowed a computer model to
account for the angle at which we are viewing the system, which turns
out to be inclined to our line of sight by 40°.
That measurement allowed the mass of the orbiting planets themselves
to be determined, and not merely lower limits as before.
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The measurements revealed that the slight discrepancy in the orbits of the
two already-measured planets could be resolved by the presence of the third
body, the newer exoplanet.
9.2a(iv) Transiting Planets
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Since 1999, astronomers have not had to resort only to periodically
changing Doppler shifts to detect exoplanets.
One planet was discovered to have its orbit aligned so that the planet
went in front of the star each time around—that is, it underwent a
transit.
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The dip in the star’s brightness of a few per cent can be measured not only
by professional but also by amateur astronomers (see figures).
9.2a(iv) Transiting Planets
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Since the planet’s orbital plane is along our line of sight,
its mass can be accurately determined, and this turns out
to be 63 per cent of Jupiter’s mass.
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The transit method has also revealed, through
spectroscopy, sodium in the exoplanet’s atmosphere.
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This result confirms our conclusion that at least some (and
probably most) of the “exoplanets” really are planets rather
than more massive brown dwarfs.
The atmosphere produced some absorption lines in the
starlight passing through it, and these were detected in very
high-quality spectra.
In the future, it is possible that astronomers will detect
evidence for life on other planets by analyzing the
composition of their atmosphere using the transit method.
9.2a(iv) Transiting Planets
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During the past few years, several additional examples of
transiting planets have been found.
Many more exoplanets will be discovered in this way from
the ground and from space.
This method is analogous to observing the transit of
Venus across the disk of our Sun (see figure).
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Progress in the search for
exoplanets is so rapid that you
should keep up by looking at the
Extrasolar Planets Encyclopaedia
at http://www.obspm.fr/planets
and the Marcy site at
http://exoplanets.org, both linked
through this book’s web pages.
9.2b The Nature of Exoplanet Systems
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We have discovered enough exoplanets to be able to
study their statistics.
About 1 per cent of nearby solar-type stars have jovian
planets in circular orbits that take between 3 and 5 Earth
days.
These are sometimes called “hot Jupiters,” since they are
so close to their parent stars (within 1/10 A.U.) that
temperatures are very high.
Another 7 per cent of these nearby stars have jovian
planets whose orbits are very eccentric.
As we observe for longer and longer periods of time, we
have better chances of discovering planets with lower
masses or with larger orbits.
9.2b The Nature of Exoplanet Systems
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The discovery of several planetary systems instead of just
our own will obviously change the models for how
planetary systems are formed.
Since giant planets couldn’t form in the torrid conditions
close in to the parent star, theorists work from the idea
that the exoplanets formed far out, as Jupiter did, and
migrated inward.
Thus it follows that these planets may be jostled loose
from their orbits and put in orbits that bring them closer
to their parent stars.
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Maybe their orbits shrink as the planets encounter debris in
the dusty disk from which they formed, or shrink along with
an overall swirling inward of the whole disk of orbiting
material.
9.2b The Nature of Exoplanet Systems
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Perhaps the highly elliptical orbits didn’t start out that
way, but were produced by gravitational interactions that
completely ejected some planets from the system.
The gravitational interactions between two planets can
lead to the ejection of one planet, leaving the other in a
very eccentric orbit.
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Another idea is that interactions between a planet and the
protoplanetary disk can cause high eccentricities.
Once a planet has part of its orbit close to its parent star,
tidal forces can circularize the orbit.
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Other planets can spiral all the way into the star and be
destroyed.
9.2b The Nature of Exoplanet Systems
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The most accepted explanation for the hot Jupiters, which
orbit so close to their stars, is that they were formed
farther out and migrated in.
But the 2005 discovery of a hot Jupiter in a triple star
system complicates matters, since the two farther-out
stars would have disrupted any protoplanetary disk.
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So this discovery is being interpreted as evidence against
the migration model.
Perhaps that model was partly based on residual prejudice
that our own Solar System’s outer giant planets are
normal.
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Perhaps the triple system had a very different type of
protoplanetary disk than the one with which we are familiar
from our own system, or the planet was captured.
9.2c Brown Dwarfs
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As mentioned in Section 9.2a(iii), some of the objects found
with the Doppler shift technique might actually be too massive
to be true “planets” (more than 13 Jupiter masses).
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Each of the brown dwarfs has less than 75 Jupiter masses (or
7.5 per cent the mass of the Sun), which is not enough for them
to become normal stars, shining through sustained nuclear
fusion of ordinary hydrogen, as we shall discuss in Chapter 12.
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If so, they are brown dwarfs, which are in some ways “failed
stars.”
Their central temperatures and pressures are just not high enough
for that. (However, they do fuse a heavy form of hydrogen known
as “deuterium,” so they are not complete failures as stars.
We discuss the origin of deuterium, all of which was formed in
the first few minutes after the Big Bang, in Chapter 19.)
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Brown dwarfs can be thought of as the previously “missing links”
between normal stars and planets.
9.2c Brown Dwarfs
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Though no detection of a brown dwarf was accepted until 1995,
hundreds have now been observed.
Many of these are in the Orion Nebula, while others are alone in
space.
The current best model for brown dwarfs is that they are
formed similarly to the way that normal stars are: in contracting
clouds of gas and dust.
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A disk forms, perhaps even with planets in it, though the material
in it contracts onto the not-quite-star.
This idea is backed up by observations with the European
Southern Observatory’s Very Large Telescope, which has
detected an excess of near-infrared radiation from many brown
dwarfs.
The scientists involved interpret their observations as showing
that the radiation is from dusty disks.
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Further, they conclude that since both regular stars and brown
dwarfs have such disks, they must also form in similar ways.
9.2c Brown Dwarfs
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Most exciting, a planet next to a brown dwarf (presumably
orbiting it) has been imaged (see figure), the first planet
to be imaged around any star.
The star with its planet is 200 light-years away from us;
observations from the ground and from space have shown
that the two objects are moving through space together.
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The planet is 55 A.U. away from its parent star.
9.2d Future Discovery Methods
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NASA’s Kepler mission, planned for a 2008 launch, is to carry a 1m telescope to detect transits of planets across stars.
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To do so, it will continuously monitor the brightness of 100,000 stars.
The 2004 transit of Venus served
as an analogue to the type of
thing Kepler will study: One of
this book’s authors (J.M.P.) and
colleagues reported on the dip in
the total amount of sunlight
reaching Earth because Venus
was blocking about 0.1 per cent
of the Sun’s disk (see figure).
9.2d Future Discovery Methods
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No telescope now in space has enough resolution to
directly detect (image) a planet closely orbiting a normal
star (rather than a brown dwarf as described in the
preceding section).
It will take an interferometry system, with two widely
separated telescopes working together, to make such a
detection.
Installation of interferometric equipment at the two 10meter Keck telescopes in Hawaii and at the four 8-meter
units of the Very Large Telescope in Chile is nearing
completion, which should lead to direct detection of some
young, giant planets that are luminous at infrared
wavelengths.
9.2d Future Discovery Methods
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Though we have mainly detected giant
planets, we are now beginning to detect
small planets in those systems.
To do even better, NASA’s Space
Interferometry Mission, now called SIM
PlanetQuest (see figure), is on the
drawing board, but it will not be
launched until at least 2011.
NASA’s Terrestrial Planet Finder is to be
able to image small planets and is
slated for launch in the third decade of
this century.
In 2005, both were delayed by an
unfortunate shift in NASA’s priorities.
9.2d Future Discovery Methods

For these missions, NASA is currently examining two approaches
to high-contrast imaging.


The first would use a huge telescope for direct imaging, though
blocking the starlight itself to reveal accompanying planets.
The second would use several infrared telescopes flying in
formation and coupled to form a “nulling interferometer,” in which
the response at the stellar position is minimized (see figure).

NASA is considering an
earlier, smaller, less
expensive mission in its
Discovery class of
spacecraft to try out
the technologies.
9.2d Future Discovery Methods

The European Space Agency (ESA) also plans spacecraft
to find exoplanets.


In 2004, they cancelled their Eddington mission to search
for Earth-like systems by looking for their transits.



But again, funding reasons have delayed or abandoned their
plans.
Their Gaia mission is to measure positions for a billion stars,
and it may discover 10,000 planets!
It should be launched by 2012.
ESA’s Darwin mission is to analyze the atmospheres of
Earth-like planets to search for signs of life, but not until
at least 2015.

It is to consist of a flotilla of three 3-m telescopes on
spacecraft in formation.
9.2d Future Discovery Methods



Also, stars are being monitored to see if their gravity
focuses and brightens the light from other stars behind
them, a process called “gravitational lensing” that we will
discuss in Chapters 16 and 17.
The hope is that not only a background star but also,
slightly before or after the star passes, a planet will cause
a brightness blip.
Some candidate events have been reported, but more
work is necessary before they will be widely accepted as
true evidence of exoplanets.
9.3 Planetary Systems in Formation

We are increasingly finding signs that planetary systems
are forming around other stars.


One of the first signs was the discovery of an apparent disk
of material around a southern star in the constellation Pictor
(see figures, right).
The best observations of it were made
with the Hubble Space Telescope, and
may show signs of orbiting planets.

An even nearer
planetary disk has been
found, enabling
observations with
higher resolution (see
figure, left).
9.3 Planetary Systems in Formation

Other images of regions in space known
as “stellar nurseries” show objects that
appear to be protoplanetary disks (see
figures, top).


Observations reported in 2005 show that
these objects contain about as much mass
as a planetary system, clinching the idea
that they are locations where planets are
forming.
Locations where there are planets in
formation glow in the infrared (see figure,
bottom), because of the wavelength of
the peak of the black-body curves for the
temperature of warm dust (see our
discussion of black bodies in Chapter 2);
thus, the Spitzer Space Telescope and,
much later, the Webb Space Telescope,
should give many insights into planetary
formation.
9.3 Planetary Systems in Formation

The Hubble Space Telescope has imaged such a ring of
dust around the nearby star Fomalhaut (see figure),
recording signs that a planet is tugging on it
gravitationally.
9.3 Planetary Systems in Formation


A Wesleyan University team has found a young Sun-like
star that winks on and off, apparently being eclipsed by
dust grains, rocks, or asteroids that are orbiting it in a
clumpy disk.
The star is near the Cone Nebula, a prolific nursery of
young stars.


No single object could provide such a long eclipse, so only
an orbiting collection of smaller objects seems to match
the observations.


It fades drastically over 2.4 days to only 4 per cent of its
maximum brightness, stays dim for another 18 days, and
then brightens for another 2.4 days out of every 48.3 days.
The actual eclipse could be from a wave of gas and dust
triggered by the masses of these objects.
The system, only 3 million years old, is changing from
month to month.