Title: From pebbles to planets. (cover story)   
Subject(s): PLANETS -- Origin; COSMOLOGY
Source: Astronomy, Feb98, Vol. 26 Issue 2, p56, 6p, 8c
Author(s): Yulsman, Tom
Abstract: Discusses the formation of planets. Overview on the
Kant-Laplace hypothesis on how planets are formed; Description of the
cosmic structure; Hypothesis on the gas runaway effect on Jupiter and
Saturn; Views by several astronomers on the issue.
AN: 186787
ISSN: 0091-6358
Note: Tucson-Pima Public Library subscribes to this magazine.
Database: MasterFILE Elite

Section: Origins

FROM PEBBLES TO PLANETS

To know whether we're alone in the universe, we must first understand
how planets form.

Until just a little more than two years ago, scientists trying to
explain the origin of planets had only our own solar system to go on
--a dust mote of information compared to the universe as a whole.

But then came the discovery in 1995 of what appeared to be a
Jupiter-like planet circling the sun-like star 51 Pegasi. The
relationship between parent star and child was unlike anything our own
solar system had led astronomers to expect. The extrasolar (meaning
outside our solar system) planet appears to be at least 150 times more
massive than Earth. Yet remarkably, it orbits 20 times more closely to
its parent than Earth circles the sun.

Since then, scientists have discovered what appear to be at least nine
additional extrasolar planets. The orbits of five, including 51
Pegasi's planet, are far smaller than the standard theory of planetary
formation deems possible. Scientists have dubbed the quintet "hot
Jupiters" because they are similar in mass to the largest planet in
our solar system and are so close to their stars that they should be
quite toasty.

So here was a discovery of solar systems so radically different from
the one we inhabit that scientists could no longer consider ours the
only model in the universe. Suddenly, they had a whole lot more to go
on than our own solar system -- and a lot of explaining to do.

But hot Jupiters have not forced scientists to throw out the
theoretical blueprint that explains how to build a solar system. That
blueprint was drawn up by the German philosopher Immanuel Kant in 1755
(just three years after Benjamin Franklin proved that lightning was
made of electricity). Later codified by the French mathematician
Pierre-Simon de Laplace, it still holds true today as the overarching
plan of planetary origins. But within that big picture, scientists
must now add new details.

And those details may just help them answer one of the most vexing
questions animating research in astronomy today: Are we earthlings
alone or are we just one tiny clan in a vastly larger cosmic
community?

Kant's 1755 work, Universal Natural History and Theories of the
Heavens, includes a simple yet enduring description of how planets
form in what today are called protoplanetary disks: ". . . we see a
region of space extending from the centre of the sun to unknown
distances, contained between two planes not far distant from each
other," Kant wrote. Within this disk, "the attraction of the
elementary matters for each other" draws particles together to form
larger and larger objects, eventually creating planets.

In 1983, observations by astronomers Bradford Smith and Rich Terrile
revealed a rotating disk of dust around the young star Beta Pictoris,
about 50 light-years away -- the first confirmation of a major part of
Kant's hypothesis. Since then, protoplanetary disks have become a
common sight through the Hubble Space Telescope.

"The Kant-Laplace hypothesis forms the basis of all modern work on
planet formation," notes Alan Boss, an astrophysicist at the Carnegie
Institution of Washington. "It has been spectacularly proven through
astronomical observations that these suspected protoplanetary disks
actually do exist. We need only to detect the presence of protoplanets
in these disks before finishing the proof."

But how do these planetary wombs form and then give birth to planets?
As scientists currently understand it, the process starts when part of
a rotating interstellar cloud consisting mostly of gas (a "molecular
cloud") begins collapsing in on itself. According to Jack Lissauer, an
expert in planet formation at NASA's Ames Research Center in
California, the material in the cloud cannot simply collapse
symmetrically into a sphere; the cloud's rotation creates angular
momentum, which prevents that.

Instead, the angular momentum causes some of the infalling material to
flatten into a disk that whirls around the waist of a young star at
the center. This disk, like the star, consists primarily of the
hydrogen and helium gas that made up the bulk of the molecular cloud,
with perhaps one to two percent heavier elements.

Relatively close to the star, the disk is cool enough for some of the
material to exist in solid form -- either tiny particles of
interstellar dust or other solids that have condensed into grains from
the disk's gas. According to Bill Ward, a planet formation expert at
NASA's Jet Propulsion Laboratory, electrostatic forces can make these
grains stick together. (This is the force that causes your clothing to
cling when you empty the drier.) The grains tend to settle into the
mid-plane of the disk and eventually grow into pebble-sized objects.

But it's still not entirely clear how a bunch of pebbles manage to
collide, stick together, and form kilometer-wide planetesimals -- the
"great big dirt balls," as Ward describes them, that are the
progenitors of planets.

Electrostatic forces operate at the surface of an object. So the
greater the surface area of nearby objects, the greater the likelihood
that this force can pull them together. But there's another part to
the equation: mass. Because the masses of neighboring pebbles are high
compared to their surface areas, the electrostatic attraction between
them cannot easily overcome their inertia. Consequently, the pebbles
don't readily move together, let alone stick. In addition, a pebble is
not massive enough for its gravitational field to attract and hold on
to other pebbles. So pebbles are in limbo -- they are too massive for
one force to get them to stick, yet they're not massive enough for
another to work either.

There are a number of hypotheses that attempt to pull pebbles out of
limbo, but each has its own problem. In one promising idea Ward
described recently at a NASA conference on cosmic origins, pebbles of
similar size become packed together at stagnant points in the
turbulent flow of material in the disk, helping them to coalesce into
larger and larger objects. The problem is that computer modeling shows
this process can build beachball-size objects, not planetesimals. "The
mystery is still as cloudy as ever," Ward concluded.

"So somehow, maybe better living through chemistry, planetesimals
form," Lissauer jokes. And once kilometer-size planetesimals form, the
next part is much easier to explain. Larger planetesimals in the
evolving disk are massive enough for their gravity to hold onto almost
all objects they collide with, and so they grow larger. Meanwhile,
smaller planetesimals are either sucked up by the bigger ones or are
stomped to dust in collisions.

In this way, Lissauer says, "the rich get richer and the poor get
poorer." And calculations show that the rich accumulate their wealth
faster than the likes of Bill Gates, growing much faster and fatter
than the rest of the swarm in a process known as runaway accretion.

Through the runaway process, a planetesimal can grow to the size of
the moon on the order of 100,000 years. At this point, the object has
sucked up most of the material within its feeding zone, so the runaway
winds down dramatically. The lunar-size ball of rock does keep growing
by accretion, just at a slower pace.

Slower, but hardly sedate. Moon-size objects collide cataclysmically,
adding their bulk together until planets with masses on the order of
Earth's are formed. At this stage in the planetary construction
process, the job is nearly complete for rocky terrestrial planets such
as Mercury, Venus, Earth, and Mars. All that remains to be added are
the peculiar characteristics that define an individual planet. With
fortuitous positioning around the star, a terrestrial planet may
acquire an atmosphere with just the right constituents in just the
right amounts to insure the cosmic equivalent of a Sun Belt climate --
one that can allow liquid water, a prerequisite for life, to exist.

But for giant planets such as Jupiter and Saturn, construction is far
from complete. These behemoths consist of a rock and ice core
containing as much as 10 or 15 Earth-masses enveloped by massive
atmospheres. The core can be built by runaway accretion, but it must
then acquire a huge envelope of gas. Just how that happens remains an
open question.

"We still don't have an acceptable model for giant planet formation,"
says Boss. "And without that, it's hard to say that we understand the
planet-formation process in its entirety."

Construction of a gas giant must be completed before the young star's
heat and radiation blow away the disk's gas. Otherwise, the growing
planet will run out of raw material. Scientists haven't come even
close to nailing down how long that takes. "Based on observations of
disks around young stars," Lissauer notes, "the total time available
is between one million and 10 million years."

But whatever number in that range is right, that's a startlingly brief
amount of time on the cosmic time scale to do something as profound as
build a giant gaseous planet. One way to beat the clock is through
another runaway process.

At first, the gravitational pull of the rock and ice core would gather
gas slowly. But modeling shows that as the planet becomes steadily
more massive, its increasing gravitational field draws in gas at an
ever-increasing rate, leading to runaway accretion of gas. According
to this model, the entire process -- from the first dust grains that
stick together to the completion of the giant gas atmosphere -- can
take eight million years.

Lissauer hypothesizes that in our own solar system, Jupiter and Saturn
experienced the full gas runaway effect. Uranus and Neptune formed
farther out from the sun, where orbital periods were longer and there
was less material, and thus accretion proceeded less rapidly.
Consequently, the gas disappeared before the runaway could occur,
explaining why these planets are smaller. At the far, cold reaches of
the disk, planetesimals grew very slowly due to the long orbital
periods, explaining why Pluto and the other objects beyond Neptune
lack substantial atmospheres.

Meanwhile, the terrestrial planets were too close to the sun to form
massive gas atmospheres. The problem: The small circumference of the
inner disk meant that there was not enough material to build a massive
planet before the disk dissipated. Also, the inner regions were "as
hot as a blast furnace," Boss says -- way too hot for hydrogen and
oxygen to form water ice, thought to be a key ingredient in building
the core of a massive planet. As a result, these cores didn't have
enough mass to grab the surrounding gas. Farther out, it was cold
enough for ice to form, providing the accreting core of a gaseous
planet much more solid material with which to grow especially large,
especially quickly.

Boss has proposed another way of making giant planets before a disk's
gas disappears. In the previous two-step models, gaseous planets such
as Jupiter initially form around a rock and ice core. But in Boss's
new computer models, gaseous planets form directly from clumps of gas
inside a disk. "When the disk is young and fairly massive, with about
10 percent of the sun's mass, it may spontaneously break up into giant
protoplanets because of the self-gravity of clumps of gas in the
disk," says Boss. Later, dust grains inside the planet would settle to
the center and form the core. If Boss's model is correct, the cores of
Jupiter and Saturn have only one-tenth to one-third as much mass as
once believed, and gas giants can form in only a few hundred thousand
years rather than a few million years. Still, this single-step method
only works in the cool regions of a disk, far from the star.

This is where the limits of the standard theory of planet formation
are reached, because it just can't explain how a Jupiter-size planet
can form within cosmic spitting distance of a star. Scientists haven't
given up on trying to find a way to build Jupiter-size planets close
in, but at the moment, hot Jupiters simply don't compute. "We still
have a very hard time imagining forming a planet half the mass of
Jupiter out at just .05 AU -- close to the hottest portion of the
protoplanetary disk," Boss says. (One AU -- astronomical unit -- is
the average distance between Earth and the sun.)

Hot Jupiters would compute if they formed farther out in the disk,
where theory predicts, and then migrated inward toward the star. "At
first it was a heresy to think that a full-grown massive planet could
migrate vast distances within a disk," Ward says. "But the discovery
of large extrasolar planets close to stars is circumstantial evidence
that giant planets have moved vast distances."

If they have, it may have been the result of a cosmic tug of war. The
migration hypothesis, first conceived in 1980 by Peter Goldreich of
Caltech and Scott Tremaine of the University of Toronto, holds that a
giant planet's gravitational field tugs strongly on the disk. But the
disk doesn't just sit there, it tugs back. The interplay of forces
exerts gravitational drag on the planet, robbing it of orbital angular
momentum and forcing it to spiral in toward the star.

The gravitational tug of war may also have a different effect,
Lissauer notes, but with the same outcome. A massive planet's
gravitational field may be so strong that it torques open a groove in
the disk, partitioning the disk into inner and outer regions.
According to this hypothesis, the inner region and the planet lose
angular momentum to the outer region, causing the planet to spiral
inward.

What applies the brakes to keep the planet from cruising into the star
and to its death? Two mechanisms have been proposed by Doug Lin of the
University of California at Santa Cruz. One invokes tidal interactions
between the star and planet and another, powerful stellar magnetic
fields. Both have their problems, Lissauer says. So scientists still
have some work to do on planet migration.

And why didn't our own Jupiter spiral into the sun? According to Boss,
the best explanation is that our protoplanetary disk was short-lived
compared to the disks in which the hot Jupiters formed. According to
this line of reasoning, right after Jupiter formed, the gas and dust
in the disk were all gone. With no disk remaining, there would have
been no weird gravitational interplay to force Jupiter to migrate
inward. Observational evidence supports this view, Boss says, showing
that protoplanetary disks exist for greatly varying amounts of time.

We were lucky that our disk dissipated just after Jupiter formed.
Otherwise, Jupiter would have acted like the death star in Star Wars:
It would have spelled doom for planets in its way -- including Earth.
Since the hot Jupiters likely did migrate, they probably killed off
quite a few terrestrial planets, including some that might have given
life a toehold.

"The discovery of hot Jupiters means that there will be some fraction
of planetary systems that are inhospitable to life," Boss points out.
"In the process of migrating inward, a future hot Jupiter will kick
any Earth-like planets it encounters out of its way, ejecting the
Earths altogether to freeze in interstellar space or vaporizing them
through a collision with the system's star." (A prospect that would no
doubt make Darth Vader envious.)

But the mere existence of hot Jupiters does not necessarily mean that
Earth-like planets are rare. Based on the number of stars surveyed, it
appears that only about five percent of solar-type stars have hot
Jupiters. They were discovered first simply because they were the
easiest planets to detect. Moreover, now that we know our solar system
is not the only one in the universe, it can be argued more
persuasively that many stars have planetary progeny. And perhaps some
of those planets are hospitable to life.

Boss believes the discovery of extrasolar planets lends more credence
to the idea of life outside our solar system than the discovery in
1996 of possible microbial fossils in the martian meteorite ALH84001.
"The very fact that we are finally beginning to find extrasolar
planets gives the whole concept of extrasolar life a tremendous
boost," he says. "Even more so than the ALH84001 discovery, which,
even if correct, might only be reflecting life that was transported
from Earth to Mars, or vice versa, by impacts."

In other words, it's very possible now that there is some larger
community out there to which we belong.

PHOTO (COLOR): PLANETS

PHOTO (COLOR): Inner planets gradually build up as particles and
pebbles coalesce into larger bodies. In the final stages of planet
formation (far right), outer planets sweep up huge amounts to become
gas giants.

PHOTO (COLOR): Planets may have already formed inside this dusty disk,
...

Planets may have already formed inside this dusty disk, which orbits
the young star Beta Pictoris. This disk is probably a remnant of the
planet formation process.

PHOTO (COLOR): Newly forming star

Newly forming star

PHOTO (COLOR): Gas with some dust mixed in

Gas with some dust mixed in

PHOTOS (COLOR): These disks, imaged by the Hubble Space Telescope,
contain mostly gas with some dust mixed in. They surround four newly
forming stars in the Orion Nebula. The disks are two to eight times
the diameter of the solar system and have enough material to form
planetary systems like our own. The planet formation process has
probably just gotten started within these disks.

~~~~~~~~

by Tom Yulsman

Tom Yulsman is a former editor of Earth. He is now a contributing
editor to Astronomy and an associate professor of journalism at the
University of Colorado in Boulder.
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Source: Astronomy, Feb98, Vol. 26 Issue 2, p56, 6p, 8c.
Item Number: 186787
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