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How Was the Solar System Formed?
By Abby Cessna
At least since the earliest recorded writings, people – scientists, astronomers, philosophers, and almost
everyone else – have been searching for the answer to how the universe was formed. There is no
authoritative model for how the universe was formed, but scientists have agreed on a most popular
model.
The most popular theory is called the nebular hypothesis. It did fall out of favor for a number of years, but
now it is back as the definitive model. According to the nebular hypothesis, our Solar System began
around 4.6 billion years ago when part of a molecular cloud of interstellar gas, which was filled with
particles of ice, dust, rock, and other particles, collapsed. These clouds collapsed from some kind of
turbulence that caused it to heat up and eventually turn into a star.
Most of the cloud formed the Sun. Other material from the cloud flattened around the Sun forming a
planetary disc. The material from the planetary disc, also known as the solar nebula, went to form the
planets and other objects in our Solar System. Some of the material forms a solid object, which gets larger
as other particles collide with it and stick together. Eventually, the object gets large enough to attract
more dust and ice with its gravitational influence. These balls then form the cores of the planets. If one of
these balls leaves the Sun early enough, it turns into a comet. Because of radiation and collision with
other objects, these balls of ice, rock, and dust turned into very different planets. Astronomers believe
that it took millions of years for the planets to form. Asteroids and other planetesimals are “failed
planets” – objects formed from the solar nebula that never got large enough to turn into planets.
Currently, astronomers are studying planets forming in other Solar Systems.
Although the nebular theory is widely accepted, there are still problems with it that astronomers have
not been able to explain away. One of these problems is the planets’ axial tilts. According to the nebular
theory, they are supposed to have the same ecliptic planet, but the inner planets and outer planets have
radically different axial tilts. With technological advancements allowing astronomers to study extrasolar
planets, scientists have noticed irregularities that cast doubt on the nebular hypothesis. Some of these
irregularities are hot Jupiters that orbit their stars in just a few days. Astronomers have adjusted the
nebular hypothesis to account for some of these problems, but so far it has not answered all the
questions.
Solar Nebular Theory
By Jerry Coffey
The solar nebula theory, also known as the nebular hypothesis, is the most widely accepted model
explaining the formation and evolution of our solar system. It was originally applied to our solar system
only but, this method of planetary system formation is now thought to hold true throughout the universe.
The modern variant that is most accepted is the Solar Nebular Disk Model (SNDM).
According to SNDM stars form in massive, dense clouds of molecular hydrogen called giant molecular
clouds(GMC). These clouds are gravitationally unstable, and matter coalesces into smaller, denser clumps
inside. These clouds collapse and form stars. This can give birth to planets under the right circumstances,
which are not fully understood. So, the formation of planetary systems is thought to be a natural result of
star formation. The process is thought to take at least 100 million years.
According to SNDM, rocky planets form in the inner part of the protoplanetary disk where the
temperature is high enough to prevent condensation of water ice and other substances into grains. This
results in the coagulation of purely rocky grains and later into the formation of rocky planetesimals. After
small planetesimals have formed, runaway accretion begins. Growth accelerates as mass accumulates.
This leads to the growth of larger bodies by the destruction of smaller bodies. This lasts between 10,000
and 100,000 years and ends when the largest bodies exceed approximately 1,000 km in diameter. Next,
oligarch accretion begins. Several hundred of the largest bodies (oligarchs), continue to accrete
planetesimals. Only the oligarchs grow. Occasionally, oligarchs impact each other and form a larger body.
The final result of the oligarchic stage is the formation of about 100 bodies between the size of the Moon
and Mars. Last is the merger phase. This begins when the oligarchs become massive enough to perturb
each other causing their orbits to become chaotic. This lasts for 10 to 100 million years and forms a
limited number of Earth sized bodies. Some of the oligarchs are thought to have brought water to Earth.
The resulting rocky planets eventually settle into nearly stable orbits.
Scientists are still trying work out how to apply the solar nebula theory to the formation of giant planets.
It is great to see that there are still challenges for our scientists. Unanswered questions mean unexplored
frontiers.
Here is a brief outline of the current theory of the events in the early history of the
solar system:
1. A cloud of interstellar gas and/or dust (the "solar nebula") is disturbed and collapses under its own gravity. The disturbance
could be, for example, the shock wave from a nearby supernova.
2. As the cloud collapses, it heats up and compresses in the center. It heats enough for the dust to vaporize. The initial collapse
is supposed to take less than 100,000 years.
3. The center compresses enough to become a protostar and the rest of the gas orbits/flows around it. Most of that gas flows
inward and adds to the mass of the forming star, but the gas is rotating. The centrifugal force from that prevents some
of the gas from reaching the forming star. Instead, it forms an "accretion disk" around the star. The disk radiates away
its energy and cools off.
4. First brake point. Depending on the details, the gas orbiting star/protostar may be unstable and start to compress under its
own gravity. That produces a double star. If it doesn't ...
5. The gas cools off enough for the metal, rock and (far enough from the forming star) ice to condense out into tiny particles.
(i.e. some of the gas turns back into dust). The metals condense almost as soon as the accretion disk forms (4.55-4.56
billion years ago according to isotope measurements of certain meteors); the rock condenses a bit later (between 4.4
and 4.55 billion years ago).
6. The dust particles collide with each other and form into larger particles. This goes on until the particles get to the size of
boulders or small asteroids.
7. Run away growth. Once the larger of these particles get big enough to have a nontrivial gravity, their growth accelerates.
Their gravity (even if it's very small) gives them an edge over smaller particles; it pulls in more, smaller particles, and
very quickly, the large objects have accumulated all of the solid matter close to their own orbit. How big they get
depends on their distance from the star and the density and composition of the protoplanetary nebula. In the solar
system, the theories say that this is large asteroid to lunar size in the inner solar system, and one to fifteen times the
Earth's size in the outer solar system. There would have been a big jump in size somewhere between the current
orbits of Mars and Jupiter: the energy from the Sun would have kept ice a vapor at closer distances, so the solid,
accretable matter would become much more common beyond a critical distance from the Sun. The accretion of these
"planetesimals" is believed to take a few hundred thousand to about twenty million years, with the outermost taking
the longest to form.
8. Two things and the second brake point. How big were those protoplanets and how quickly did they form? At about this time,
about 1 million years after the nebula cooled, the star would generate a very strong solar wind, which would sweep
away all of the gas left in the protoplanetary nebula. If a protoplanet was large enough, soon enough, its gravity would
pull in the nebular gas, and it would become a gas giant. If not, it would remain a rocky or icy body.
9. At this point, the solar system is composed only of solid, protoplanetary bodies and gas giants. The "planetesimals" would
slowly collide with each other and become more massive.
10.
Eventually, after ten to a hundred million years, you end up with ten or so planets, in stable orbits, and that's a solar
system. These planets and their surfaces may be heavily modified by the last, big collision they experience (e.g. the
largely metal composition of Mercury or the Moon).
Note: this was the theory of planetary formation as it stood before the discovery of extrasolar planets. The discoveries don't
match what the theory predicted. That could be an observational bias (odd solar systems may be easier to detect from Earth)
or problems with the theory (probably with subtle points, not the basic outline.)
Did Our Solar System Start with a “Little Bang?”
By Nancy Atkinson
What prompted the formation of our little corner of the universe – our sun and planetary system? For
several decades, scientists have thought that the Solar System formed as a result of a shock wave from an
exploding star—a supernova—that triggered the collapse of a dense, dusty gas cloud, which then
contracted to form the Sun and the planets. But detailed models of this formation process have only
worked under the simplifying assumption that the temperatures during the violent events remained
constant. That, of course, is very unlikely. But now, astrophysicists at the Carnegie Institution’s
Department of Terrestrial Magnetism (DTM) have shown for the first time that a supernova could indeed
have triggered the Solar System’s formation under the more likely conditions of rapid heating and
cooling. So have these new findings resolved this long-standing debate?
“We’ve had chemical evidence from meteorites that points to a supernova triggering our Solar System’s
formation since the 1970s,” remarked lead author, Carnegie’s Alan Boss. “But the devil has been in the
details. Until this study, scientists have not been able to work out a self-consistent scenario, where
collapse is triggered at the same time that newly created isotopes from the supernova are injected into
the collapsing cloud.”
Short-lived radioactive isotopes—versions of elements with the same number of protons, but a different
number of neutrons—found in very old meteorites decay on time scales of millions of years and turn into
different (so-called daughter) elements. Finding the daughter elements in primitive meteorites implies
that the parent short-lived radioisotopes must have been created only a million or so years before the
meteorites themselves were formed. “One of these parent isotopes, iron-60, can be made in significant
amounts only in the potent nuclear furnaces of massive or evolved stars,” explained Boss. “Iron-60 decays
into nickel-60, and nickel-60 has been found in primitive meteorites. So we’ve known where and when
the parent isotope was made, but not how it got here.”
Previous models by Boss and former DTM Fellow Prudence Foster showed that the isotopes could be
deposited into a pre-solar cloud if a shock wave from a supernova explosion slowed to 6 to 25 miles per
second and the wave and cloud had a constant temperature of -440 °F (10 K). “Those models didn’t work
if the material was heated by compression and cooled by radiation, and this conundrum has left serious
doubts in the community about whether a supernova shock started these events over four billion years
ago or not,” remarked Harri Vanhala, who found the negative result in his Ph.D. thesis work at the
Harvard-Smithsonian Center for Astrophysics in 1997.
Using an adaptive mesh refinement hydrodynamics code, FLASH2.5, designed to handle shock fronts, as
well as an improved cooling law, the Carnegie researchers considered several different situations. In all of
the models, the shock front struck a pre-solar cloud with the mass of our Sun, consisting of dust, water,
carbon monoxide, and molecular hydrogen, reaching temperatures as high as 1,340°F (1000 K). In the
absence of cooling, the cloud could not collapse. However, with the new cooling law, they found that after
100,000 years the pre-solar cloud was 1,000 times denser than before, and that heat from the shock front
was rapidly lost, resulting in only a thin layer with temperatures close to 1,340°F (1000 K). After 160,000
years, the cloud center had collapsed to become a million times denser, forming the protosun. The
researchers found that isotopes from the shock front were mixed into the protosun in a manner
consistent with their origin in a supernova.
“This is the first time a detailed model for a supernova triggering the formation of our solar system has
been shown to work,” said Boss. “We started with a Little Bang 9 billion years after the Big Bang.”
New Theory Proposed for Solar System Formation
By Fraser Cain
Like most creation stories, this one is dramatic: we began, not as a mere glimmer buried in an obscure cloud, but
instead amidst the glare and turmoil of restless giants.
Or so says a new theory, supported by stunning astronomical images and hard chemical analysis. For years most
astronomers have imagined that the Sun and Solar System formed in relative isolation, buried in a quiet, dark
corner of a less-than-imposing interstellar cloud. The new theory challenges this conventional wisdom, arguing
instead that the Sun formed in a violent nebular environment – a byproduct of the chaos wrought by intense
ultraviolet radiation and powerful explosions that accompany the short but spectacular lives of massive, luminous
stars.
The new theory is described in a “Perspectives” article appearing in the May 21 issue of Science. The article was
written by a group of Arizona State University astronomers and meteorite researchers who cite recently
discovered isotopic evidence and accumulated astronomical observations to argue for a history of development of
the Sun, the Earth and our Solar System that is significantly different from the traditionally accepted scenario.
If borne out by future work, this vision of our cosmic birth could have profound implications for understanding
everything from the size and shape of our solar system to the physical makeup of the Earth and the development of
the chemistry of life.
“There are two different sorts of environment where low-mass stars like the Sun form,” explained ASU astronomer
Jeff Hester, the essay’s lead author. “In one kind of star-forming environment, you have a fairly quiescent process in
which an undisturbed molecular cloud slowly collapses, forming a star here, a star there. The other type of
environment in which Sun-like stars form is radically different. These are more massive regions that form not only
low-mass stars, but luminous high-mass stars, as well.”
More massive regions are very different because once a high-mass star forms, it begins pumping out huge amounts
of energy that in turn completely changes the way Sun-like stars form in the surrounding environment. “People
have long imagined that the Sun formed in the first, more quiescent type of environment,” Hester noted, “but we
believe that we have compelling evidence that this is not the case.”
Critical to the team’s argument is the recent discovery in meteorites of patterns of isotopes that can only have been
caused by the radioactive decay of iron-60, an unstable isotope that has a half-life of only a million and a half years.
Iron-60 can only be formed in the heart of a massive star and thus the presence of live iron-60 in the young Solar
System provides strong evidence that when the Sun formed (4.5 billion years ago) a massive star was nearby.
Hester’s coauthors on the Science essay include Steve Desch, Kevin Healy, and Laurie Leshin. Leshin is a
cosmochemist and director of Arizona State University’s Center for Meteorite Studies. “One of the exciting things
about the research is that it is truly transdisciplinary, drawing from both astrophysics and the study of meteorites
– rocks that you can pick up and hold in your hand – to arrive at a new understanding of our origins,” noted Leshin.
When a massive star is born, its intense ultraviolet radiation forms an “HII region” – a region of hot, ionized gas
that pushes outward through interstellar space. The Eagle Nebula, the Orion Nebula, and the Trifid Nebula are all
well-known examples of HII regions. A shock wave is driven in advance of the expanding HII region, compressing
surrounding gas and triggering the formation of new low-mass stars. “We see triggered low-mass star formation
going on in HII regions today,” said Healy, who recently completed a study of radio observations of this process at
work.
The star does not have much time to get its act together, though. Within 100,000 years or so, the star and what is
left of its small natal cloud will be uncovered by the advancing boundary of the HII region and exposed directly to
the harsh ultraviolet radiation from the massive star. “We see such objects emerging from the boundaries of HII
regions,” Hester said. “These are the ‘evaporating gaseous globules’ or ‘EGGs’ seen in the famous Hubble image of
the Eagle Nebula.”
EGGs do not live forever either. Within about ten thousand years an EGG evaporates, leaving behind only the lowmass star and its now-unprotected protoplanetary disk to face the brunt of the massive star’s wrath. Like a chip of
dry ice on a hot day, the disk itself now begins to evaporate, forming a characteristic tear-drop-shaped structure
like the “proplyds” seen in Hubble images of the Orion Nebula. “Once we understood what we were looking at, we
realized that we had a number of images of EGGs caught just as they were turning into proplyds,” said Hester. “The
evolutionary tie between these two classes of objects is clear.”
Within another ten thousand years or so the proplyd, too, is eroded away. All that remains is the star itself,
surrounded by the inner part of the disk (comparable in size to our Solar System), which is able to withstand the
continuing onslaught of radiation. It is from this disk and in this environment that planets may form.
The process leaves a Sun-like star and its surrounding disk sitting in the interior of a low density cavity with a
massive star close at hand. Massive stars die young, exploding in violent events called “supernovas.” When a
supernova explodes it peppers surrounding infant planetary systems with newly synthesized chemical elements –
including short-lived radioactive isotopes such as iron-60.
“This is where the meteorite data come in,” said Hester. “When we look at HII regions we see that they are filled
with young, Sun-like stars, many of which are known to be surrounded by protoplanetary disks. Once you ask the
question, ‘what is going to happen when those massive stars go supernova?’, the answer is pretty obvious. Those
young disks are going to get enriched with a lot of freshly-made elements.”
“When you then pick up a meteorite and find a mix of materials that can only be easily explained by a nearby
supernova, you realize that you are looking at the answer to a very longstanding question in astronomy and
planetary science,” Desch added.
“So from this we now know that if you could go back 4.5 billion years and watch the Sun and Solar System forming,
you would see the kind of environment that you see today in the Eagle or Trifid nebulas,” said Hester.
“There are many aspects of our Solar System that seem to make sense in light of the new scenario,” notes Leshin.
“For example, this might be why the outer part of the Solar System – the Kuiper Belt – seems to end abruptly.
Ultraviolet radiation would also have played a role in the organic chemistry of the young solar system, and could
explain other peculiar effects such as anomalies in the abundances of isotopes of oxygen in meteorites.”
One of the most intriguing speculations is that the amount of radioactive material injected into the young solar
system by a supernova might have profoundly influenced the habitability of Earth itself. Heat released by the decay
of this material may have been responsible for “baking out” the planetesimals from which the earth formed, and in
the process determining how much water is on Earth today.
“It is kind of exciting to think that life on Earth may owe its existence to exactly what sort of massive star triggered
the formation of the Sun in the first place, and exactly how close we happened to be to that star when it went
supernova,” mused Hester. “One thing that is clear is that the traditional boundaries between fields such as
astrophysics, meteoritics, planetary science, and astrobiology just got less clear-cut. This new scenario has a lot of
implications, and makes a lot of new predictions that we can test.”
If it is accepted, the new theory may also be of use in looking for life in the universe beyond. “We want to know
how common Earth-like planets are. The problem with answering that question is that if you don’t know how
Earth-like planets are formed – if you don’t understand their connection with astrophysical environments – then
all you can do is speculate,” Hester said.
“We think that we’re starting to see a very specific causal connection between astrophysical environments and the
things that have to be in place to make a planet like ours.”