Figure
1.
Lunar craters: Von Karman, Leibniz, and Oppenheimer. Credit: NASA/Michael
Benson
The ongoing (and exciting!) Dawn mission to the Asteroid Belt is just the latest in a long line of efforts to understand the nature and origin of our Solar System. The modern era of system exploration began more than half a century ago, back in the 1950s. That’s when C.C. Patterson of Caltech used measurements of lead isotopes in meteorites associated with Meteor Crater, Arizona, to estimate that the Solar System formed 4.55 billion years ago (Patterson 1956). Within error margins, his date still stands today.
Other crucial evidence came from manned and unmanned
missions to the Moon, beginning in 1959 with the Luna program of the
Soviet Union and quickly followed by the Surveyor and Apollo programs
of the United States. These and later efforts mapped the entire Lunar surface,
enabling a detailed study of the Moon’s history of cratering
(Figure 1). They also brought back rocks from Lunar impact sites that provided
a range of dates, none of which were older than 3.9 billion years. This was the
first indication of a discrete, system-wide cataclysm now known as the Late
Heavy Bombardment.
Since those pioneering space programs, ever larger amounts
of data have been accumulating from robotic missions to various parts of the
inner and outer Solar System, as well as from ground- and space-based studies
of star-forming regions in our neighborhood of the Milky Way. Beginning in the
mid-1990s, data on extrasolar planetary systems have contributed strikingly new
perspectives, since most exoplanetary systems bear little resemblance to ours,
even though they must have formed from similar building blocks.
Figure 2. The eight Solar
planets at their approximate relative sizes.
As a result of all this cosmic archaeology over the past six
decades by some thousands of researchers, it’s now possible for a geeky
layperson (like myself) to sketch a surprisingly detailed history of our Solar
System (Figure 2). This review of current findings is principally informed by
the work of Alessandro Morbidelli and the growing number of astronomers and
planetary scientists who have collaborated on successive iterations of the
so-called Nice model of system evolution. (As
a theoretical framework, this model very nice indeed, but its name actually
derives from the French city (Nice) that
houses the institution (l’Observatoire de la Côte
d’Azur) with which Dr. Morbidelli is affiliated.)
The story proceeds in seven sections:
- primordial chaos: Birth environment of the Solar System
- structure emerges: Structure and early evolution of the Sun’s protoplanetary disk
- accretion everywhere: Assembly of planetesimals and planetary cores
- the grand tack: Migration of the four giant planets through the primordial gas nebula
- worlds in collision: Formation of the four terrestrial planets by giant impacts
- late heavy bombardment: Asteroids and comets scatter throughout the system
- life: Biogenesis on Earth and potentially elsewhere in our system
primordial chaos
All stars form in clusters. Our Sun was no exception. All
clusters originate in huge dark nebulae (Figure 3) composed of molecular
hydrogen swirling in supersonic turbulence (McKee
& Ostriker 2007). Such a vast dark cloud, extending for tens of light
years, was the primordial chaos out of which you and I and everything else on
Earth originated.
Figure 3. Dark molecular clouds in Taurus.
Credit: European Southern Observatory/Davide de Martine
Credit: European Southern Observatory/Davide de Martine
Fragmentation and clumping within the birth nebula produced
hydrogen “cores” that collapsed into protostars. Evidence from short-lived
radioactive isotopes in the present-day Solar System indicate that our native
cloud produced at least one extremely massive O-type star (~25 times Solar mass),
which enriched its surroundings either by exploding as a supernova or by
producing fierce stellar winds. A star so massive can form only in a relatively
large cluster with more than 1,000 members (Adams
2010).
Accordingly, various analyses have suggested an initial membership
of 1,000 to 5,000 stars for the Solar System’s birth cluster. This is an
unusually large population, since the median membership for birth clusters in
the Milky Way is only about 300 protostars (Lada
& Lada 2003, Adams et al. 2006). The powerful gravitational
influence of member stars would have kept our home cluster bound together for
tens or even hundreds of millions of years – a span of ages comparable to those
of the Pleiades (100
million; see Figure 7) and Hyades
Cluster (625 million). Our huge cohort of sibling stars was originally confined
to a volume only a few parsecs in diameter (~10 light years), but the cluster steadily
expanded over time as individual stars began to spill out of its tidal radius.
Stars and substellar objects are currently classified into
10 types according to their spectra. They
range from the very brightest, hottest, and bluest objects (types O and B, which
include the three stars of Orion’s Belt), through bright white stars (type A,
represented by such familiar backyard stars as Sirius, Vega, and Altair),
through the yellowish Sun-like stars (types F, G, and K, represented by the nearby
planet-hosting stars Upsilon Andromeda, our Sun, and Epsilon Eridani,
respectively), through the red dwarfs (type M, which includes GJ 581, another nearby
planetary host), down to the dim brown dwarfs (types L, T, and now Y), which
are not massive enough to sustain nuclear fusion, and thus are not stars.
Remarkably, star clusters tend to produce objects at the
full range of spectral types (except for type O, which as we have seen is
confined to larger clusters). True stars are born in numbers that correspond
inversely to their mass and temperature, so that red dwarfs are the most
numerous and O stars are the rarest. This correlation does not extend into the
substellar regime, however, as brown dwarfs are apparently much less common
than red dwarfs (Kirkpatrick et al. 2012).
Figure 4. The Iris Nebula (NGC 7023), a stellar
nursery undergoing
dispersal. Credit: Wikimedia/Hunter Wilson
dispersal. Credit: Wikimedia/Hunter Wilson
Within the depths of our primordial cloud, star formation
proceeded in waves – sometimes called starburst episodes – extending over
thousands or even millions of years. As soon as the brightest stars ignited,
their intense radiation and stellar winds began to disperse the parent nebula
(Figure 4). Complete dispersal of the gas envelope required 1 to 10 million
years (Lada & Lada 2003, Throop & Bally
2008), whereupon the cluster of newborn stars was unveiled for the
universe to see. By this time the process of star formation had ended and
cluster evaporation had already begun.
Generations of newborn stars arrived in cocoons of hydrogen
and dust known as protoplanetary nebulae or protoplanetary disks (“proplyds”).
Like the vast, encompassing parent cloud, these smaller clouds also dispersed
within a few million years, with dissipation proceeding faster around hotter,
more massive stars and slower around smaller, cooler stars. Typical lifetimes of
protoplanetary disks fall in the same range as the lifetimes of molecular
clouds – i.e., 1-10 million years – with a median age between 2 and 3 million
years (Williams & Cieza 2011). Stars older
than 6 million years rarely preserve their primordial disks (Williams 2010).
structure emerges
Despite the Copernican principle of mediocrity, which
asserts that there is nothing special about Earth, many factors indicate that
we humans occupy a favored piece of real estate. First, we originated in an
unusually large birth cluster with a substantial cohort of very bright stars.
Second, our native star belongs to spectral class G2, meaning that it is hotter
and more massive than 90% of the stars in the Milky Way. Notably, stars of
higher mass (such as our Sun) are more likely to host planets than stars of
lower mass (Johnson et al. 2010). Third,
our Sun has a higher than average enrichment in elements heavier than hydrogen
(known to astronomers as metals). All members of our birth cluster must have
shared this endowment, which like high mass encourages the evolution of
planets, especially gas giants like Jupiter and Saturn (Johnson & Li 2012). Thus, our native environment showed
clear promise of future greatness.
Figure 5. Proplyds in the Orion Nebula.
Credit: NASA/ESA/Luca Ricci
Credit: NASA/ESA/Luca Ricci
The protoplanetary nebula surrounding the newly-formed Sun
was probably fluffy and irregular at first, like many of the proplyds in Figure
5. In short order, nevertheless, the angular momentum of our rapidly spinning
parent star caused this puffy cloud to flatten into a thin disk of gas and dust,
typically known as the Solar nebula. The disk midplane was aligned with the
Sun’s equator, and its spin naturally followed the Sun’s rotation (Figure 6).
The ratio of gas to solids in the Solar nebula was originally
about 100 to 1 (Andrews & Williams 2005),
and its overall mass must have been about 1% to 3% of the Sun’s (Williams 2010, Adams 2010), corresponding to 10-35
Jupiter masses (Mjup; the Sun is 1048 Mjup). Since the total mass of all
surviving planets, dwarf planets, moons, asteroids, and comets in the
present-day Solar System is less than 2 Mjup, the evolutionary process was
markedly inefficient: 80% to 95% of our original patrimony of gas and dust has
vanished.
Considerable evidence indicates that the primordial disk of
solids in our system had a sharp outer edge at a radius of 30-35 astronomical
units (AU; Levison et al. 2003, Tsiganis et al. 2005, Adams 2010). This is
notably smaller than the observed dimensions of many nearby dust disks,
suggesting that our native nebula was truncated in some way, perhaps by sibling
stars in the birth cluster. A likely culprit would be a nearby O star, since observations
of the Orion Nebula demonstrate that such massive luminaries can burn off the outer
layers of the protoplanetary nebulae of smaller stars that venture within a few
light years of their highly energetic photospheres (Williams
& Cieza 2011).
Figure 6. Artist’s impression of a young
protoplanetary disk.
Credit: NASA/JPL-CalTech
Credit: NASA/JPL-CalTech
Our own Sun’s magnetospheric activity created an inner
cavity in the disk at an approximate radius of 0.05-0.10
AU (Laine et al. 2008). Between this inner edge and the outer limit at ~30 AU,
the temperature, surface density, and composition of the disk varied
systematically. At small radii, close to the blazing Sun, only the refractory
particles (metals and silicates) suspended in the gas cloud could survive,
while icy particles (water, methane, ammonia) dominated the mix on wider
orbits.
The boundary between the two
regimes of ice and rock is known as the ice line or snow line – the
distance from the Sun where free-floating molecules of water and other
volatiles condense into ice (Ida
& Lin 2004, Kennedy & Kenyon 2008).
This occurs at a temperature of 170 K, which in the contemporary Solar
System corresponds to a radius of 2.7 AU. At primordial times, however, the
situation was different. On the one hand, our Sun was about 30% cooler than it
is now, so it added less heat to the circumstellar environment; on the other
hand, the viscous flow of gases through the Solar nebula created a powerful
alternative source of heat. The net result is that our system’s ice line may
have been located around 4 or 5 AU when the Sun was ~100,000 years old,
shrinking to 2 AU by the time the Sun reached its one-millionth birthday (Kennedy & Kenyon 2008).
Another key structural feature of
the Solar nebula was its flared profile, such that its vertical dimension
increased along with its radius. This configuration is typical of
protoplanetary disks (Williams
& Cieza 2011), and is visible in the
proplyd at the very center of Figure 5.
Figure 7. The Pleiades star cluster. Image by Antonio
Fernández Sánchez.
Credit: NASA/courtesy of nasaimages.org
Credit: NASA/courtesy of nasaimages.org
accretion everywhere
The Solar
System’s primordial ice line has been compared to a standing blizzard (Lin 2008). Just beyond this limit, the
presence of ice particles increases the density of solid materials by a factor
of 3 or 4 (Kennedy & Kenyon 2008).
This “bump” in density and solid mass has crucial implications for the
evolution of planetary systems.
The abundance of icy particles,
along with the favorable collisional velocities obtaining on long, chilly
orbits, combined to make this region the “sweet spot” of planet formation (Mordasini et al. 2012). Conditions for the
coagulation of solids were more favorable here than anywhere else in the disk,
so it is no surprise that our system’s most massive planet, Jupiter, assembled
just beyond this boundary.
The formation of gas giant planets has been described as a
threshold phenomenon, as it requires a specific set of initial conditions –
sufficient density of solids in the protoplanetary nebula to assemble large
planetary cores, sufficient persistence of nebular hydrogen to support runaway
gas accretion – that apparently obtain around less than half of all stars (see How Weird Is Our Solar System?). As Andrew Mann
and colleagues have argued, “a substantial fraction, and probably the majority
of stars do not host [gas] giant planets” (Mann
et al. 2010). Low-mass stars are especially likely to be giant-free (Laughlin et al. 2004).
The Solar nebula was moderately massive, with a modest
enhancement of metals. These initial conditions enabled it to generate two
relatively small gas giants. Although Jupiter has triple the mass of Saturn, it
is still a relative lightweight in the exoplanetary population. The median mass
of extrasolar gas giants is 1.6 Mjup, and many systems host giants of 5 or even
10 Mjup. Regardless of their mass, however, gas giants seem more likely to come
in pairs than in triplets or quadruplets; only four systems (47 Ursae Majoris,
HD 37124, HIP 14810, Mu Arae) are known to contain three, and only Upsilon
Andromedae has so far been proposed as the home of four.
Yet proto-Jupiter and proto-Saturn
were not alone on the cold orbits of the outer nebula. At least two and
possibly three or more objects in the range of the gas dwarfs formed on the far
side of the sweet spot – Uranus and Neptune for sure, and perhaps a smaller gas
dwarf or Super Earth nicknamed “Amphitrite” (Desch
2007, Desch & Porter 2010). Their cores assembled somewhere between
5 and 10 AU.
Figure 8. The South Pole of Vesta, oldest rocky sphere
in the Solar System.
Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA
Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA
A single favored region of the
Solar nebula, then, produced all of the most massive planets in our system. Nevertheless,
accretion – the process of growth
by gradual accumulation – was active throughout the nebula as soon as it flattened
into a disk. Accretion is the route by which primordial dust grains
conglomerated into planetesimals,
the building blocks of planets. Current theories propose that a steady,
stepwise process of growth was punctuated by the much more rapid collapse of
refractory particles into full-sized planetesimals, with an approximate range
in diameter of 1 to 100 kilometers (Johansen et
al. 2007, Morbidelli et al. 2009b).
In the inner Solar System,
accretion proceeded quickly because of the relatively high velocities of
orbiting solids. The result was most likely a large population of so-called planetary embryos traveling in concentric
orbits. The presence of nebular gas damped the eccentricity of their orbits –
i.e., prevented any deviations from circularity – so that their growth stopped
once they exhausted their local “feeding zones.” Isolation masses for these
rocky embryos have been estimated at 1%-10% of Earth mass, equivalent to a
population of Moon- to Mars-sized objects (Morbidelli
et al. 2012).
Some sense of the time required for
embryo gestation is provided by the rocky asteroid Vesta (Figure 8), which
evidently attained its mature mass and spherical shape just 2 to 3 million
years after the condensation of the Solar nebula (Russell
et al. 2012). Vesta must have had numerous siblings of roughly similar
mass and composition in the young Solar System, though very few survive as
isolated objects today.
. . . . continued in next blog post, with list of references . . . .
This comment has been removed by the author.
ReplyDeleteGood evening, Mr. Alley.
ReplyDeleteCan I use your post material on "Solar System Archeology, parts 1 and 2" on my site?
And... of course, promote your site, also.
The material is excellent !!!
Well, I also publish Astronomy, Cosmology and Physics.
The site is: http://roberto-furnari.blogspot.com/