Solar System - Summary and History
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Sun and Planets
The following table compares major features of the Sun and planets, and relates many planetary characteristics to Earth's own:
NASA (Jupiter's moon Ganymede is larger than Mercury or Earth's Moon but much less dense)
The following table compares additional orbital and atmospheric characteristics of the planets:
|Neptune||0.009||1.8||28.3||-201||-193 to -253||1.3|
See an animation of the orbits of the nine planets around the Sun (and of satellites and rings around certain planets), with a table of basic orbital and physical characteristics. In addition, Sylvain G. Korzennik -- an astronomer at the Harvard-Smithsonian Center for Astrophysics working on the Advanced Fiber Optic Echelle AFOE) spectrometer -- has developed the following animations of planetary orbits for the Solar System:
Lastly, try the Orbit Viewer also, originally written by Osamu Ajiki of AstroArts and modified by Ron Baalke of NASA's Jet Propulsion Laboratory, to see real-time orbit animations of the planets, and known asteroids, Edgeworth-Kuiper ice bodies, and comets.
History of Planetary Development
On October 4, 2006, a team of astronomers announced the finding of evidence that Sol formed in a fragment (Solar nebula) of a giant molecular cloud (e.g., the Orion Cloud) of gas and dust that gave birth to a large open star cluster with hundreds to thousands of members. According to astronomer Leslie Looney, the evidence for Sol's stellar sisters was found in decayed particles from radioactive isotopes of iron trapped in meteorites, which can be studied as fossil traces of early Solar System conditions. The isotopic evidence indicates that a supernova from a massive star with the mass of at least 20 Solar-masses (probably a very rare, hot, and blue O-type star like Anitak Aa) exploded near the early Sun when it formed 4.6 billion years ago. Measured abundances of the isotopic particle species indicate that the supernova was located only about 0.32 to 5.22 light-years from Sol. Where there are supernovae or any massive star, there should have been hundreds to thousands of low-mass stars like the Sun that were born of the same nebula of gas and dust. Due to insufficient gravitational pull, Sol's surrounding cluster of stars dispersed over the past five billion years as they moved around the developing Milky Way galaxy, and members escaped the cluster due to velocity changes from close encounters with each other, tidal forces in the galactic gravitational field, and encounters with field stars and interstellar clouds crossing their way (press release).
Dave Devine, and
Ralph Sutherland, STScI, NASA
Larger false-color image.
Sol was probably born in a large
open star cluster near a massive
but rare, O-type star (like Theta1
Orionis C, the brightest of the
four central stars of the Trapezium
Cluster in the Orion nebula, at
left) that exploded as a supernova.
On May 24, 2007, a team of astronomers announced that the presence of an isotope of aluminium suggests the Sun was born when an extremely massive star with around 30 Solar-masses released a great amount of energy in winds loaded with aluminium-26. The strong winds of the massive star may have buffeted the Solar nebula sufficiently to initiate the development of the Solar System (Zeeya Merali, New Scientist, May 24, 2007; and Bizzarro et al, 2007; and Shukolyukov and Lugmair, 1993).
The high average abundance of gold in the Solar System suggests that large amounts of the element was injected into matter that eventually coalesced into the Solar nebula by the collision of two neutron stars in a short-duration gamma ray burst (more from Astronomy Picture of the Day).
© Werner Benger,
Albert Einstein Institute
(Artwork used with permission)
Larger illustration from a movie.
The high abundance of gold in the
Solar System may have come mostly
from the collision of two neutron stars
in a close binary system (more from
Insights Magazine and the movie).
The Solar nebula of gas and dust became cold and dense enough to collapse under the force of gravity and form our Sun, Sol, by initiating nuclear fusion of core hydrogen gas.
The developing Sun was surrounded by a rotating disk of leftover gas (mostly hydrogen and helium) and dust.
et al, 1999)
Herbig-Haro object 30 (HH 30)
The Sun collapsed out
of the Solar Nebula
ringed with a disk of
infalling gas and dust,
much of which it spewed
back out in polar jets,
like those shown in this
video clip of young HH 30.
Year 0.1 to 1 Million - planetesimals form
Micron-sized dust grains or their more volatile components (such as water) were vaporized in the hot and denser region closer to the Sun, while those located in the cooler and more tenuous outer region of the disk grew from the condensation of vaporized heavier elements around them. Particles located beyond the "snow line" (between 2 and 4 AUs out from the Sun) grew faster from the condensation of volatiles into icy grains, while inner grains became rocky particles.
Dust particles are stirred by nearby gas and collide with each other. Some particles grew and spiraled inward, so that meter-sized bodies grew and moved halfway towards the Sun within a thousand years.
Kilometer-sized bodies called planetesimals gathered up most of the original dust within one million years.
Pat Rawlings, NASA
Within the circum-Solar disk of gas and dust, micron-sized
dust grains agglomerated into ever larger particles,
meter-sized boulders, planetesimals, then planetary embryos.
Some planetesimals survived and grew from gravity-assisted collisions into ever larger planetary embryos until run out of planetesimals within narrow orbital bands.
The size of planetary embryos grew with distance from the Sun, due to the geometric growth in the volume of surrounding planetesimals within gravitational reach.
Evidence of an isotope of iron found in meteorites indicate that a supernova exploded at around Year 1 million to inject iron-60 into the young Solar System which later planetesimals and embryos incorporated into their iron rich cores (Zeeya Merali, New Scientist, May 24, 2007; and Bizzarro et al, 2007).
According to astronomer Alan P. Boss (Astronomy, October 2006), many astronomers now believe that the development of planetesimals into protoplanets as large as Earth's Moon (0.123 Earth-mass) was a runaway process, where a young Solar System may have developed a swarm of hundreds of Lunar-mass protoplanets in as little as 100,000 years. In a May 2008 update, astronomer Douglas N. C. Lin (Scientific American, May 2008) notes that within around 1 AU from the Sun, embryos grew to about 0.1 Earth-mass within 100,000 years.
Year 1 to 10 Million - planetary embryos form
At 5 AUs, embryos grew to around 4 Earth-masses within a few million years, but embryos could grow larger near the snow line or on the edges of gaps in the Sun's circum-Solar disk where planetesimals also tend to accumulate.
to Saturn and Titan
Jupiter probably formed first
while the disk was still rich
in gas, acquiring half its mass
in as little as a thousand years
(shown here with Europa).
According to the latest theoretical models, the gas-rich giant planets formed first. Jupiter probably began as a planetary "seed" of some multiple (10 according to some early models) of an Earth-mass located near the planetesimal-rich snow line that then accumulated some 300 Earth-masses of hydrogen and helium gas at an accelerating rate while the Solar disk was still gas rich, perhaps acquiring half its mass within only a thousand years. It probably swept out the first generation of asteroids by Year 2 million (where dating of radioactive isotopes found in meteorites indicate that the present generation of asteroids -- which include fragments from the iron cores of large differentiated planetesimals and planetary embryos destroyed in massive collisions -- was formed around four million years after the birth of the Sun). Saturn's development may have stalled at a lower mass than Jupiter simply because it formed a few million years later than Jupiter, after the first gas giant to develop had already depleted the disk's abundance of hydrogen and helium gas.
Cassini Imaging Team,
SSI, JPL, ESA, NASA
While Jupiter helped Saturn
to form, there was less gas
left for Saturn to bulk up with
after the first two million years
of planetary development.
The development of the gas giants from planetary embryos was regulated by competing processes involving the cooling of gas falling into the Sun, how fast such infalling gas was expelled by the Sun into interstellar space, and type I and II planetary migration towards the Sun. Once a gas giant like Jupiter formed, however, its presence probably facilitated the development of more gas giants. It probably have flung nearby planetesimals to farther out reaches of the Solar System where they can form. Then Jupiter probably helped Saturn, Uranus, and Neptune to growth by gravitationally pulling in gas and planetary material that helped them form.
In the case of Uranus and Neptune, the accumulation of planetesimals (assisted by Jupiter) progressed farther to some 10 to 20 Earth-masses which delayed the onset of gas accretion until too little gas was left so that each has only around 2 Earth-masses of gas (more from Alan P. Boss, 2002). Uranus and Neptune are "ice giants," with thick icy mantles around their rocky cores but only a relatively thin atmosphere of gas compared with Jupiter and Saturn.
Although the fast formation of gas giants appears to involve a balance of competing processes, the development of the four rocky inner or "terrestrial" planets (Mercury, Venus, Earth, and Mars) within the snow line inward of Jupiter's orbit was slower but appears to have avoided significant orbital migration. At the range of the terrestrial planets' orbital distances with the snow line (0.4 to 1.5 AU from the Sun, embryos that were composed of mostly iron and silicate rocks grow to around 0.1 Earth-mass within 100,000 years.
After the circum-Solar disk was depleted of gas, the inner planetary embryos gradually destabilized each other's orbits by making them more elliptical (eccentric) over a few million years to eventually caused more collisions. While astronomers are not yet in agreement as to how orbital stability was subsequently achieved among the inner planets, it is known that the four terrestrial planets eventually merged with some of the remaining planetesimals while deflecting the rest into the Sun over the following 100 million years, which made their orbits more circular and stable. As the planetary embryos grew into Mars-sized protoplanets, however, these objects interacted gravitationally over many orbits so that their initially circular orbits became increasingly elliptical and they collided and merged into larger bodies over tens of millions of years. Colliding at speeds up to 22,000 miles per hour (36,000 kilometers per hour), such a collision may have stripped most of the rocky mantle from the protoplanet that became Mercury with its iron-rich core.
Dating of radioactive isotopes indicate that Mars may have formed some 10 million years after the Sun, based on the dating of Martian meteorites. Earth developed with most of its final mass at around 50 million years when the proto-Earth collided with a Mars-sized planetary embryo ("Theia") around Year 50 million. Much of the Mars-sized embryo's core merged with the proto-Earth's own, while the lighter ("mantle") materials of the collision reformed as the Moon around 10 million years later (more discussion, illustrations, and links and 2007 update).
By around Year 50 million, the dispersion of the Solar System's larger star cluster may have perturbed some planetary orbits. As previously discussed, some orbital instabilities may have also developed as the Sun cleared out most of the gas within its disk. In addition, the formation of the giant planets also led to gravitational scattering of leftover planetesimals and planetary embryos. Uranus and Neptune are now thought to have hurled planetesimals toward the Edgeworth-Kuiper Belt as well as into the Sun. Jupiter's greater gravitational might enabled it to sling planetary objects far out into the Oort Cloud, which may contain as much as 100 Earth-masses.
Further planetary migrations of the giant planets and
their gravitational scattering of leftover planetesimals
may have caused the craters of the "late heavy
bombardment" found on Earth's Moon.
Gravitational scattering of planetesimals and embryos led to further orbital migration of the remaining planets. While Neptune and Pluto developed orbital synchrony, Saturn may have moved closer to Jupiter before moving back outward, and caused the so-called "late heavy bombardment" of the inner Solar System as dated by craters on Earth's Moon around Year 500 to 800 million (Bottke and Levison, 2007; and Gomes et al, 2005.
Year 1 to 4.7 Billion - major post-formation cataclysms
Extended Scattered Disk objects include 2000 CR105 (which moves inward to within 44 AUs of the Sun but then outward beyond 500 AUs) and Sedna, which never comes closer to the Sun than 76 AUs before moving to around 900 AUs away (into the realm of the inner Oort Cloud). Given the large orbital eccentricities of these two objects (which move beyond 500 AUs of the Sun), some astronomers have argued that they were likely to have been strongly perturbed by a massive celestial object (which is unlikely to have been Neptune as they do not come close enough to its gravitational influence) such as a rogue planet or passing star, which could have dragged the two objects farther out after initial orbital perturbation by Neptune. (More discussion on such scenarios with illustrations from computer models are available from a 2005 Powerpoint presentation by Brett Gladman and Collin Chan.)
Although inclined by only around 11.9 degrees from the ecliptic where the eight major planets orbit, Sedna's distant orbit is extremely elliptical indicating that its formation and orbit may have been influenced by a passing nearby star during the early years of the Solar System, when Sol formed out of a molecular cloud with many other closeby stars around 4.6 billion years ago. Like 2000 CR105, Sedna may have been perturbed by a Solar-mass star at around 800 AUs from Sol more than 100 million years after its birth, given today's observed numbers of Oort Cloud comets (Morbidelli and Levison, submitted 2004).
Unlike the other planets, Uranus' axis of rotation is lies mostly in the plane of the Solar System, as if some some titanic collision had tipped the planet over on its side, but a new theory suggests that its extreme tilt could have been created by a series of smaller shifts through orbital migrations and interactions between the giant planets during the earliest stages of Solar System formation (Adrián Brunini, 2006).
At a meeting of the American Astronomical Society on October 9, 2006, two planetary scientists (Alex Alemi and David Stevenson) described model results which suggest that Venus may have once had a moon that was subsequently destroyed. Under current theories of solar system formation, Venus is unlikely to have avoided theia a protoplanetary collision large enough to create a moon, and computer simulations suggest that most large collisions create a debris disk from which a moon forms. Alemi and Stevenson suggest that a sequence of two large collisions within around 10 million years could have first created a moon but then destroyed it and sent debris crashing onto Venus; this eventually created the planet's slow retrograde rotation today, which is otherwise difficult to explain (Alemi and Stevenson, 2006; and George Musser, Scientific American Science News, October 10, 2006).
Oddly enough, four of Neptune's moons are orbiting the planet within its Roche limit. Inside that limit, Neptune's gravitation pull is so strong that, in theory, no Solar nebular material could have agglomerated into those satellites, and so the satellites must have been captured or dragged within the Roche limit by tidal forces on the planet's surface (Rist, 2000). According to Carolyn Porco of the University of Arizona's Lunar and Planetary Laboratory, these satellites may eventually disintegrate to create a ring system as striking as those of Saturn.
Over time, the Milky Way's spiral arms have cast the Sun far from its birthplace, according to new numerical simulations. As a result, stars near the Solar System vary widely in their chemical composition because a number of them appear to have been perturbed by the galaxy's arms into "wild" or elongated orbits that move them far from their birthplace. However, this perturbation by the spiral arms still preserves the orbital circularity of many stars (including the Sun) around the galactic center (more from Rachel Courtland, New Scientist, September 17, 2008; Rok Roškar et al, 2008; and the Roškar team's an animation of how the creation and destruction of galactic spiral arms can causes stars to migrate far from their birthplace).
Fact sheets on major objects in the Solar System are available from NASA's National Space Science Data Center.
David Seal (a mission planner and engineer at NASA's Jet Propulsion Laboratory at CalTech) has a web site that generates simulated images of the Sun, planets, and major moons from different perspectives and at different times of the year. Try his Solar System Simulator.
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