NASA (from left: Neptune, Uranus, Saturn, and Jupiter)
Giant Gas Balls
Jupiter, Saturn, Uranus, and Neptune are often called gas giants. They are basically gigantic gas balls compared to Earth and the other three rocky inner planets, or to the icy "dwarf planets" (Main-Belt asteroid Ceres and Pluto and its ice moon Charon, which are also classed as large Edgeworth-Kuiper (E-K) Belt objects). The four giant planets are comprised mostly of an outer layer of molecular hydrogen and helium and a much thicker layer of metallic hydrogen. However, each may have a small solid core as large as three to 20 Earth masses at their center. Sometimes they are called "Jovians" because Saturn, Uranus, and Neptune are considered to be very similar to Jupiter ("Jove" is a variation of Jupiter in Latin).
NASA ("omitted" is misspelled)
See an animation of the orbits of these outer planets around the Sun,
with a table of basic orbital and physical characteristics. A real-time
Plot of the Outer Solar System showing current planetary positions
within their orbits as well as comets, asteroids, and related objects
is also available from the Minor Planet Center.
In the several decades old, "bottoms-up" or "core-accretion" model, the primordial disk of gas and dust that coalesced into the planetesimals and formed the planets and other planetary bodies was colder in the outer Solar System, being much farther away from the developing Sun. As a result, water and other substances that existed as volatile gases and liquids in the inner nebula were frozen into ices and supplemented the mass of disk materials being accreted into large protoplanets. Because of the greater mass concentration from ices available beyond five times the Earth-Sun distance (AU) from Sol, these protoplanets are thought to have grown much more quickly to greater size than those in warmer, inner orbits. Once these ice-rock planetary bodies grew to a critical threshold size somewhere between five to 10 times the mass of the Earth, their gravity became so great that they began to pull in large amounts of gas directly from the surrounding Solar nebula to form giant protoplanets. Finally, as happened with the inner planets, the largest protoplanet in each local swarm drew in smaller ones to form the planets now observed through collisions, or threw them out of their original orbits into the Oort Cloud, interstellar space, or into collisions with other Solar System objects such as the Sun.
Larger illustration.
The core accretion model describes fails to
explain how the largest gas giants could have
formed quickly from Sol's protoplanetary disk.
According to astronomer Alan P. Boss (Astronomy, October 2006), there's little agreement among astronomers on how the largest gas giants in the Solar System (Jupiter and Saturn) and even larger, recently discovered extra-solar giant planets may have formed. According to the "top-down" or "disk-instability" model, concentrations of a protoplanetary disk's hydrogen and helium gas (which makes up most of its mass) can grow by pulling more gas onto themselves through gravitational attraction. Hence, in a few orbital periods, spiral arms can form and collide within the disk like those in spiral galaxies in a runaway process leading to clumps of gas within a thousand years. If dense amd cool enough, these gas clumps quickly contract and collapse into gas-giant protoplanets which attract dust particles that fall into planetary cores. Between the polar extremes of the tops-down and bottoms-up models, however, astronomers have also developed hybrid mechanisms that incorporate elements of both models.
Carnegie Institution
Larger computer image.
Disk instability can lead to the
rapid formation of spiral arms and
collapse of gas clumps into giant
planets (more).
On January 8, 2007, astronomers using NASA's Spitzer Space Telescope announced their collection of evidence indicating that gas-giant planets either form within the first 10 million years of a Sol-type star's life, or not at all. They conclude that gas giant formation occurs relatively quickly after a star is born, as the lifespan for stars like the Sun with a spectral type around G2 is about 10 billion years. This hypothesis is based on a search for traces of gas around 15 very young Sun-like stars, most with ages ranging from 3 million to 30 million years, where all appear to have less than 10 percent of Jupiter's mass in gas remaining around them. The astronomers also suspect that orbital drag from abundant gas around a star may also be important for nudging rocky inner ("terrestrial") planets like Earth into relatively circular orbits as they form. Using Spitzer's Infrared Spectrometer, the astronomers searched for relatively warm gas in the inner regions of these star systems, in a zone comparable to the region between Earth and Jupiter in the Solar System. They also used ground-based radio telescopes to search for cooler gas in the outer regions of those systems, a zone comparable to the region around Saturn and beyond (Spitzer news release; and Meyer et al, 2006). Their observations were made as part of a Spitzer Legacy Science Program dealing with the Formation and Evolution of Planetary Systems (FEPS).
NASA
Larger illustration
of newly formed
gas giant around
10-million-year-
old Sol-type star,
as imagined by
Pyle with two moons
and white gas traces.
Recent research
supports theories
of quick gas-giant
planet formation
around Sun-like
stars with sufficient
gas (more).
As the gas giants developed, their gravitational pull began to perturb the orbits of the planetesimals in the inner E-K Belt. Neptune began by grabbing many nearby planetesimals from their orbits and passing them to Uranus, which then handed many to Saturn and so on to Jupiter. As such planetesimals moved closer towards the Sun into smaller orbits, each lost orbital energy and angular momentum that was absorbed by each passing gas giant. As a result, Neptune, Uranus, and Saturn began to move outward. While Neptune may have moved out as much as 30 percent, however, Jupiter lost orbital energy because it absorbed or ejected the incoming planetesimals in its reach and so its orbit may have shrunk by about two percent from this cause alone (Curtis Rist, Discover, September 2000; and Hahn and Malhotra, 2000).
© Petr Scheirich
(Used with permission)
Larger image.
The eccentric orbits of the
Hilda asteroids suggest that
Jupiter may have migrated
inward towards Sol by about
0.45 AUs (see orbit animation
and Franklin et al, 2004).
According to computer simulations inspired by the finding of "hot" Jupiter-class planets found in inner orbits around nearby stars, the Solar System's own Jupiter may have formed 10 percent farther from the Sun than it is now, and then spiralled in by about 0.45 AUs (70 million kilometers or 44 million miles) over at least 100,000 years as it lost angular momentum to drag within the thick dust disk that surrounds young stars. Supporting evidence of this in-migration comes from an unusual group of 700 or so rocky bodies known as the Hilda asteroids, which orbit the Sun three times for every two made by Jupiter, and of which the vast majority have slightly elongated elliptical orbits, whereas many other asteroids have near-circular orbits. Computer simulations (led by Fred Franklin's team at the Harvard-Smithsonian Center for Astrophysics) indicate that Jupiter's early in-migration would have ejected any proto-Hilda asteroids with circular orbits from the Solar System and would have further elongated the orbits of those that remained (Franklin et al, 2004). Luckily, Sol's dust disk was probably thin compared with those stars that dragged their outer gas giants into inner orbits closer than Mercury or into the stars themselves, perturbing the orbits of any developing, inner terrestrial planets. Indeed, astronomer Phil Armitage speculates that the in-migration of Jupiter could also have disturbed the proto-planetary bodies of the inner Solar System so that they collided more frequently, to spur the formation and growth of Earth itself. (More discussion is available on planetary migration models from Phillip K. Armitage and W.K.M. Rice, 2005 and Matsumura et al, 2006.)
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