Differentiation into Inner and Outer Planets

At this stage the individual accreting objects in the disk show differences in their growth and composition that depend on their distances from the hot central mass. Close to the nascent Sun, temperatures are too high for water to condense from gaseous form to ice, but, at the distance of present-day Jupiter (778 million kilometres [km], 5.2 astronomical units [AU], or 483 million miles) and beyond, water ice can form. The significance of this difference is related to the availability of water to the forming planets. Because of the relative abundances in the universe of the various elements, more molecules of water can form than of any other compound. (Water, in fact, is the second most abundant molecule in the universe, after molecular hydrogen.) Consequently, objects forming in the solar nebula at temperatures at which water can condense to ice are able to acquire much more mass in the form of solid material than objects forming closer to the Sun.

Once such an accreting body achieves approximately 10 times the present mass of Earth, its gravity can attract and retain large amounts of even the lightest elements, hydrogen and helium, from the solar nebula. These are the two most abundant elements in the universe, and so planets forming in this region can become very massive indeed. Only at distances of 5 AU or more is there enough mass of material in the solar nebula to build such a planet.

This simple picture can explain the extensive differences observed between the inner and outer planets. The inner planets formed at temperatures too high to allow the abundant volatile sub-stances—those with comparatively low freezing temperatures—such as water, carbon dioxide, and ammonia to condense to their ices. They therefore remained small rocky bodies. In contrast, the large low-density, gas-rich outer planets formed at distances beyond what astronomers have dubbed the "snow line"—i.e., the minimum radius from the Sun at which water ice could have condensed, at about 150 K (Kelvin; -120 °C, or -190 °F). The effect of the temperature gradient in the solar nebula can be seen today in the increasing fraction of condensed volatiles in solid bodies as their distance from the Sun increases. (See the table of compositional data for selected objects.) As the nebular gas cooled, the first solid materials to condense from a gaseous phase were grains of metal-containing silicates, the basis of rocks. This was followed, at larger distances from the Sun, by formation of the ices. In the inner solar system, Earth's Moon, with a density of 3.3 g/cm3 (1.91 oz/in3), is a satellite composed of silicate minerals. In the outer solar system are low-density moons such as Saturn's Tethys. With a density of about 1 g/cm3 (0.58 oz/in3), this object must consist mainly of water ice. At distances still farther out, the satellite densities rise again but only slightly, presumably because they incorporate denser solids, such as frozen carbon dioxide, which condense at even lower temperatures.

Despite its apparent logic, this scenario has received some strong challenges since the early 1990s. One has come from the discovery of other solar systems, many of which contain giant planets orbiting very close to their stars. Another has been the unexpected finding from the Galileo spacecraft mission that Jupiter's atmosphere is enriched with volatile substances such as argon and molecular nitrogen. For these gases to have condensed and become incorporated in the icy bodies that accreted to form Jupiter's core required temperatures of 30 K (-240 °C, or -400 °F) or less. This corresponds to a distance far beyond the traditional snow line where Jupiter is thought to have formed. On the other hand, certain later models have suggested that the temperature close to the central plane of the solar nebula was much cooler (25 K [-248 °C, or -415 °F]) than previously estimated.

COMPOSITIONAL DATA FOR SELECTED SOLAR SYSTEM OBJECTS

object

distance from sun (au)*

mean density (g/cm3)

general composition

Sun

1.4

hydrogen, helium

Mercury

0.4

5.4

iron, nickel, silicates

Venus

0.7

5.2

silicates, iron, nickel

Earth

1

5.5

silicates, iron, nickel

Moon

1

3.3

silicates

Mars

1.5

3.9

silicates, iron, sulfur

asteroids

2-4.5 (main and outer belts)

typically 2-4

silicates, iron, nickel

Jupiter

5.2

1.3

hydrogen, helium

Io

5.2

3.6

silicates, sulfur

Europa

5.2

3.0

silicates, water ice (crust)

Ganymede

5.2

1.9

water ice, silicates

Callisto

5.2

1.8

water ice, silicates

Saturn

9.5

0.7

hydrogen, helium

Tethys

9.5

1.0

water ice

Titan

9.5

1.9

water ice, silicates, organics

Centaur objects

5-30 (mainly between orbits of Jupiter and Neptune)

possibly less than 1

presumed similar to that of comets: water ice, other ices, traces of silicates

Uranus

19.2

1.3

ices, silicates, hydrogen, helium

Neptune

30.1

1.6

ices, silicates, hydrogen, helium

Triton

30.1

2.0

water ice, silicates, organics

Pluto

39.5

2.0

water ice, silicates, organics

Kuiper belt objects

30-50 (main concentration)

possibly less than 1

presumed similar to that of comets: water ice, other ices, traces of silicates, organics

Oort cloud objects

20,000-100,000

possibly less than 1

presumed similar to that of comets: water ice, other ices, traces of silicates, organics

*One astronomical unit (AU) is the mean distance of Earth from the Sun, about 150 million km.

*One astronomical unit (AU) is the mean distance of Earth from the Sun, about 150 million km.

Although a number of such problems remain to be resolved, the solar nebula model of Kant and Laplace appears basically correct. Support comes from observations at infrared and radio wavelengths, which have revealed disks of matter around young stars. These observations also suggest that planets form in a remarkably short time. The collapse of an interstellar cloud into a disk should take about one million years. The thickness of this disk is determined by the gas it contains, as the solid particles that are forming rapidly settle to the disk's midplane, in times ranging from 100,000 years for 1-micrometre (0.00004-inch) particles to just 10 years for 1-cm (0.4-inch) particles. As the local density increases at the mid-plane, the opportunity becomes greater for the growth of particles by collision. As the particles grow, the resulting increase in their gravitational fields accelerates further growth. Calculations show that objects 10 km (6 miles) in size will form in just 1,000 years. Such objects are large enough to be called planetesimals, the building blocks of planets.

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