The Solar System

414 PART 4^ |^ THE SOLAR SYSTEM

19-4 Planets Orbiting

Other Stars

Are there other planetary systems? Th e evidence says yes. Do

they contain planets like Earth? Th e evidence so far is

incomplete.

Planet-Forming Disks

Both visible- and radio-wavelength observations detect dense

disks of gas and dust orbiting young stars. For example, at least

50 percent of the stars in the Orion nebula are surrounded by

such disks (Figure 19-3). A young star is detectable at the center

of most disks, and astronomers can measure that the disks con-

tain many Earth masses of material in a region a few times larger

in diameter than our solar system. Th e Orion star-forming

region is only a few million years old, so planets may not have

formed in these disks yet. Furthermore, the intense radiation

from nearby hot stars is evaporating the disks so quickly that

planets may never have a chance to grow large. Th e important

point for astronomers is that so many young stars have disks.

Evidently, disks of gas and dust are a common feature around

stars that are forming.

Th e Hubble Space Telescope can detect dense disks around

young stars in a slightly diff erent way. Th e disks show up in sil-

houette against the nebulae that surround the newborn stars

(■ Figure 19-12). Th ese disks are related to the formation of

19-4

a bb

Visual-wavelength imagesVisual-wavelength imagesVisual-wavelength images

■ Figure 19-11

Every old, solid surface in the solar system is scarred by craters. (a) Earth’s moon has craters ranging from basins hundreds of kilometers in diameter down to
microscopic pits. (b) The surface of Mercury, as photographed by a passing spacecraft, shows vast numbers of overlapping craters. (NASA)

SCIENTIFIC ARGUMENT

Why are there two kinds of planets in our solar system?

This is an opportunity for you to build an argument that closely

analyzes the solar nebula theory. Planets begin forming from solid

bits of matter, not from gas. Consequently, the kind of planet that

forms at a given distance from the sun depends on the kind of

substances that can condense out of the gas there to form solid

particles. In the inner parts of the solar nebula, the temperature

was so high that most of the gas could not condense to form

solids. Only metals and silicates could form solid grains, and the

innermost planets grew from this dense material. Much of the mass

of the solar nebula consisted of hydrogen, helium, water vapor, and

other gases, and they were present in the inner solar nebula but

couldn’t form solid grains. The Terrestrial planets could grow only

from the solids in their zone, not from the gases, so the Terrestrial

planets are small and dense.

In the outer solar nebula, the composition of the gas was the

same, but it was cold enough for water vapor, and other simple mol-

ecules containing hydrogen, to condense to form ice grains. Because

hydrogen was so abundant, there was lots of ice available. The outer

planets grew from large amounts of ice combined with small amounts

of metals and silicates. Eventually the outer planets grew massive

enough that they could begin to capture gas directly from the neb-

ula, and they became the hydrogen- and helium-rich Jovian worlds.

The condensation sequence combined with the solar nebula

theory gives you a way to understand the difference between the

Terrestrial and Jovian planets. Now expand your argument: Why do

some astronomers argue that the formation of the Jovian plan-

ets is a problem that needs further explanation?