Chapter 4Exploring Our Evolving Solar System

Comparing the Planets: Orbits

• The Solar System to Scale* – The four inner

planets are crowded in close to the Sun.

– The four outer planets orbit the Sun at much greater distances.

*Planets are not to scale!

Comparing the Planets:Size and Composition

• Inner planets: rocky materials with dense iron cores and high average densities

• Outer planets: primarily light elements such as hydrogen and helium, low average densities

Moons are Natural Satellites

• All planets have moons, except Mercury and Venus.

• The outer planets have many more moons than the inner planets.

• Seven satellites are almost as big as the inner planets.

Determining Composition:Bodies with Surrounding Atmospheres

Determining Composition of Titan• Dips in the spectrum of sunlight reflected from Titan

are due to absorption by hydrogen atoms (H), oxygen molecules (O2), and methane molecules (CH4).

• Only methane is actually present in Titan’s atmosphere.

• Astronomers must account for the absorption that takes place in the atmospheres of the Sun and Earth.

Determining Composition:Planets without Atmospheres

• Spectra of reflected light is compared to known substances.• Infrared light from the

Sun, reflected from the surface of Europa, has almost exactly the same spectrum as sunlight reflected from water ice.

The Jovian Planets Are Made of Light Elements

Asteroids

• 100,000+ rocky objects within the orbit of Jupiter • Also called minor planets• The largest, Ceres, has a diameter of about 900 km

(560 mi) • Orbit the Sun in the same direction as the planets• Most orbit the Sun at distances of 2 to 3.5 AU, in the

asteroid belt

Trans-NeptunianObjects

• 1,000+ small bodies orbiting beyond the orbit of Neptune

• The largest of these are known as dwarf planets • Include Pluto, Eris, Charon, Makemake, etc.• Orbit the Sun in the same direction as the planets• Most orbit within the Kuiper belt at 30 AU to 50 AU

Comets

• Objects that result when Kuiper belt objects collide• Fragments a few kilometers across, diverted into

new and elongated orbits • The Sun’s radiation vaporizes ices, producing tails

of gas and dust particles• Astronomers deduce composition by studying the

spectra of these tails created by reflected sunlight• Oort cloud comets orbit out to 50,000 AU

Cosmic “Recycling”

• The Big Bang produced H and He (some Li and Be)―still common

• All heavier elements created by massive stars, dispersed when stars die

• Our solar system is recycled “star dust”

The SolarNebular

Hypothesis

• A cloud of interstellar gas and dust contracts because of its own gravity.

• The cloud flattens and spins more rapidly around its axis.

• A central condensation develops that evolves into a glowing protosun.

• The planets form out of the surrounding disk of gas and dust.

Protoplanetary Disks• Rapid rotation flattens the nebula.• ~100,000 years after contraction begins, a rotating,

flattened disk surrounds what will become the protosun. • Also called a proplyd, planets form from its material.• Explains why orbits all lie in the same plane, in the same

direction.

Temperatures in the Solar

Nebula

• Temperatures varied across the solar nebula as the planets were forming. • A general decline in temperature with increasing distance

from the center of the nebula. • Beyond 5 AU from the center of the nebula, temperatures

were low enough for water to condense and form ice.• Beyond 30 AU, methane (CH4) could also condense into ice.

Planetesimals Become Protoplanets,then Rocky Planets

Outer Planet Formation:

Capturing an Envelope of Gas

Cold, slow moving gases were gravitationally attracted to the Jovian planet cores.

Final Stages of Solar System

Evolution

• Our unstable young Sun ejected its thin outermost layers into space―a brief but intense burst of mass loss called a T Tauri wind.

• The T Tauri wind swept the solar system nearly clean of gas and dust.

• The planets stabilized at roughly their present-day sizes.

Searching for Extrasolar Planets:Three Methods