Chapter 26: The Solar System

The Solar System and Its Formation

• The solar system consists of:
  • Sun
  • System of planets
  • Asteroids
  • Comets
• Planets are divided into two classes:
  • Inner planets:
    • Mercury
    • Venus
    • Earth
    • Mars
  • Outer planets:
    • Jupiter
    • Saturn
    • Uranus
    • Neptune
• The Nebular theory:
  • Theory that the Sun and planets formed together from a cloud of gas and dust—a nebula.
• Nebular theory formation:
  • Gravitation between materials in the cloud pulled it inward.
  • When pulled inward, spin increased in accord with the conservation of angular momentum.
  • The spinning cloud conformed to the shape of a spinning disk.
• Nebular theory formation:
  • The center of the disk is the protosun.
  • Away from the center, planetesimals formed.
  • Planetesimals accreted more matter to become planets.
• Solid Particles Formed by Condensation
  • Inside the frost line: too hot for hydrogen compounds to form ices (by condensation from gas to solid)
  • Outside the frost line: cold enough for ices to form
• As a nebula shrinks under the influence of gravity, it spins faster.
  • Explanation: In accordance with the conservation of angular momentum, as the radius of the nebula decreases, its spin rate increases (like a skater who pulls her arms inward in a spin).

The Sun

• Sun:
  • nearest star to Earth
  • composed of mostly hydrogen in the plasma phase
  • hydrogen is fused to helium by thermonuclear fusion in its core
  • 4.5 million tons of mass are converted to energy each second
  • a tiny fraction of this energy reaches and sustains Earth
• Nuclear fusion requires that like-charged nuclei get close enough to each other to fuse.
  • This can happen only if the temperature is extremely high – over 10 million K.
• The process that powers most stars is a three-step fusion process
• The Sun generates energy by converting hydrogen to helium in its core by the process of nuclear fusion.
  • When four protons are converted to a helium nucleus in the proton–proton chain, some mass is lost. The law of conservation of mass and energy requires that this mass appear as energy, eventually resulting in the light we see.
• Interior structure of the Sun:
  • Outer layers are not to scale.
  • The core is where nuclear fusion takes place.
• Energy transport
  • The radiation zone is relatively transparent; the cooler convection zone is opaque:
• The apparent surface of the sun is called the photosphere.
  • It is the layer in the sun’s atmosphere that is dense enough to emit plenty of light, but not so dense that the light can’t escape.
  • So, it is the source of most of the sunlight received by Earth.
• The photosphere is less than 500 km (300 mi) deep.
  • If the sun magically shrank to the size of a bowling ball, the photosphere would be no thicker than a layer of tissue paper wrapped around the ball.
• The presence of granulation is clear evidence that energy is flowing upward through the photosphere by a process known as convection.
• Sunspots: appear dark because slightly cooler than surroundings
• Sunspots come and go, typically in a few days.
  • Sunspots are linked by pairs of magnetic field lines:
• The Sun has an 11-year sunspot cycle, during which sunspot numbers rise, fall, and then rise again:
• The rotation of the Sun drags magnetic field lines around with it, causing kinks
• Three, the Zeeman effect gives astronomers a way to measure the strength of magnetic fields on the sun.
• Four, characteristics of the sunspot cycle vary over centuries and appear to affect Earth's climate.
• The figure shows a filtergram made at the wavelength of the H-alpha Balmer line.
  • The image shows complex structure in the chromosphere—including long, dark filaments silhouetted against the brighter surface.
• The outermost part of the sun’s atmosphere is called the corona—after the Greek word for ‘crown.’
  • The corona is so dim that it is not visible in Earth’s daytime sky—because of the glare of scattered light from the sun’s brilliant photosphere.
• Solar wind escapes Sun mostly through coronal holes, which can be seen in X-ray images
• Strictly speaking, in every second that passes, the mass of the Sun decreases.
  • It is this decrease that bathes the solar system with radiant energy.
  • Solar mass is converted to energy via the celebrated equation, E = mc^2.

The Inner Planets

• The inner planets—four nearest to the Sun composed of high-density solid rock:
  • Mercury
  • Venus
  • Earth
  • Mars
• Orbital speeds of planets around the Sun decrease with increasing distance from the Sun.
• Mercury:
  • closest to the Sun
  • slightly larger than our Moon
  • almost no atmosphere due to small size
  • daytime is long and hot (up to 430ºC)
  • nighttime is long and cold (about –170ºC)
• Venus:
  • next closest to the Sun
  • diameter about 0.95 that of Earth
  • very dense atmosphere, mostly carbon dioxide
  • volcanically active
  • very harsh place
• Earth:
  • third planet from the Sun—our home
  • at a distance where most of its water is neither solid nor gas, but liquid
• Mars:
  • fourth planet from the Sun—a potential away- from-home habitat
  • little more than half Earth's size
  • thin atmosphere—95% carbon dioxide and 0.15% oxygen (A planet with a thin atmosphere is ineffective in reducing the temperature difference between day and night!)
  • equatorial temperatures range from 30ºC in day to –130ºC at night
  • presently the focus of planetary exploration

The Outer Planets

• Outer planets:
  • gaseous, low-density worlds
  • appreciably larger than Earth
  • more widely spaced than the inner planets
  • in order of distance from Sun:
    • Jupiter
    • Saturn
    • Uranus
    • Neptune
• Jupiter:
  • first of the outer planets, beyond Mars
  • more than 11 times Earth's diameter—giant of the solar system
  • composition more liquid than gaseous or solid
  • atmospheric pressure more than a million times that of Earth's
• Jupiter:
  • atmosphere is 82% hydrogen, 17% helium, 1% methane, ammonia, and other molecules—cough!
  • no definite surface as occurs on the inner rocky planets
  • solid core of iron, nickel, and other minerals
  • Because of its thick atmospheric blanket, daytime and nighttime temperatures are about the same for equal altitudes above its "surface."
• Jupiter's moon Europa has an ice-capped ocean, which may hold extraterrestrial life
• Saturn:
  • most remarkable for its easily seen rings
  • twice as far from Earth as Jupiter
  • diameter about ten times that of Earth, excluding the rings
  • lowest density of all planets—could float in giant bathtub (density is less than that of water)
• Saturn:
  • surrounded by rings—hypothesized to be bits of a moon never formed, or remnants of a moon torn apart by tidal forces
  • inner part of rings, like any satellite, travels faster than outer part of the ring system
  • Rocks that make up the rings orbit independently of other rocks.
• Saturn's largest moon, Titan, was visited by the Cassini spacecraft.
• Uranus:
  • twice as far from Earth as Saturn is
  • diameter about four times that of Earth
  • 98º tilt to the orbital plane— a most unusual feature
  • faint ring system
  • methane atmosphere
  • very cold place
• Neptune:
  • lies beyond Uranus
  • diameter almost four times that of Earth, somewhat smaller than Uranus
  • atmosphere mainly hydrogen and helium
  • highly elongated elliptical path about the Sun
• Pluto:
  • since 2006, classified as a dwarf planet
  • very unlike other planets in composition, size, and orbit
  • highly elliptical orbit, like comets
  • spends most of its orbital time well beyond Neptune, in the Kuiper Belt
  • composition like that of Kuiper-Belt objects
  • look-alike neighbors not classified as planets
  • former planetary status was more historical than astronomical

Earth's Moon

• Earth's Moon:
  • more is known about the Moon than any other celestial body
  • diameter about one quarter that of Earth
  • no atmosphere—no weather and erosion to conceal past scarring of its surface (wears no "makeup")
• Twelve people have stood on the Moon. Here we see Buzz Aldrin, one of the three Apollo 11 astronauts.
• The Moon spins about its polar axis as it revolves around Earth.
• During the time of a new Moon, the Moon is between the Sun and Earth.
  • Explanation: A new Moon is mainly in the daytime sky, between Earth and the Sun. When it is exactly between them, we have a solar eclipse.
• During the time of a full Moon, the Earth is between the Sun and the Moon.
  • Explanation: A full Moon occurs when Earth is between the Sun and Moon, while in Earth's view, the hemisphere of the Moon is fully in sunshine. When Earth is exactly between the Sun and the Moon, we have a lunar eclipse.
• A magnetic compass aligns with a magnetic field.
  • Like a compass in a magnetic field, the Moon aligns with Earth's gravitational field.
• Eclipses occur when the Moon's shadow falls on part of Earth.
  • This is a solar eclipse.
• A lunar eclipse occurs when Earth's shadow falls on the Moon.
• Eclipse:
  • The red light of sunrises and sunsets all around Earth is refracted onto the Moon's surface during a lunar eclipse.

Failed Leftovers from Planet Formation

• Asteroids:
  • small rocky bodies that orbit the Sun
  • most are located between Mars and Jupiter
  • some encounter Earth
  • unnoticed on ground—conspicuous on ice (the reason many are found in Antarctica)
• Comets:
  • differ from asteroids in chemical composition
  • are masses of water, methane, and ice—dirty snowballs
  • most located in Kuiper Belt and Oort Cloud
  • highly elliptical (highly eccentric) orbital paths
  • tail of comets swept outward from Sun by solar wind
• The tails of comets point in a direction away from the Sun.
  • Explanation: The solar wind blows the tails in a direction away from the Sun.
• Meteoroids
  • are relatively small (sand- grain to boulder size) pieces of debris chipped off asteroids or comets
• Meteor:
  • a meteoroid that strikes Earth’s atmosphere
  • often called a "falling star"
• Meteorite
  • a meteoroid that survives the trip through the atmosphere and reaches Earth's surface
• A meteorite has changed its status from meteoroid to meteor to meteorite.