Lecture 31:

The Family of the Sun

Astronomy 161 – Winter 2004

Key Ideas:The Solar System contains:

• The Sun• Terrestrial Planets• Jovian Planets• Pluto• Giant Moons• Asteroids, Icy Bodies, Comets, & Meteoroids

The planets all lie in nearly the same plane and orbit in the same general direction.

The Golden Age of ExplorationThe Solar System has been explored with robotic

spacecraft & astronauts:• Landed men on the Moon• Robotic landers on Moon, Venus, & Mars• Returned rocks from the Moon (~382 kg)• Probed Atmospheres of Venus, Mars, & Jupiter• Flown spacecraft by all planets except Pluto• Mapped Venus with radar• Flown by asteroids & comets, landing on one

asteroid

Magellan Venus

Cassini & Huygens Mars SojournerPioneer 10 & 11

Voyager 1 & 2

Apollo 11-17

The Family of the SunThe Sun: a middle-aged, average sized starThe Terrestrial Planets:

• Rocky Planets: Mercury, Venus, Earth & Mars

The Jovian Planets:• Gas Giants: Jupiter, Saturn, Uranus & Neptune

Pluto: fits into neither categorySmall Icy & Rocky Bodies:

• Icy: Icy Moons, Kuiper Belt Objects, & Comets• Rocky: Giant Moons, Asteroids & Meteoroids

The 9 Planets, in order:Planets:MercuryVenusEarthMarsJupiterSaturnUranusNeptunePluto

Mnemonic:My Very Educated Mother Just Served Us Nine Pizzas

Relative Sizes of the Planets

Basic Properties of the PlanetsLocations:

• Terrestrial in the inner solar system: 0.4-1.5AU• Jovian in the outer solar system: 5-30 AU

All orbit in the same direction & same plane:• Orbit counterclockwise, in the same sense as the

rotation of the Sun.• All except Pluto orbit very near the Ecliptic plane.

Provides clues to Solar System formation.

Pluto

Neptune

Uranus

Saturn

Jupiter

Mars

EarthVenus

Mercury

Pluto

EclipticPlane

The Solar System, by mass

The contents of the Solar System, ordered by their total mass in Earth masses:• Sun: 330,000 MEarth

• 4 Jovian Planets: 447 MEarth total

• 4 Terrestrial Planets: 1.985 MEarth total

• Giant Moons: 0.105 MEarth total

• Pluto, the icy oddball planet: 0.002 MEarth

Rest of the contents make a tiny contribution.

The SunThe Sun is a middle-aged, average-sized star.

• Mostly Hydrogen & Helium• Contains 99.8% the mass of the Solar System• about 4.6 Gyr old

The Sun shines because it is hot:• Surface (photosphere) is ~6000 K• Radiates mostly Visible light plus UV & IR

Kept hot by nuclear fusion in its core:• Builds Helium from Hydrogen fusion.

The Jovian Planets

Jupiter, Saturn, Uranus & Neptune• Largest Planets: at least 15 times mass of Earth.• Jupiter, largest, is 318 Earth Masses• Only in the outer solar system (5 to 30 AU)

Gas Giants (“Jupiter-like”):• No Solid Surfaces (mostly atmosphere)• Mostly Hydrogen & Helium• Rocky/icy inner cores• Low density: 0.7 to 1.7 g/cc (water is 1 g/cc)

The Jovian Planets

Jupiter(318 M)

Uranus(15 M)

Saturn(95 M)

Neptune(17 M)

Terrestrial Planets

Mercury, Venus, Earth & Mars• “Earth-Like” Rocky Planets• Largest is Earth• Only in the inner solar system (0.4 to 1.5 AU)

Rocky Planets:• Solid Surfaces• Mostly silicates and iron • High Density: 3.9-5.5 g/cc (rock & metal)• Earth, Venus, & Mars have atmospheres

The Terrestrial Planets

Earth(1 M)

Mars(0.11 M)Venus

(0.82 M)

Mercury(0.055 M)

Solid inner core

Liquid outer core

The Giant MoonsNatural satellites orbiting planets.Giant Moons:

• Earth: The Moon• Jupiter: Io, Europa, Ganymede, & Callisto (the

Galilean moons) • Saturn: Titan • Neptune: Triton

Many smaller moons, both rocky & icy.Only Mercury & Venus have no moons.

The Giant Moons

Pluto: The Planet that Doesn’t Fit

Pluto is neither a Terrestrial nor Jovian Planet.• Smallest of the planets• Intermediate Density: 1.8 g/cc (mostly icy)

Pluto’s orbit is also odd:• The most elliptical orbit of all the planets• The most highly inclined: ~17º from the Ecliptic.

Largest of a distinct class of objects, but still a “planet”.

Distant Pluto

Pluto

CharonPluto’s Moon

Hubble Reconstruction of Pluto

Small Icy Bodies

Pluto is the largest of a class of icy bodies:• Found only in the outer solar system• Densities of 1.2 to 2 g/cc (like ices)

Examples:• Triton, large moon of Neptune• Charon, Pluto’s large moon• Trans-Neptunian Objects (Kuiper Belt Objects &

Plutinos)

Triton

The Leftovers (Minor Bodies)Asteroids:

• Range from 500km (Ceres) to large boulders• Made of rock (density 2-3 g/cc)

Meteoroids:• Bits of rock and metal• Range in size from grains of sand to boulders

Comets:• Composite rock & ice “dirty snowballs”• Longs tails of gas & dust are swept off them when

they pass near the Sun.

Asteroids

253 Mathilde

951 Gaspra243 Ida

Meteor burning up in the atmosphere.

Comet Hale-Bopp

5 kmNucleus of Comet Halley