Inventory of the Solar System

Astr 221 2015Nightwatch Ch. 7 & 10

• There are eight major planets with nearly circular orbits.

• Pluto and Eris are smaller than the major planets and have more elliptical orbits.

What are the major features of the Sun and planets?

Sun and planets to scale

Planets are very tiny compared to

distances between them.

• Over 99.9% of solar system’s mass• Made mostly of H/He gas (plasma)• Converts 4 million tons of mass into energy each second

Sun

• Made of metal and rock; large iron core • Desolate, cratered; long, tall, steep cliffs• Very hot and very cold: 425C (day)–170C (night)

Mercury

• Nearly identical in size to Earth; surface hidden by clouds • Hellish conditions due to an extreme greenhouse effect• Even hotter than Mercury: 470C, day and night

Venus

• An oasis of life• The only surface liquid water in the solar system• A surprisingly large moon

Earth and Moon with sizes shown to scale

Earth

• Looks almost Earth-like, but don’t go without a spacesuit!• Giant volcanoes, a huge canyon, polar caps, more• Water flowed in distant past; could there have been life?

Mars

“Canals” on Mars

• Percival Lowell misinterpreted surface features seen in telescopic images of Mars.

Swarms of Smaller Bodies

• Many rocky asteroids and icy comets populate the solar system.

Asteroids are cratered and not round.

Origin of Asteroid Belt

• Rocky planetesimals between Mars and Jupiter did not accrete into a planet.

• Jupiter’s gravity, through influence of orbital resonances, stirred up asteroid orbits and prevented their accretion into a planet.

• Much farther from Sun than inner planets

• Mostly H/He; no solid surface

• 300 times more massive than Earth

• Many moons, rings

Jupiter

Saturn

• Giant and gaseous like Jupiter• Spectacular rings• Many moons, including cloudy Titan

Rings are NOT solid; they are made of countless small chunks of ice and rock, each orbiting like a tiny moon.

Artist’s conception

• Smaller than Jupiter/Saturn; much larger than Earth

• Made of H/He gas and hydrogen compounds (H2O, NH3, CH4)

• Extreme axis tilt• Moons and rings

Uranus

• Similar to Uranus (except for axis tilt)

• Many moons (including Triton)

Neptune

Pluto (and Other Dwarf Planets)

• Much smaller than major planets• Icy, comet-like composition• Pluto’s main moon (Charon) is of similar size

Other Icy Bodies

• There are many icy objects like Pluto on elliptical, inclined orbits beyond Neptune.

• The largest of these, Eris, was discovered in summer 2005, and is even larger than Pluto.

• What are the major features of the Sun and planets?– The planets are very small compared to

the distances between them.– The planets of the inner solar system are

rocky and have few moons.– The planets of the outer solar system are

gaseous and have many moons and rings.

– Pluto is unlike either the inner or outer planets.

Anatomy of a Comet

• A coma is the atmosphere that comes from a comet’s heated nucleus.

• A plasma tail is gas escaping from coma, pushed by the solar wind.

• A dust tail is pushed by photons.

Kuiper belt:on orderly orbits from 30–100 AU in disk of solar system

Oort cloud:on random orbits extending to about 50,000 AU

Only a tiny number of comets enter the inner solar system. Most stay far from the Sun.

Comets eject small particles that follow the comet around in its orbit and cause meteor showers when Earth crosses the comet’s orbit.

Processed Meteorites

Where did the solar system come from?

Evidence from Other Gas Clouds

• We can see stars forming in other interstellar gas clouds, lending support to the nebular theory.

Was our solar system destined to be?

• Formation of planets in the solar nebula seems inevitable.

• But details of individual planets could have been different.

Giant Impact