Copernicus and the heliocentric system (1473-1543 AD)
Replaces the geocentric system of Ptolemy
Nearly circular orbit of planets
The earth spins on its own axis, confirmed by Foucault’s pendulum in 1851
Moon orbits the Earth
Solar System as a coherent dynamical system: Kepler’s Laws (1571-1630)
Planets go around the Sun in nearly circular orbits - ellipses with small eccentricities. The Sun is at one focus of the ellipse (Kepler’s First Law)
All orbits are located close to a single plane perpendicular to the axis of rotation of the Sun (Plane of the Ecliptic). Most planets are within 3 degrees of the Plane of the Elliptic (exceptions - Mercury and Pluto)
Planetary motion around the Sun follows unchanging rhythm - faster when they are closer to Sun and slower when they are farther away (Kepler’s Second Law)
Period of revolution of the planets is dependent on their distance from the Sun. The farther out they are the slower is the revolution period (Kepler’s Third Law). The square of the orbital period in years is proportional to the cube of the distance from the Sun in AU
Planets revolve in their orbits in the same sense as the rotation of the Sun.
The spin direction of the planets is also in the same direction (exceptions are Venus and Uranus)
The planets' axis of rotation is nearly perpendicular to the plane of the elliptic.
The present Solar System is the end product of a history that is common to all the planets and the Sun.
The Planetary System: Inner Planets
Mercury, Venus, Earth and Mars, the Earth’s Moon
The Asteroidal Belt
The Outer or Giant Planets
Jupiter, Saturn, Uranus and Neptune and Pluto
Over 99.85% of the mass of the Solar System is in the Sun. All planets put together make up less than ) 0.15% of the mass.
Modern observations: Comparative Planetology
impacts on all planetary surfaces of all ages
volcanism on planets - indicative of internal energy sources
General characteristics of terrestrial planets
Mercury: no atmosphere, densely cratered, surface temperature from -280° F to 800° F, revolution period 88 days
Venus: Earth’s sister planet in size and density; thick atmosphere (~75 km) about 100 times more pressure than on the earth; mostly of CO2; surface temperature ~900° F; highlands and vast rolling plains
Mars: atmosphere 1% of the earth, mostly CO2; polar ice caps (-193° F); no liquid water now, permafrost; Large volcanoes (Olympus Mtns, as big as Ohio and 75,000 ft. high), canyons several times bigger than Grand Canyon; two moons- Phobos and Deimos
Asteroidal Belt: Debris of small bodies of rock and metal, source of meteorites
The Formation of the Solar System and the Planets: Nebular hypothesis of Laplace (~1800)
Initial contraction of a cloud of dust and gas
Formation of a disk with a central mass
Temperature gradient in the disk
Condensation of elements
The nuclear phase of the Sun
Solar wind and loss of gases from the inner solar system
Planetesimals and Planetary embryos and accretion
Consequences for the earth
Early molten earth and core formation
Early atmosphere
Factors in subsequent evolution of terrestrial planets
Cooling rate due to heat loss depends on the size of the planet
Venus and earth, the largest planets are cooling slowly - volcanism till today
Mars, volcanic activity until a billion years ago
Moon and Mercury dead for the last three billion years-no activity since
distance from the Sun and presence of liquid water
Mercury and Venus, closest to Sun have no water.
Venus has water as steam in its atmosphere
Earth has liquid water
Mars , farthest from Sun is too cold, and has water as ice
Presence of biosphere
Biosphere-liquid water interactions affect the atmospheric evolution
If Venus had life, it could have developed an atmosphere like the earth