Modern astronomers believe that the sun and planets condensed out of a nebula or large cloud of gas and dust.
This idea is named the Nebular Hypothesis. It was first presented by the German philosopher Immanuel Kant in the late 1700’s.
Such clouds have been observed around stars other than our sun (e.g., Beta Pictoris)
The Formation of the Solar System Our solar system began as a rotating gas cloud or nebula that collapsed toward its center under the influence of gravity. A condensation formed at the center, which is called a protostar. A flattened disk of matter surrounded the protostar, which begtan to shine and become a star, our sun.
The rising temperature from the sun removed the gas from the inner regions, leaving dust and larger debris
Inner planets formed from solid debris
Outer planets retained original gases
Planets established dominance in their regions of the solar system. After almost all of the remaining gas, dust, and small debris was collected by the larger objects, the solar system took on the form we recognize today.
Other Star Systems Forming
We can look at young star systems developing today.
The planets orbiting these stars are formed from the surrounding disks of gas and dust, called protoplanetary disks or proplyds.
Proplyd in the Orion Nebula
May represent new solar system in formation
Disk of gas, dust seen edge-on, star Beta in center (covered to reveal faint outer disk)
Solar System Composite http://www2.jpl.nasa.gov/galileo/sepo/education/nav/ss2.gif
Common Properties of Planet Orbits in Our Solar System As viewed from above, all of the planets orbit the Sun in a counterclockwise direction. The planets orbit in nearly the same plane (ecliptic). All planets except Pluto have an orbital inclination of less than 7°.
Inner Solar System http://www.nineplanets.org/overview.html
Outer Solar System http://www.nineplanets.org/overview.html
Pluto’s Odd Orbit
Pluto’s orbit, or plane of revolution, is tilted by 17 ° to the general solar system orbits (ecliptic)
Pluto can also cut across Neptune’s orbit (but they can never collide)
Plane of the Ecliptic : The orbits of the planets are mostly in the same plane.
This plane is called the ecliptic and is defined by the plane of the earth’s orbit.
The exception is Pluto, which is tilted quite a bit in comparison to the rest of the planets.
The ecliptic plane is a remnant of the original, rotating nebular disk that formed the sun and planets
Ecliptic: Artist’s View
Directions of Motion : The planets orbit in a counterclockwise direction around the sun (when looking down upon the solar system from the sun’s north pole).
All the planets, except for Venus, Uranus, and Pluto, rotate in the same direction as their orbits.
All planets revolve counter-clockwise when viewed from above
The planets nearest to the Sun (Mercury, Venus, Earth, and Mars) are relatively close together, while those farther away (Jupiter, Saturn, Uranus, and Neptune) are more spread out.
Most of the planets are in nearly circular orbits.
Solar System Orbits (AU)
The astronomical unit (AU) is useful in measuring distances in the solar system
One AU equals the average earth-to-sun distance of 93 million miles
AU distances allow direct comparison to the earth which is equal to 1.0
Planet Distance in AU Mercury 0.4 Venus 0.7 Earth 1.0 Mars 1.5 Jupiter 5.2 Saturn 9.5 Uranus 19.2 Neptune 30 Pluto 39.5
In mass, the sun represent 99% of the solar system
The smallest planet, Mercury, has a diameter of 3031 mi
Pluto, the previous smallest planet, has a diameter of 1457 mi
The largest planet, Jupiter, has a diameter of 88,700 mi
Earth = 7926 mi
Ganymede, the largest moon of Jupiter, is larger than Mercury, yet Ganymede is not considered a planet because it revolves around Jupiter
Two Basic Groups of Planets TERRESTRIAL (earth-like) Small size, low Mass Higher density Mostly rock Mercury, Venus, Earth, Mars JOVIAN (Jupiter-like) Large size, massive Low density Mostly gas Jupiter, Saturn, Uranus, Neptune
Terrestrial or Rocky planets —Composed of rock and metal: Mercury, Venus, Earth, Mars
Jovian or Gas planets —Composed primarily of the gases hydrogen and helium: Jupiter, Saturn, Uranus, Neptune
Small Planets —Diameters less than 13,000 km: Mercury, Venus, Earth, Mars
Inferior —Closer to the sun than earth: Mercury and Venus
Superior —Farther from the sun than earth: Mars, Jupiter, Saturn, Uranus, Neptune
Classical —Known since prehistoric times, visible to the unaided eye: Mercury, Venus, Mars, Jupiter, Saturn
Modern —Discovered in modern times, visible only with telescopes: Uranus, Neptune, (Pluto)
Classification Table Planet Rocky or Gas? Small or Giant? Inner or Outer? Inferior or Superior? Classical or Modern? Mercury R S I I C Venus R S I I C Earth R S I N/A ? Mars R S I S C Jupiter G G O S C Saturn G G O S C Uranus G G O S M Neptune G G O S M Pluto ? S O S M
New Solar System (> 2006)
Two Categories: Planets and Dwarf Planets (dwarfs beyond Neptune are “Plutoids”)
Today’s astronomers recognize only 8 planets
Solar System Model
If the sun were an orange, the earth would be a grain of sand thirty feet away.
Jupiter would be a cherry pit located one block from the sun.
Saturn would be another cherry pit located one block from Jupiter.
Pluto would be a grain of sand 10 blocks from the sun.
The nearest star to our sun (Alpha Centauri) would be represented as another orange 2000 miles from the sun.
The planets have been given the Roman names of gods from ancient Greece.
Mercury, Venus, Mars, Jupiter, Saturn
Hermes, Aphrodite, Ares, Zeus , Kronos
The modern planets, Uranus, Neptune, and Pluto, are also Roman gods