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The Solar Nebula Hypothesis Basis of modern theory of planet formation:
• Planets form at the same time from the same cloud as the star.
• Planet formation sites can be observed today as dust disks of T Tauri stars.
• The sun and our solar system formed ~5 billion years ago.
Evidence for Ongoing Planet Formation
right now!
Many young
stars in the
Orion Nebula
are surrounded
by dust disks:
Probably
sites of
planet
formation
Survey of the Solar System Relative Sizes of the Planets
Assume we reduce all bodies in the solar system so that the Earth has diameter of 0.3 mm.
• Sun: ~ size of a small plum
• Mercury, Venus, Earth, Mars: ~ size of a grain of salt
• Jupiter: ~ size of an apple seed
• Saturn: ~ slightly smaller than Jupiter’s “apple seed”
• Uranus, Neptune: larger salt grains
Planetary Orbits
Earth
Venus
Mercury
All planets in
almost circular
( elliptical) orbits
around the Sun, in
approximately the
same plane
( ecliptic ).
Sense of revolution:
counter-clockwise
Sense of rotation:
counter-clockwise
( with exception of
Venus, and
Uranus)
Orbits generally
inclined by no
more than 3.4 °
Exceptions:
Mercury (7 ° )
( distances and times reproduced to scale )
Two Kinds of Planets
Planets of our solar system can be divided
into two very different kinds:
1 . Terrestrial
( earthlike) planets:
Mercury, Venus,
Earth, Mars
2 . Jovian (Jupiter-like)
planets: Jupiter, Saturn,
Uranus, Neptune
Terrestrial Planets
• Four inner
planets of the
solar system
• Relatively
small in size
and mass
( Earth is the
largest and
most massive)
• Rocky surface
•The surface of Venus can not be
seen directly from Earth because of
its dense cloud cover.
The Jovian Planets
Much larger in mass and size than terrestrial planets
Much lower
average density
All have rings
( not only Saturn !)
Mostly gas;
no solid surface
Space Debris
In addition to planets, small bodies orbit the Sun:
Asteroids, comets, meteoroids
Asteroid
Eros,
imaged by
the NEAR
spacecraft
Comets
Mostly in highly elliptical
orbits, occasionally coming
close to the Sun
Icy nucleus, which
evaporates and gets blown
into space by solar wind
pressure
Meteoroids
Small ( m to mm
sized) dust grains
throughout the solar
system
If they collide with
Earth, they evaporate
in the atmosphere.
→ visible as streaks
of light: meteors
The Age of the Solar System • Sun and planets should have about the same age.
• Ages of rocks can be measured through radioactive dating:
measure abundance
of a radioactively decaying element to find the time since formation of the rock.
• Dating of rocks on Earth, on the Moon, and meteorites all give ages of ~4.6 billion years.
The Story of Planet Building The planets were formed from the same protostellar
material as the Sun, still found in the Sun’s atmosphere.
Rocky planet material was formed from the clumping together of dust grains in the protostellar cloud.
Mass of less than Mass of more than ~15 Earth masses: ~15 Earth masses:
Planets can not grow Planets can grow by by
gravitational gravitationally attracting collapse material from the
protostellar cloud
• To compare densities of planets, compensate for compression due to the planet’s gravity:
only condensed
materials could stick together to form planets.
• Temperature in the protostellar cloud decreased outward.
• Further out →protostellar cloud cooler → metals with lower melting point condensed → change of chemical composition throughout solar system
Formation and Growth of Planetesimals
• Planet formation starts with clumping together of grains of solid matter:
planetesimals
• Planetesimals (few cm to km in size) collide to form planets
• Planetesimal growth through condensation and accretion
•Gravitational instabilities may have helped in the growth of planetesimals into protoplanets.
Simplest form of planet growth:
• Unchanged composition of accreted matter over time
• As rocks melted, heavier elements sink to the center
→ differentiation
• This also produces a secondary atmosphere
→ outgassing
• Improvement of this scenario: gradual change of grain composition due to cooling of the nebula and storing of heat from potential energy
The Jovian Problem Two problems for the theory of planet formation:
1) Observations of extrasolar planets indicate that Jovian planets are common.
2) Protoplanetary disks tend to be evaporated quickly
(typically within ~100,000 years) by the radiation of nearby massive stars.
→ Too short for Jovian planets to grow!
Solution:
Computer simulations show that Jovian planets can grow by direct gas accretion without forming rocky
planetesimals.
Clearing the Nebula
Surfaces of the Moon and Mercury show evidence for
heavy bombardment by asteroids.
Remains of the protostellar nebula were cleared away by:
• Radiation pressure of the Sun
• Solar wind
• Sweeping-up of space debris by planets
• Ejection by close encounters with planets
Mercury • Very similar to
Earth’s Moon in several ways:
• small; no atmosphere
• lowlands flooded by ancient lava flows
• heavily cratered surfaces
• Most of our knowledge based on measurements by Mariner 10 spacecraft (1974 - 1975) and the current MESSENGER mission.
The Rotation of Venus • Almost all planets rotate counterclockwise, i.e. in the same
sense as orbital motion.
• Exceptions:
Venus and Uranus
Venus rotates clockwise, with period slightly longer than its orbital period.
• Possible reasons:
• off-center collision with massive protoplanet
• tidal forces of the sun on molten core
Mars • Diameter ≈ ½ Earth’s diameter• Very thin atmosphere,
mostly CO2
• Rotation period = 24 h, 40 min.
• Axis tilted against orbital plane by 25o, similar to Earth’s inclination (23.5o)
• Seasons similar to Earth → growth and shrinking of polar ice cap
• Crust not broken into tectonic plates
History of Mars’ Atmosphere Atmosphere probably initially produced through outgassing.
• Loss of gasses from a planet’s atmosphere:
• Compare typical velocity of gas
molecules to escape velocity.
• Gas molecule velocity greater than escape velocity: → gasses escape into space.
No liquid water on the surface:
• would evaporate due to low pressure
But evidence for liquid water in the past:
• outflow channels from sudden, massive floods
• collapsed structures after withdrawal of sub-surface water
• splash craters and valleys resembling meandering river beds
• gullies, possibly from debris flows
Volcanism on Mars
• Tharsis rise (volcanic bulge):
• Nearly as large as the U.S.
• Rises ~10 km above mean radius of Mars
• Rising magma has repeatedly broken through crust to form volcanoes
The Moons of Mars • Two small moons:
Phobos and Deimos
• Too small to pull themselves into spherical shape
• Typical of small, rocky bodies: dark grey, low density
• Phobos: very close to Mars; orbits around Mars faster than Mars’ rotation
• Probably captured from the outer asteroid belt
Mars Exploration:
The Mars Rovers
Two identical
rovers, Spirit and Opportunity, have been moving across the surface of Mars since 2005.
Curiosity joined them in
2012
Jupiter
• Largest and most massive planet in the solar system
• Contains almost ¾ of all planetary matter in the solar system
• Most striking features: visible from Earth, and multi-colored cloud belts
• Rotation period: 10 hours
Jupiter’s Interior From radius and mass → average density of Jupiter ≈ 1.34 g/cm3
=> Jupiter cannot be made mostly of rock, like earthlike planets.
→ Jupiter consists mostly of hydrogen and helium.
Due to the high pressure, hydrogen is compressed into a liquid, and even metallic state.
The Cloud Belts on Jupiter Just like on Earth, high-and low-pressure
zones are bounded by high-pressure winds.
Jupiter’s Cloud belt structure has remained
unchanged since humans began mapping
them
Jupiter’s Ring Not only Saturn, but all four gas giants have rings.
• Galileo spacecraft
image of Jupiter’s • Jupiter’s ring: dark
ring, illuminated and reddish; only
from behind discovered by
Voyager 1 spacecraft
• Composed of microscopic
particles of rocky material
• Rings must be constantly re-supplied with new dust
• Location: Inside Roche limit, where larger bodies (moons) would be destroyed by tidal forces
• Ring material can’t be old because radiation pressure and Jupiter’s magnetic field force dust particles to spiral down into the planet
Jupiter’s Family of Moons • Over two dozen moons known now; new
ones are still being discovered
• Four largest moons already discovered by Galileo: The Galilean moons
Io Europa Ganymede Callisto
• Interesting and diverse individual geologies
Saturn • Mass: ~1/3 of mass of Jupiter
• Radius: ~16% smaller than Jupiter
• Average density: 0.69 g/cm3 → would float in water!
• Rotates about as fast as Jupiter, but is twice as oblate → no large core of heavy elements
• Mostly hydrogen and helium; liquid hydrogen core
• Saturn radiates ~1.8 times the energy received from the
Sun
Composition of Saturn’s Rings
Rings are composed
of ice particles
moving at large velocities around Saturn, but at small relative velocities (all moving in the same direction)
Discovery of Uranus
• Chance discovery by
William Herschel in 1781
– While scanning sky for nearby objects with measurable parallax, discovered Uranus as slightly extended object, ~3.7 arc seconds in diameter
Uranus 1/3 the diameter of Jupiter
The Motion of Uranus
• Orbit slightly elliptical; orbital period ≈ 84 years
• Very unusual
1 /20 the mass of Jupiter
No liquid
metallic
hydrogen
Deep
hydrogen +
helium
atmosphere
The Atmosphere of Uranus • Like other gas giants: no surface
• Gradual transition from gas phase to fluid interior
orientation of
rotation axis:
almost in the orbital
plane 97.9 o
•Large portions of
the planet
exposed to
“eternal” sunlight
for many years,
then complete
darkness for many
years!
•Possibly result of
impact of a large
planetesimal during
the phase of planet
formation
• Mostly H; 15% He, a few % methane, ammonia and water vapor
• Optical view from Earth: blue color due to methane, absorbing longer wavelengths
• Cloud structures are only visible after artificial computer enhancement of optical images taken from Voyager spacecraft.
Uranus’s Moons
• Five largest moons visible from Earth
– All tidally locked to Uranus
• 22 more have since been found
– 13 near rings
– 9 in large irregular orbits
VLT/ESO
Neptune
• Similar in size and density to Uranus, but has significant amount of heat flowing from interior
• Blue-green color from methane in the atmosphere
• 4 times Earth’s diameter; 4% smaller than Uranus
Discovery of Neptune
• Discovered in 1846 at position predicted from gravitational disturbances on
Uranus’s orbit by J. C. Adams and U. J.
Leverrier
– Galileo had recorded it when observing Jupiter and its moons, but never recognized it as a planet
The Atmosphere of Neptune
• Cloud-belt structure with high-velocity winds; origin not well understood
• Blue-green color from CH4 in the H-rich atmosphere with clouds of methane ice
• Darker cyclonic disturbances, similar to Great Red Spot on Jupiter, but not long-lived
• White cloud features of methane ice crystals
The Moons of Neptune Two moons (Triton and Nereid) visible from Earth;
6 more discovered by Voyager 2
Unusual orbits:
• Triton: The only satellite in the solar system orbiting clockwise, i.e. “backward”
• Nereid: Highly eccentric orbit; very long orbital period (359.4 d)
Neptune’s Moons • At least 14 moons, two larger moons
(Triton and Nereid) visible from Earth
– Peculiar orbits
Definition of a Planet
(2006, International Astronomical Union)
1. Is in orbit around the Sun
2. Has sufficient mass to assume a nearly round shape
3. Is not a satellite (moon)
4. Has cleared the neighborhood around its orbit
Pluto – The First Dwarf Planet
• Virtually no surface features visible from Earth
• ~65% of size of Earth’s Moon
• Highly elliptical orbit; coming occasionally closer to the sun than Neptune
• Orbit highly inclined (17°) against other planets’ orbits
→ Neptune and Pluto will never collide
• Surface covered with nitrogen ice; traces of frozen methane and carbon monoxide
• Daytime temperature (50 K) enough to vaporize some N and CO to form a very tenuous atmosphere
Dwarf Planets
Dwarf planets are defined as objects that are similar to planets but do not meet all planet criteria.
Pluto Eris
Ceres
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