Formation and

content of the solar

system.

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

Dust Disks around

Forming Stars

Dust disks around some T Tauri stars can be

imaged directly (HST).

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

Earthlike planets

Jovian planets (gas giants)

The Condensation of Solids

• 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.

The Growth of 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

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.

Venus

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

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

• Volcanic activity (including highest volcano in the solar system)

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.

• Mars has lost all lighter gasses; retained only heavier gasses (CO2)

Hidden Water on Mars

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

• central channel in a valley suggests long-term flowing water

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

• Probably heated by liquid helium droplets falling towards center

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)

Uranus

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

Pluto

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

Pluto’s Moons

NASA

Dwarf Planets

Dwarf planets are defined as objects that are similar to planets but do not meet all planet criteria.

Pluto Eris

Ceres