chapter 6: our solar system and its origin...4/8/2009 habbal astro110-01 lecture 29 4 the scale of...

4/8/2009 Habbal Astro110-01 Lecture 29 1

Chapter 6: Our Solar System and Its Origin


What does our solar system look like?


•  The planets are tiny compared to the distances between them (a million times smaller than shown here), but they exhibit clear patterns of composition and motion.

•  The patterns are far more important and interesting than numbers, names, and other trivia !!


The scale of the solar system •  On a 1-to-10 billion scale:

–  Sun is the size of a large grapefruit (14 cm). –  Earth is the size of a ball point, 15 meters away.

The average distance from the Earth to the Sun defined to be one Astronomical Unit (about 150 million kilometers).

–  Pluto (the most distant planet in our solar system) is about 600 meters away (1/3 of a mile).


The Sun •  The Sun is the closest star. •  Accounts for over 99.9% of mass in the solar system. •  Composition: 70% H, 28% He, 2% heavier elements.

•  Radius ~ 7 x 105 km (110x Earthʼs radius)

•  Mass ~ 2 x 1030 kg (300,000x Earthʼs mass)

•  Surface temp ~ 5800 K

•  Luminosity ~ 4 x1026 Watts


Mercury (0.4 AU from the Sun) •  made of metal and rock; large iron core •  no atmosphere •  very hot and very cold: 425°C (day), –170°C (night)


Venus (0.7 AU) •  nearly identical in size to Earth •  extreme greenhouse effect

•  even hotter than Mercury: 470°C, both day and night •  atmospheric pressure equiv. to 1 km deep in oceans •  no oxygen, no water, …

•  how did it end up so different from Earth?


Earth (1 AU) •  An oasis of life •  The only surface liquid water in the solar system

•  about 3/4 of surface covered by water •  A surprisingly large moon

Earth and Moon to scale


Mars (1.5 AU) •  Looks Earth-like, but … •  Cold rocky planet with little atmosphere •  Water flowed in the distant past: could there have been life?


Jupiter (5.2 AU)

•  Distant: >2x as far from the Sun as Mars.

•  Big ball of gas, mostly H/He: no solid surface

•  300× Earth mass! >1000× Earth volume!

•  Many moons, rings…


The moons are miniature planets and as interesting as Jupiter itself

Iohas active volcanos

Europaicy surface +subsurface

ocean?

Ganymedelargest moonin the S.S. Larger than Mercury

Callistolarge ice

ball w/craters

The four Galilean (first seen by Galileo) moons


Saturn (9.5 AU) •  Giant and gaseous like Jupiter •  Most spectacular rings of the 4 Jovian planets •  Many moons, including cloud-covered Titan


Saturnʼs rings

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

Artistʼs conception


Saturn

Cassini probe arrived in July 2004.

Dropped Huygens probe onto the surface of Titan.


Uranus (19.2 AU)

•  much smaller than Jupiter/Saturn, but still much larger than Earth

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

•  extreme axis tilt: nearly tipped on its “side”


Neptune (30.1 AU)

•  Very similar to Uranus (but much smaller axis tilt)

•  Many moons, including unusual Triton: orbits “backward


Pluto (39.5 AU) •  A “misfit” among the planets: far from Sun like large jovian planets, but much smaller than any terrestrial planet. •  Comet-like composition (ices, rock) and orbit (eccentric, inclined, 248 years period).


Planetary data table

63

50

27

3



63

50

27

3


What are the clues to our solar systemʼs formation?

  Patterns of motion (organized)

  Composition (differentiated between terrestrial and Jovian)

  Asteroids and comets (remnants of the formation process)

  Anomalies (massive, random impacts in early solar system)


1. The Sun, all planets, and all large moons orbit and rotate in an organized way.

Counterclockwise, as seen from above the north pole (right hand rule)


2. Terrestrial planets are small, rocky, and close to the Sun. Jovian planets are large, gas-rich, and far from the Sun. (What about Pluto?)


Asteroids: big rocks between Mars & Jupiter, in the Asteroid Belt

Comets: dirty snowballs past Neptune (mostly ice, some rock). Come from the Kuiper Belt & beyond.

These objects far outnumber the planets and their moons.

3. Asteroids & Comets:“Leftovers” from planet formation


Sun-grazing comets

Three frames taken hours apart on October 23rd, show bright SOHO comet number 367 plunging toward the fiery solar surface, its tail streaming away from the Sun located just beyond the left hand border.

From bottom to top, the comet's tail grows as the intensifying solar radiation heats the frozen comet material and increases the outflow of gas and dust. Comet number 367 was not seen to survive its close solar encounter.

Because of their orbits, sungrazers are believed to belong to a family of comets produced by the breakup of a single much larger comet.

Comet 367

106 km

time


4. A successful theory of solar system formation must

explain the major trends, but also allow for exceptions to

rules.


Summary: Four Major Features of our Solar System


How did the solar system form?


According to the nebular theory, our solar system formed from the gravitational collapse of a giant cloud of interstellar gas.

(nebula = “cloud” in Latin)

First conceived in 1755 by the German philosopher Immanuel Kant.


The cloud of gas that gave birth to our solar system resulted from the recycling of gas through many

generations of stars within our galaxy.


Stars are born in molecular clouds •  Clouds are very cold: ~10-30 K. (273 K = water freezes) •  Stars form when gravity overcomes thermal pressure. •  Then gas clumps begin to collapse.


Earliest stages of star birth

•  Dense cores of gas in the larger molecular cloud collapse due to self-gravity.

•  Cloud heats up as it contracts due to conservation of energy: gravitational potential energy is converted to thermal energy (heat).


Rotation is an important factor during the star birth

process (part 1) •  As gravity forces a dense core to

become smaller, it spins faster and faster.

•  This is due to conservation of angular momentum. –  Dense cores have a small

amount of initial rotation. –  As the cores get smaller, they

must spin up to conserve angular momentum.


Rotation is an important factor during the star birth

process (part 2) •  Collisions between gas particles in

cloud gradually reduce random motions and up+down motions.

•  Collisions flatten the cloud into a disk.

•  The result is a rotating protostar with a rotating disk of gas & dust.

•  The orderly motions of our solar system today are a direct result of the solar systemʼs birth in a spinning, flattened cloud of gas.


As gravity causes cloud to shrink, its spin increases

(conservation of angular momentum).

Spinning cloud also flattens as it shrinks.


Collisions between gas particles in cloud gradually reduce random motions.

Initial gas cloud has motions of all different ellipticities. But at the end, only circular orbits remain.


Spinning cloud flattens as it shrinks.


Orderly motions of our solar system today are a direct result of the solar

systemʼs birth in a spinning, flattened cloud of gas.


Nearby star-forming regions have 1000ʼs of young (few Myr) stars.

Most of them (~2/3) have disks of gas & dust around them, which are the birthplaces for other solar systems.

Disks around other stars:Solar systems in the making


Plenty of evidence for spinning disks of gas and dust around other stars, especially around newly formed (few Myr) stars.


chapter 6: our solar system and its origin...4/8/2009 habbal astro110-01 lecture 29 4 the scale of...

Documents