1. Formation of Our Solar System
By the Lunar and Planetary Institute For Use in Teacher Workshops
Image: Lunar and Planetary Laboratory:

2. Some data to explain: 1. Planets isolated
2. Orbits ~circular / in ~same plane
3. Planets (and moons) travel along orbits in same direction…. same direction as Sun rotates (counter- clockwise viewed from above)
Data we must explain:
1. Planets are isolated from each other.
2. Planetary orbits are nearly circular.
3. All orbits lie in almost the same plane.
4. All planets travel along their orbits in the same direction…. same direction of Sun rotation - CCW
5. Most planets rotate in this same direction - CCW
a.Venus - slowly CW
b.Uranus - on its side
c.Pluto - on its side - captured asteroid?
d.Moons go CCW around planets (few exceptions)
6. The revolution of most of the moons is in this same direction.
7.The Solar System is highly differentiated: rocky (slow rotators, few or no moons) vs. gaseous planets (fast rotators, many moons).
8.Asteroids: old, different from rocky or gaseous planets.
9.Comets: old, icy, do not move on the same plane as the planets. Exist in a swarm called the Oort cloud, very far from the Sun.
The planets, most of the satellites of the planets and the asteroids revolve around the Sun in the same direction, in nearly circular orbits. When looking down from above the Sun's north pole, the planets orbit in a counter-clockwise direction. The planets orbit the Sun in or near the same plane, called the ecliptic. Pluto is a special case in that its orbit is the most highly inclined (18 degrees) and the most highly elliptical of all the planets. Because of this, for part of its orbit, Pluto is closer to the Sun than is Neptune. The axis of rotation for most of the planets is nearly perpendicular to the ecliptic. The exceptions are Uranus and Pluto, which are tipped on their sides.
Lunar and Planetary Institute image at

3. Some more data to explain:
4. Most planets rotate in this same direction
Mercury 0° Venus 177° Earth 23° Mars 25°
Jupiter 3° Saturn 27° Uranus 98° Neptune 30°
NASA images edited by LPI

4. And some more data to explain:
5. Solar System highly differentiated:
Terrestrial Planets (rocky, dense with density ~4-5 g/cm3)
Jovian Planets (light, gassy, H, He, density 0.7-2)
Terrestrial Planets
This image shows the terrestrial planets Mercury, Venus, Earth and Mars approximately to scale. The terrestria lplanets are compact, rocky, Earth-like planets. (Copyright Calvin J. Hamilton)
Jovian Planets
This image shows the Jovian planets Jupiter, Saturn, Uranus and Neptune approximately to scale. The Jovian planets are named because of their gigantic Jupiter-like appearance. (Copyright Calvin J. Hamilton)
Planets
Terrestrial Small Dense ( g/cm3) Rocky + Metals
Jovian Large Low density ( g/cm3) Gaseous
Images: Lunar and Planetary Laboratory:

5. How Did We Get a Solar System?
Image: LPI
ABOUT THIS IMAGE:
Strangely glowing dark clouds float serenely in this remarkable and beautiful image taken with NASA's Hubble Space Telescope. These dense, opaque dust clouds - known as "globules" - are silhouetted against nearby bright stars in the busy star-forming region, IC These globules were first found in IC 2944 by astronomer A.D. Thackeray in 1950.
Although globules like these have been known since Dutch-American astronomer Bart Bok first drew attention to such objects in 1947, little is still known about their origin and nature, except that they are generally associated with areas of star formation, called "HII regions" due to the presence of hydrogen gas.
The largest of the globules in this image is actually two separate clouds that gently overlap along our line of sight. Each cloud is nearly 1.4 light-years (50 arcseconds) along its longest dimension, and collectively, they contain enough material to equal over 15 solar masses. IC 2944, the surrounding HII region, is filled with gas and dust that is illuminated and heated by a loose cluster of O-type stars. These stars are much hotter and much more massive than our Sun. IC 2944 is relatively close by, located only 5900 light-years (1800 parsecs) away in the constellation Centaurus.
Thanks to the remarkable resolution offered by the Hubble Space Telescope, astronomers can for the first time study the intricate structure of these globules. The globules appear to be heavily fractured, as if major forces were tearing them apart. When radio astronomers observed the faint hiss of molecules within the globules, they realized that the globules are actually in constant, churning motion, moving supersonically among each other. This may be caused by the powerful ultraviolet radiation from the luminous, massive stars, which also heat up the gas in the HII region, causing it to expand and stream against the globules, leading to their destruction. Despite their serene appearance, the globules may actually be likened to clumps of butter put onto a red-hot pan.
It is likely that the globules are dense clumps of gas and dust that existed before the massive O-stars were born. But once these luminous stars began to irradiate and destroy their surroundings, the clumps became visible when their less dense surroundings were eroded away, thus exposing them to the full brunt of the ultraviolet radiation and the expanding HII region. The new images catch a glimpse of the process of destruction. Had the appearance of the luminous O-stars been a bit delayed, it is likely that the clumps would actually have collapsed to form several more low-mass stars like the Sun. Instead they are now being toasted and torn apart.
The hydrogen-emission image that clearly shows the outline of the dark globules was taken in February 1999 with Hubble's Wide Field Planetary Camera 2 (WFPC2) by Bo Reipurth (University of Hawaii) and collaborators. Additional broadband images that helped to establish the true color of the stars in the field were taken by the Hubble Heritage Team in February The composite result is a four-color image of the red, green, blue and H-alpha filters.
Object Names: Thackeray's Globules, IC 2944
Concentrations of dust and gas in huge cloud of cold, thinly dispersed interstellar gas and dust ; material starts to collect (gravity > magnetic forces)
Hubble image at

6. How Did We Get a Solar System?
Gravity concentrates most stuff near center
Heat and pressure increase
Collapse – central proto-sun rotates faster (probably got initial rotation from the cloud)
Forms solar nebula
1.Interstellar gas and dust cloud: diameter 1 ly. Gravitational collapse.
2.Solar nebula: 100 AU diameter, rotating disk. Dust particles in inner nebula are broken down because of warmer temperatures. Dust particles from 10 AU on out remain. Inner nebula starts to cool.
Image: LPI

7. How Did We Get a Solar System?
4.Gravitational accretion: planetesimals have sufficient gravity to attract material gravitationally. Larges bodies dominate and grow rapidly. The largest bodies form protoplanets. Protomoons form around them. Smaller bodies suffer numerous collisions and fragment. Some planetesimals, through gravitational interactions with the large protoplanets, are ejected into the outer solar system forming the Oort cloud of comets.
Fusion ignition of the Sun followed by 1 my period of violent solar activity. Solar winds sweep lighter materials (H, He, H2O, Ammonia, etc) outward from the Sun, leaving the inner solar system enriched in refractory materials such as silica and iron.
After ~10 million years, material in center of nebula hot enough to fuse H
“...here comes the sun…”
NASA/JPL-Caltech Image at

8. How Did We Get a Solar System?
4.Gravitational accretion: planetesimals have sufficient gravity to attract material gravitationally. Larges bodies dominate and grow rapidly. The largest bodies form protoplanets. Protomoons form around them. Smaller bodies suffer numerous collisions and fragment. Some planetesimals, through gravitational interactions with the large protoplanets, are ejected into the outer solar system forming the Oort cloud of comets.
Fusion ignition of the Sun followed by 1 my period of violent solar activity. Solar winds sweep lighter materials (H, He, H2O, Ammonia, etc) outward from the Sun, leaving the inner solar system enriched in refractory materials such as silica and iron.
Gravitational accretion: planetesimals attract stuff
Large protoplanets dominate, grow rapidly, clean up area ( takes ~10 to 25 My)
Image: LPI

9. Early in the Life of Planets
Planetesimals swept up debris
Accretion + Impacts = HEAT
Eventually begin to melt materials
Iron, silica melt at different temperatures
Iron sank – density layering
Terrestrial Planet Interiors Mercury Mercury has an average density of 5430 kilograms per cubic meter, which is second only to Earth among all the planets. It is estimated that the planet Mercury, like Earth, has a ferrous core with a size equivalent to two-thirds to three-fourths that of the planet's overall radius. The core is believed to be composed of an iron-nickel alloy covered by a mantle and surface crust. Venus It is believed that the composition of the planet Venus is similar to that of Earth. The planet crust extends to around kilometers below the surface, under which the mantle reaches to a depth of some 3000 kilometers. The planet core comprises a liquid iron-nickel alloy. Average planet density is 5240 kilograms per cubic meter. Earth The Earth comprises three separate layers: a crust, a mantle, and a core (in descending order from the surface). The crust thickness averages 30 kilometers for land masses and 5 kilometers for seabeds. The mantle extends from just below the crust to some 2900 kilometers deep. The core below the mantle begins at a depth of around 5100 kilometers, and comprises an outer core (liquid iron-nickel alloy) and inner core (solid iron-nickel alloy). The crust is composed mainly of granite in the case of land masses and basalt in the case of seabeds. The mantle is composed primarily of peridotite and high-pressure minerals. Average planet density is 5520 kilograms per cubic meter. Mars Mars is roughly one-half the diameter of Earth. Due to its small size, it is believed that the martian center has cooled. Geological structure is mainly rock and metal. The mantle below the crust comprises iron-oxide-rich silicate. The core is made up of an iron-nickel alloy and iron sulfide. Average planet density is 3930 kilograms per cubic meter. Pluto The structure of Pluto is not very well understood at present. Nevertheless, spectroscopic observation from Earth in the 1970s has revealed that the planet surface is covered with methane ice. Surface temperature is -230?C (-382?F), and the frozen methane exhibits a bright coloration. However, with the exception of the polar caps, the frozen methane surface is seen to change to a dark red when eclipsed by its moon Charon. Average planet density is 2060 kilograms per cubic meter. The low average density requires that the planet must be a mix of ice and rock. Image Credit: Lunar and Planetary Institute
Image from LPI:

10. How Did We Get a Solar System?
1. The smaller protoplanets ( inner solar nebula) are unable to accrete gas because of their higher temperature.
5.Gravitational accretion of gas: the largest protoplanets in the coolest parts of the solar nebula accrete gas. The smaller protoplanets in the inner solar nebula are unable to accrete gas because of their higher temperature.
Jupiter and Saturn form first? Sweep up H and He – leave C, O, and N for the others?
6.Sweep of debris: over about 1 billion years, the material left over from the solar system formation is cleared. The Sun enters its T-Tauri phase and blows the remaining gas out of the solar system. The Jovian planets strongly influence the orbits of the remaining planetesimals (comets), either throwing them out into the Oort cloud or inward, where they collide with the terrestrial planets. Terrestrial planets obtain their atmospheres, and particularly water, from the constant impact of cometary material. In a process that remains uncertain, Jupiter influences the region of the asteroid belt so that some material remains in stable solar orbits but is unable to coalesce into a larger body.
WHEN?
Refractory elements (heavier elements stable at higher temperatures) condensed in the inner nebula. Terrestrial Planets.
Volatile elements (lighter elements stable at lower temperatures) condensed in the outer nebula. Jovian Planets.
Outer Solar System
Cold – ices, gases – 10x more particles than inner
May have formed icy center, then captured lighter gases (Jupiter and Saturn first? Took H and He?)
Image: LPI

11. How Did We Get a Solar System?
1. The smaller protoplanets ( inner solar nebula) are unable to accrete gas because of their higher temperature.
5.Gravitational accretion of gas: the largest protoplanets in the coolest parts of the solar nebula accrete gas. The smaller protoplanets in the inner solar nebula are unable to accrete gas because of their higher temperature.
6.Sweep of debris: over about 1 billion years, the material left over from the solar system formation is cleared. The Sun enters its T-Tauri phase and blows the remaining gas out of the solar system. The Jovian planets strongly influence the orbits of the remaining planetesimals (comets), either throwing them out into the Oort cloud or inward, where they collide with the terrestrial planets. Terrestrial planets obtain their atmospheres, and particularly water, from the constant impact of cometary material. In a process that remains uncertain, Jupiter influences the region of the asteroid belt so that some material remains in stable solar orbits but is unable to coalesce into a larger body.
WHEN?
Refractory elements (heavier elements stable at higher temperatures) condensed in the inner nebula. Terrestrial Planets.
Volatile elements (lighter elements stable at lower temperatures) condensed in the outer nebula. Jovian Planets.
Early burst of solar wind - sweeps debris out of system
Gravitational accretion of gas for protoplanets in the coolest nebular parts
Image: LPI

12. When Did the Solar System Form?
4.56 billion years ago
How do we know? (evidence for formation)
Lunar samples to 4.6 Ga
Meteorites Ga
Earth – 3.9 (or 4.4 Ga)
The meteorite "offers us a snapshot of the original composition of the entire solar system before the planets formed," said Michael Zolensky, a scientist at NASA's Johnson Space Center (JSC). "It tells us what the initial materials were like that went into making up the Earth, the Moon and the Sun.“
Photo: Sikhote-Alin meteorite This is one fragment of the Sikhote-Alin meteorite. It is about 15 cm across. The photograph shows the original meteorite surface, melted into thumb-print shapes during its flight through our atmosphere. (Photo by Carl Allen, NASA JSC photo S )
Lunar Meteorite: The fusion crust on the 663-gram meteorite is evident in this view. The small cube in the corner is 1 cm on each side. (from NASA photo S )
Lunar meteorite at
Meteorite photo by Carl Allen at

13. We Can Also Look Around ….
A Hubble Space Telescope view of a small portion of the Orion Nebula (1600 LY away) reveals five young stars (probably a few million years old). Four of the stars are surrounded by gas and dust trapped as the stars formed, but were left in orbit about the star. These are possibly protoplanetary disks, or "proplyds," that might evolve on to agglomerate planets. The proplyds which are closest to the hottest stars of the parent star cluster are seen as bright objects, while the object farthest from the hottest stars is seen as a dark object. The field of view is only 0.14 light-years across.
The Orion Nebula star-birth region is 1,500 light-years away, in the direction of the constellation Orion the Hunter.
The image was taken on 29 December 1993 with the HST's Wide Field and Planetary Camera 2.
Close-up of "Proplyds" in Orion
Thanks Hubble!
Hubble images at
and

14. The Wall of Time