The Solar System: II — The Formation of the Solar System

Solar System Origins

• Introduction
• Gravitational Condensation
• Determining Planetary Composition
• Extrasolar Planets
• Discussion Questions
• Optional Website Reading

Introduction

Gravitational Condensation

The formation of the solar system and to a great extent, the star at the heart of the solar system, is a story of opposing forces. Since any given object with some measurable amount of mass attracts all other mass in the universe, the instant a clump of mass concentrates, it starts pulling in matter from the most even distributions of mass. As a cloud of matter condenses, irregularities will cause the matter falling in to fall in one direction more than another, and after some period of time, the entire condensing cloud will begin to spin. The velocity of the spinning particles will throw them away from the center to which gravity is pulling them. As the particles condense and density grows, pressure --which is a measure of the forces of collision between the collapsing particles, increases. The collisions get more energetic, kinetic motion increases as the average velocity goes up, and the overall measure of average kinetic energy increases. This pressure pushes the particles out while gravity pulls them in.

The spinning sphere of matter turns into a disk. Matter spinning at the poles moves out from the axis of rotation -- and gravity pulls it back towards the center. The two forces are in different directions, and gravity wins, pulling the spinning polar matter closer to the center.

Matter spinning along the equatorial plane counters the gravitational pull directly, so it doesn't tend to move closer to the center of the mass. Over time, continual flattening at the poles and continual rotation at a fixed distance at the equator pull the particles most distance from the center to a plane, a disk rotating in the equatorial plane of the cloud.

This matter isn't evenly distributed, or coherent. Lumps form, which attract smaller masses more than the other smaller masses around. The lumps grow by accretion, by gathering bits of stuff together, kind of like a sticky candy rolling across the floor picks up small bits of dirt. If you roll the candy across the floor enough times, you can get the floor clean, and you wind up with a big wad of candy and dirt.

Where the lumps -- planetesimals -- can remain in stable orbits, they continue to draw in more and more matter, cleaning out the lanes between the lumps, until most of the matter in the disk is now accreted into planets.

Time Frame (Roughly, in years)	Event
4.6 * 109	Solar nebula begins to contract, forming a proto-sun.
4.5 * 109	Proto-sun stabilizes with surface temperature near 6000K. Proto-planetary disk forms. Solar nebula temperature climbs from 50K to 2000K near the core.
4.0 * 109	Proto-planetesimal lumps beging to merge together into fewer lumps with nearly circular orbits in the proto-planetary disk plane. Proto-sun continues to condense; thermonuclear burning begins.
3.8 * 109	Atmospheres form on terrestrial planets by outgassing, and on Jovian planets by accretion.

Computer simulations give us an estimate that the time necessary for planets and the sun to form from a primordial gas cloud with sufficient mass was about 500 000 000 years, assuming the laws of conservation of momentum, the gravitational constant of the universe, and the laws of thermodynamics haven't changed.

Studies of the composition of our solar system with its high level of heavy elements indicate that it was formed from the detritus of a population I star, rather than the collapse of a primordial purely hydrogen-helium mix typical of the early universe. More recently, it has been suggested that such a cloud would condense and produce the isotopic radioactive nucleotides observed in our universe if a shock wave — like that from a nearby supernova type II star — triggered the collapse of the cloud.

Determining Planetary Composition

Determining planetary composition is a logical game played out using evidence from several different techniques and observing platforms. The table below is by no means complete, and lists some of the challenges and methods used to identify chemical composition of atmospheres, planetary surfaces, and core structures.

Planet	Earth-based observation	Fly-by/orbiter	Landing (direct)	Conclusions
All Objects	Orbital data: mass/size → density. Transmission spectroscopy: Atmospheres at transit Reflective spectroscopy: Atmospheres at opposition.	Magnetosphere mapping Surface mapping	Sample analysis	Produces models of core/mantle proportions, atmosphere composition, and surface composition.
Mercury	Close to sun: only observe at low altitudes (denser atmosphere). Reflectance spectrophotometry → heterogeneous SiO2 39-57. V, near-IR, IR: pyroxene, low-iron alkali basalt.	MESSENGER: Reflectance spectrophotometry: lower Fe than expected. Surface features show crust apparently shrunk as planet cooled.	NONE	Atmosphere: trace hydrogen and helium. Iron core ~42% of volume.
Venus	Cloud-cover: gas chromatography. No surface features visible.	Magellan used stereoscopic photography to map surface features (98% of planet). Surface shows no plate tectonics; lower heat loss resulting in partially liquid core and no significant magnetic field.	Venera 4 detected 95% or more CO2 content and 18 BAR pressure before loss of signal. Magellan was dropped spinning into the atmosphere with its solar panels extended. Torque changes measured atmospheric density.	Atmosphere is 96.4% CO2, 3.41% N, 0.135% H2O vapor, trace oxygen, argon, neon, sulfur dioxide. Core possibly liquid or solid. Based on magnetic field measurements, assuming a silicate crust and molten mantle, iron-nickel core.
Earth	LANDSAT, weather satellites. Extensive mapping of land features, radiation belts.	Orbiters measure solar wind interaction with magnetosphere.	Direct soil, atmosphere sample testing, core drilling, earthquake wave measurements, surface magnetic field measurements..	Inner core, outer core, mantle, upper mantle, and crust density measurements.
Mars	Earth-based observations largely superseded by unmanned mission observations nearly continuous for last four decades.	Mariners 6 and 7, Viking Orbiter, Odyssey: Surface feature mapping, γ-ray studies of atmosphere and surface detect hydrogen spectra, thought to indicate water ice.	Viking Lander, Phoenix (stationary); Pathfinder, Spirit and Opportunity (rovers): Soil samples, direct seismic measurements, evidence of former surface liquid water.	Silicate crust with permafrost; silicate mantle, iron-sulfide core.
Asteroids and Comets	Optical observations of asteroids problematic: usually low albedo, rapid motion, and tumbling. Analysis of meteorite fragments thought representative of asteroid composition. Spectrographic studies of comet tails successful.	Stardust and similar expeditions passed near comets or asteroids, sending back pictures and spectrographic data.	Stardust collected materials from interplanetary space (solar wind) and the tail of Comet Wild-2, returning samples to earth. Hayabusa (Japanese), returning with samples (if collection was successful) of 25143 Itokawa asteroid in 2010. Deep Impact successfully rendezvous and observation of collision of smart impactor to Comet Tempel 1; mission extended to comet Hartley 2 (Oct 10, 2010). Detected water ice on comet Tempel surface.	Asteroids: Homogenous composition: iron silicates. Comets: Water ice, frozen CO2, silicate rocks.
Jupiter	Occultation of stars and analysis of light transmission (e.g., α Arietis). Radio observations continue to yield new information.	Pioneer, Voyager (fly-by); Galileo (orbiter). Galileo sent back surface studies of Jupiter, all four moons, and Comet Shoemaker-Levy impact.	Galileo atmospheric probe collected data for 57.6min before being crushed by atmospheric pressure 22X Earth-normal. Galileo itself sent into Jupiter to prevent crashing/contaminating Europa (which may have liquid water and life).	Atmosphere: 97.2% He, 2.3% H2, traces of Ne, CH4, A, NH3. Lack of hydrogen in atmosphere indicates probable solid hydrogen core. Hot interior radiating 1.4X solar input.
Saturn	Monitoring using HST; no significant new earth-based information.	Cassini: Surface mapping in visual/IR. Plasma spectrometry, magnetosphere mapping. Primary mission now extended; continues flybys of Saturn's moons. Has mapped liquid seas on Titan.	Huygens Probe dropped into Titan's atmosphere, returning direct sampling data.	Less dense than water: assumed rock core surrounding by hydrogen, helium, liquid metallic layer. Hot interior radiating 2.5X solar input. Significant storm turbulence and lightning near south polar region.
Uranus	Generally no visible features from earth-based telescopes. Accidental discovery of ring system while using occultation of start SAO 158687 to examine planetary atmosphere. Subsequent observation using HST and the Keck telescopes identified additional rings.	Voyager 2 direct observation of rings, atmosphere, measurements of magnetosphere.	NONE	Planet tilted; axis of rotation at 98°. Magnetic field axis at 59° to axis of rotation. Assumed small rocky core, "icy" mantle (bulk of planet, mostly water, ammonia, methane), and hydrogen/helium atmosphere.
Neptune	Earth-based observation limited. HST able to identify dark blue spot. Radio telescope monitoring useful to measure Neptune's magnetic field rotation; IR observations track storm progression.	Voyager 2 flyby returned surface pictures of Neptune and images of moon Triton. Direct measurement of magnetic field similar to Uranus.	NONE	Based on spectroscopic analysis, assumed
Kuiper Belt (Pluto, Eris, Makemake, Sedna, Quaoar, etc)	Extremely limited observations even using Keck and HST; able to determine orbital features, size and mass.	NONE: New Horizons launched 2006, arrived at Pluto in July, 2015.	NONE	Pluto is currently assumed to have a rocky core, surrounded by a water ice "mantle" and a "crust" of frozen nitrogen.

Extrasolar Planets

Astronomers had long postulated something similar to the process outlined above, and computer simulations merely filled in details. One of the arguments in favor of the model is that it doesn't propose any special circumstances necessary for the formation of planets as part of the normal formation of stars within a certain range. Astronomers realized that if the model was accurate, there should be many stars with planets. Over the last two decades, a diligent search has turned up evidence that many nearby stars have planets large enough to create deviations in the star's path through space. While only a few planets have been observed directly (through starlight reflected from their primary), over 300 exoplanets have been identified by other means. Such evidence supports the hypothesis that planetary formation is likely to be part of normal star condensation patterns.

Astrometry	"Wobble" in primary position due to orbit of massive planet.
Radial Velocity	Variation in radial velocity (Doppler shift) due to stellar velocity changes in its orbit caused by massive planetary gravitational field. Limited to planetary orbits whose planes lie in the line of sight from earth.
Pulsar Timing	Variation in pulsar frequency due to pulsar orbital changes under the influence of a massive planet's gravitational field; limited to detecting stars around (relatively rare) pulsars.
Transit	Variation in stellar magnitude caused by planetary transit. Limitations: planet must cover a significant portion of the primary star's disk and be in line of site from earth.
Gravitational microlensing	Changes to the lensing effect of a massive star in front of a distant background star. Limited to stars with planets in a precise orientation between the distant star and Earth.
Circumstellar Disks	Detection of dust disks generated by collisions between comets and asteroids — suggesting planets may also orbit the star.
Direct Imaging	Telescopes detect reflected starlight from the primary on the planets's surface. Discovered: 2M1207b, companion to brown dwarf 2M1207.

All extrasolar planetary methods have drawbacks and most can be used only in limited, special situations, making detection of such planets difficult. New projects to detect extrasolar planets include the Kepler satellite, which will use the transit method to scan stars in a limited sector (the Cygnus region) for variations in stellar emissions.

Discussion Questions

• Why are there two methods of gas accumulation, one for terrestrial and one for Jovian planets? What factors control whether a planet can create, capture, or retain an atmosphere?
• What factors dictate that the composition of the planets is different as we travel out from the sun?

Optional Web Reading

• There's a good source of information on objects in the solar system at Views of the Solar System

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