Our Planetary System & the Formation of the Solar System Chapters 7 & 8
Comparative Planetology • We learn about the planets by comparing them and assessing their similarities and differences • Similarities and differences help us understand solar system formation • ... tell us about Earth • ... help us make sense of exo-planetary systems. • ... help us understand what the conditions for life are on other planets. • ... and allow us to understand trends and processes rather than memorize facts.
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What features of our solar solar system provide formation clues?
Inventory of the Solar System
• The Sun, planets and large
moon generally orbit and rotate in an organized way
• 1 Star • 8 Planets + at least 5 dwarf planets • 4 Planetary Ring Systems
• There are two major types of planets
• Asteroids and comets:
numerous, and their composition varies with location in the solar system
• Exceptions... 3
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Inventory of the Solar System • > 100 Natural Satellites (i.e., • • • • •
moons) > 4000 Numbered Asteroids ~ 1012 comets Zodiacal Dust Cloud Solar Wind / Solar Magnetic Field 70,000 Kuiper Belt Objects (with diameters > 100 km)
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Jovian Planets
Terrestrial Planets
Mercury
Earth
Venus
Mars
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Jupiter
Saturn
Uranus 1781
Neptune 1846
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Density • Density: Measure of the amount of mass contained in a given volume. • Density is an indicator of the composition of a planet • Density is not correlated with size
Examples • The Earth’s density is 5.5 g / cm3, but the density of the crust is 2.5 – 3.5 g / cm3. The core is comprised of iron & nickel compressed to abnormal densities • The Jovian planets are very large, but have low densities. These planets are comprised mostly of hydrogen, helium, & methane (CH4) 9
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Robotic missions: four types • Flyby: a spacecraft flies by a world just once
• Orbiter: orbits the world it is
studying, and collects long-term data
• Lander or probe: lands on the planet’s surface or probes the atmosphere while descending through it
• Sample return mission: returns samples of target source to Earth for further study
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Solar System: Major Characteristics • Orbits of planets are co-planar • Orbits of planets are nearly circular (exceptions – Mercury, Pluto, & comets) • Motion of Planets are prograde • Planetary spins are prograde, with periods of 10-20 hours (exceptions – Venus, Uranus, & Pluto) • Terrestrial planets (Mercury→Mars) have refractory (bits of rocks) compositions, and the Jovian planets are gaseous • Jupiter, Saturn, & Uranus resemble mini-solar systems (many satellites) • Asteroids and comets are numerous, and their composition varies with location in the solar system • Solar system is transparent (i.e., dust free)
Nebular Theory •
Nebular Theory: Our solar system formed from the gravitational collapse of an interstellar cloud of gas
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theory credited to Immanuel Kant (1755 A.D.), and PierreSimon Laplace (~ 1795 A.D.)
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Where does the Solar System come from?
Where does the Solar System come from?
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It comes from gas clouds enriched by prior episodes of star formation (production of heavy elements)
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It comes from gas clouds enriched by prior episodes of star formation (production of heavy elements)
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The Orion Nebula is an example of such enrichment
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The Orion Nebula is an example of such enrichment
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The Orion Nebula
What caused the orderly patterns of motion in our solar systems? • Heating: as the nebula collapsed, gravitational potential energy → kinetic energy→heat. The Sun formed in the center
• Spinning: conservation of angular
momentum ensured that everything didn’t collapse into the center
• More than 3000 stars are in this image amongst the gas and dust in the nebula
• Flattening: random motion dampened
out through collisions, leaving flattened rotating disk
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An example of a disk: β Pictoris
Another Example
disk star
Central star has been blocked by a Coronagraph 19
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Circumstellar disks (optical)
Surrounding Gas Star
!!!!??
Dust Disk
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Ionization of surrounding gas Jets: removal of mass reduces Angular Momentum (= mass x velocity x radius)
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Circumstellar disks (optical)
Circumstellar disks (optical)
What an What an infrared telescope optical telescope sees sees
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Artist’s conception of collapsing stellar disk
More Examples
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Four Types of Nebular Material • Gas: what makes up planetary atmospheres • Ice (Volatiles): molecules that are liquid or gaseous at moderate temperatures but form solids/crystals at low temperatures (e.g., Water – H2O, Carbon dioxide – CO2, Methane – CH4) • Rock: objects such as silicates that can be left behind after ice mixed with heavier elements are heated (e.g., silicates – molecules of oxygen combined with either silicon, magnesium, or aluminum) • Metal: material, such as iron, nickel, & magnesium that separate out from the rest of the material that make up rock when temperatures get extremely high
Why are there two major types of planets? • planets formed out of
material that was able to condense at particular distances from the Sun.
• The condensation of
hydrogen, hydrogen compounds, rock and metal is temperature dependent
Heat 29
Why are there two major types of planets?
compounds can only condense beyond the frost • Hydrogen line, which lies between the orbits of Mars and Jupiter 31
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How did the terrestrial planets form?
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grains stick together, forming planetesimals planetesimals attract each other gravitationally (accretion) Protoplanets form, sweeping up grains in their path 32
How did the Jovian planets form?
Composition • The composition of Jupiter and Saturn will reflect the materials that are available there.
ALMA Observations of HL Tau
• They build up 10 M cores • Which then gravitationally attract hydrogen and helium • Their satellites and ring system form out of a surrounding disk • Alternate theory: They formed from collapse (like the Sun) earth
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Solar System Formation
Solar System Formation
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F IG . 2.— Panels (a), (b), and (c) show 2.9, 1.3, and 0.87 mm ALMA continuum images of HL Tau. Panel (d) shows the 1.3 mm ps
other panels, as well asof an inset an enlarged view of the inner 300 mas centered on the psf’s peak (the other bands show similar High resolution ALMA image thewith star HL Tau (f) show the image and spectral index maps resulting from the combination of the 1.3 and 0.87 mm data. The spectral index (α) ma α/αerror < 4. The synthesized beams are shown in the lower left of each panel, also see Table 1. The range of the colorbar shown Dust disk with dark ringsto −2×rms to 0.9× the image peak, using the values in Table 1. The colorscales for panels (a), (c) and (e) are the same corresponds rms and image peak corresponding to each respective wavelength in Table 1.
reconcile with a simple disk/outflow scenario, suggesting that the blue-shifted outflow 36 has broken out of the parental core (Monin et al. 1996), or that there is another – as yet unidentified – driving source. Unfortunately, the 12 CO (1-0) data are missing significant flux (due to a lack of short spacings), and have insufficient sensitivity in the outer portions of the field of view to warrant deeper analysis of its properties. Figs. 1b, and c show zoomed in views of our serendipitous detections of XZ Tau (A and B), and LkHα358; no other continuum sources
3.1.1. Position and Proper
The fitted position for HL Tau in images is given in Table 1. The ph tions are accurate to < 1 mas and the p tent between the three observed bands (consistent with dedicated LBC astrome ALMA partnership et al. 2015); thus, we absolute ALMA position uncertainty. T
Solar System Formation
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What ended planetary formation? •
the clearing of gas and dust through radiation pressure from the Sun
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... and through streams of charged particles (solar wind) from the Sun
The rings are carved out by orbiting planets
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What ended planetary formation?
Where did asteroids and comets come from?
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the clearing of gas and dust through radiation pressure from the Sun
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... and through streams of charged particles (solar wind) from the Sun
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leftover planetesimals
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...or were ejected to the outer solar system by planets
the result of fragmentation as protoplanets grow in size Note that many asteroids and comets crashed into planets
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Creation of layers of the “rocky” parts of Planets •
Differentiation: The gravitational separation or segregation of different densities of material into different layers in the interior of a planet, as a result of heating
The Process 1) E.g., the Earth was struck by large rocks in the early days of the solar system 2) Kinetic energy from these rocks was converted into heat 3) Central temperature rose, & the core of the planet became liquid 4) Denser material migrated to the center
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Atmospheres • How does a planet obtain an atmosphere? - it forms with one (capture/primordial) -it produces one from the material in which the planet is made (outgassing)
• Exosphere: layer from which escape can occur • k T ~ β m v2
Hydrogen Atom (atomic mass = 1)
Argon Atom (atomic mass = 40)
Mass
• How does a planet hold an atmosphere? - must be massive enough - …or the gas will escape - must be cool enough • Why is the composition of atmospheres different for different planets? - Large planets: massive enough to capture hydrogen & helium early on in their formation - Small planets: outgassing (made up of what the planet formed with) 43
Velocity Temperature Atmosphere
Exosphere (layer from which escape can occur)
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• For a fixed T, lighter atoms escape more readily than heavier atoms because they have higher velocities
Age-Dating • Solidification Age: Time since the material became solid • Gas Retention Age: A measure of the age of a rock, defined in terms of its ability to retain radioactive argon (which is the daughter product of potassium)
Radioactive Dating • Half-Life: Given a quantity of material, the half-life is the time which half the material will have decayed into the daughter product Examples -
• Radioactive Decay U-238 (92p+,146n) → Pb-206 (82p+,124n) + (10p+,22n) K-40 (19p+,21n) → Ar-40 (18p+,22n) ter daugh • The Decay Rates ent par
U-238 → 4.5 billion years K-40 → 1.25 billion years
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Radioactive Dating • Half-Life: Given a quantity of material, the half-life is the time which half the material will have decayed into the daughter product
Radioactive decay of Potassium-40 to Argon-40
Examples -
• Radioactive Decay U-238 (92p+,146n) → Pb-206 (82p+,124n) + (10p+,22n) K-40 (19p+,21n) → Ar-40 (18p+,22n) ter daugh • The Decay Rates ent par
U-238 → 4.5 billion years K-40 → 1.25 billion years
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Radioactive Decay
Radioactive Decay • To measure the age of the rock,
Initial amount of Parent product
Present amount
• We first determine λ in terms of the half-life time τhl,
Inverse Fraction of Parent product left
• And thus, Age of rock
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Radioactive Decay • The number of ‘daughter atoms’ after τ is,
• And thus,
• The ratio Dτ / Nτ can be measured, and τhl is known from laboratory measurements. • Age-dating (via U-238) of lunar rocks show the moon to be ~ 4.5 billion years old
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Summary: Formation and Condensation of the Solar Nebula • Stars form out of clouds of molecular gas & dust • Collapse occurs when the gas is dense enough to collapse under its own weight • Central parts of collapsing cloud become heated, & the shrinking nebulae begin to spin faster Angular Momentum = Mass x Velocity x Radius • Results - center becomes star - spinning disk ultimately gives rise to planets - angular momentum decreased through mass loss (jets)
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Summary: Disk Evolution • Temperature gradient develops in the disk - outer disk cools - inner disk is heated by proto-Sun • Grains, whose composition depends on the local temperature, begin to condense - stick together initially, building up planetesimals - planetesimals attract each other gravitationally (a process called accretion) - Protoplanets form, sweeping up grains in their path - As protoplanets grow in size, fragmentation becomes important for the production of meteoroids & asteroids (as well as for heating the interior of the planets)
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