Chapter 8 Survey of Solar Systems
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Upon completing this chapter you should be able to:
• Identify the primary components of the Solar System, and describe their distinctive properties. • Discuss the differences between terrestrial, Jovian, and dwarf planets, and their satellites, and recount why astronomers reclassified Pluto. • Explain how astronomers measure masses and radii for bodies in the Solar System, and carry out a calculation to find a body's density from these measurements. • Describe the densities of different classes of objects in the Solar System and relate this to their composition.
Upon completing this chapter you should be able to:
• Recall the age of the Solar System and explain how it is determined. • Describe the steps in the formation of the Solar System according to the nebular theory and relate these to the properties of the planets and other bodies. • Explain why disks are expected to form around stars, and describe the observations that indicate disks are present around yound stars. • Describe the role of planetesimals in planet formation and modification, and where some can still be found.
Upon completing this chapter you should be able to:
• Discuss the roles of rocky, icy, and gaseous materials in the formation of planets and their atmospheres. • Explain the various methods currently being used to detect exoplanets, the information each provides about an exoplanet, and the limitations of these methods. • Discuss the ways in which exoplanetary systems appear to differ from the Solar System.
The Solar System consists of the Sun and the bodies in its gravitational domain: the eight planets, dozens of dwarf planets, and swarms of moons, asteroids, and comets. Although we have only walked on the Earth and the Moon, we have detailed pictures sent to us from spacecraft of most of the planets and their satellites. Some are baren balls of rock; others are mostly ice. Some have thin, frigid atmospheres so cold that ordinary gases crystallize as snow on their cratered surfaces; others have thick atmospheres that constistency of molten lava and no solid surface at all.
All the planets circle the Sun in the same direction, and most of them spin in the same direction. The moons also form flattened systems, generally orbiting in the same direction.
The Solar System • The Solar System is occupied by a diversity of objects, but shows an underlying order in the dynamics of their movements • The planets form two main families: – solid rocky inner planets (terrestrial) – gaseous/liquid outer planets (Jovian)
• Astronomers deduce that the Solar System formed some 4.5 billion years ago out of the collapse of a huge cloud of gas and dust
The Sun: the only star in our solar system Image of the Sun made with ultraviolet telescope that reveals high temperature gases in the Sun's atmosphere.
• The Sun is a star, a ball • Composed mainly of of incandescent gas hydrogen (71%) and helium whose output is (27%), it also contains generated by nuclear traces of nearly all the other reactions in its core chemical elements
The Sun • It is the most massive object in the Solar System – 700 times the mass of the rest of the Solar System combined • Emits its own • Its large mass provides the light gravitational force to hold all the Solar System bodies in their orbital patterns around the Sun
The Planets
• Orbits are almost circular lying in nearly the same plane • They emit no visible light of their own but shine by reflected sunlight • Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune
The Planets
The orbits are shown in the correct relative scale in the 2 drawings. Because of the great difference in scale, the inner and outer Solar System are displayed separately.
The Planets • All of the planets travel counterclockwise around the Sun (as seen from high above the Earth’s north pole) • Six planets rotate counterclockwise; Venus rotates clockwise (retrograde rotation), and Uranus appears to rotate on its side
The Planets
The planets' orbits from the side. The drawf planets Ceres, Pluto, and Eris are also shown, illustrating their highly inclined orbits. This view also shows that while Pluto sometimes gets closer to the Sun than Neptune, its orbit actually remains well separated from Neptune's.
According to the IAU, a planet is a celestrial body that is in orbit around the sun, is massive enought for its self-gravity to create a nearly round shape, and has cleared the neighborhood around its orbit. By this definition, our solar system has eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Some astronmers call these the major planets.
Instead of "inner" and "outer" planets, astronomers sometimes use "terrestrial" and "Jovian" to describe the two types of planets. The terrestrial planets (Mercury to Mars) are so named because of their resemblance to the Earth.
The Jovian planets (Jupiter to Neptune) are named for their resemblance to Jupiter.
Inner Planets • Mercury, Venus, Earth, Mars • Small rocky (mainly silicon and oxygen) bodies with relatively thin or no atmospheres • Also known as terrestrial planets
• It is believed that only the rocky material could condense at the higher temperatures of the inner part of the solar nebula.
Outer Planets • Jupiter, Saturn, Uranus, and Neptune • Gaseous, liquid, or icy (H2O, CO2, CH4, NH3) • Also referred to as Jovian planets • Jovian planets are much larger than terrestrial planets and do not have a well-defined surface • Compared to the Terrestrial planets, the Jovian planets have many satellites.
A drawf planet shares all the characteristics of a planet except that it has not cleared the neighborhood around its orbit. Ceres, Pluto, Haumea, Makemake, and Eris fall into this category...
Dwarf Planets • Pluto (it has not cleared its neighborhood) and similar objects fail to fit into either family • Recently, scientists have discovered more than 200 similar objects orbiting the Sun at the same distance as Pluto • In 2008, dwarf planets that orbit beyond Neptune are called Plutoids. • In 2006, a new family was introduced – the dwarf planets – Massive enough to pull themselves spherical – Orbits have not been swept clear of debris
Spins and Axial Tilts
• Most planets spin (rotate) in the same direction, clounterclockwise as viewed from above the north pole. • Venus and Uranus are exceptions • Axial tilts range from 0º to 98º.
Asteroids and Comets • Composition and size – Asteroids are rocky or metallic bodies ranging in size from a few meters to 1000 km across (about 1/10 the Earth’s diameter) – Comets are icy bodies about 10 km or less across that can grow very long tails of gas and dust as they near the Sun and are vaporized by its heat – Although the majority of comets probably originate in the Oort cloud, some come from the Kuiper belt. Together they probably contain more than 1 trillion comet nuclei, only a few of which get close enough to the Sun to be detected.
Asteroids and Comets • Their location within Solar System – Most asteroids are in the asteroid belt between Mars and Jupiter indicating that these asteroids are the failed building-blocks of a planet – Some comets may also come from a disk-like swarm of icy objects that lies beyond Neptune and extends to perhaps 1000 AU, a region called the Kuiper ( KY -per) Belt
Asteroids and Comets
Most comets orbit the Sun far beyond Pluto in the Oort cloud, a spherical shell extending from 40,000 to 100,000 AU from the Sun
Bode's Rule: The search for order In the 1700's astronomers noticed that the spacing between the orbits of the planets seems to follow a fairly regular progression. This mathematical progression may indicate something about the natural spacing between orbits of large bodies, or it may be a chance pattern ...
Bode’s Rule • Very roughly, each planet is about twice as far from the Sun as its inner neighbor. The progression of distance from the Sun can be expressed by a simple math relation...Bode's Rule. • First noted in 1766, formalized mathematically by J. E. Bode in 1778 – 0, 3, 6, 12 ,24, 48, 96, 192, 384 (nine numbers...) – 4, 7, 10, 16, 28, 52, 100, 196, 388 " – 0.4, 0.7, 1.0, 1.6, 2.8, 5.2, 10.0, 19.6, 38.8 " • Does a pretty good job, up to a point...
Measuring Composition: Density
• A planet’s average density is determined by dividing a planet’s mass by its volume – Mass determined from Kepler’s modified third law – Volume derived from a planet’s measured radius
Measuring Composition • Since the inner and outer planets differ dramatically in composition, it is important to understand how composition is determined • A planet’s reflection spectrum can reveal a planet’s atmospheric contents and the nature of surface rocks • Seismic activity has only been measured on Earth for the purposes of determining interior composition
Measuring Composition: Density • Once average density known, the following factors are taken into account to determine a planet’s interior composition and structure: – Densities of abundant, candidate materials – Variation of these densities as a result of compression due to gravity – Surface composition determined from reflection spectra – Material separation by density differentiation – Mathematical analysis of equatorial bulges
Analysis Concludes:
• The terrestrial planets, with average densities ranging from 3.9 to 5.5 g/cm3, are largely rock and iron, have iron cores, and have relative element ratios similar to the Sun except for deficiencies in lightweight gasses
Analysis Concludes:
• The Jovian planets, with average densities ranging from 0.71 to 1.67 g/cm3, have relative element ratios similar to the Sun and have Earth-sized rocky cores
Satellites • The number of planetary satellites changes frequently as more are discovered! – – – – – – – –
Jupiter 63 Saturn 60 Uranus 27 Neptune 13 Mars 2 Earth 1 Mercury and Venus are moonless Even Pluto and Eris have moons!
Age of the Solar System • All objects in the Solar System seem to have formed at nearly the same time, out of the same original cloud of gas and dust • Radioactive dating of rocks from the Earth, Moon, and some asteroids suggests an age of about 4.5 billion yrs • A similar age is found for the Sun based on current observations and nuclear reaction rates
Formation of Planetary Systems
• A theory of the Solar System’s formation must account for the following: – Planets orbit in the same direction and in the same plane – Rocky inner planets and gaseous/liquid/icy outer planets – Compositional trends in the solar system – All Solar System bodies appear to be less than 4.5 billion years old – Other details – structure of asteroids, cratering of planetary surfaces, detailed chemical composition of surface rocks and atmospheres, etc.
The Solar Nebula Hypothesis • Derived from 18th century ideas of Laplace and Kant • Proposes that Solar System evolved from a rotating, flattened disk of gas and dust (an interstellar cloud), the outer part of the disk becoming the planets and the inner part becoming the Sun
The Solar Nebula Hypothesis
• This hypothesis naturally explains the Solar System’s flatness and the common direction of motion of the planets around the Sun • Interstellar clouds are common between the stars in our galaxy and this suggests that most stars may have planets around them
Interstellar Clouds
• Come in many shapes and sizes – one that formed Solar System was probably a few light years in diameter and 2 solar masses • Typical clouds are 71% hydrogen, 27% helium, and traces of the other elements
Interstellar Clouds • Clouds also contain tiny dust particles called interstellar grains – Grain size from large molecules to a few micrometers – They are a mixture of silicates, iron and carbon compounds, and water ice
Formation of the Solar Nebula • Triggered by a collision with another cloud or a nearby exploding star, rotation forces clouds to gravitationally collapse into a rotating disk
The Solar Nebula • A few million years pass for a cloud to collapse into a rotating disk with a bulge in the center • This disk, about 200 AU across and 10 AU thick, is called the solar nebula with the bulge becoming the Sun and the disk condensing into planets • The jovian planets has a composition that most resembles the original solar nebula.
Disk Observations
A) The small blobs in this are stars in the process of formation and their surrounding disk of dust and gas. These are in the Orion Nebula, a huge gas cloudabout 1500 ly from Earth. B) Dust ring that surounds the star Fomalhaut. A small mask in the telescope blots out the star's direct light, which would otherwise overexpose the image.
Temperatures in the Solar Nebula
• Before the planets formed, the inner part of the disk was hot, heated by gas falling onto the disk and a young Sun – the outer disk was colder than the freezing point of water
Condensation • Condensation occurs when gas cools below a critical temperature at a given gas pressure and its molecules bind together to form liquid/solid particles • The process in which a gas cools and its molecules stick together to form liquid particles ....
Condensation in the Solar Nebula – Iron vapor will condense at 1300 K, silicates will condense at 1200 K, and water vapor will condense at room temperature in air – In a mixture of gases, materials with the highest vaporization temperature condense first – Condensation ceases when the temperature never drops low enough – Sun kept inner solar nebula (out to almost Jupiter’s orbit) too hot for anything but iron and silicate materials to condense – Outer solar nebula cold enough for ice to condense
Formation of Planets
An artist's dipiction of how the planets may have formed in the nebula. nebular hypothesis: Interstellar cloud, solar nebula, accretion, collisions between planetesimals, planets.
Accretion – Next step is for the tiny particles to stick together, perhaps by electrical forces, into bigger pieces in a process called accretion – As long as collisions are not too violent, accretion leads to objects, called planetesimals, ranging in size from millimeters to kilometers – Astronomers believe that the satellites of the Jovian planets were planetesimals orbiting the growing planet.
Planetesimals • Planetesimals in the inner solar nebula were rockyiron composites, while planetesimals in the outer solar nebula were icyrocky-iron composites • Planets formed from “gentle” collisions of the planetesimals, which dominated over more violent shattering collisions
Formation of the Planets • Simulations show that planetesimal collisions gradually lead to approximately circular planetary orbits • As planetesimals grew in size and mass their increased gravitational attraction helped them grow faster into clumps and rings surrounding the Sun
Formation of the Planets • Planet growth was especially fast in the outer solar nebula due to: – Larger volume of material to draw upon – Larger objects (bigger than Earth) could start gravitationally capturing gases like H and He
Final Stages • Rain of planetesimals cratered surfaces • Remaining planetesimals became small moons, comets, and asteroids
Continuous Bombardment • Continued planetesimal bombardment and internal radioactivity melted the planets and led to the density differentiation of planetary interiors
Formation of Moons • Moons of the outer planets were probably formed from planetesimals orbiting the growing planets • Not large enough to capture H or He, the outer moons are mainly rock and ice giving them solid surfaces
Formation of Atmospheres • Atmospheres were the last planet-forming process • Outer planets gravitationally captured their atmospheres from the solar nebula • Inner planets created their atmospheres by volcanic activity and perhaps from comets and asteroids that vaporized on impact • Objects like Mercury and the Moon are too small – not enough gravity – to retain any gases on their surfaces
Exosolar Planets
• Evidence exists for planets around other nearby stars • The new planets are not observed directly, but rather by their gravitational effects on their parent star • These new planets are a surprise - they have huge planets very close to their parent stars • The recent discovery that planets more massive than Jupiter orbit nearby stars in small orbits surprising because according to the nebular hypothesis, massive planets should only form away from their star.
Exosolar Planets • Idea: The huge planets formed far from their stars as current theory would project, but their orbits subsequently shrank • This migration of planets may be caused by interactions between forming planets and leftover gas and dust in the disk
Detecting Exoplanets • We can detect planets around other stars by using the doppler shift method. – A planet and its stars revolve around a common center of mass. – As the star approaches us in its motion around the center of mass, its spectrum will be blueshifted. – As it recedes, the spectrum will be redshifted.
Gravitational Lensing Method • Another method utilizes a prediction of General Relativity called gravitational lensing. • Gravity bends the path of light when it passes near a massive object. • If that object is a star with companion planets, we can use the lensing effect to detect them. • We have not found many planetary systems using this method, but it has the potential to find lowmass planets!
Transit Method • We can also use the transit method for detection. – Look for dimming of light from the central star.
A Sample of Exoplanets
Comparison of the orbital radii and relative sizes of expoplanetary system...
