Chapter 6: Formation of Planetary Systems
Our Solar System and Beyond
6.1 A brief tour of the Solar System
What does the solar system look like?
-Planets fall into two major categories:
-Planets fall into two major categories:
Small, rocky terrestrial planets and large, hydrogen-rich jovian planets.
-Swarms of asteroids and comets populate the solar system.
-Large bodies in the solar system have orderly motions.
6.2 Clues to the formation of our Solar System
What features of our solar system provide clues to how it formed?
Hydrogen compounds
Compounds containing hydrogen, such as water, ammonia, and methane.
Asteroid
Rocky bodies that orbit the sun much like planets, but they are much smaller.
Asteroid belt
Between the orbits of Mars and Jupiter.
(Rocky asteroids and icy comets far outnumber the planets and their moons.)
Comets
Small objects that orbit the Sun, but they are made largely of ices mixed with rock.
Kuiper belt
Donut shaped region beyond the orbit of Neptune. Contains at least 100,000 icy objects, Pluto and Eris are the largest known.
Oort Cloud
The second cometary region. Much farther from the Sun and may contain a trillion comets.
What theory best explains the features of our solar system?
The nebular theory holds that our solar system formed from the gravitational collapse of a great cloud of gas, and it explains all the general features of our solar system.
6.3 The Birth of the Solar System
Where did the solar system come from?
Solar nebula
A cloud of gas that collapsed under its own gravity.
-The gas that made up the solar nebula contained hydrogen and helium from the Big Bang and heavier elements produced by stars.
What cause the orderly patterns of motion in our solar system?
Heating, Spinning, and flattening.
-As the solar nebula shrank in size, three important processes altered its density, temperature, and shape changing it from a large, diffuse cloud to a much smaller spinning disk.
-The orderly motions of our solar system today are a direct result of the solar system's birth in a spinning, flattened cloud of gas.
(Observations of disks around other stars support the idea that our own solar system was once a spinning disk of gas.)
6.4 The Formation of Planets
Why are there two major types of planets?
Condensation
The general process in which solid (or liquid) particles form in a gas.
-Hydrogen and helium gas (98% of the solar nebula)
-Hydrogen Compounds (1.4% of the solar nebula)
-Rock (0.4% of the solar nebula)
-Metal (0.2% of the solar nebula)
Frost line
The minimum distance at which is cold enough for ice to condense.
Accretion
The process by which small "seeds" grew into planets.
Terrestrial Planets
Made from the solid bits of metal and rock that condensed in the inner solar system.
Jovian Planets
Began as large icy planetismals, which then captured hydrogen and helium gas from the solar nebula.
-Remaining gas in the Solar Nebula was cleared away into space, ending the era of planet formation.
Where did asteroids and comets come from?
Rocky asteroids and icy comets are leftover planetismals from the era of planet formation.
Heavy bombardment
The period where leftover planetismals battered the planets during the solar system's first few hundred million years.
How do we explain the existence of our Moon and other exceptions to the rules?
Our Moon is probably the result of a giant impact that blasted Earth's outer layers into orbit, where the material accreted to form the Moon.
When did the planets form?
Age dating of meteorites that are unchanged since they condensed and accreted tells us the solar system is about 4 1/2 billion years old.
Radiometric dating
Relies on careful measurement of the proportions of various atoms and isotopes in the rock.
Radioactive
Isotope has a nucleus prone to spontaneous change, or decay, such as breaking apart or having one of its protons turn into a neutron.
Half-life
The length of time it would take for half the nuclei in the collection to decay.
6.5 Other planetary systems
How do we detect planets around other stars?
Directly: Pictures or spectra of the planets themselves constitute direct evidence of their existence.
Indirectly: Precise measurements of a star's properties may indirectly reveal the effects of orbiting planets.
(Almost all extrasolar planets detected to date have been found indirectly rather than through direct imagine.)
-Orbiting planets exert gravitational tugs on their star, so we can detect the planets by observing the stars resulting "wobble" around its average position in the sky.
Astrometric Technique
Make very precise measurements of stellar positions in the sky.
Doppler Technique
Searches for a star's orbital movement around the center of mass by looking for changing Doppler shift in a star's spectrum.
Hot Jupiter
Has a Jupiter-like mass but a much higher surface temperature.
-Alternation Doppler shifts in a star's spectrum, indication back-and-forth motion, can also reveal the influence of orbiting planets.
Transit
The planet appears to move across the face of the star, causing a small, temporary dip in the star's brightness.
How do extrasolar planets compare with planets in our solar system?
-Most known extrasolar planets are probably jovian, but we've also found "Super Earths" likely made of metal and rock.
-Most of the known extrasolar planets are more massive than Jupiter.
-Most extrasolar planets have different orbital characteristics than in our solar system.
Do we need to modify our theory of solar system formation?
Our theory has presented challenges, but more than a decade of re-examination hasn't turned up any obvious flaws in the basic theory.
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Always on time!
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