19.S: Summary

Origin of the Universe

The universe is an incredibly vast and complex expanse, containing an infinite number of galaxies and solar systems, with our solar system occupying just a small part of it. The origins of the universe, often explained by the big bang theory, provide the context for understanding the Earth’s formation and early history. The big bang theory, supported by Einstein's theory of general relativity and various empirical observations, suggests that the universe began from an infinitely dense and hot core, followed by an explosive expansion that has been ongoing for 13.8 billion years. Key evidence supporting this theory includes the redshift of galaxies, cosmic microwave background radiation, and the process of stellar evolution, which explains the formation of elements and the life cycles of stars. The universe's expansion and the death of stars lead to the creation of celestial phenomena such as white dwarfs, neutron stars, and black holes, highlighting the dynamic and ever-changing nature of the cosmos.

Origin of the Solar System--The Nebular Hypothesis

The formation of our solar system is explained by the nebular hypothesis, which suggests that a spinning cloud of dust, or nebula, composed mostly of light elements, flattened into a protoplanetary disk, eventually forming a star with orbiting planets. As this nebula condensed, the majority of its material accumulated at the center, creating the Sun, which accounts for over 99% of the solar system's mass. Temperature variations within the nebula led to the differentiation of planets: the inner, rocky planets formed in hotter regions, while the outer, gas giants formed in cooler areas. The planets grew as particles collided and merged, with gas giants like Jupiter and Saturn accumulating more hydrogen and helium, while ice giants like Uranus and Neptune developed from a mix of gases and ices. Jupiter's immense gravity further influenced the solar system's structure, contributing to the formation of the asteroid belt and impacting the development of nearby planets. The outermost regions of the solar system, including the Kuiper belt and Oort cloud, contain icy bodies and dwarf planets like Pluto, which was reclassified in 2006 due to new criteria defining what constitutes a planet.

Overview of Our Planetary System

Our solar system currently consists of the Sun, eight planets, five dwarf planets, nearly 200 known moons, and a host of smaller objects. The planets can be divided into two groups: the inner terrestrial planets and the outer giant planets. Pluto, Eris, Haumea, and Makemake do not fit into either category; as icy dwarf planets, they exist in an ice realm on the fringes of the main planetary system. The giant planets are composed mostly of liquids and gases. Smaller members of the solar system include asteroids (including the dwarf planet Ceres), which are rocky and metallic objects found mostly between Mars and Jupiter; comets, which are made mostly of frozen gases and generally orbit far from the Sun; and countless smaller grains of cosmic dust. When a meteor survives its passage through our atmosphere and falls to Earth, we call it a meteorite.

Composition and Structure of Planets

The giant planets have dense cores roughly 10 times the mass of Earth, surrounded by layers of hydrogen and helium. The terrestrial planets consist mostly of rocks and metals. They were once molten, which allowed their structures to differentiate (that is, their denser materials sank to the center). The Moon resembles the terrestrial planets in composition, but most of the other moons—which orbit the giant planets—have larger quantities of frozen ice within them. In general, worlds closer to the Sun have higher surface temperatures. The surfaces of terrestrial planets have been modified by impacts from space and by varying degrees of geological activity.

The ages of the surfaces of objects in the solar system can be estimated by counting craters: on a given world, a more heavily cratered region will generally be older than one that is less cratered. We can also use samples of rocks with radioactive elements in them to obtain the time since the layer in which the rock formed last solidified. The half-life of a radioactive element is the time it takes for half the sample to decay; we determine how many half-lives have passed by how much of a sample remains the radioactive element and how much has become the decay product. In this way, we have estimated the age of the Moon and Earth to be roughly 4.5 billion years.