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The Origin Of The Solar System Observation and Theory Like any theory, a theory of the origin of the solar system must comply with observations of its various properties and characteristics. The suggested theory must explain why: 1. All the planets lie within approximately one plane. 2. All the planets revolve around the Sun in a counterclockwise direction. 3. The Sun and most of the planets rotate in a counterclockwise direction, and explain the reason for the planets that do not. 4. There are two categories of planets, terrestrial (Mercury, Venus, Earth, and Mars) and Jovian (Jupiter, Saturn, Uranus, and Neptune). 5. All the objects making up the solar system presumably formed at about the same time, about 4.6 billion years ago. Solar Nebula Theory (Nebular Hypothesis) – The most accepted theory of the origin of the solar system is the solar nebula theory. The solar nebula is defined as the cloud of gas and dust from which the solar system formed. Stages of Development Stage One 1 – Initially, a slowly counterclockwise-rotating cloud of gas and dust, which may be part of a much larger nebula (a giant molecular cloud), began to gravitationally collapse. This explains why all planets revolve around the Sun counterclockwise and why the Sun and most of the planets rotate counterclockwise 2. This collapse may have been triggered in a number of ways: 1. Supernova Outburst – A nearby star exploded as a supernova, sending out shockwaves that initiated the collapse of the cloud of interstellar gas and dust. 2. Stellar Winds and Radiation Pressure – Stellar winds (ejected streams of charged particles, mostly protons and electrons) and radiation pressure (the pressure exerted by the emitted electromagnetic radiation) from hot stars near the cloud of interstellar gas and dust initiated compression. 1 3. Density Waves – Density waves are the result of gravitational variations within a galaxy. These waves spiral outward from the center at a constant rotational rate somewhat slower than the rotation of stars and clouds of interstellar gas and dust. Density waves do not consist of moving matter, but are a moving pattern of compression that is overtaken by the faster moving interstellar material. When a cloud of interstellar gas and dust moves through the compressional waves, it piles up and becomes dense enough to gravitationally collapse. Stage Two – As the nebula collapsed under its own gravity, three things occurred: 1. Increase Rotation Rate 3 – An increase in rotation rate of the nebula resulting from the conservation of angular momentum. This is an important physical quantity because all experimental evidence indicates that angular momentum is rigorously conserved in our Universe – it can be transferred, but it cannot be created or destroyed. Moreover, angular momentum is a constant, and velocity and radius are inversely related. Therefore, the decrease in the radius of the solar nebula due to its collapse, must spin faster in order to conserve angular momentum. 2. Disk-Shaped Formation 4 – The more rapid spin increased centrifugal forces, and caused the nebula to flatten into a disk. This explains why all the planets lie in approximately one plane 5. 3. Temperature and Pressure Increase – Gravitational energy caused an increase in temperature. At the center, a spherical assemblage developed, where the temperature and pressure continued to rise until it finally reached the point where nuclear fusion of hydrogen occurred. At this stage, the Sun was “born” and this spherical assemblage of matter began its “life” as a star. Stage Three 6 – Temperature differences between the warm inner regions and the cool outer regions of the disk determined what kinds of material could condense to form planets – this is referred to as the temperature-condensation sequence. (Condensation is the formation of solid or liquid particles from a cloud of gas.) Thus, the higher temperature in the inner region where the terrestrial planets eventually formed produced solids characterized by metals, silicates, and high temperature oxides. Volatiles – elements or compounds with low vaporizing temperatures – in this region remained in the form of gases. In contrast, the lower temperature in the outer region where the Jovian planets eventually formed (beyond the frost line) was characterized by volatiles in the solid phase. More specifically, ices of water, ammonia, methane, carbon dioxide, etc. Although the outer region of the disk contained condensates of all kinds (metals, rocks, and ices), ices were nearly three times more abundant than metals and rocks because of the overall greater abundance of hydrogen compounds in the solar nebula. This temperature-dependent condensation primarily explains why there are two major categories of planets, terrestrial 7 and Jovian 8. 2 Stage Four 9 – The solid grains resulting from the condensation then built up by “gentle” low velocity collisions and sticking together to form larger objects called planetesimals. Initially, the particles stuck together through electrostatic forces, but as the particles grew in mass, gravity began to aid in their bonding together, accelerating their growth. This process of growing by colliding and sticking is called accretion. Asteroids and comets are now recognized as surviving examples of planetesimals. Stage Five 10 – Planetesimals continued to collide and gradually built up into larger bodies. When a planetesimal diameter exceeded several hundred miles, it would have been massive enough to dominate its orbit and gravitationally attract other approaching planetesimals, thereby continuing to rapidly grow in size. In this way, the terrestrial planets and the cores of Jovian planets formed. Stage Six 11 – As the planets grew, their gravitational reach extended farther and farther, causing an increase in violent impacts. The late stages of the solar system’s formation may have been characterized by planet-sized objects with non-established orbits. Some of these objects were probably ejected from the solar system. However, a few of these planet-sized objects may have impacted the planets and can explain some of their peculiarities. In particular, such catastrophic events may explain Mercury’s deficiency in silicate material (rock), the highly inclined rotation axis of Uranus, and the formation of the Earth’s moon. (All discussed later.) These violent impacts explain the many craters observed on the Moon and other planetary objects of the solar system. Stage Seven – The slow-orbiting massive cores of the proto-Jupiter and proto-Saturn gravitationally attracted large quantities of gas (mainly hydrogen and helium) left over from the solar nebula. As a result, both planets became quite massive, developing thick atmospheres, and with increasing depth and thus pressure, layers of liquid. However, since Uranus and Neptune reside in a region where apparently the solar nebula’s density was significantly less, the formation of their cores at their current distance from the Sun is highly implausible. According to the so-called Nice model, Uranus and Neptune initially accreted in the Jupiter-Saturn region (where more material was available), and through their gravitational interaction with a large number of remaining planetesimals, migrated outward to their current positions over hundreds of millions of years. In addition, Uranus and Neptune are thought to have formed after Jupiter and Saturn did, when the strong solar wind had blown away much of the disc material. As a result, they accumulated little hydrogen and helium. 3 Hot Jupiters – Planetary migration is the most likely explanation for hot Jupiters – exoplanets (see below) with Jovian masses, but with orbits very close to their parent stars. A hot Jupiter is thought to form at a distance from its parent star beyond the frost line, where the planet can accrete from rock, ice and gases. The planet then migrates inward to the star where they eventually forms a stable orbit. Over time, its atmosphere and outer layers are stripped away 12, and its remaining core may become a classification of planet called chthonian. Stage Eight – In roughly 5 billion years, the Sun will have fused almost all of the hydrogen fuel in its core into helium, beginning its evolution into the red giant phase. Though the Sun’s core will contract, the outer envelope will expand outward many times its current diameter (engulfing Mercury, Venus, and most likely the Earth 13), before casting off its outer layers as a planetary nebula and leaving behind a stellar remnant known as a white dwarf. In time, the gravity of passing, nearby stars can disrupt the orbits of the remaining planets, potentially ejecting them into interstellar space. Ultimately, the Sun may be left with none of the original objects in orbit around it. The division of the origin and evolution of the solar system into eight stages is my interpretation; that is, it does not represent an actual number of phases in the development of the solar system. Other Solar Systems T Tauri stars – The first indirect evidence for planet-forming disks came from studies of T Tauri stars (named after the prototype star in the constellation Taurus), which are similar in mass to the Sun, but very young (about one million years old). In the 1980s, astronomers realized that about a third of T Tauri stars emit an excess of infrared radiation. This can be explained if these particular stars are surrounded by dust heated by short-wavelength radiation from the stars. Protoplanetary Disks (proplyds) 14 – In the 1990s, astronomers using the Hubble Space Telescope discovered many examples of disks around newly formed stars in the great nebula in Orion. As the name suggests, protoplanetary disks (proplyds) are thought to contain the material from which planets form around stars. Exoplanets – An exoplanet (or extrasolar planet) is a planet that orbits a star other than the Sun. A few thousand candidate planets have been detected. There is at least one planet on average per star 15. 4 Extraterrestrial Life – The discovery of exoplanets has intensified interest in the search for extraterrestrial life, particularly for those that orbit in the host star’s habitable zone where it is possible for liquid water (and therefore life) to exist on the surface. However, the study of planet habitability also considers a wide range of other factors in determining the suitability of a planet for hosting life. Approximately 1 in 5 Sun-like stars have an Earth-sized planet in the habitable zone, with the nearest expected to be within 12 light-years distance from Earth. Assuming 200 billion stars in the Milky Way, that would be 11 billion potentially habitable Earth-sized planets in the Milky Way, rising to 40 billion if red dwarfs (small and relatively cool stars) are included. Kepler 452b 16 – Kepler 452b is the first near-Earth-size planet discovered orbiting within the habitable zone of a star (about 1,400 light-years away) very similar to the Sun. It is 60% larger than the Earth, and is considered a “super-Earth” size planet. While Kepler 452b’s mass and composition are not yet determined, research suggests that planets its size have a good chance of being rocky. Kepler 452b is older than the Earth (6 billion years), giving a substantial opportunity for life to arise, should all the necessary ingredients and conditions for life exist on this planet. Detection Methods – Planets are extremely faint compared to their parent stars. At visible wavelengths, they usually have less than a millionth of their host star’s brightness. It is difficult to detect such a faint light source, and furthermore the parent star causes a glare that tends to wash it out. Direct Imaging – All exoplanets that have been directly imaged are both large (more massive than Jupiter) and widely separated from their parent star. One of the clearest images of an exoplanet with its parent star (similar in mass to our Sun, but much younger) is about eight times the mass of Jupiter, and over 30 billion miles out 17. Current models of how planets form around stars have a very tough time putting a planet this massive that far out. It’s possible an encounter with a more massive planet too close to the star to see gravitationally flung the planet to its current distance. Indirect Methods – The vast majority of exoplanets were detected through various indirect methods rather than actual imaging. Most confirmed exoplanets have been found using space-based telescopes. Many of the detection methods can work more effectively with space-based telescopes that avoid atmospheric haze and turbulence. Radial Velocity or Doppler Method – As a planet orbits a star, the star also moves in its own small orbit around the system’s center of mass 18. Variations in the star’s radial velocity – that is, the speed with which it moves towards or away from Earth – can be detected from displacements in the star’s spectral lines due to the Doppler Effect 19. 5 Astrometry 20 – Astrometry consists of precisely measuring a star’s position in the sky and observing the ways in which that position changes over time. If the star has a planet, then the gravitational influence of the planet will cause the star itself to move in a tiny circular or elliptical orbit about their common center of mass. The Gaia space telescope uses this method. Transit Method 21 – If a planet crosses (or transits) in front of its parent star’s disk, then the observed brightness of the star drops by a small amount. The amount by which the star dims depends on its size and on the size of the planet. The transit method reveals the radius of a planet, and it has the benefit that it sometimes allows a planet’s atmosphere to be investigated through spectroscopy. A number of space telescopes (Kepler telescope) use this method. Gravitational Microlensing 22 – Microlensing occurs when the gravitational field of a star acts like a lens, magnifying the light of a distant background star. Possible planets orbiting the foreground star can cause detectable anomalies in the lensing event light curve. This method is most sensitive to detecting planets around 1to10 AU away from stars like the Sun. 6