Overview of the Solar System
The solar system was formed when an enormous cloud of gas and dust contracted by gravity to create a protoplanetary disk, which then grew into planetesimals, which ultimately formed the planets.? The solar system consists of 8 planets, and in general these are categorized into terrestrial planets that are composed of rocks and metal components on the interior and Jovian planets (or giant planets) composed of gas on the exterior. This is because the temperature became lower toward the exterior of the solar nebula. The orbits of the planets are ellipses that are nearly circular, with the exception of Pluto and Mercury. Between Mars and Jupiter, there is an asteroid belt with a distribution of asteroids with rock and metal components, and asteroids exist in the Kuiper belt outside Neptune. Comets are found all across the solar system, and are emitted from Oort clouds that exist as a heavy spherical exterior on the outer boundary of the solar system. The surfaces of planets have diverse appearances, but almost all members exhibit craters made from collisions with asteroids or comets. Areas with few craters are where the surface has recently changed, indicating that the area is a young region. On inner planets, we see traces of volcanoes and lava flow.
The Distribution and Orbits of Planets
The distance from the sun to a planet is calculated according to the Titius-Bode Law. This law is astoundingly accurate when applied to the seven interior planets, but the difference becomes larger beginning with Neptune. Planetary movement is characterized in revolutions in direct motion counter clockwise around the sun. Also, the orbital planes of the planets’ revolutions are all within the zodiac in a 16 degree range from the ecliptic plane, with the exception of Pluto. The revolution orbit is all close to a circle, with the exceptions of Mercury and Pluto. Pluto exhibits a high degree of eccentricity, and hence there are cases when it enters inside Neptune’s orbit. The rotational axes of the planets are around 25 degrees tilted in relation to the rotation orbital plane in the case of the earth, Mars, Saturn and Neptune, but all other planets have an axis that is nearly vertical. The direction of rotation is the same as the direction of revolution for all planets with the exceptions of Venus and Uranus. Venus exhibits a retrograde motion by rotating in the opposite direction, while Uranus has a rotational axis that lies on the equatorial plane. The rotational velocity of Mercury and Venus is very slow, and this is explained as the result of the combination of rotation and revolution.
The Physical Characteristics of Planets
The mass of a planet can be determined by applying Kepler’s Third Law to the satellites that are attached to the planet in question.
Here, P refers to the revolution period, a to the semi-major axis of the orbit, and m1 and m2 respectively to the mass of the planet and the mass of the satellite. In general, the mass of the satellite is extremely small compared to that of the planet, and therefore we are able to obtain the mass of the planet using the formula above.
Also, in cases where no satellite exists, we can obtain the planetary mass by measuring the perturbation effect of gravity that the planet has on the orbital movement of other planets, asteroids, comets or our space probes. The size of a planet can be obtained by directly measuring the size of the visible disk of the planet, by measuring the accurate time intervals at which the planet obscures stars, its own satellite, or a space probe, or, in the case of planets close to the earth, by measuring the time it takes for a radar pulse emitted from the earth to return by reflecting from various points on the planet in question. Though we are unable to directly explore the interior of the planets, we can build a model of the estimated interior based on the average density, chemical composition, oblateness, etc., factors that have been determined based on observation.
From the planet’s surface, we can obtain information regarding color, albedo and temperature.
From the color, we learn the chemical composition of the surface and the atmosphere. The oceans and the land on earth makes the earth appear to be a blue sphere mottled with green, brown and orange, while regions covered in clouds or snow appear white. The desert regions on Mars make this planet appear brown, while the surface of Io, a satellite of Jupiter, is observed to be yellow due to the sulfur emitted from volcanic eruptions. Meanwhile, the albedo of a celestial body is defined as the ratio of reflected radiation from the surface to incident radiation upon it. Planets with no atmosphere or very little have extremely low albedo, and this is because their surfaces are composed of low rocks that have low albedo. The surfaces of the Jovian planets or Venus reflect a lot of the light from clouds, and therefore have high albedo.
The temperature of planet surfaces can be estimated based on Stefan’s law by assuming the planet to be a blackbody. This law is expressed as , where E stands for the total amount of energy emitted per unit area per unit of time from the surface of the blackbody, T for the effective temperature, and s for the proportional constant. If the total energy received by the planet from the sun is equal to the total energy emitted by the planet, the planet is in equilibrium, and we can determine the temperature of the planet by measuring these two quantities. However, we must take into consideration that we have omitted factors such as the planet’s atmospheric circulation, convection, ?the atmosphere’s heat conductivity, the existence of an interior heat source, and greenhouse effects. Also, by measuring the wavelength corresponding to the maximum amount of radiation according to Wien’s law regarding blackbodies whereby λmax=(0.002898m)/T, we are able to determine the temperature. Also, by applying the escape velocity Ve = (2Gm/R)0.5 to the planet’s surface, we can find out what components are included in the planet’s atmosphere. Here, R stands for the radius of the planet.
The Unsolved Questions Regarding the Solar System
There remain many questions regarding the solar system’s structure and various aspects of its members, and among these the most fundamental question is how the solar system originated and evolved to reach its current state. Meanwhile, the distribution of angular momentum in the solar system is another issue that is challenging to explain.
The Earth’s Motion
To understand the movement of the earth, we need to apply the system of coordinates. For the celestial coordinate system, we use the horizontal coordinate system, the equatorial coordinate system, the ecliptic coordinate system, and the galactic coordinate system, etc. with each having its respective advantages and drawbacks. We therefore select one that is most convenient depending on our purpose. The revolutionary and rotational movements of the earth serve as the standard of time, and the mean solar time that we currently use has been obtained by eliminating the problems inherent in the use of true solar time. When the mean sun twice consecutively culminates, then we consider one mean solar day to have passed. Sidereal time is based on the vernal equinox, while in the mean solar time, because the transit time of the mean sun differs according to the longitude, we have established standard times for each country for our usage. All observation data is expressed in terms of the Universal Time. One year is the time it takes for the earth to revolve once around the sun, but depending on the definition of the base point, the sidereal year, the tropical year, and the anomalistic year are formed. The calendar that we currently use is the Gregorian calendar that is made to approximately coincide with the changes of the seasons. The earth’s varying seasons occur because the earth’s equatorial plane is 23.5 degrees tilted in relation to the ecliptic plane.
The evidence of the earth’s rotation can be found in the Coriolis effect, Foucault’s pendulum, and the oblate spheroid shape of the earth, etc. while the revolution of the earth is evidenced by the aberration of starlight, stellar parallax, and the Doppler effect, etc. Tidal friction due to differential gravity reduces the energy of the earth’s rotation and therefore the length of the day increases at a rate of approximately 0.002 seconds per century and causes the synchronous rotation of the moon and tidal evolution, thereby increasing the distance between the moon and the earth. In the distant future, the length of a day and one month will become the same, approximately 50 times the present length. Meanwhile, the differential gravity that exercises on the rise along the earth’s equator creates torques and gives rise to precession motion, causing the vernal equinox to move in 26,000 year periods. As a result, the values of the right ascension and the declination of celestial bodies change and the stellar constellations also change, so that around A.D. 14,000, Vega will be located on the North Pole. Also, since the moon and the sun? move above and beneath the earth’s equatorial plane, the torques that are exercised in the rise of the earth’s equator cause periodic changes, resulting in nutation, a phenomenon in which the rotational axis of the earth shakes.
The Earth and the Moon
The size of the earth was first determined by Eratosthenes of Ancient Greece. When compared to the parent planet, the moon is an immense satellite that is the largest in the solar system. Considering that the mass of the moon is very small compared to the mass of the earth, we can apply Kepler’s Law of Harmonization to obtain the earth’s mass from the orbit of an artificial satellite, and the mass of the moon can be determined by observing the earth’s movement in relation to the earth and moon’s center of mass. Today, we can deduce the distribution of the moon’s interior mass based on the orbit of artificial satellites that revolve around the moon, and we can measure the accurate mass of the moon. The interior of the moon was investigated by Apollo’s seismic waves, and we have thus learned that a mascon exists and that the nucleus is not in the center of the shape but instead located closer toward the earth.
The elements of the earth’s motion include its revolutionary motion in relation to the sun that is the center of mass for the earth-moon system, the earth’s rotational movement, the rotational motion in relation to the? center of mass of the earth’s center, the precession motion of the earth’s rotational axis, nutation, and the decrease of the earth’s rotational period due to tidal friction, etc. The movements of the moon are even more complex. ?The phases of the moon are created by an eclipse phenomenon, because the size of the area that receives sunlight varies when viewed from the earth depending on the elongation of the moon.
The surface of the moon is modeled by collision craters of varying sizes, and these have been formed by collision with meteoroids. The surface of the moon is divided into ?the highlands and the Maria. The highlands is an area covered with countless craters that is around 3 km higher than the Maria and aged approximately 4.6 billion years, making it the oldest area on the earth’s surface. The Maria is a large black region, a plain of black basaltic lava that is nearly circular in shape.? This circular basin was been created by lava that filled the area upon eruption due to the impact of a meteor.
The Earth’s Atmosphere
The earth’s atmosphere absorbs and scatters stellar light and causes refraction effects. The atmospheric effects that affect visible light include scattering, extinction, refraction, seeing, dispersion, etc.
The light scatters when interacting with particles according to Rayleigh’s scattering law determined by the wavelength of the light and the size of the particle. The earth’s magnetic field is distorted by the solar wind and forms the earth magnetosphere: the magnetosphere is the zone affected by the earth’s magnetic field. Solar winds change direction at the magnetopause and disappear far away from the earth, but nonetheless many protons and electrons seep in and are caught in the annular Van Allen belt that is symmetrical to the earth’s magnetic axis and here the motions of the particles follow the Lorentz Force Law, (F = q (VxB)). The light of an aurora occurs when low energy electrons escape the inner belt and collide with atmospheric gases, causing them to arise or become ionized and emitted.
The interior structure of the earth is known from various direct and indirect evidences. The lithosphere that includes the crust and the upper part of the mantle is known based on our analysis of the types of rocks, and the lower part of the mantle and the structure and components of the nucleus can be investigated through seismic waves. The asthenosphere just below the lithosphere are zones with sufficiently high pressure and temperature so that the matter that composes the asthenosphere can have fluidity even in a solid state. Ultimately, in this zone heat is usually transferred through convection. The lithosphere is the exterior layer of solids in which convection is impossible, and the internal heat is transferred through conduction. In cases such as the earth or Io with high geological activity, the heat may sometimes be transferred through the processes of volcanic activities or the circulation across the entire lithosphere. This phenomenon causes the plate tectonics, which is the movement of the plates. The earth’s lithosphere is composed of the crusts of the oceans and the continents as well as the upper mantle. Heat transfer through convection and conduction can occur to a similar degree in objects that are mainly composed of rocky matter and ice.
The evolutionary patterns of terrestrial planets are all quite similar. Primordial heat is the remainder from the early stages of a planet’s formation, and is one of the important sources of heat for terrestrial planets and other similar celestial bodies. Two other types of heat sources are radiation decay heat and tidal heating. Generally, decay occurs in 235U, 238U, 232Th, 40K, etc. that are located in the mantle and crust which contain a lot of silicon. Radiation decay occurred frequently in the early evolutionary stages of a planet when there was a lot of radiation matter, while tidal heat is a major heat source with strong tidal effects as in the case of large planets such as Jupiter that have large satellites. Among terrestrial planets, the amount of heat that the planet possessed from the beginning of that is continually generated determines the condition of the planet’s surface.
The internal structure of Jovian planets can be estimated based on factors that have been determined by observation, including their density, gravity, gravitational field strength, radiation and the chemical components of the atmosphere. The surfaces of Jupiter and Saturn are thought to probably lack clear liquid or solid states. According to a theoretical model, their interiors form five layers. The two layers near the center comprise the nucleus of rock and ice matter, and their nuclei are surrounded by hydrogen and helium, which constitutes the majority of these planets’ masses. The hydrogen near the nucleus is believed to be in a metal state. The central zones of Jupiter and Saturn have high temperatures, exceeding 15,000K in the case of Jupiter.
Uranus and Neptune also lack surfaces of clear liquid or solid states, and are believed to have rocky nucleus. The results of the most recent model indicate that the rock and ice matter remain incompletely separated. The mantle that is mostly composed of ice matter surrounds the nucleus, and the mantle is surrounded by a layer composed mostly of hydrogen and helium. Overall, compared to Jupiter and Saturn, the roles of hydrogen and helium are significant in Uranus and Neptune, and the layers are chemically separated to a lesser degree. Jupiter, Saturn and Neptune exhibit an excess heat phenomenon, whereby they emit more energy than they receive from the sun. In the case of Jupiter, the excess heat is caused by the continuing emission of heat that remained from the early formation period in addition to the heat arising from radiation decay, and in Saturn, it is believed that the heat is caused by helium droplets sinking after separating from metallic hydrogen.
The magnetic fields of Jupiter and Saturn are thought to be created in the metallic hydrogen layer that is in a liquid state, while in Uranus and Neptune, the magnetic field arises from the layer of ice matter in a liquid state containing ions such as H3O+, OH-, and NH4+. The atmosphere of a Jovian planet is mostly composed of H, H2, and helium, and other molecules include CH4, NH3, etc. The majority of molecules contained in the atmosphere are detected through ultraviolet and infrared spectral observation. The Galileo spacecraft was equipped with a mass spectrometer and was able to research the relative content of molecules in the region upon entering the atmosphere. The cloud layer at the outermost of Jupiter and Saturn is ammonia, and in Uranus and Neptune, methane clouds have been detected. In Jovian planets, the atmospheric layer can be divided into two according to the changes in temperature along depth: in the lower troposphere, the temperature drops when moving upward, but in Uranus, the drop in temperature occurs more slowly. ?In the upper zone known as the thermosphere, the temperature increases when moving upward.
The wind velocity on Jovian planets is identified by observing the movements of spots such as clouds. The value obtained by this method includes the planet’s rotational effect, and therefore we must subtract the rotational speed. In Jupiter and Saturn, we verified the evidence that convection cells exist in a series in a deep location, based on the patterns of the observed wind velocity. In Jupiter, the major change in wind velocity occurs at the borders of bands with alternating colors, but this does not apply to Saturn. ?Jovian planets all have large scale spherical magnetic fields, and the overall appearance is similar to that of the earth. In Uranus and Neptune, the rotational axis and the magnetic axis differ significantly, and therefore there appear changes in the strength of the magnetic field over the course of time.
Small Solar System Bodies
The ring of Jupiter is composed of minute particles, and was formed by the influx of gas erupting from a volcano on Io, the nearest satellite. This differs markedly from the ring of Saturn, which is composed of chunks of ice that are tens of centimeters in size. The ring of Uranus is presumed to have been formed in the early stages of the solar system’s creation. Among the planets, an object of our keenest interest is Europa in Jupiter, where we have recently discovered the existence of an ice sea, making Jupiter emerge as the most promising site in the solar system in terms of searching for potential life forms. Meanwhile, Miranda, located in the innermost area of Uranus, shows large regional variations in surface features, leading to questions over its origin. Computer simulations on Pluto and its satellite Charon have indicated that they were originally the satellites of Neptune, but escaped due to the gravitation perturbation by a third celestial body.
Asteroids are widely distributed across the asteroid belt between Mars and Jupiter, and the location of the Trojan Group has been identified as one of the Lagrangian points. The majority of asteroids follow orbits with large inclination angles and eccentricity and in terms of their composition, asteroids are categorized into the C type containing a lot of carbon and the S type which contain a lot of silicates or stony matter. Recently, we have identified asteroids that approach the earth as NEO (Near Earth Object) and are conducting international monitoring campaign for early detection of potentially hazardous asteroids.
Comets consist of the nucleus, the coma, the hydrogen cloud, the dust tail and the ion tail, and can be categorized by the orbital period with a criterion of 100 years into long period comets and short period comets. The ion tail interacts with the solar wind, so that it extends in the direction opposite to the solar wind. Originally, the orbit of a comet is a hyperbola or a parabola and leaves the solar system after once approaching the sun, but in cases where the comet has approached Jupiter, its orbit changes and it becomes a periodic comet. Comets are dirty chunks of ice and are believed to come from the Oort cloud. Research into comets began in earnest when an exploration satellite was able to observe Haley’s comet when it approached in 1998 and we learned that the nucleus of the comet is peanut-shaped and that the nucleus is made of the darkest matter in the solar system.
Meteoroids include all celestial bodies that exist in the interplanetary space of the solar system with the exception of planets, satellites, asteroids, and comets. Meteors are those that are drawn by the attractive force of the earth and? emit light due to the friction with the earth’s atmosphere, and meteorites are parts that remain without burning off entirely and end up falling to the surface of the earth. Meteorites are categorized into stony meteorites, iron meteorites, and stony iron meteorites, and they provide important information that allows us to explain the age and origin of the solar system.
The Origin of the Solar System
The nebular hypothesis and the collision hypothesis have been presented as models for explaining the origin of the solar system, and ultimately the validity of these models are determined based on how well they are able to explain the information that we currently know about our solar system. Currently, the models accepted to be the most plausible are the proto-nebular hypothesis or the planetesimal hypothesis, which claims that planetesimals are created through an accretion process. The sun was created through the process of star formation, and many hypotheses have also been presented regarding the origin of the moon, including the fission hypothesis, the capture hypothesis and the binary accretion model, but currently we have obtained calculation results of quantitative simulations based on the giant impact model.