The Solar System in Close-Up Read online

  With the invention of the telescope in 1608 the Italian, Galileo Galilei, was able to gather data to support Copernicus’s model for the Sun and planets. Galileo discovered four moon’s orbiting the planet Jupiter; he also observed sunspots moving across the surface of the Sun and craters on the Moon. Galileo’s discovery that the planet Venus had phases just like Earth’s Moon confirmed that Venus orbited the Sun closer than Earth and provided support for Copernicus’s sun-centered model.

  One major problem restricting the full acceptance of Kepler’s and Galileo’s theories was that it was not known what kept the planets in orbit. People did not know how planets, once they started orbiting the Sun, could keep moving. Isaac Newton put the explanation of this motion forward in the seventeenth century. Newton put forward the idea that the Sun must be exerting a force on the planets to keep them in orbit. This force was called gravity and it exists between any two masses (such as a planet and a star like our Sun). Using his law of gravity, Newton was able to prove the validity of Kepler’s three laws of planetary motion. Newton also showed that other types of orbits around the Sun were also possible. For example, the orbits could also be parabolas or hyperbolas. Newton also developed a Universal Law of Gravitation, which states:Two bodies attract each other with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.

  This law means that the more mass a planet or star has, the greater its gravitational pull. This pull decreases with increasing distance from the object.

  Discovering New Planets

  Towards the end of the eighteenth century, only six planets were known—Mercury, Venus, Earth, Mars, Jupiter, and Saturn. In 1781, British astronomer, William Herschel accidentally discovered the seventh planet Uranus. In 1846, Urbain Leverrier in France, and John Adams in England used Newton’s gravitational laws to independently predicted that variations in the orbit of Uranus were due to the influence of an eighth planet. Soon after, the Berlin observatory found the predicted planet and named it Neptune. In the early twentieth century Percival Lowell and William Pickering predicted that another planet should exist beyond Neptune. In 1930, Clyde Tombaugh found a body, which was named Pluto, close to where Lowell and Pickering predicted it to be. Between 1930 and 2006 Pluto was regarded as the ninth planet of the solar system. However, in 2006 a meeting of the International Astronomical Union (IAU) decided on a definition of a planet that excluded Pluto as a planet, making it a ‘dwarf planet’ along with a number of other newly discovered bodies. As a result we now have what many call, ‘the new solar system’.

  What Is a Planet?

  Traditionally, a planet has been regarded as a spherical body that orbits a star and is visible because it reflects light from the star. The spherical shape is only possible when the object has enough mass that gravity is able to pull it into a spherical shape. All planets, and many large moons and large asteroids are spherical.

  In August 2006 the IAU decided on the following definition of a planet:

  To be a planet a body must1.be in orbit around the Sun,

  2.have sufficient mass for self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly spherical) shape, and

  3.have cleared the neighborhood around its orbit.

  What made this definition suddenly critical was the discovery of a number of objects in the outer solar system beyond Pluto. Before the definition was accepted, there could have been as many as 50 planets orbiting the Sun.

  The IAU also introduced a new classification—that of a ‘dwarf planet’. A dwarf planet is a body that orbits the Sun, has sufficient mass for self-gravity to have pulled it into a spherical shape, and has NOT cleared the neighborhood around its orbit, and is NOT a satellite. All other objects orbiting the Sun are collectively known as ‘small solar system bodies’. Currently, there are a number of bodies regarded as dwarf planets and more are expected to be added to the list over the next few years when more information about them is known. Examples of dwarf planets include, Pluto, Eris and the large asteroid Ceres. Dwarf planets are not considered to be true planets mainly because they do not have the ability to clear their orbital path of other material.

  Currently there are eight major planets in the solar system. In order of distance from the Sun they are—Mercury, Venus, Earth, Mars (the inner planets), Jupiter, Saturn, Uranus and Neptune (the outer planets). There are also five dwarf planets—Ceres, Pluto, Haumea, Makemake and Eris; but more are likely to be added to this list as details of them are better determined (Fig. 1.3).

  Fig. 1.1Stars develop everywhere we look in space. In our region of the universe they form mostly in the arms of spiral galaxies. The solar system we live in is part of the Milky Way Galaxy. The Milky Way has a spiral structure like that shown in this photograph of M83 taken by the Hubble Space Telescope (Photo: NASA/HST).

  Fig. 1.2Astronomers use optical telescopes to explore the universe. Pictured is the 3.9 m diameter Australian telescope on Siding Springs Mountain, Australia (Credit: Australian Astronomical Observatory/David Malin).

  Fig. 1.3(a) The inner planets and (b) outer planets orbiting the Sun.

  Many of the planets in the solar system have natural satellites or moons orbiting them. The planets Jupiter and Saturn have the most moons orbiting them. True moons are large enough for gravity to have pulled their mass into a spherical shape, while smaller moons do not have enough mass and gravity and are irregular in shape. To be a moon, a body must be naturally orbiting a planet and be smaller than the planet. The planets have captured most of their moons during the formation of the solar system.

  The four largest planets (Jupiter, Saturn, Uranus, Neptune) are also surrounded by planetary rings of varying size and complexity. These rings are composed primarily of dust or particulate matter. Saturn has the most prominent rings and these can be easily seen through small telescopes on Earth. The origin of such rings is not known, but they may be left over debris from moons which have been torn apart by tidal forces (see Table 1.1).Table 1.1Major planets and dwarf planets in the solar system

  Note: Distances are given in astronomical units (AU) where one AU is the average distance between the Earth and Sun

  Difference Between a Planet and Dwarf Planet

  Dwarf planets are not just smaller than a planet. The main difference is that a dwarf planet does not have the ability to clear its orbital region of other matter, while a planet has. Planets are able to remove smaller bodies near their orbits by flinging them away, sweeping them up or by holding them in stable orbits, whereas dwarf planets lack the mass to do so. To make this clearer some planetary scientists have developed mathematical methods that help us to distinguish between a planet and a dwarf planet.

  American planetary scientists Alan Stern and Harold Levison introduced a parameter Λ (lambda) to express the likelihood of a body “clearing the neighborhood around its orbit”. The value of this parameter is proportional to the square of the mass M (which determines the gravitational reach of the massive body for a given amount of deflection) and inversely proportional to the time to orbit the Sun T (which governs the rate at which the encounters occur). If Λ > 1 then the body will have already or will eventually clear its orbital path of other matter. If Λ < 1 then the body is unlikely to clear its orbital path and would therefore classed as a dwarf planet.

  where k = 0.0043.

  Steven Soter (another USA planetary scientist) and other astronomers developed another method for distinguishing between planets and dwarf planets also based on their ability to clear the neighborhood of their orbital path. Soter developed a parameter μ (mu) called the planetary discriminant, as a measure of the actual degree of cleanliness of the orbital region.

  where M = mass of the candidate body, m = total mass of the other objects that share its orbital region.

  If μ > 100 then the body has cleared its orbital path and is a planet. If μ < 100 then the body can
not clear its orbital path and would be classed as a dwarf planet.

  There are several other schemes that try to differentiate between planets and dwarf planet.

  Table 1.2 shows the value of Λ and μ for each planet and dwarf planet. It is clear that dwarf planets are different to planets as they do not have the ability to clear their orbital region of other matter (i.e. they have very low values of Λ and μ).Table 1.2Distinguishing between planets and dwarf planets (*)

  The IAU has not specified the upper and lower size and mass limits of dwarf planets. The size and mass at which an object attains a hydrostatic equilibrium shape depends on its composition and thermal history.

  As of 2015 the IAU recognizes five bodies as dwarf planets: Ceres, Pluto, Haumea, Makemake, and Eris. Ceres and Pluto are known to be dwarf planets through direct observation. Eris is generally accepted as a dwarf planet because it is more massive than Pluto, whereas Haumea and Makemake qualified to be assigned names as dwarf planets based on their absolute magnitudes. There are other bodies in the outer solar system that some astronomers like Mike Brown (USA), believe are also dwarf planets, for example, Orcus, Salacia, Quaoar, and Sedna. No space probes have ever visited these bodies.

  Moons and Dwarf Planets

  Many of the planets and dwarf planets in the solar system have smaller bodies called “moons” orbiting them. For example, Earth has one moon; Mars has two moons, while Jupiter has at least 63 moons. To be a moon, an object must be in orbit around a planet or dwarf planet. True moons have been pulled into a near spherical shape by self-gravity. Small moons that are irregular in shape are often captured asteroids and are better referred to as ‘moonlets’.

  Nineteen moons are known to be massive enough to have relaxed into a near spherical shape under their own gravity, and seven of them are more massive than either Eris or Pluto. Moons are not physically distinct from the dwarf planets, but are not members of that class because they do not directly orbit the Sun. The seven that are more massive than Eris are: Earth’s Moon, the four Galilean moons of Jupiter (Io, Europa, Ganymede, and Callisto), one moon of Saturn (Titan), and one moon of Neptune (Triton). The others are six moons of Saturn (Mimas, Enceladus, Tethys, Dione, Rhea, and Iapetus), five moons of Uranus (Miranda, Ariel, Umbriel, Titania, and Oberon), and one moon of Pluto (Charon). Some people refer to these moons as “satellites” of the planet they orbit.

  Features of the Solar System

  The Sun, planets, dwarf planets and their moons form a family of bodies called the solar system. We now know that the Sun is indeed at the centre of the solar system and that the major planets orbit the Sun in nearly circular orbits. Our understanding of the solar system has changed dramatically over the centuries as bigger and better telescopes were developed and more data of planetary motions was collected. Although people knew the planets of the solar system existed, little was known about the nature of these worlds until space probes containing scientific instruments were sent to explore these objects.

  The Sun fits the definition of a star because it emits its own heat and light through the process of thermonuclear fusion. Planets differ to stars in that they do not emit their own heat and light because they do not have enough mass for fusion to occur in their core. We see planets because they reflect sunlight. Stars and planets are spherical in shape and are held in that shape by gravity.

  The planets orbit the Sun in much the same plane, and because of this they all appear to move across the sky through a narrow band of constellations called the zodiac. Observed from a position above the Sun’s north pole, all the planets orbit the Sun in an anti-clockwise direction. The orbits of the planets in the solar system are often described as elliptical, but for most of the planets these ellipses are close to being circular. Eccentricity (e) is a measure of how far an orbit deviates from circularity. If e = 0 then the orbit if a perfect circle; while e becomes more elliptical as it approaches 1 (see Fig. 1.5).

  Fig. 1.4This strange looking object is of Phobos—one of the two moons of the planet Mars. The crater Stickney (lower right) dominates this side of Phobos (Credit: NASA).

  Fig. 1.5Shapes of planetary orbits (e = eccentricity).

  The time taken by a planet to orbit the Sun is called its period of revolution and is also the length of its year. A planet’s year depends on its distance from the Sun: the further a planet is from the Sun, the slower its speed and the longer its year.

  As planets orbit the Sun, they also spin or rotate on an axis, which is an invisible line through their centre, from their north to south pole. A planet’s rotation period is known as its day. Planets also have varying degrees of axial tilt. Axial tilt is the angle between a planet’s axis of rotation and the vertical (Fig. 1.6).

  Fig. 1.6Approximate axial tilt of each planet.

  Each planet has its own gravitational field, which tends to pull objects towards its centre. The strength of a gravitational field is measured in newtons per kilogram (N/kg) at the surface of a planet. It takes a lot of energy to overcome gravity and escape from the surface of a planet. The minimum speed that an object (such as a rocket) must attain in order to travel into space from the surface of a planet, moon or other body is called the escape velocity. If the rocket’s velocity is too low, gravity will pull it back down.

  Planets also have a density, which is a measure of the amount of mass in a given volume. Density is measured in kilograms per cubic metre (kg/m3) but is often quoted in grams per cubic centimeter (g/m3) as well. The density of water is 1.0 g/cm3 while that of iron is 7.87 g/cm3.

  Planets often have a magnetic field. Such fields are produced by churning motions of metallic liquids in a planet’s core that conduct electricity and have an electric charge. The magnetic fields act like a giant bar magnet and can be offset from the rotation axis of a planet. For example, the Earth’s magnetic field is tilted about 11° to its axis of rotation.

  The liquid conducting material in a planet’s interior can be made to swirl about if the planet is rotating quickly enough. The faster a planet rotates, the more the material gets stirred up and the stronger the generated magnetic field. If the liquid interior becomes solid or if the rotation slows down, the magnetic field will weaken.

  Magnetic fields protect a planet from the charged particles streaming out from the Sun in the form of the solar wind. When solar wind particles run into a magnetic field, they are deflected and spiral around the magnetic field lines. Most solar wind particles are deflected past a planet, but a few leak into the magnetosphere to get trapped in radiation belts that surround the planet. Some particles gain enough energy to interact with atoms and molecules in the atmosphere of the planet to create a light show of auroras (see Fig. 1.8).

  Fig. 1.7The planets compared to the size of the Sun, with all bodies drawn to the same scale.

  Fig. 1.8The magnetic field around Earth. Charged particles in the solar wind can get trapped in the magnetic field near the poles and create ‘aurora’. Some charged particles also get trapped in the Van Allen Radiation Belts.

  The four ‘inner planets’ (Mercury, Venus, Earth and Mars) are called terrestrial planets. They are smaller, denser and rockier than the ‘outer planets’ (Jupiter, Saturn, Uranus, and Neptune). The inner planets are also warmer and rotate more slowly than the outer planets. The outer planets are gaseous planets, containing mostly hydrogen and helium with some methane and ammonia. These rapidly rotating planets are cold and icy with deep atmospheres.

  During the 1800s, astronomers discovered a large number of small, rocky bodies orbiting the Sun between Mars and Jupiter. Bodies such as Ceres, Pallas and Vesta, which had been thought of as small planets for almost half a century, became classified as asteroids.

  Formation of the Solar System

  The solar system is thought to have formed about 4.5 billion years ago from a vast cloud of very hot gas and dust called the solar nebula. This cloud of interstellar material began to condense under its own gravitational forces. As a result, density and pressure at the
centre of the nebula began to increase, producing a dense core of matter called the protosun. Collisions between the particles in the core caused the temperature to rise deep inside the protosun.

  The planets and other bodies in the solar system formed because the solar nebula was rotating. Without rotation, everything in the nebula would have collapsed into the protosun. The rotating material formed a flat disc with a warm centre and cool edges. This explains why nearly all the planets now rotate in much the same plane.

  As the temperature inside the protosun increased, light gases like hydrogen and helium were forced outward while heavy elements remained closer to the core. The heavier material condensed to form the inner planets (which are mainly rock containing silicates and metals), while the lighter, gaseous material (methane, ammonia and water) condensed to form the outer planets. Thus a planet’s composition depends on what material was available at different locations in the rotating disc and the temperature at each location (Fig. 1.9).

  Fig. 1.9Stages in the formation of the solar system: (a) A slowly rotating cloud of interstellar gas and dust begins to condense under its own gravity. (b) A central core begins to form a protosun. A flattened disc of gas and dust surrounds the protosun, and begins to rotate and flatten. (c) The planets begin to condense out of the flattened disc as it rotates. (d) The planets have cleared their orbit of debris.