Solar System

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Solar System

The solar system is defined as all celestial bodies that orbit the sun, including the sun itself. It is comprised of the sun, eight major planets, many dwarf planets, the moons that orbit planetary bodies, some 100,0000 asteroids larger than 0.6 mi (1 km) in diameter, and perhaps 1 trillion cometary nuclei. (As of 2006, Pluto was demoted to a dwarf planet by the International Astronomical Union, reducing the number of planets in the solar system to eight.) While the major planets lie within 30 astronomical units (AU) of the sun, the outermost boundary of the solar system stretches to one million AU, one third the way to the nearest star (Proxima Centauri), excluding the Sun. It is believed that the solar system was formed through the collapse of a spinning cloud of interstellar gas and dust. (The solar system can also be used to describe any star system, however, traditionally it is reserved for the sun’s system.)

What and where is the solar system?

The central, and most important object in the solar system is the sun. It is the largest and most massive object in the solar system—its diameter is 109 times that of the Earth, and it is 333,000 times more massive. The extent of the solar system is determined by the gravitational attraction of the sun. Indeed, the boundary of the solar system is defined as the surface within which the gravitational pull of the sun dominates over that of the Milky Way galaxy. Under this definition, the solar system extends outwards from the sun to a distance of about 100,000 AU. The solar system is much larger, therefore, than the distance to the remotest known (and largest) dwarf planet, Eris which orbits the sun at a furthest distance of 97.56 AU.

The sun and the solar system are situated some 26,000 light-years (the distance that light travels in one year while in a vacuum) from the center of the Milky Way galaxy. Traveling at a velocity of 137 mph (220 km/h), the sun takes about 240 million years to complete one orbit about the galactic center, and since its formation the sun has completed about 19 such trips. As it orbits the center of the galaxy, the sun also moves in an oscillatory fashion above and below the galactic plane (the sun’s motion is similar to that of a carousel fair-ground ride) with a period of about 30 million years. During their periodic sojourns above and below the plane of the galaxy, the sun and solar system suffer gravitational encounters with other stars and giant molecular clouds. These close encounters result in the loss of objects (essentially dormant cometary nuclei located in the outer Oort cloud) that are on, or near, the boundary of the solar system. These encounters also nudge some cometary nuclei toward the inner solar system, where they may be observed as long-period comets.

Solar system inventory

One of the central and age-old questions concerning the solar system is, “How did it form?’ From the very outset, cosmologists (scientists that deal with the age of the universe, galaxy, solar system, etc.) know that such a question has no simple answer, and rather than attempting to explain specific observations about the solar system, scientists have tried to build-up a general picture of how stars and planets might form. Therefore, scientists do not try to explain why there are eight major planets within the sun’s solar system, or why the second planet is 17.8 times less massive than the seventh one. Rather, they seek to explain, for example, the compositional differences that exist between the planets. Indeed, it has long been realized that it is the chemical and dynamical properties of the planets that place the most important constraints on any theory that attempts to explain the origin of the solar system.

The objects within the solar system demonstrate several essential dynamical characteristics. When viewed from above the sun’s North Pole, all of the planets orbit the sun along near-circular orbits in a counterclockwise

manner. The sun also rotates in a counterclockwise direction. With respect to the sun, therefore, the planets have prograde orbits. The major planets, asteroids and short-period comets all move along orbits only slightly inclined to one another. This is why, for example, that when viewed from Earth, the asteroids and planets all appear to move in the narrow zodiacal band of constellations. All of the major planets, with two exceptions, spin on their central axes in the same direction that they orbit the sun. That is, the planets mostly spin in a prograde motion. The planets Venus and Uranus are the three exceptions, having retrograde (backwards) spins. (In addition, the dwarf planet Pluto is an example of a dwarf planet with a retrograde orbit.)

The distances at which the planets orbit the sun increase geometrically, and it appears that each planet is roughly 64% further from the sun than its nearest inner neighbor. This observation is reflected in the so-called Titius-Bode rule, which is a mathematical relation for planetary distances. The formula for the rule is d(AU) = (4 + 3× 2ⁿ) / 10, where n = 0, 1, 2, 3,... represents the number of each planet, and d is the distance from the sun, expressed in astronomical units. The formula gives the approximate distance to Mercury when n = 0, and the other planetary distances follow in sequence. It should be pointed out here that there is no known physical explanation for the Titius-Bode rule, and it may well be just a numerical coincidence.

Certainly, the rulepredictswoefully inaccurate distances for the planet Neptune and dwarf planet Pluto.

One final point on planetary distances is that the separation between successive planets increases dramatically beyond the orbit of Mars. While the inner, or terrestrial, planets are typically separated by distances of about four-tenths of an AU, the outer, or Jovian, planets are typically separated by five to ten AU. This observation alone suggests that the planetary formation process was different somewhere beyond the orbit of Mars.

While the asteroids and short-period comets satisfy, in a general sense, the same dynamical constraints as the major planets, scientists have to remember that such objects have undergone significant orbital evolution since the solar system formed. The asteroids, for example, have undergone many mutual collisions and fragmentation events, and the cometary nuclei have suffered from numerous gravitational perturbations from the planets. Long-period comets in particular have suffered considerable dynamical evolution, first to become members of the Oort cloud, and second to become comets visible in the inner solar system.

The compositional make-up of the various solar system bodies offers several important clues about the conditions under which they formed. The four interior planets—Mercury, Venus, Earth, and Mars—are

classified as terrestrial and are composed of rocky material surrounding an iron-nickel metallic core. On the other hand, Jupiter, Saturn, Neptune, and Uranus are classified as the gas giants and are large masses of hydrogen in gaseous, liquid, and solid form surrounding Earth-size rock and metal cores. Dwarf planet Pluto fits neither of these categories, having an icy surface of frozen methane. Pluto more greatly resembles the satellites of the gas giants, which contain large fractions of icy material. This observation suggests that the initial conditions under which ices might have formed only prevailed beyond the orbit of Jupiter.

In summary, any proposed theory for the formation of the solar system must explain both the dynamical and chemical properties of the objects in the solar system. It must also be sufficient flexibility to allow for distinctive features such as retrograde spin, and the chaotic migration of cometary orbits.

The solar nebula hypothesis

Astronomers almost universally believe that the best descriptive model for the formation of the solar system is the solar nebula hypothesis. The essential idea behind the solar nebula model is that the sun and planets formed through the collapse of a rotating cloud of interstellar gas and dust. In this way, planet formation is thought to be a natural consequence of star formation.

The solar nebula hypothesis is not a new scientific proposal. Indeed, German philosopher Immanuel Kant (1724–1804) first discussed the idea in 1755. Later, French mathematician, Pierre Simon de Laplace (1749–1827) developed the model in his text, The System of the World, published in 1796. The model is still under development today.

The key idea behind the solar nebula hypothesis is that once a rotating interstellar gas cloud has commenced gravitational collapse, then the conservation of angular momentum will force the cloud to develop a massive, central condensation that is surrounded by a less massive flattened ring, or disk of material. The nebula hypothesis asserts that the sun forms from the central condensation, and that the planets accumulate from the material in the disk (Figure 1). The solar nebula model naturally explains why the sun is the most massive object in the solar system, and why the planets rotate about the sun in the same sense, along nearly circular orbits and in essentiallythesameplane(Figure2).

During the gravitational collapse of an interstellar cloud, the central regions become heated through the

release of gravitational energy. This means that the young solar nebular is hot, and that the gas and (vaporized) dust in the central regions is well mixed. By constructing models to follow the gradual cooling of the solar nebula, scientists have been able to establish a chemical condensation sequence. Near to the central proto-sun, the nebular temperature will be very high, and consequently no solid matter can exist. Everything is in a gaseous form. As one moves further away from the central proto-sun, however, the temperature of the nebula falls off. At distances beyond 0.2 AU from the proto-sun, the temperature drops below 2,000 K (3,100°F; 1,700°C). At this temperature, metals and oxides can begin to form. Still further out (at about 0.5 AU), the temperature will drop below 1,000 K (1,300°F; 730°C), and silicate rocks can begin to form. Beyond about 5 AU from the proto-sun, the temperature of the nebula will be below 200 K (-100°F; -73°C), and ices can start to condense. The temperature and distance controlled sequence of chemical condensation in the solar nebula correctly predicts the basic chemical make-up of the planets (Figure 3).

The angular momentum problem

Perhaps the most important issue to be resolved in future versions of the solar nebula model is that of the distribution of angular momentum. The problem for the solar nebula theory is that it predicts that most of the mass and angular momentum should be in the sun. In other words, the sun should spin much more rapidly than it does. A mechanism is therefore required to transport angular momentum away from the central proto-sun and redistribute it in the outer planetary disk. One proposed transport mechanism invokes the presence of magnetic field in the nebula, while another mechanism proposed the existence of viscous stresses produced by turbulence in the nebular gas.

Building the planets

Precise dating of meteorites and lunar rock samples indicate that the solar system is 4.6 billion years old. The meteorites also indicate an age spread of about 20 million years, during which time the planets themselves formed.

The standard solar nebula model suggests that the planets were created through a multi-step process. The first important step is the coagulation and sedimentation of rock and ice grains in the mid-plain of the nebula. These grains and aggregates, 0.4 in (1 cm) to 3 ft (1 m) in size, continue to accumulate in the mid-plain of the nebula to produce a swarm of some 10 trillion larger bodies, called planetesimals, that are some 0.6 mi (1 km), or so in size. Finally, the planetesimals themselves accumulate into larger, self-gravitating bodies called proto-planets. The proto-planets were probably a few hundred kilometers in size. Finally, growth of proto-planet-sized objects results in the planets.

The final stages of planetary formation were decidedly violent—it is believed that a collision with a Mars-sized proto-planet produced Earth’s moon. Likewise, it is thought that the retrograde rotations of Venus and Uranus may have been caused by glancing proto-planetary impacts. The rocky and icy planetesimals not incorporated into the proto-planets now orbit the sun as asteroids and cometary nuclei. The cometary nuclei that formed in the outer solar nebula were mostly ejected from the nebula by gravitational encounters with the large Jovian planets and now reside in the Oort cloud.

One problem that has still to be worked-out under the solar nebula paradigm concerns the formation of Jupiter. The estimated accumulation time for Jupiter is about 100 million years, but it is now known that the solar nebula itself probably only survived for between 100,000 to 10 million years. In other words, the accumulation process in the standard nebula model is too slow by a least a factor of 10 and maybe 100. Indeed, much has yet to be learned of how the solar system formed.

Active study of the solar system is ongoing. Several probes and robots—such as the Galileo spacecraft, the Cassini mission, New Horizons, and Mars Pathfinder mission—have been launched towards other planets, dwarf planets, and their moons. The information they send back will help further explain the evolution of the solar system. In fact, the Near Earth Asteroid Rendezvous-Shoemaker (NEAR-Shoemaker) spacecraft flew by the asteroid Mathilde and found it to have a surprisingly low density. Later, NEAR-Shoemaker made a soft landing (on February 12, 2001) onto the asteroid Eros, the first time a human instrument has soft-landed on an asteroid. Its mission provided a tremendous amount of information about asteroids.

Of great importance to the study of solar systems in general was the discovery in 1999 of an entire solar system around a star besides the sun. Forty-four light-years from Earth, three large planets were found

KEY TERMS

Accretion —The process by which the mass of a body increases by the gravitational attraction of smaller objects.

Angular momentum —The product of orbital distance, orbital speed, and mass. In a closed system, angular momentum is a conserved quantity—it can be transferred from one place to another, but it cannot be created or destroyed.

Oort cloud —A vast, spherical cloud of some one trillion cometary nuclei that orbit the Sun. The cloud, named after Dutch astronomer Jan Oort who first suggested its existence, extends to a distance of 105 AU from the Sun.

Planetesimal —Small, 0.6 mi (1 km) sized objects made of rock and/or ice that accrete to form proto planets.

Prograde rotation —Rotational spin in the same sense as the orbital motion. For solar system objects, the orbital motion is counterclockwise, and prograde spin results in the object revolving from east to west.

Retrograde rotation —Axial spin that is directed in the opposite sense to that of the orbital motion.

circling the star Upsilon Andromedae. Astronomers suspect the planets are similar to Jupiter and Saturn— huge spheres of gas without a solid surface. The discovery of other solar systems in the Milky Way galaxy is yielding important insight into the formation of evolution of solar systems in general.

As of 2006, astronomers estimate that at least 10% of all stars similar in size and characteristics to the sun have one or more planets orbiting them. In 2005 and 2006, some of the extrasolar planets discovered have included: Gliese 876d, around the red dwarf star Gliese 876; HD 149026 b, the largest planet core ever found; HD 188753 Ab, a planet within a triple star system; and OGLE-2005-BLG-390Lb, the most distant and coldest extrasolar planet ever discovered.