The solar system has been a topic of study from the beginning of history. For nearly all that time, people have had to rely on long-range and indirect measurements of its objects. For all of human history and pre-history, observations were based on visible light. Then in the 20th century people discovered how to use additional parts of the spectrum. Radio waves, received here on Earth, have been used since 1931 to investigate celestial objects. Starting with the emergence of space flight in 1957, instruments operating above Earth's obscuring atmosphere could take advantage not only of light and radio, but virtually the whole spectrum (the electromagnetic spectrum is the subject of a later chapter). At last, with interplanetary travel, instruments can be carried to many solar system objects, to measure their physical properties and dynamics directly and at very close range. In the 21st century, knowledge of the solar system is advancing at an unprecedented rate.

The solar system consists of an average star we call the sun, the planets Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. It includes the satellites of the planets, numerous comets, asteroids, meteoroids, and the interplanetary medium, which permeates interplanetary space. The sun is the richest source of electromagnetic energy in the solar system. The sun's nearest known stellar neighbor is a red dwarf star called Proxima Centauri, at a distance of about 4.2 light years. (A light year is the distance light travels in a year, at about 300,000 km per second.)

In Cosmic Perspective

Our whole solar system, together with all the local stars you can see on a clear dark night, orbits the center of our home galaxy. This spiral disk we call the Milky Way includes some 200 billion stars, thousands of gigantic clouds of gas and dust, and enormous quantities of mysterious dark matter.

Interstellar space is the term given to the space between stars in the galaxy. We are beginning to find that many stars besides the sun harbor their own planets, called extrasolar planets. As of January 2004 astronomers have detected about 100 planets orbiting other stars. They are all giant, Jupiter-like planets, made mostly of gas, since current detection methods cannot reveal smaller worlds. Their formation process is still unclear.

The image at right shows a galaxy similar to the Milky Way, known as M100 (click the image for details). The Milky Way has two small galaxies orbiting it nearby, which are visible from the southern hemisphere. They are called the Large Magellanic Cloud and the Small Magellanic Cloud. Our galaxy, one of billions of galaxies known, is travelling through intergalactic space. On a cosmic scale, all galaxies are generally receding from each other, although those relatively close together may exhibit additional local motion toward or away from each other.

Motions Within the Solar System

The sun and planets each rotate on their axes. Because they formed from the same rotating disk, the planets, most of their satellites, and the asteroids, all revolve around the sun in the same direction as it rotates, and in nearly circular orbits. The planets orbit the sun in or near the same plane, called the ecliptic (because it is where eclipses occur). Pluto is a special case in that its orbit is the most highly inclined (17 degrees) and the most highly elliptical of all the planets. Because its orbit is so eccentric, Pluto sometimes comes closer to the sun than does Neptune. It's interesting to note that most planets rotate in or near the plane in which they orbit the sun, since they formed, rotating, out of the same dust ring. Uranus must have suffered a whopping collision, though, that set it rotating on its side.

The Sun

The sun is a typical star. Its spectral classification is "G2 V." G2 basically means it's a yellow-white star, and the roman numeral V means it's a "main sequence" dwarf star (by far the most common) as opposed to supergiant, or sub-dwarf, etc. You can view current images of the sun as seen today by the suite of instruments aboard SOHO (Solar & Heliospheric Observatory) as it views the sun from the L1 Lagrange point between the Earth and the sun.

The sun dominates the gravitational field of the solar system; it contains about 99.85% of the solar system's mass. The planets, which condensed out of the same disk of material that formed the sun, contain only about 0.135% of the mass of the solar system. Satellites of the planets, comets, asteroids, meteoroids, and the interplanetary medium constitute the remaining 0.015%. Even though the planets make up only a small portion of the solar system's mass, they do retain the vast majority of the solar system's angular momentum. This storehouse of momentum can be utilized by interplanetary spacecraft on so-called "gravity-assist" trajectories.

The sun's gravity creates extreme pressures and temperatures within itself, sustaining a thermonuclear reaction fusing hydrogen nuclei and producing helium nuclei. This reaction yields tremendous amounts of energy, causing the material of the sun to be plasma and gas. These thermonuclear reactions began about 5 x 10⁹ years ago in the sun, and will probably continue for another 5 x 10⁹ years. The apparent surface of the sun has no clean physical boundary, as solid planets do, although it appears as a sharp boundary when seen from the distance of Earth. Click the SOHO solar image at right for more details about the image.

The sun rotates on its axis with a period of approximately 25.4 days. This is the adopted value at 16° latitude. Because the sun is a gaseous body, not all its material rotates together. (This solar fact sheet describes how rotation varies with latitude). Solar matter at very high latitudes takes over 30 days to complete a rotation. Our star's output varies slightly over an 11-year cycle, during which the number of sunspots changes.

The sun's axis is tilted about 7.25 degrees to the axis of the Earth's orbit, so we see a little more of the sun's northern polar region each September and more of its southern region in March. As viewed from the Earth's surface, the Sun subtends roughly half a degree of arc upon the sky (as does the Moon, at this period in cosmic time.)

The sun has strong magnetic fields that are associated with sunspots and coronal mass ejections, CMEs (also called solar flares). A sunspot is a relatively cool area that appears dark against the hotter face of the sun. Sunspots are formed when magnetic field lines just below the sun's visible surface are twisted, and reach though the photosphere. CMEs are huge magnetic bubbles of plasma that erupt from the sun's corona and travel through space at high speed. View this spectacular 5-Mbyte SOHO movie of CMEs in August, 1999.

The solar magnetic field is not uniform, and it is very dynamic. Solar magnetic field variations and dynamics are targets of major interest in the exploration of the solar system.

These and many other aspects of the sun are the subjects of ongoing research.

Our Bubble of Interplanetary Space

The "vacuum" of interplanetary space includes copious amounts of energy radiated from the sun, some interplanetary and interstellar dust (microscopic solid particles) and gas, and the solar wind. The solar wind, discovered by Eugene Parker in 1958, is a flow of lightweight ions and electrons (which together comprise plasma) thrown from the sun. The solar wind inflates a bubble, called the heliosphere, in the surrounding interstellar medium (ISM).

The solar wind has a visible effect on comet tails. It flows outward from our star at about 400 km per second, measured in the vicinity of Earth's orbit, and the Ulysses spacecraft found that it approximately doubles its speed at high solar latitudes.

The boundary at which the solar wind meets the ISM, containing the collective "solar" wind from other local stars in our galaxy, is called the heliopause. This is where the solar wind and the sun's magnetic field stop. The boundary is theorized to be roughly teardrop-shaped, because it gets "blown back" to form a heliotail, as the sun moves through the ISM (toward the right in the diagram above). The sun's relative motion may also create an advance bow shock, analogous to that of a moving boat. This is a matter of debate and depends partly on the strength of the interstellar magnetic field.

But before it gets out to the heliopause, the solar wind is thought to slow to subsonic speeds, creating a termination shock. This appears at the perimeter of the green circle in the diagram. Its actual shape, whether roughly spherical or teardrop, depends on magnetic field strengths, as yet unknown.

In the diagram above, temperatures are theorized; none have been actually measured beyond the termination shock. Note that even with the high particle temperatures, their density is so low that massive objects like spacecraft remain very cold (as long as they are shaded, or distant, from the sun).

The white lines in the diagram represent charged particles, mostly hydrogen ions, in the interstellar wind. They are deflected around the heliosphere's edge (the heliopause). The pink arrow shows how neutral particles penetrate the heliopause. These are primarily hydrogen and helium atoms, which are mostly not affected by magnetic fields, and there are also heavier dust grains. These interstellar neutral particles make up a substantial fraction of the material found within the heliosphere. The little black + in the green area represents the location of Voyager 1 at 80 AU in January of 2001. Voyager 1 is humanity's most distant object, and in 2004 the spacecraft is returning evidence that it is probably beginning to encounter the termination shock now, at a distance of just over 90 AU from the Sun.

The solar wind changes with the 11-year solar cycle, and the interstellar medium is not homogeneous, so the shape and size of the heliosphere probably fluctuate.

The solar magnetic field is the dominating magnetic field within the heliosphere, except in the immediate environment of planets which have their own magnetic fields. It can be measured by spacecraft throughout the solar system, but not here on earth, where we are shielded by our planet's own magnetic field.

The actual properties of the interstellar medium (outside the heliosphere), including the strength and orientation of its magnetic field, are important in determining the size and shape of the heliopause. Measurements that the two Voyager spacecraft will make in the region beyond the termination shock, and possibly beyond the heliopause, will provide important inputs to models of the termination shock and heliopause. Even though the Voyagers will sample these regions in discrete locations, this information will result in more robust overall models.

For further information on this vast subject and its many related topics, search the web for "heliosphere," "Alfven waves," "pickup ions," and "local interstellar cloud."