The Sun And The Solar Wind

The Sun is a typical star of intermediate size and luminosity. Sunlight and other radiation are produced by the conversion of hydrogen into helium in the Sun’s hot, dense interior (see Nuclear Energy). Although this nuclear fusion is transforming 600 million metric tons of hydrogen each second, the Sun is so massive (2 × 10³⁰ kg, or 4.4 × 10³⁰ lb) that it can continue to shine at its present brightness for 6 billion years. This stability has allowed life to develop and survive on Earth.

For all the Sun’s steadiness, it is an extremely active star. On its surface, dark sunspots bounded by intense magnetic fields come and go in 11-year cycles and sudden bursts of charged particles from solar flares can cause auroras and disturb radio signals on Earth. A continuous stream of protons, electrons, and ions also leaves the Sun and moves out through the solar system. This solar wind shapes the ion tails of comets and leaves its traces in the lunar soil, samples of which were brought back from the Moon’s surface by piloted United States Apollo spacecraft (see Space Exploration; Apollo program).

The Sun’s activity also influences the heliopause, a region of space that astronomers believe marks the boundary between the solar system and interstellar space. The heliopause is a dynamic region that expands and contracts due to the constantly changing speed and pressure of the solar wind.

In November 2003 a team of astronomers reported that the Voyager 1 spacecraft appeared to have encountered the outskirts of the heliopause at about 86 AU from the Sun. They based their report on data that indicated the solar wind had slowed from 1.1 million km/h (700,000 mph) to 160,000 km/h (100,000 mph). This finding is consistent with the theory that when the solar wind meets interstellar space at a turbulent zone known as the termination shock boundary, it will slow abruptly. However, another team of astronomers disputed the finding, saying that the spacecraft had neared but had not yet reached the heliopause.

Eight major or classical planets are currently recognized by the International Astronomical Union (IAU), the body that gives official names to objects in the solar system. The planets are commonly divided into two groups: the inner planets (Mercury, Venus, Earth, and Mars) and the outer planets (Jupiter, Saturn, Uranus, and Neptune). The inner planets are small and are composed primarily of rock and iron. The outer planets are much larger and consist mainly of hydrogen, helium, and ice. Pluto, historically counted as the ninth planet, does not belong to either group, and was reclassified as a dwarf planet by the IAU in 2006. Some astronomers have objected to the reclassification of Pluto and the new IAU definition of a planet. Others feel strongly that classifying Pluto as one of the largest members of a family of similar icy bodies orbiting the Sun beyond Neptune provides a much better explanation of Pluto’s existence.

The IAU has defined a classical planet as a body that orbits the Sun, that has a rounded shape from the effects of its own gravity, and that is the dominant object in its region of space and has cleared the neighborhood of its orbit of other objects. A dwarf planet is also a rounded body orbiting the Sun—but one that is not massive enough to have cleared its region of space of other objects.

Sun

Sun, closest star to Earth. The Sun is a huge mass of hot, glowing gas. The strong gravitational pull of the Sun holds Earth and the other planets in the solar system in orbit. The Sun’s light and heat influence all of the objects in the solar system and allow life to exist on Earth.

Mercury  

Mercury orbits closer to the Sun than any other planet, making it dry, hot, and virtually airless. Although the planet’s cratered surface resembles that of the Moon, it is believed that the interior is actually similar to Earth’s, consisting primarily of iron and other heavy elements. This composite photograph was taken in 1974 by Mariner 10, the first probe to study Mercury in detail.

Venus

Venus is the brightest object in our sky, after the sun and moon. Swirling clouds of sulfur and sulfuric acid obscure Venus’s surface and inhibited study of the planet from Earth until technology permitted space vehicles, outfitted with probes, to visit it. These probes determined that Venus is the hottest of the planets, with a surface temperature of about 460° C (about 860° F). Scientists believe that a greenhouse effect causes the extreme temperature, hypothesizing that the planet’s thick clouds and dense atmosphere trap energy from the sun.  

Earth

An oxygen-rich and protective atmosphere, moderate temperatures, abundant water, and a varied chemical composition enable Earth to support life, the only planet known to harbor life. The planet is composed of rock and metal, which are present in molten form beneath its surface. The Apollo 17 spacecraft took this snapshot in 1972 of the Arabian Peninsula, the African continent, and Antarctica (most of the white area near the bottom).  

Mars

The most detailed information available about Mars has come from unpiloted spacecraft sent to the planet by the United States. From this data, scientists have determined that the planet’s atmosphere consists primarily of carbon dioxide, with small amounts of nitrogen, oxygen, water vapor, and other gases. Because the atmosphere is extremely thin, daily temperatures can vary as much as 100 Celsius degrees (190 Fahrenheit degrees). In general, surface temperatures are too cold and surface pressures too low for water to exist in a liquid state on Mars. The planet resembles a cold, high-altitude desert.  

Jupiter and its Moons

Jupiter is the largest of the planets, with a volume more than 1,300 times greater than that of Earth. Jupiter’s colorful bands are caused by strong atmospheric currents and accentuated by a dense cloud cover. The massive planet, upper right, is shown here with its four largest satellites: Io, upper left, Ganymede, lower left, Europa, center, and Callisto, lower right

Saturn

Saturn, distinguished by its rings, is the second largest planet in the solar system. This processed Hubble Space Telescope image shows the planet’s cloud bands, storms, and rings as they would appear to the human eye.

Uranus

Uranus’s blue-green color comes from the methane gas present in its cold, clear atmosphere. The dark shadings at the right edge of the sphere correspond to the day-night boundary on the planet. Beyond this boundary, Uranus’s northern hemisphere remains in a four-decade-long period of darkness because of the way the planet rotates. Scientists compiled this view of Uranus from images returned from Voyager 2 in 1986, when the probe was 9.1 million km (5.7 million mi) away from the planet.

Neptune

This image of Neptune, taken by the Voyager 2 spacecraft, shows the planet’s most prominent features. The large, dark oval surrounded by white clouds near the planet’s equator is the Great Dark Spot, a storm similar to Jupiter’s Great Red Spot. The smaller dark oval with a bright core below and to the right of the Great Dark Spot is another storm known as Dark Spot 2.

Pluto

Pluto is farther from the Sun than the major planets in the solar system, although it occasionally moves in closer than Neptune due to an irregular orbit. The small, rocky, and cold world takes 247.7 years to revolve around the Sun. This artist's rendition depicts Pluto, foreground; its moon, Charon, background; and the distant Sun, upper right.