Planetesimals

Asteroids

The Asteroid Ida

Rocky and metallic, asteroids are minor planets mostly found in the asteroid belt between the orbits of Mars and Jupiter. Apollo and Aten asteroids cross Earth’s orbit and Trojan asteroids in the Lagrangian points are within 60 degrees of Jupiter’s orbit. If asteroids are far from the Sun, their compositions are similar to that of comets (carbonaceous ice). If asteroids are closer to the Sun, their compositions are molten ice and iron core. Asteroids that crash into other cosmic objects are called killer asteroids. The three major types of asteroids are the C-type (carbonaceous, 75% of known asteroids), the S-type (silicaceous, 17%), and the M-type (metallic, 8%). Asteroids and comets are now classified as “small solar-system bodies.”

Meteoroids

Meteoroid

Meteoroids are debris in space from comets or asteroids; meteors are shooting stars or fire balls in air; meteorites are meteoroids that invade Earth’s atmosphere and impact the ground; micrometeorites are perfect shiny spheres microscopic in size and the major cause of small-scale erosion on the moon. The three major types of meteoroids are stony, stony iron, and iron.

Comets

Halley’s Comet

A comet has a head (nucleus and coma) and tails (dust, iron, and sodium). The nucleus is a few miles in diameter and composed of ice with dust mixed in, hence the term dirty snow ball. The coma is gases vaporized from the nucleus by sublimation (solid to gas). The two types of tails are dust tails (sunlight reflected off particles) and ion tails (sunlight emitted by ions blown back by solar wind). Comets either come from the Oort Cloud (50,000 to 150,000 AU in radius, a billion swarming comet nuclei) or the Kuiper Belt (30 to 100 AU beyond Neptune). Nuclei detached by gravity are caught by the Sun’s gravitational field and pulled into orbits. Short period comets are less than 30 AU away and long period comets are thousands of AU away. The most famous comet, of course, is Halley’s Comet, which is only visible every 75 or 76 years.

DWARF PLANETS

Pluto and Its Moons

PLUTO: Named for the Roman god of the Underworld, Pluto, originally a planet, is now classified by astronomers as a dwarf planet. With the most eccentric orbit and the greatest inclination to the ecliptic, Pluto has a revolution period of 249 years. For 20 of those 249 years, Pluto is closer to the Sun than Neptune. Pluto has density similar to satellites of outer planets, a rocky core with an icy surface of water and methane, and a tenuous atmosphere of methane. Pluto has four moons— Charon, Nix, Hydra, and P4; the latter three, however, are much smaller than the former.

THE DEBATE OVER PLUTO: Why Pluto IS a Planet— 1) Massive enough to be spherical; 2) Orbits of objects in Kuiper belt affected; 3) Mercury’s orbit is elliptical; 4) Asteroid hunters mistakenly discovered Neptune; 5) Each planet is unique; 6) Earth-moon system. Why Pluto IS NOT a Planet— 1) Smaller than other planets; 2) Gravity too weak to affect other planets; 3) Wildly elliptical orbit; 4) Nearly always mistaken for an asteroid in searches; 5) Properties do not follow the pattern; 6) Charon closely resembles Pluto and is large as moon

DEFINITION OF AND THREE CRITERIA OF PLANETS: 1) large enough that, when it is formed, condenses under its own gravity to be shaped like a sphere; 2) orbits a star directly and is not a moon of another planet; 3) clear its path of all debris in its neighborhood. To be a dwarf planet, the first two criteria must be met.

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The Sun

THE STAR OF THE SOLAR SYSTEM

The Sun’s Anatomy

The Sun: Very Close, Very Average, Very Spherical, Very Active, Very Energetic, Prime of Life

VERY CLOSE: At about 93 million miles (1 astronomical unit/AU) or 8 light minutes from Earth, the Sun is up close and personal. The closest stars from the Sun are Proxima Centauri (4.25 ly) and Sirius (8.55 ly).

VERY AVERAGE: Of the spectral type G2V, the Sun’s surface temperature is about 5800 Kelvin. It is average in size (1 million miles in diameter), average in life cycle (5 billion years with 5 billion years more to go), and average in composition (76% Hydrogen and 22% Helium). Unlike most stars that are in a binary star system, the Sun is lonely.

VERY SPHERICAL: Due to an inward pushing force due to gravitational contraction and outward pushing force due to high pressure from high temperature, the sun achieves hydrostatic equilibrium.

VERY ACTIVE: Constantly changing, the Sun’s surface contains sunspots, solar storms or solar flares (AKA Corona Mass Ejection). The Sun constantly produces energy with nuclear fusion and oscillates like a bell.

VERY ENERGETIC: The Sun emits 1026 watts into space with solar winds. Nuclear fusion occurs with 657 million tons of H2 (hydrogen) fused into 653 million tons of He (helium) every second.

LAYERS OF THE SUN: The four parts of the Sun are: core, photosphere, chromosphere, and corona. Inside the core at 15 million Kelvin, nuclear fusion produces energy by fusing 2 hydrogen atoms into 1 helium atom. The energy is then carried from the core by radiation and convection. The photosphere (5,800 Kelvin) is the outermost/lowest level of solar atmosphere. In 1814, German physicist Joseph von Fraunhofer discovered the set of spectral lines (AKA Fraunhofer Lines), or dark features, in the optical spectrum of the Sun. Active with plages, prominences, filaments, sunspots, and solar flares, the photosphere has granulation, or pockets (granules) of hot gases rising and pockets of cooler gases sinking. Hotter than the photosphere at 15,000 Kelvin, the chromosphere is a thin diffuse layer composed of spicules, or dynamic jets. The corona at 2 million Kelvin is only observed during a total solar eclipse. A thin layer, the corona emits mostly X-rays.

SUNSPOTS: Sunspots are areas of unusually strong magnetic fields, of relatively dark and cool areas with umbra and penumbra, usually in pairs, and of sizes greater than several Earth’s in diameter. The number of sunspots varies with an 11-year cycle (Sunspot Cycle). During the Maunder Minimum from 1645 to 1715, the number of sunspots was unusually low. Moreover, during the Solar Cycle (22-year cycle), the polarity of the sunspots reverses and returns to the original state.

Solar Flares in 2012

Solar Flare

2012 marks the height of another 11-year solar cycle as the number of solar flares peak. Throughout 2012, solar flares will disrupt satellite transmissions and radio signals. A solar flare is a high-energy ejection from the sun’s corona, emitting up to 6 × 1025 Joules and reaching up to the edge of the solar system!

The Planets: Part I

“The Planets” sub-page under the “The Solar System” page has now been complete and updated with pictures and additional information. Part I includes the terrestrial planets: Mercury, Venus, Earth, and Mars! See the page here.