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.