point in a planet's orbit where it is farthest from the Sun.
astronomical unit (AU)
Average distance between Earth and the Sun; approximately 150 million km.
ratio of the distance between the foci to the length of the major axis; defines the shape of a planet's elliptical orbit.
point in a planet's orbit where it is closest to the Sun
a planet's backward motion in the sky.
gas giant planet
large, gaseous planets that are very cold at their surfaces, have ring systems, many moons, and lack solid surfaces—Jupiter, Saturn, Uranus, and Neptune.
wobble in Earth's rotational axis
rocky-surfaced, relatively small, dense inner planets closest to the Sun—Mercury, Venus, Earth, and Mars
low, warm, dark-colored cloud that sinks and flows rapidly in the Jovian atmosphere.
liquid metallic hydrogen
form of hydrogen with both liquid and metallic properties that exists as a layer in the Jovian atmosphere.
high, cool, light-colored cloud that rises and flows rapidly in the Jovian atmosphere.
metallic or silica-rich object, 1 km to 950 km in diameter, that bombarded early Earth, generating heat energy; rocky remnant of the early solar system found mostly between the orbits of Mars and Jupiter in the asteroid belt.
extended volume of glowing gas that forms when a comet's nucleus is heated.
small, eccentrically orbiting body made of rock and ice that consists of a nucleus, a coma, and one or more tails that point away from the Sun.
streak of light produced when a meteoroid falls toward Earth and burns up in Earth's atmosphere.
occurs when Earth intersects a cometary orbit and comet particles burn up as they enter Earth's upper atmosphere.
small asteroid or astoid fragment that fell on early Earth, generating heat; meteoroid that does not completely burn up in Earth's atmosphere and strikes Earth's surface, sometimes causing an impact crater.
piece of interplanetary material that falls toward Earth and enters its atmosphere.
positively charged center of an atom, made up of protons and neutrons and surrounded by electrons in energy levels; small, solid core of a comet.
Space object built of solid particles that can form planets through collisions and mergers.