1. Comets are essentially large, dirty snowballs
1. They are composed of solidified compounds that exist on Earth as gases (like water, Ammonia, Methane, and CO2)
2. Comets are in stable orbit around the sun, beyond Pluto
1. A region known as Oort cloud
2. Asteroids
1. They are composed of minerals and metallic elements. Characteristics of Earth and other inner-solar-system planets (Mars, Mercury, Venus)
2. Orbiting around the sun in a region called Asteroid Belt between Mars and Jupiter
3. They represent a remains of inner planet that failed to form or was destroyed by a large meteorite impact
4. Iridium is associated with mineral and metallic material in extraterrestrial materials
5. Sometimes, the asteroids and comets are disturbed by Jupiter’s gravitational force, resulting in some being sent to inner planets
6. Mass extinction accurse in 26 million years time intervals
3. Three hypotheses explaining why there is 26 million time interval
1. Galactic plane hypothesis
1. The oscillation of our solar system back and forth through the dense central plane and the spiral arm of the milky way galaxy-this leads to interaction with interstellar clouds of gas and dust which could disturb the orbits of asteroids and comets in the Oort cloud sending some on Earth-crossing orbit
2. Companion-star hypothesis
1. Sun has a companion star equal to 10 2/0 of its mass, (Nemesis) it presumed to be highly elliptical orbit that passes through the Oort cloud every 26 million years
3. The planet X hypothesis
1. A disturbance of another cluster of comets called Kuiper disk, its thought to exist outside the orbit of Neptune where planet X refers to any planet outside Pluto

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