Chapter 18: Origin of the Solar System and Extrasolar Planets

asteroids

Small, rocky world; most asteroids lie between Mars and Jupiter in the asteroid belt.

Kuiper Belt

The collection of icy planetesimals that orbit in a region from just beyond Neptune out to about 50.

Kuiper Belt Objects (KBOs)

An object in the Kuiper Belt, a region beyond Neptune’s orbit containing planetesimals remaining from the formation of the Solar System. Pluto is one of the largest Kuiper Belt objects.

comets

One of the small, icy bodies that orbit the Sun and produce tails of gas and dust when they near the Sun.

meteors

A small bit of matter heated by friction to incandescent vapor as it falls into Earth’s atmosphere.

meteoroid

A meteor in space before it enters Earth’s atmosphere.

meteorite

A meteor that has survived its passage through the atmosphere and strikes the ground.

half-life

The time required for half of the atoms in a radioactive sample to decay.

passing star hypothesis

The proposal that our Solar System formed when two stars passed near each other and material was pulled out of one to form the planets.

evolutionary hypothesis

Explanation for natural events that involves gradual changes as opposed to sudden catastrophic changes—for example, the formation of the planets in the gas cloud around the forming Sun.

catastrophic hypothesis

Explanation for natural processes that depends on dramatic and unlikely events, such as the collision of two stars to produce our Solar System.

nebular hypothesis

The proposal that the Solar System formed from a rotating cloud of gas.

solar nebula theory

The proposal that the planets formed from the same cloud of gas and dust that formed the Sun.

condensation

The growth of a particle by addition of material from surrounding gas, one atom or molecule at a time.

frost line

In the solar nebula, the boundary beyond which water vapor and other compounds could form ice particles.

condensation sequence

The sequence in which different materials condense from the solar nebula at increasing distances from the Sun.

planetesimals

One of the small bodies that formed from the solar nebula and eventually grew into protoplanets.

accretion

The sticking together of solid particles to produce a larger particle.

protoplanets

Massive object resulting from the coalescence of planetesimals in the solar nebula and destined to become a planet.

gravitational collapse

The stage in the formation of a massive planet when it grows massive enough to begin capturing gas directly from the nebula around it.

heat of formation

In planetology, the heat released by the infall of matter during the formation of a planetary body.

Differentiation

The separation of planetary material according to density.

outgassing

The release of gases from a planet’s interior.

Jovian problem

The puzzle that protoplanetary disks around young stars don’t seem to survive long enough to form Jovian planets by condensation, accretion, and gravitational collapse, yet Jovian-mass extrasolar planets are common. See also extrasolar planet.

direct collapse

The hypothetical process by which a Jovian planet might skip the accretion of a solid core, instead forming quickly and directly from the gases of the solar nebula.

heavy bombardment

The period of intense meteorite impacts early in the formation of the planets, when the Solar System was filled with debris.

debris disks

A disk of dust found by infrared observations around some stars. The dust is debris from collisions among asteroids, comets, and Kuiper Belt objects.

extrasolar planet

A planet orbiting a star other than the Sun.

hot Jupiters

A massive and presumably Jovian planet that orbits close to its star and consequently has a high temperature.

transit

The passage of an extrasolar planet across the disk of its parent star as observed from Earth, partially blocking the light from the star and allowing detection and study of the planet.

microlensing

Brightening of a background star due to focusing of its light by the gravity of a foreground extrasolar planet, allowing the planet to be detected and some of its characteristics measured.