What kinds of objects are in the solar system?

Gravity and energy influence the formation of objects in the solar system, including planets, dwarf planets, comets, asteroids, and other small solar system bodies.

What is the solar system? Almost all of the specks of light you can see in the night sky are stars. A few of the tiny lights are part of our solar system. Stars are much farther away than objects in our solar system.

Objects in the Solar System The largest object in the solar system is the Sun, a star. Planets orbit the Sun and have nearly spherical shapes. The mass of a planet must be much larger than the total mass of all other objects whose orbits are close by.

The Motion of the Planets (cont.) A planet’s orbit is an ellipse— a stretched-out circle. Focus points, or foci, determine the shape of the ellipse.

The Motion of the Planets The time it takes an object to travel once around the Sun is its period of revolution. The time it takes an object to complete one rotation is its period of rotation.

Objects in the Solar System (cont.) Eight of the objects in the solar system are planets. Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune

Mercury, Venus, Earth, and Mars are the inner planets. The inner planets are mostly solid, rocky material.

Jupiter, Saturn, Uranus, and Neptune are the outer planets. The outer planets are mostly ice and gases, such as hydrogen and helium.

Dwarf Planets According to the International Astronomical Union (IAU), a dwarf planet is an object that orbits a star and has enough mass and gravity to pull itself into a spherical shape.

Objects in the Solar System (cont.) A dwarf planet is a spherical object that orbits the Sun (a star) and is not a moon or another planet. Unlike a planet, a dwarf planet has objects similar in mass orbiting nearby or crossing its orbital path.

Dwarf Planets (cont.) Ceres is the smallest dwarf planet with a diameter of about 950 km. Pluto is so far from the Sun that it takes about 248 years to complete one orbit. Eris is the largest dwarf planet.

All of the dwarf planets are smaller than Earth’s moon. Pluto: Dr. R. Albrecht, ESA/ESO Space Telescope European Coordinating Facility; NASA Ceres: NASA, ESA, and J. Parker (Southwest Research Institute) Eris: NASA, ESA, and M. Brown (California Institute of Technology)

Ceres, a dwarf planet, orbits the Sun as planets do.

Objects in the Solar System (cont.) Millions of small, rocky objects called asteroids orbit the Sun in the asteroid belt between the orbits of Mars and Jupiter. Asteroids vary in size and are usually not spherical.

Most asteroids orbit the Sun in the asteroid belt between the orbits of Mars and Jupiter. Asteroid: NASA/JPL/JHUAPL Ida: NASA/JPL/USGS Vesta: Ben Zellner (Georgia Southern University), Peter Thomas (Cornell University), NASA/ESA Eros: NASA/Goddard Space Flight Center Scientific Visualization Studio

Asteroids Asteroids are chunks of rock and ice that never clumped together to form a planet. Some astronomers think the strength of Jupiter’s gravitational field might have caused the chunks to collide so violently that they broke apart instead of sticking together.

Comets A comet is made of gas, dust, and ice and moves around the Sun in an oval-shaped orbit. The particles in a comet are loosely held together by the gravitational attractions among the particles. Comets orbit the Sun in stretched out elliptical orbits. The solid, inner part of a comet is its nucleus. As a comet moves closer to the Sun, it heats up and can develop a bright tail.

Comets are mixtures of rock, ice, and dust. Comet: Roger Ressmeyer/Getty Images Wild 2: NASA/JPL-Caltech

Meteoroids A meteoroid is a small rocky particle that moves through space. A meteor is a streak of light in Earth’s atmosphere made by a glowing meteoroid. A meteorite is a meteoroid that strikes a planet or a moon. An impact crater is a round depression formed on the surface of a planet, moon, or other space object by the impact of a meteorite. A Moon is an object that revolves around a planet (not a star).

Objects in the Solar System (cont.) Distances between objects in the solar system are extremely large. Astronomers do not use meters or kilometers to describe these distances. A more convenient unit is used— the astronomical unit (AU). One AU is Earth’s average distance from the Sun— about 150,000,000 km.

It is easier to express very large distances using astronomical units rather than kilometers.