The inner planets are the ones closest to the sun.
inner planet definition by Webster's New World
Webster's New World College Dictionary Copyright © 2010 by Wiley Publishing, Inc., Cleveland, Ohio. Used by arrangement with John Wiley & Sons, Inc.
any of the four planets closest to the sun; Mercury, Venus, Earth, or Mars
inner planet definition by American Heritage Dictionary
nounThe American Heritage® Dictionary of the English Language, 4th edition Copyright © 2010 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Any of the four planets, Mercury, Venus, Earth, and Mars, whose orbits are closest to the sun.
inner planet - Science Definition
Any of the four planets Mercury, Venus, Earth, and Mars, whose orbits lie nearest the Sun. The inner planets are of similar size and have high densities compared to the larger gas giants among the outer planets. They are composed mostly of rock and metal and are relatively slow to rotate, with solid surfaces, no rings, and few moons. Also called terrestrial planet. Compare outer planet. See also inferior planet.