Mars has two satellites, Phobos and Deimos , but these are more akin to large asteroids than actual moons. Unlike the gas giants, terrestrial planets also have no planetary ring systems. Each are composed primarily of silicate rock and metal, which is differentiated between a dense, metallic core and a silicate mantle. The Moon is similar, but has a much smaller iron core. Io and Europa are also satellites that have internal structures similar to that of terrestrial planets.
Europa, on the other hand, is believed to have an iron core that is surrounded by an outer layer of water. Dwarf planets, like Ceres and Pluto , and other large asteroids are similar to terrestrial planets in the fact that they do have a solid surface. However, they differ in that they are, on average, composed of more icy materials than rock.
Most of the planets detected outside of the Solar System have been gas giants, owing to the fact that they are easier to spot. However, since , hundreds of potentially terrestrial extrasolar planets have been found — mainly by the Kepler space mission.
Examples of extrasolar terrestrial planets include Gliese d , a planet that has a mass 7 to 9 times that of Earth. This planet orbits the red dwarf Gliese , which is located approximately 15 light years from Earth. The existence of three or possibly four terrestrial exoplanets was also confirmed between and in the Gliese system, another red dwarf roughly 20 light years from Earth. The smallest of these, Gliese e , is only about 1. Two others, Gliese c and Gliese d , as well as a proposed fourth planet Gliese g are more-massive super-Earths orbiting in or close to the habitable zone of the star.
If true, this could mean that these worlds are potentially habitable Earth-like planets. The first confirmed terrestrial exoplanet, Keplerb — a planet with between 3 and 4 Earth masses and located some light years from Earth — was found in by the Kepler space mission.
Since then, Kepler has discovered hundreds of planets ranging from Moon-sized to super-Earths, with many more candidates in this size range. As of January, , planet candidates have been discovered. Scientists have proposed several categories for classifying terrestrial planets. Silicate planets are the standard type of terrestrial planet seen in the Solar System, which are composed primarily of a silicon-based rocky mantle and a metallic iron core.
Iron planets are a theoretical type of terrestrial planet that consists almost entirely of iron and therefore has a greater density and a smaller radius than other terrestrial planets of comparable mass.
Planets of this type are believed to form in the high-temperature regions close to a star, and where the protoplanetary disk is rich in iron. Coreless planets are another theoretical type of terrestrial planet, one that consists of silicate rock but has no metallic core. In other words, coreless planets are the opposite of an iron planet. Coreless planets are believed to form farther from the star where volatile oxidizing material is more common.
Though the Solar System has no coreless planets, chondrite asteroids and meteorites are common. Venus has no known moons. Much of the planet's surface is marked with volcanoes and deep canyons. The biggest canyon on Venus stretches across the surface for 4, miles nearly 6, kilometers. And it's possible that at least some of the planet's volcanoes are still active. Few spacecraft have ever penetrated Venus' thick atmosphere and survived.
And it's not just spacecraft that have trouble getting through the atmosphere — there are fewer crater impacts on Venus than other planets because only the largest meteors can make it. The planet is hostile to life as we know it.
Of the four terrestrial planets, Earth is the largest, and the only one with extensive regions of liquid water. Water is necessary for life as we know it, and life is abundant on Earth — from the deepest oceans to the highest mountains. Like the other terrestrial planets, Earth has a rocky surface with mountains and canyons, and a heavy-metal core.
Earth's atmosphere contains water vapor, which helps to moderate daily temperatures. The planet has regular seasons for much of its surface; regions closer to the equator tend to stay warm, while spots closer to the poles are cooler and in the winter, icy. The Earth's climate, however, is warming up due to climate change associated with human-generated greenhouse gases, which act as a trap for escaping heat.
Earth has a northern magnetic pole that is wandering considerably, by dozens of miles a year; some scientists suggest it might be an early sign of the north and south magnetic poles flipping. The last major flip was , years ago. Earth has one large moon that astronauts visited in the s and s.
Mars has the largest mountain in the solar system, rising 78, feet nearly 24 km above the surface. Much of the surface is very old and filled with craters, but there are geologically newer areas of the planet as well. At the Martian poles are polar ice caps that shrink in size during the Martian spring and summer. Mars is less dense than Earth and has a smaller magnetic field, which is indicative of a solid core, rather than a liquid one. While scientists have found no evidence of life yet, Mars is known to have water ice and organics — some of the ingredients for living things.
Evidence of methane has also been found in some parts of the surface. Methane is produced from both living and non-living processes. Mars has two small moons, Phobos and Deimos. The Red Planet is also a popular destination for spacecraft, given that the planet may have been habitable in the ancient past. During its lifetime, NASA's Kepler space observatory discovered more than 2, confirmed alien planets — and thousands more possibilities — as of January Kepler ran out of fuel in , but many of its possible planet discoveries still need to be confirmed with follow-up observations from other telescopes.
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