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LECTURE 1: INTRODUCTION

A terrestrial planet, telluric planet or rocky planet is a planet that is primarily composed of silicate rocks. They are also the closest planets to the Sun. The terms are derived from Latin words for Earth (Terra and Tellus), so an alternative definition would be that these are planets which are, in some notable fashion, "Earth-like". Terrestrial planets are substantially different from gas giants, which might not have solid surfaces and are composed mostly of some combination of hydrogen, helium, and water existing in various physical states.

Terrestrial planets all have roughly the same structure: a central metallic core, mostly iron, with a surrounding silicate mantle. The Moon is similar, but lacks an iron core. Terrestrial planets have canyons, craters, mountains, and volcanoes. Terrestrial planets possess secondary atmospheres — atmospheres generated through internal vulcanism or comet impacts, as opposed to the gas giants, which possess primary atmospheres — atmospheres captured directly from the original solar nebula.

Theoretically, there are two types of terrestrial or rocky planets, one dominated by silicon compounds and another dominated by carbon compounds, like carbonaceous chondrite asteroids. These are the silicate planets and carbon planets (or "diamond planets") respectively.

Earth's solar system has four terrestrial planets: Mercury, Venus, Earth and Mars, and one terrestrial dwarf planet, Ceres. Objects like Pluto are similar to terrestrial planets in the fact that they do have a solid surface, but are composed of more icy materials (see Ice dwarf). During the formation of the solar system, there were probably many more (planetesimals), but they have all merged with or been destroyed by the four remaining worlds in the solar nebula. Only one terrestrial planet, Earth, is known to have an active hydrosphere.

In addition, Earth's moon and Jupiter's satellites Io and Europa can also be regarded as terrestrial worlds, though since they orbit planets they cannot be considered planets themselves. Io and Europa have mainly rocky compositions despite forming beyond the snow line. This may be because the region of the circum-Jovian disc in which they formed was kept too warm by radiation from the proto-Jupiter to contain large quantities of icy material.

The uncompressed density of the solar terrestrial planets, Ceres and the two largest asteroids generally trends towards lower densities as the distance from the sun increases.

The main exception to this rule is the density of the moon, which owes its smaller density to its unusual origin. It remains to be seen whether extrasolar terrestrial planets will also follow this trend.

The majority of planets found outside our solar system to date have been gas giants, simply because gas giants are larger and therefore easier to see or infer from observation. However, a number of extrasolar planets are known or suspected to be terrestrial.

Aleksander Wolszczan detected the first extrasolar terrestrial planets. The three planets orbit the pulsar PSR B1257+12 with masses of 0.02, 4.3, and 3.9 times that of Earth's. They were discovered by accident: their transit caused interruptions in the pulsar's radio emissions (had they not been orbiting around a pulsar, they would not have been found).

When 51 Pegasi b, the first extrasolar planet found around a fusing star, was discovered, many astronomers assumed it must be a gigantic terrestrial, as it was assumed no gas giant could exist as close to its star (0.052 AU) as 51 Pegasi b did. However, subsequent diameter measurements of a similar extrasolar planet (HD 209458 b), which transited its star showed that these objects were indeed gas giants.

In June 2005, the first planet around a fusing star that is almost certainly terrestrial was found orbiting around the red dwarf star Gliese 876, 15 light years away. That planet has a mass of 5 to 7 times that of earth and an orbital period of just two Earth days.

On 10 August 2005, Probing Lensing Anomalies NETwork/Robotic Telescope Network (PLANET/RoboNet) and Optical Gravitational Lensing Experiment (OGLE) observed the signature of a cold planet designated OGLE-2005-BLG-390Lb, about 5.5 times the mass of Earth, orbiting a star about 21,000 light years away in the constellation Scorpius. The newly discovered planet orbits its parent star at a distance similar to that of our solar system's asteroid belt. The planet revealed its existence through a technique known as gravitational microlensing, currently unique in its capability to detect cool planets with masses down to that of Earth.

In April 2007, a team of 11 European scientists announced the discovery of a planet outside our solar system that is potentially habitable, with Earth-like temperatures. The planet was discovered by the European Southern Observatory's telescope in La Silla, Chile, which has a special instrument that splits light to find wobbles in different wave lengths. Those wobbles can reveal the existence of other worlds. What they revealed is a planet circling the red dwarf star, Gliese 581. The discovery of the new planet, named Gliese 581 c, is sure to fuel studies of planets circling similar dim stars. About 80 percent of the stars near Earth are red dwarfs. The new planet is about five times heavier than Earth, classifying it as a super-earth. Its discoverers aren't certain if it is rocky, like Earth, or if it is a frozen ice ball with liquid water on the surface. If it is rocky like Earth, which is what the prevailing theory proposes, it has a diameter about 1 1/2 times bigger than our planet. If it is an iceball, it would be even bigger.

A number of telescopes capable of directly imaging extrasolar terrestrial planets are on the drawing board. These include the Terrestrial Planet Finder, Space Interferometry Mission, Darwin, New Worlds Mission, the kepler mission, and Overwhelmingly Large Telescope.

The four inner or terrestrial planets have dense, rocky compositions, few or no moons, and no ring systems. They are composed largely of minerals with high melting points, such as the silicates which form their crusts and mantles, and metals such as iron and nickel, which form their cores. Three of the four inner planets (Venus, Earth and Mars) have substantial atmospheres; all have impact craters and tectonic surface features such as rift valleys and volcanoes. The term inner planet should not be confused with inferior planet, which designates those planets which are closer to the Sun than Earth is (i.e. Mercury and Venus).

Mercury

Mercury (0.4 AU) is the closest planet to the Sun and the smallest planet (0.055 Earth masses). Mercury has no natural satellites, and its only known geological features besides impact craters are "wrinkle-ridges", probably produced by a period of contraction early in its history.^[20] Mercury's almost negligible atmosphere consists of atoms blasted off its surface by the solar wind.^[21] Its relatively large iron core and thin mantle have not yet been adequately explained. Hypotheses include that its outer layers were stripped off by a giant impact, and that it was prevented from fully accreting by the young Sun's energy.^[22][23]

Venus

Venus (0.7 AU) is close in size to Earth, (0.815 Earth masses) and like Earth, has a thick silicate mantle around an iron core, a substantial atmosphere and evidence of internal geological activity. However, it is much drier than Earth and its atmosphere is ninety times as dense. Venus has no natural satellites. It is the hottest planet, with surface temperatures over 400 °C, most likely due to the amount of greenhouse gases in the atmosphere.^[24] No definitive evidence of current geological activity has been detected on Venus, but it has no magnetic field that would prevent depletion of its substantial atmosphere, which suggests that its atmosphere is regularly replenished by volcanic eruptions.^[25]

Earth

Earth (1 AU) is the largest and densest of the inner planets, the only one known to have current geological activity, and the only planet known to have life. Its liquid hydrosphere is unique among the terrestrial planets, and it is also the only planet where plate tectonics has been observed. Earth's atmosphere is radically different from those of the other planets, having been altered by the presence of life to contain 21% free oxygen.^[26] It has one natural satellite, the Moon, the only large satellite of a terrestrial planet in the Solar System.

Mars

Mars (1.5 AU) is smaller than Earth and Venus (0.107 Earth masses). It possesses a tenuous atmosphere of mostly carbon dioxide. Its surface, peppered with vast volcanoes such as Olympus Mons and rift valleys such as Valles Marineris, shows geological activity that may have persisted until very recently. Its red color comes from rust in its iron-rich soil.^[27] Mars has two tiny natural satellites (Deimos and Phobos) thought to be captured asteroids.^[28]

Asteroids are mostly small Solar System bodies composed mainly of rocky and metallic non-volatile minerals.

The main asteroid belt occupies the orbit between Mars and Jupiter, between 2.3 and 3.3 AU from the Sun. It is thought to be remnants from the Solar System's formation that failed to coalesce because of the gravitational interference of Jupiter.

Asteroids range in size from hundreds of kilometres across to microscopic. All asteroids save the largest, Ceres, are classified as small Solar System bodies, but some asteroids such as Vesta and Hygieia may be reclassed as dwarf planets if they are shown to have achieved hydrostatic equilibrium.

The asteroid belt contains tens of thousands, possibly millions, of objects over one kilometre in diameter.^[29] Despite this, the total mass of the main belt is unlikely to be more than a thousandth of that of the Earth.^[30] The main belt is very sparsely populated; spacecraft routinely pass through without incident. Asteroids with diameters between 10 and 10^-4 m are called meteoroids.^[31]

Ceres

Ceres (2.77 AU) is the largest body in the asteroid belt and is classified as a dwarf planet. It has a diameter of slightly under 1000 km, large enough for its own gravity to pull it into a spherical shape. Ceres was considered a planet when it was discovered in the 19th century, but was reclassified as an asteroid in the 1850s as further observation revealed additional asteroids.^[32] It was again reclassified in 2006 as a dwarf planet.

Asteroid groups

Asteroids in the main belt are divided into asteroid groups and families based on their orbital characteristics. Asteroid moons are asteroids that orbit larger asteroids. They are not as clearly distinguished as planetary moons, sometimes being almost as large as their partners. The asteroid belt also contains main-belt comets which may have been the source of Earth's water.^[33]

Trojan asteroids are located in either of Jupiter's L₄ or L₅ points (gravitationally stable regions leading and trailing a planet in its orbit); the term "Trojan" is also used for small bodies in any other planetary or satellite Lagrange point. Hilda asteroids are in a 2:3 resonance with Jupiter; that is, they go around the Sun three times for every two Jupiter orbits.

The inner Solar System is also dusted with rogue asteroids, many of which cross the orbits of the inner planets.

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     Prof. Drygalski