Planets

Planets are celestial bodies which are larger than an Asteroid or Comet, illuminated by light from a Star, such as the Sun, around which it revolves. A Planet is massive enough to be rounded by its own gravity but is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of Planetesimals.

According to the International Astronomical Union (IAU), 2006, any celestial body in the Solar System can be considered a Planet if it:

• is in orbit around the Sun,
• has sufficient mass to assume Hydrostatic Equilibrium (a nearly round shape), and
• has "cleared the neighbourhood" around its orbit.

In the conventional sense Planets are one of the 8 celestial bodies. Earlier there were 9 Planets which included Pluto, however scientific studies do not conform to it being as as a 'True Planet'. Now the Planets include Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. These are in the order of increasing distance from the Sun. Six of the Planets are orbited by one or more Natural Satellites. Since 1992, hundreds of Planets around other stars (extrasolar planets or exoplanets) in the Milky Way Galaxy have been discovered. As of May 2010, there are 453 known Extrasolar Planets.

Types / Categories of Planets

1.  Terrestrial Planets - Planets which are primarily composed of silicate rocks are known as Terrestrial Planets. These Planets are the Inner Planets closest to the Sun.

Earth's Solar System has 4 Terrestrial Planets -

• Mercury
• Venus
• Earth
• Mars

They are also known as Telluric Planets or Rocky Planets because of their composition. All Terrestrial Planets have roughly the same structure which includes a central metallic core composed mostly of 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. These Atmospheres are generated through internal Volcanism or Comet impacts, as opposed to the Gas Giants, which possess primary Atmospheres - Atmospheres captured directly from the original solar Nebula.

Theoretically, there are 2 types of Terrestrial Planets

1. Silicate Planets - These Planets are dominated by silicon compounds.
2. Carbon Planets - These Planets are dominated by carbon compounds, like carbonaceous chondrite Asteroids. They are also known as Diamond Planets.

2.  Gas Giants - Planets which are largely composed of gaseous material are known as Gas Giants. These Planets are significantly larger than Terrestrial Planets. These are also known as Jovian Planets or Giant Planets.

There are 4 Gas Giants in our Solar System -

• Jupiter
• Saturn
• Uranus
• Neptune

Giant Planets have mass which is above 10 Earth masses. A Gas Giant has a thick Atmosphere and a solid core. Jupiter and Saturn, are composed primarily of hydrogen and helium. Uranus and Neptune are sometimes called Ice Giants, as they are mostly composed of water, ammonia, and methane ices.

3.  Dwarf Planets - Dwarf Planets are celestial bodies orbiting the Sun and are massive enough to be rounded by its own gravity but have not cleared their neighbouring region of Planetesimals and is not a Satellite. Dwarf Planets lack the mass to remove smaller bodies near their orbits by collision, capture, or gravitational disturbance. There are only 5 Dwarf Planets - Ceres, Pluto, Haumea, Makemake and Eris. Ceres and Pluto, are known to qualify as Dwarf Planets through direct observation. While Eris, Haumea and Makemake are considered to be Dwarf Planets from mathematical modelling.

4.  Extrasolar Planets - Extrasolar Planets are those Planets which are found outside the Solar System. These are also known as Exoplanets. The vast majority of Extrasolar Planets have been detected through radial velocity observations and other indirect methods rather than actual imaging. Most are Giant Planets thought to resemble Jupiter. Any known Exoplanet therefore is also known as 'Hot Jupiter'. Several relatively lightweight Exoplanets, only a few times more massive than Earth, have also been detected. The first confirmed detection was made in 1992, with the discovery of several terrestrial-mass Planets orbiting the pulsar PSR B1257+12. The first confirmed detection of an Exoplanet orbiting a main-sequence star was made in 1995, when a Giant Planet, 51 Pegasi b, was found in a four-day orbit around the nearby G-type Star 51 Pegasi. Most known Exoplanets orbit Stars roughly similar to Earth's Sun. The vast majority of Exoplanets detected so far have high masses. As of January 2010, all but 25 of Exoplanets have more than 10 times the mass of Earth. As of 30th July 2010, astronomers have made confirmed detections of 473 such Planets.

Extrasolar Planets can be divided into:

• Extrasolar Gas Giants - Planets which are largely composed of gaseous material and are located outside the Solar System are known as Extrasolar Gas Giants. Many of the Extrasolar Gas Giants are much closer to their parent stars and hence much hotter than Gas Giants in the Solar System.

• Extrasolar Terrestrial Planets - Planets which are primarily composed of silicate rocks and are located outside the Solar System are known as Extrasolar Terrestrial Planets. In the early 1990s, the first Extrasolar Planets were discovered orbiting the Pulsar PSR B1257+12 with masses of 0.02, 4.3, and 3.9 times that of Earth's.

Some of the Extrasolar Planets

• 51 Pegasi b.
• Epsilon Eridani b.
• PSR B1257+12 B.
• OGLE-TR-56 b.
• OGLE-2003-BLG-235L b.
• 2M1207 b.
• Fomalhaut b.
• 55 Cancri b.
• PSR B1620-26 b.
• Gliese 876 b.
• Iota Draconis b.
• Mu Arae c.
• WASP-17b.

Theoretical Types of Planets

Silicate Planets - These are made primarily of silicon-based rocky mantle with a metallic (iron) core. Terrestrial Planets are also known as Silicate Planets.

Iron Planet - This type of theoretical Terrestrial Planet consists almost entirely of iron. It is because of this reason that it has a smaller radius than other Terrestrial Planets of comparable mass. Mercury is an example of this type. It has a metallic core equal to 60-70% of its Planetary mass. Like Mercury, Iron Planets are believed to form in the high temperature regions close to a Star, and if the Protoplanetary disk is rich in iron.

Coreless Planet - This type of Planet consists of silicate rock but has no metallic core. The Solar System contains no Coreless Planets but chondrite Asteroids and meteorites are common. Coreless Planets are believed to form farther from the star where volatile oxidizing material is more common.

Carbon Planet or Diamond Planet - Another theoretical type of Terrestrial Planet which is composed primarily of carbon-based minerals. The Solar System contains no Carbon Planets, but does have Carbonaceous Asteroids.

Super-Earth - Super-Earth is any Extrasolar Rocky Planet several times larger than Earth. Super-Earths represent the upper-end of the Terrestrial Planet mass range.

Characteristics and Physical Features of Planets

• Spherical Shape - A Planet is is massive enough for the force of its own gravity to dominate over the electromagnetic forces binding its physical structure, leading to a state of Hydrostatic Equilibrium. This translates the shape of Planets into Spherical or Spheroidal. Up to a certain mass, an object can be irregular in shape, but beyond that point, which varies depending on the chemical make-up of the object, gravity begins to pull an object towards its own centre of mass until the object collapses into a sphere.

• Presence of Atmosphere - All of the Solar System Planets have Atmospheres. Planets have large masses which translates into strong gravity and this gravity keeps large amount of gaseous particles like hydrogen and helium close to the surface of the Planet. Mercury is the only Solar Planet without a substantial Atmosphere.

• Presence of a Magnetic Field - All Planets of Solar System except Venus and Mars have a Magnetic Field. This Magnetic Field is also known as Magnetosphere. Planets have flows of electrically conducting  material in their interiors, which generate their magnetic fields. The magnetic field of Mercury is the weakest, and is barely able to deflect the solar wind. Ganymede's magnetic field is several times larger, and Jupiter's is the strongest in the Solar System.

• Presence of Natural Satellites - All Planets of the Solar System have their own Natural Satellites, with the exception of Mercury and Venus. These Natural Satellites are often known as 'Moons'. For example Earth has 1 Moon , Mars has 2 and the Gas Giants have numerous Moons.

• Planet Mass - Planets have large mass composed of either gaseous material or rocky materials. The upper mass limit for Planet-hood is roughly 13 times Jupiter's mass, beyond which it achieves conditions suitable for nuclear fusion. The smallest known Planet PSR B1257+12 a, one of the first Extrasolar Planets discovered, has a mass is roughly half that of the Planet Mercury.

• Axial Tilt and Weather - All Planets have varying degrees of Axial tilt; they lie at an angle to the plane of their Stars' equators. This causes the amount of light received by each Hemisphere to vary over the course of its year; when the Northern Hemisphere points away from its Star, the Southern Hemisphere points towards it, and vice versa. Each Planet therefore possesses seasons which are changes in the climate over the course of its year. The varying amount of light and heat received by each hemisphere creates annual changes in weather patterns for each half of the Planet.

• Orbit - All Planets revolve around Stars. All the Planets orbit the Sun in the same direction as the Sun rotates. The period of one revolution of a Planet's Orbit is known as its Sidereal Period or Year.  A Planet's year depends on its distance from its star; the farther a Planet is from its Star, not only the longer the distance it must travel, but also the slower its speed, as it is less affected by the Star's gravity.

• Orbital Clearing - Another characteristic of a Planet is that it has cleared its neighbourhood. A Planet that has cleared its neighbourhood has accumulated enough mass to gather up or sweep away all the Planetesimals in its orbit. In effect, it orbits its Star in isolation, as opposed to sharing its orbit with a multitude of similar-sized objects.

• Planetary Rotation - The Planets rotate around invisible axes through their centres. A Planet's rotation period is known as its Day. Most of the Planets in the Solar System rotate in the same direction as they orbit the Sun, which is counter-clockwise as seen from above the sun's north pole, the exceptions being Venus and Uranus which rotate clockwise; Uranus has a retrograde rotation relative to its orbit. There is a great variation in the length of day between the Planets. Venus takes 243 Earth days to rotate while the Gas Giants take only a few hours.

• Occurrence of Geological Processes - Planets also share certain Geological Processes like the occurrence of Volcanoes, Hurricanes, Tectonics and even Hydrology. Planetary Atmospheres are affected by the varying degrees of energy received from either the Sun or their interiors, leading to the formation of dynamic weather systems such as Hurricanes on Earth, Planet-wide Dust Storms on Mars, an Earth-sized Anticyclone on Jupiter called the Great Red Spot, and holes in the Atmosphere on Neptune.

Size - Planet Jupiter is the largest, at 318 Earth masses, while Mercury is smallest, at 0.055 Earth masses.

Origin and Evolution of Planets

It is not clear as to how Planets formed. According to the prevalent theory - Planets form during the collapse of a Nebula (interstellar cloud of dust) into a thin disk of gas and dust. A Protostar forms, which actually is a large mass that forms by contraction out of the gas of a giant molecular cloud in the interstellar medium. It is surrounded by a rotating Protoplanetary Disk. Through Accretion  (a process of sticky collision) dust particles in the disk steadily accumulate mass to form ever-larger bodies. Local concentrations of mass known as Planetesimals form, and these accelerate the Accretion process by drawing in additional material by their gravitational attraction. These concentrations become ever denser until they collapse inward under gravity to form Protoplanets. A Protoplanet is an earliest form of a Planet, approximately the size of the Moon. After a Planet reaches a diameter larger than the Earth's Moon, it begins to accumulate an extended Atmosphere, greatly increasing the capture rate of the Planetesimals (solid objects formed of dust and cloud) by means of Atmospheric drag.

The Protostar then grows in a manner which enables it's ignition to form a Star. The surviving disk is removed from the inside outward by Photo evaporation, the solar wind, Poynting-Robertson drag and other effects. Thereafter there still may be many Protoplanets orbiting the Star or each other, but over time many will collide, either to form a single larger Planet or release material for other larger Protoplanets or Planets to absorb. Those objects that have become massive enough will capture most matter in their orbital neighbourhoods to become Planets. The energetic impacts of the smaller Planetesimals (as well as radioactive decay) will heat up the growing Planet, causing it to at least partially melt. The interior of the Planet begins to differentiate by mass, developing a denser core. Smaller Terrestrial Planets lose most of their Atmospheres because of this Accretion, but the lost gases can be replaced by out gassing from the mantle and from the subsequent impact of Comets. Smaller Planets will lose any Atmosphere they gain through various escape mechanisms.