Galaxy, Planets etc

The defining dynamic characteristic of a planet is that it has cleared its neighborhood. A planet that has cleared its neighborhood 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. This characteristic was mandated as part of the IAU’s official definition of a planet in August, 2006. This criterion excludes such planetary bodies as Pluto, Eris and Ceres from full-fledged planethood, making them instead dwarf planets.Although to date this criterion only applies to our Solar System, a number of young extrasolar systems have been found in which evidence suggests orbital clearing is taking place within their circumstellar discs.

Planets also 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; changes to the climate over the course of its year. The point at which each hemisphere is farthest or nearest from its star is known as its solstice. Each planet has two in the course of its orbit; when one hemisphere has its summer solstice, when its day is longest, the other has its winter solstice, when its day is shortest. Jupiter’s axial tilt is very small, so its seasonal variation is minimal; Uranus, on the other hand, has an axial tilt so extreme it is virtually on its side, which means that its hemispheres are either perpetually in sunlight or perpetually in darkness around the time of its solstices.Among extrasolar planets, axial tilts are not known for certain, though most hot Jupiters are believed to possess negligible to no axial tilt, as a result of their proximity to their stars.

The planets also rotate around invisible axes through their centres. A planet’s rotation period is known as its day. All planets in the Solar System rotate in a counter-clockwise direction, except for Venus, which rotates clockwise(Uranus is generally said to be rotating clockwise as wellthough because of its extreme axial tilt, it can be said to be rotating either clockwise or anti-clockwise, depending on whether one states it to be inclined 82° from the ecliptic in one direction, or 98° in the opposite direction). There is great variation in the length of day between the planets, with Venus taking 243 Earth days to rotate, and the gas giants only a few hours.The rotational periods of extrasolar planets are not known; however their proximity to their stars means that hot Jupiters are tidaly locked (their orbits are in sync with their rotations). This means they only ever show one face to their stars, with one side in perpetual day, the other in perpetual night.

Minor planets, or asteroids or planetoids, are minor celestial bodies of the Solar system orbiting the Sun (mostly Small solar system bodies) that are smaller than major planets, but larger than meteoroids (commonly defined as being 10 meters across or less, and that are not comets. The distinction is made on visual appearance when discovered: comets must show a perceptible coma, and they get listed in their own catalogs. Minor planets in contrast appear star-like (”asteroid”, from Greek αστεροειδής, asteroeidēs = star-like, star-shaped, from ancient Greek Aστήρ, astēr = star); they get a provisional designation by year in the order of discovery, and a designation (a sequential number) and name if their existence is well established and an orbit has been determined. Their physical nature often remains poorly known.

The first named minor planet was Ceres, discovered in 1801 by Giuseppe Piazzi which was originally considered a new planet, and is now classified as a dwarf planet. Sir William Herschel (discoverer of Uranus), coined the term asteroid for the first objects discovered in the 19th century, all of which orbit the sun between Mars and Jupiter, and generally in relatively low-eccentricity (i.e., not very elongated) orbits. But since then, minor planets have been found to cross the orbits of planets, from Mercury to Neptune — with hundreds of trans-Neptunian objects (TNOs) now known to exist well past Neptune’s orbit.

Though the main distinction between a minor planet and a comet lies in the fact that comets show a coma (or atmosphere) and/or a tail, due primarily to sublimation of ices by solar radiation, one can justifiably consider comets to be a subset of the large group known as minor planets. A few objects have ended up being dual-listed because they were first classified as minor planets but later showed evidence of cometary activity. Conversely, some (perhaps all) comets eventually are depleted of their volatile ices and then appear as pointlike objects, i.e. asteroids. The outermost regions of the solar system are also believed to contain a cloud of dormant comets, and the closer Trans-neptunian objects that have been discovered may not be fundamentally different from giant proto-comets.

Minor planets are divided into groups and families based on their orbital characteristics. Apart from the broadest divisions, it is customary to name a group of asteroids after the first member of that group to be discovered (often the largest). While so-called groups are relatively loose dynamical associations, families are much “tighter” and result usually from the catastrophic breakup of a large parent asteroid sometime in the past. Families have only been recognized within the main asteroid belt. They were first recognised by Kiyotsugu Hirayama in 1918 and are often called Hirayama families in his honor. The term planetule was also coined by the geologist Conybeare to describe minor planets

Planets in our Solar System possess orbital resonances in their own right. All except Mercury and Venus have natural satellites, often called “moons.” Earth has one, and Mars has two, and the gas giants have numerous moons in complex planetary systems. Many gas giant moons have similar features to the terrestrial planets and dwarf planets, and some have been studied for signs of life (especially Europa).The four gas giants are also orbited by planetary rings of varying size and complexity. The rings are composed primarily of dust or particulate matter, but can host tiny ‘moonlets’ whose gravity shapes and maintains their structure. Although the origins of planetary rings is not precisely known, they are believed to be the result of natural satellites that fell below their parent planet’s Roche limit and were torn apart by tidal forces.No secondary characteristics have been observed around extrasolar planets. However the sub-brown dwarf Cha 110913-773444, which has been described as a rogue planet, is believed to be orbited by a tiny protoplanetary disc.

One important characteristic of the planets is their intrinsic magnetic moments which in turn give rise to magnetospheres. The presence of a magnetic field indicates that the planet is still geologically alive. In other words, magnetized planets have flows of electrically conducting material in their interiors, which generate their magnetic fields. These fields significantly change the interaction of the planet and solar wind. A magnetized planet creates a cavity around itself called magnetosphere, which the solar wind can not penetrate. The size of the magnetosphere can be much larger than that of the planet itself. In contrast, non-magnetized planets have only small magnetospheres induced by interaction of the ionosphere with the solar wind, which can not effectively protect the planet.Of the eight planets in our Solar System, only Venus and Mars lack such a magnetic field.In addition, the moon of Jupiter Ganymede also has one. Of the magnetized planets Mercury has the weakest magnetic field, and is barely enough to deflect the solar wind. Ganymede’s magnetic field is several times larger, and Jupiter’s is the strongest in the Solar System. The magnetic fields of other giant planets are roughly similar in strength of that of Earth but their magnetic moments are significantly larger than the Earth’s magnetic moment. The magnetic fields of Uranus and Neptune are strongly tilted relative the rotational axis and displaced from the centre of the planet.In 2004, a team of astronomers in Hawaii observed an extrasolar planet around the star HD 179949, which appeared to be creating a sunspot on the surface of its parent star. The team hypothesised that the planet’s magnetosphere was transferring energy onto the star’s surface increasing its already high 14,000 degree surface temperature by an additional 750 degrees.

A rogue planet is an object which has equivalent mass to a planet and is not gravitationally bound to any star, and that therefore moves through space as an independent object. Several astronomers claim to have detected such objects (for example, S Ori 70), but those detections remain unconfirmed.Some astronomers refer to these objects as “planets”, usually because they believe such objects were planets that were ejected from orbit around a star. However, others believe that the definition of ‘planet’ should depend on current observable state, and not origin. Additionally, these objects may form on their own through gas cloud collapse similar to star formation; in which case they would never have been planets.

In astronomy, the naked-eye planets are the five planets of our solar system that can be discerned with the naked eye without much difficulty. Hence, they were the only planets known to the ancients prior to the invention of the telescope. They are Mercury, Venus, Mars, Jupiter, and Saturn, the five planets closest to Earth. The name planet comes from the Greek term πλανήτης, planētēs, meaning “wanderer”, as ancient astronomers noted how certain lights moved across the sky in relation to the other stars. They called these objects asteres planetai, or wandering stars. In some cultures, the Sun and Moon have also been counted as planets, to make the seven classical planets.Mercury and Venus are only visible in twilight hours as their orbits are interior to the Earth’s orbit. Venus is the most prominent planet, being the third brightest object in the sky after the Sun and the Moon. Mercury is more difficult to see due to its unwavering proximity to the Sun. Lengthy twilight and an extremely low angle at maximum elongations make optical filters necessary to see Mercury from extreme northerly locations like Scandinavia or Alaska.Mars is at its brightest when it is in opposition to the Earth, which occurs approximately every two years. Jupiter and Saturn are the largest of the five planets, but are further from the sun, and therefore receive less sunlight. Nonetheless, Jupiter is often the next brightest object in the sky after Venus. Saturn’s luminosity is often enhanced by its rings, which reflect light back toward the Earth to varying degrees depending on their inclination to the ecliptic; however, the rings themselves are not visible to the naked eye from the Earth. Uranus and the asteroid Vesta are visible to the naked eye in principle on very clear nights, but unlike the true naked-eye planets they are always less luminous than several thousands of stars, and as such, do not stand out enough for their existences to be noticed without the aid of a telescope.

With the discovery during the latter half of the 20th century of more objects within the Solar System and large objects around other stars, disputes arose over what should constitute a planet. There was particular disagreement over whether an object should be considered a planet if it was part of a distinct population such as a belt, or if it was large enough to generate energy by the thermonuclear fusion of deuterium.In 2003, The International Astronomical Union (IAU) Working Group on Extrasolar Planets made a position statement on the definition of a planet that incorporated a working definition:
1. Objects with true masses below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 times the mass of Jupiter for objects with the same isotopic abundance as the Sun) that orbit stars or stellar remnants are “planets” (no matter how they formed). The minimum mass and size required for an extrasolar object to be considered a planet should be the same as that used in our Solar System.
2. Substellar objects with true masses above the limiting mass for thermonuclear fusion of deuterium are “brown dwarfs”, no matter how they formed or where they are located.
3. Free-floating objects in young star clusters with masses below the limiting mass for thermonuclear fusion of deuterium are not “planets”, but are “sub-brown dwarfs” (or whatever name is most appropriate).

This definition has since been widely used by astronomers when publishing discoveries in academic journals. Although temporary, it remains an effective, working definition until a more permanent one is formally adopted. Nevertheless, it did not address the dispute over the lower mass limit,and steered clear of the controversy regarding objects within the Solar System.

This matter was finally addressed during the 2006 meeting of the IAU’s General Assembly. After much debate and one failed proposal, the assembly voted to pass a resolution that defined planets within the Solar System.

A planet, as defined by the International Astronomical Union (IAU) for the Solar System, is a body that orbits the Sun, is massive enough to be rounded by its own gravity, and has cleared its neighbouring region of planetesimals.No formal definition has been made for extrasolar planets.The term planet is an ancient one having ties to history, science, myth, and religion. The planets were originally seen as a divine presence; as emissaries of the gods. Even today, many people continue to believe the movement of the planets affects their lives, although such a causation is rejected by the scientific community. As scientific knowledge advanced, the human perception of the planets changed over time, incorporating a number of disparate objects. Even now there is no uncontested definition of what a planet is. In 2006, the IAU officially adopted a resolution defining planets within the Solar System. This definition has been both praised and criticized, and remains disputed by some scientists.The planets were initially thought to orbit the Earth in circular motions; after the development of the telescope, the planets were determined to orbit the Sun, and their orbits were found to be elliptical. As observational tools improved, astronomers saw that, like Earth, the planets rotated around tilted axes and shared such features as ice-caps and seasons. Since the dawn of the Space Age, close observation by probes has found that Earth and the other planets share characteristics such as volcanism, hurricanes, tectonics and even hydrology. Since 1992, through the discovery of hundreds of extrasolar planets (planets around other stars), scientists are beginning to observe similar features throughout the Milky Way Galaxy.Under IAU definitions, there are eight planets in the Solar System (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune) and 277 known extrasolar ones.[3] The Solar System also contains at least three dwarf planets (Ceres, Pluto, and Eris). Many of these planets are orbited by one or more moons, which can be larger than small planets. Planets are generally divided into two main types: large, low-density gas giants and smaller, rocky terrestrials.