A proplyd in the Orion Nebula. The Orion Nebula (also known as Messier 42, M42, or NGC 1976) is a Diffuse nebula situated south of Orion 's Belt

The protoplanetary disk HH-30 in Taurus, about 450 light years away. Herbig-Haro objects are small patches of nebulosity associated with newly-born Stars and are formed when Gas ejected by young Stars collides Taurus (it looks like a bull (ˈtɔrəs bull, symbol, Unicode ♉ is one of the Constellations of the Zodiac. The disk emits the reddish stellar jet, a common structure of these formations. Herbig-Haro objects are small patches of nebulosity associated with newly-born Stars and are formed when Gas ejected by young Stars collides

An artist's impression of protoplanetary disk.

A protoplanetary disk (or proplyd) is a rotating disk of dense gas surrounding a young newly formed star, a T Tauri star or Herbig star. T Tauri stars ( TTS) are a class of variable Stars named after their prototype – T Tauri. Herbig Ae/Be stars are Pre-main sequence stars - young ( Spectral types A and B The protoplanetary disk may be considered an accretion disk because gaseous material may be falling from the inner edge of the disk onto the surface of the star, but this process should not be confused with the accretion process thought to build up the planets themselves. An accretion disc (or accretion disk) is a structure (often a Circumstellar disk) formed by diffuse material in orbital motion around a central body

Protoplanetary disks around T Tauri stars differ from the discs surrounding the primary components of close binary systems in their size and temperature. Protoplanetary discs have radii up to 1000 astronomical units and are rather cool. The astronomical unit ( AU or au or au or sometimes ua) is a unit of Length based on the distance from the Earth to the Only their innermost parts reach temperatures above 1000 kelvin. The kelvin (symbol K) is a unit increment of Temperature and is one of the seven SI base units The Kelvin scale is a thermodynamic They are very often accompanied by jets. Herbig-Haro objects are small patches of nebulosity associated with newly-born Stars and are formed when Gas ejected by young Stars collides

Protostars typically form from molecular clouds consisting primarily of molecular hydrogen. See also Solar nebula A molecular cloud, sometimes called a stellar nursery if Star formation is occurring within is a type of Interstellar Hydrogen (ˈhaɪdrədʒən is the Chemical element with Atomic number 1 When a portion of a molecular cloud reaches a critical size, mass, or density, it begins to collapse under its own gravity. The Jeans instability causes the collapse of interstellar gas clouds and subsequent star formation Gravitation is a natural Phenomenon by which objects with Mass attract one another As this collapsing cloud, called a solar nebula, becomes more dense, random gas motions originally present in the cloud average out in favor of the direction of the nebula's net angular momentum. The formation and evolution of the Solar System is estimated to have begun Conservation of angular momentum causes the rotation to increase as the nebula becomes smaller. In Physics, the angular momentum of a particle about an origin is a vector quantity equal to the mass of the particle multiplied by the Cross product of the position This rotation causes the cloud to flatten out - much like forming a flat pizza out of dough - and take the form of a disk. Pizza (ˈpiːtsə, in Italian:) is a popular dish made with an Oven -baked flat generally round Bread that is covered with tomatoes or a tomato-based Dough is a paste made out of any Cereals (grains or leguminous crops by mixing the Flour with a small amount of Water. The initial collapse takes about 100,000 years. After that time the star reaches a surface temperature similar to that of a main sequence star of the same mass and becomes visible. It is now a T Tauri star. Accretion of gas onto the star continues for another 10 million years^[1], before the disk disappears, perhaps being blown away by the young star's solar wind, or perhaps simply ceasing to emit radiation after accretion has ended. The oldest protoplanetary disk ever discovered is 25 million years old^[2].

The nebular hypothesis of solar system formation describes how protoplanetary disks are thought to evolve into planetary systems. The formation and evolution of the Solar System is estimated to have begun Electrostatic and gravitational interactions may cause the dust and ice grains in the disk to accrete into planetesimals. Planetesimals are solid objects thought to exist in Protoplanetary disks and in Debris disks A widely accepted theory of planet formation the so-called planetesimal This process competes against the stellar wind, which drives the gas out of the system, and accretion, which pulls material into the central T Tauri star. A stellar wind is a flow of neutral or charged gas ejected from the upper atmosphere of a Star.

Protoplanetary disks have been observed around several young stars in our galaxy. Recent observations by the Hubble Space Telescope have shown proplyds and planetary discs to be forming within the Orion Nebula. The Hubble Space Telescope ( HST; also known colloquially as "the Hubble" or just "Hubble" is a space telescope that was carried into The Orion Nebula (also known as Messier 42, M42, or NGC 1976) is a Diffuse nebula situated south of Orion 's Belt

Astronomers have discovered large discs of material, which may themselves be protoplanetary discs, around the stars Vega, Alphecca and Fomalhaut, all of which are very close to the Sun. Vega (α Lyr / α Lyrae / Alpha Lyrae ( or) is the brightest Star in the Constellation Lyra, the fifth brightest star in the night Alpha Coronae Borealis (α CrB / α Coronae Borealis is a Binary star in the Constellation Corona Borealis. Fomalhaut (α PsA / α Piscis Austrini / Alpha Piscis Austrini is the brightest star in the Constellation Piscis Austrinus and one of the brightest

The Castor co-moving group of stars containing Vega and Fomalhaut has recently been isolated. Using data from the Hipparcos satellite telescope the Castor group was found to have an estimated age of 200 ± 100 million years. This indicates that the infrared excesses seen around Vega and Fomalhaut are likely due to a disk of debris from colliding planetesimals rather than a protoplanetary disk. An infrared excess is a measurement of an astronomical source typically a Star, that has a greater measured Infrared flux than expected by assuming the Successful imaging of Fomalhaut's disk by the Hubble Space Telescope confirms this.

Naming confusion

The name protoplanetary nebula is sometimes employed (for example in Davis 2006) when discussing protoplanetary disks. However, this can lead to confusion with the same term subsequently being also employed when discussing the unrelated concept of planetary nebulae. A planetary nebula is an Emission nebula consisting of a glowing shell of Gas and plasma formed by certain types of Stars when they die (See protoplanetary nebula for more information on naming. A protoplanetary nebula or preplanetary nebula (PPN is an Astronomical object which is at the short-lived episode during a Star 's rapid Stellar )

Water

Water is the only known substance found in planetary disks in both the solid (ice) and gaseous phases together in large quantities. Consequently, determining their relative proportions is useful in characterizing the physical state of a nebula and the planet formation process. Typical protoplanetary disks are composed of a wide range of densities and temperatures which results in an array of gas/ice ratios. Ice predominates far and mid nebula while gaseous water tends to dominate the centerplane area of the near nebula and above the disk photosphere. (Davis 2006)

See also

• Planetary formation
• Formation and evolution of the Solar System
• Debris disk

References