A protoplanet is a large planetary embryo that originated within a

protoplanetary disc A protoplanetary disk is a rotating circumstellar disc of dense gas and dust surrounding a young newly formed star, a T Tauri star, or Herbig Ae/Be star. The protoplanetary disk may also be considered an accretion disk for the star itself, be ...

and has undergone internal melting to produce a differentiated interior. Protoplanets are thought to form out of kilometer-sized

planetesimal Planetesimals are solid objects thought to exist in protoplanetary disks and debris disks. Per the Chamberlin–Moulton planetesimal hypothesis, they are believed to form out of cosmic dust grains. Believed to have formed in the Solar System a ...

s that gravitationally perturb each other's orbits and collide, gradually coalescing into the dominant

planet A planet is a large, rounded astronomical body that is neither a star nor its remnant. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud collapses out of a nebula to create a you ...

The planetesimal hypothesis

is an object formed from dust, rock, and other materials, measuring from meters to hundreds of kilometers in size. According to the Chamberlin–Moulton planetesimal hypothesis and the theories of Viktor Safronov, a protoplanetary disk of materials such as gas and dust would orbit a star early in the formation of a planetary system. The action of

gravity In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stro ...

on such materials form larger and larger chunks until some reach the size of planetesimals. It is thought that the collisions of planetesimals created a few hundred larger planetary embryos. Over the course of hundreds of millions of years, they collided with one another. The exact sequence whereby planetary embryos collided to assemble the planets is not known, but it is thought that initial collisions would have replaced the first "generation" of embryos with a second generation consisting of fewer but larger embryos. These in their turn would have collided to create a third generation of fewer but even larger embryos. Eventually, only a handful of embryos were left, which collided to complete the assembly of the

s proper. Early protoplanets had more

radioactive Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is consid ...

elements, the quantity of which has been reduced over time due to

radioactive decay Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is consid ...

. Heating due to radioactivity, impact, and gravitational pressure melted parts of protoplanets as they grew toward being planets. In melted zones their heavier elements sank to the center, whereas lighter elements rose to the surface. Such a process is known as

planetary differentiation In planetary science, planetary differentiation is the process by which the chemical elements of a planetary body accumulate in different areas of that body, due to their physical or chemical behavior (e.g. density and chemical affinities). The ...

. The composition of some

meteorite A meteorite is a solid piece of debris from an object, such as a comet, asteroid, or meteoroid, that originates in outer space and survives its passage through the atmosphere to reach the surface of a planet or Natural satellite, moon. When the ...

s show that differentiation took place in some

asteroid An asteroid is a minor planet of the inner Solar System. Sizes and shapes of asteroids vary significantly, ranging from 1-meter rocks to a dwarf planet almost 1000 km in diameter; they are rocky, metallic or icy bodies with no atmosphere. ...

Evidence of protoplanets in the Solar System

In the case of the

Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar S ...

, it is thought that the collisions of planetesimals created a few hundred planetary embryos. Such embryos were similar to

Ceres Ceres most commonly refers to: * Ceres (dwarf planet), the largest asteroid * Ceres (mythology), the Roman goddess of agriculture Ceres may also refer to: Places Brazil * Ceres, Goiás, Brazil * Ceres Microregion, in north-central Goiás ...

and

Pluto Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of trans-Neptunian object, bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the S ...

with masses of about 10²² to 10²³ kg and were a few thousand kilometers in diameter. According to the

giant impact hypothesis The giant-impact hypothesis, sometimes called the Big Splash, or the Theia Impact, suggests that the Moon formed from the ejecta of a collision between the proto-Earth and a Mars-sized planet, approximately 4.5 billion years ago, in the Hadean ...

the

Moon The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...

formed from a colossal impact of a hypothetical protoplanet called

Theia In Greek mythology, Theia (; grc, Θεία, Theía, divine, also rendered Thea or Thia), also called Euryphaessa ( grc, Εὐρυφάεσσα) "wide-shining", is one of the twelve Titans, the children of the earth goddess Gaia and the Sky deity ...

with Earth, early in the Solar System's history. In the inner Solar System, the three protoplanets to survive more-or-less intact are the

Pallas Pallas may refer to: Astronomy * 2 Pallas asteroid ** Pallas family, a group of asteroids that includes 2 Pallas * Pallas (crater), a crater on Earth's moon Mythology * Pallas (Giant), a son of Uranus and Gaia, killed and flayed by Athena * Pa ...

, and Vesta.

Psyche Psyche (''Psyché'' in French) is the Greek term for "soul" (ψυχή). Psyche may also refer to: Psychology * Psyche (psychology), the totality of the human mind, conscious and unconscious * ''Psyche'', an 1846 book about the unconscious by Car ...

is likely the survivor of a violent hit-and-run with another object that stripped off the outer, rocky layers of a protoplanet. The asteroid

Metis Metis or Métis may refer to: Ethnic groups * Métis, recognized Indigenous communities in Canada and America whose distinct culture and language emerged after early intermarriage between First Nations peoples and early European settlers, prima ...

may also have a similar origin history to that of Psyche. The asteroid

Lutetia The Gallo-Roman town of ''Lutetia'' (''Lutetia Parisiorum'' in Latin, in French ''Lutèce'') was the predecessor of the modern-day city of Paris. It was founded in about the middle of the 3rd century BCE by the Parisii, a Gallic tribe. Trac ...

also has characteristics that resemble a protoplanet. Kuiper-belt

dwarf planet A dwarf planet is a small planetary-mass object that is in direct orbit of the Sun, smaller than any of the eight classical planets but still a world in its own right. The prototypical dwarf planet is Pluto. The interest of dwarf planets to p ...

s have also been referred to as protoplanets. Because

iron meteorite Iron meteorites, also known as siderites, or ferrous meteorites, are a type of meteorite that consist overwhelmingly of an iron–nickel alloy known as meteoric iron that usually consists of two mineral phases: kamacite and taenite. Most iron met ...

s have been found on Earth, it is deemed likely that there once were other metal-cored protoplanets in the

asteroid belt The asteroid belt is a torus-shaped region in the Solar System, located roughly between the orbits of the planets Jupiter and Mars. It contains a great many solid, irregularly shaped bodies, of many sizes, but much smaller than planets, called ...

that since have been disrupted and that are the source of these meteorites.

Observed protoplanets

In February 2013 astronomers made the first direct observation of a protoplanet forming in a disk of gas and dust around a distant star,

HD 100546 HD 100546, is a star 316.4 light-years from Earth. It is orbited by an approximately exoplanet at 6.5 AU, although further examination of the disk profile indicate it might be a more massive object such as a brown dwarf or more th ...

. Subsequent observations suggest that several protoplanets may be present in the gas disk. Another protoplanet, AB Aur b, may be in the earliest observed stage of formation for a gas giant. It is located in the gas disk of the star

AB Aurigae AB Aurigae is a young Herbig Ae star in the Auriga constellation. It is located at a distance of approximately 531 light years from the Sun based on stellar parallax. This pre-main-sequence star has a stellar classification of A0 ...

. AB Aur b is among the largest exoplanets identified, and has a distant orbit, three times as far as Neptune is from the Earth's sun. Observations of AB Aur b may challenge conventional thinking about how planets are formed. It was viewed by the Subaru Telescope and the

Hubble Space Telescope The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the first space telescope, but it is one of the largest and most versa ...

References

External links

Thread on the definition of a protoplanet
(Minor Planet Mailing List : July 15, 2011) {{Portal bar, Astronomy, Stars, Spaceflight, Outer space, Solar System Types of planet

The planetesimal hypothesis

Evidence of protoplanets in the Solar System

Observed protoplanets

See also

References

External links