A protoplanet is a large planetary embryo that originated within a protoplanetary disk and has undergone internal melting to produce a differentiated interior. Protoplanets are thought to form out of kilometer-sized planetesimals that gravitationally perturb each other's orbits and collide, gradually coalescing into the dominant

planet A planet is a large, Hydrostatic equilibrium, rounded Astronomical object, astronomical body that is generally required to be in orbit around a star, stellar remnant, or brown dwarf, and is not one itself. The Solar System has eight planets b ...

The planetesimal hypothesis

A planetesimal 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 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 elements, the quantity of which has been reduced over time due to radioactive decay. 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. The composition of some meteorites show that differentiation took place in some

asteroid An asteroid is a minor planet—an object larger than a meteoroid that is neither a planet nor an identified comet—that orbits within the Solar System#Inner Solar System, inner Solar System or is co-orbital with Jupiter (Trojan asteroids). As ...

Evidence in the Solar System - surviving remnant protoplanets

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 "Sola ...

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

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

Moon The Moon is Earth's only natural satellite. It Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...

formed from a colossal impact of a hypothetical protoplanet called Theia 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

s Ceres, Pallas, and Vesta. Psyche 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 may also have a similar origin history to that of Psyche. The asteroid Lutetia also has characteristics that resemble a protoplanet. Kuiper-belt dwarf planets have also been referred to as protoplanets. Because iron meteorites 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, centered on the Sun and roughly spanning the space between the orbits of the planets Jupiter and Mars. It contains a great many solid, irregularly shaped bodies called asteroids ...

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

Extrasolar protoplanets - observed protoplanets

The first directly imaged exoplanet candidates were confirmed in 2005. Several of them are very young, DH Tauri b, GQ Lupi b, 2M1207b and show signs of accretion. However, all these candidates either lack in confirmation of a planetary mass or in confirmation that they formed within the protoplanetary disk of the host object. In January 2012 astronomers made the first direct observation of a candidate protoplanet forming in a disk of gas and dust around a distant star, LkCa 15. Subsequent observations, however, refuted the existence of this candidate. In February 2013 astronomers made the first direct observation of a candidate protoplanet, that is still a candidate, forming in a disk of gas and dust around a distant star, HD 100546. 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 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 (HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the Orbiting Solar Observatory, first space telescope, but it is one of the largest and most ...

. Rings, gaps, spirals, dust concentrations and shadows in

protoplanetary disks 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 not be considered an accretion disk; while the two are si ...

could be caused by protoplanets. These structures are not completely understood and are therefore not seen as a proof for the presence of a protoplanet. One new emerging way to study the effect of protoplanets on the disk are molecular line observations of protoplanetary disks in the form of gas velocity maps. HD 97048 b is the first protoplanet detected by disk

kinematics In physics, kinematics studies the geometrical aspects of motion of physical objects independent of forces that set them in motion. Constrained motion such as linked machine parts are also described as kinematics. Kinematics is concerned with s ...

in the form of a kink in the gas velocity map.

Unconfirmed protoplanets

The confident detection of protoplanets is difficult. Protoplanets usually exist in gas-rich protoplanetary disks. Such disks can produce over-densities by a process called disk fragmentation. Such fragments can be small enough to be unresolved and mimic the appearance of a protoplanet. A number of unconfirmed protoplanet candidates are known and some detections were later questioned.

References

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