Aprotoplanet is a large planetary embryo that originated within aprotoplanetary disk and has undergone internal melting to produce a differentiated interior. Protoplanets are thought to form out of kilometer-sizedplanetesimals that gravitationally perturb each other's orbits and collide, gradually coalescing into the dominantplanets.
Aplanetesimal is an object formed from dust, rock, and other materials, measuring from meters to hundreds of kilometers in size. According to theChamberlin–Moulton planetesimal hypothesis and the theories ofViktor 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 ofgravity on such materials form larger and larger chunks until some reach the size of planetesimals.[1][2]
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 theplanets proper.[3]
Early protoplanets had moreradioactive elements,[4] the quantity of which has been reduced over time due toradioactive decay. Heating due to radioactivity, impact, and gravitational pressure melted parts of protoplanets as they grew toward being planets. In melted zones their heavierelements sank to the center, whereas lighter elements rose to the surface. Such a process is known asplanetary differentiation. The composition of somemeteorites show that differentiation took place in someasteroids.
Evidence in the Solar System - surviving remnant protoplanets
In the case of theSolar System, it is thought that the collisions of planetesimals created a few hundred planetary embryos. Such embryos were similar toCeres andPluto with masses of about 1022 to 1023 kg and were a few thousand kilometers in diameter.[citation needed]
In the inner Solar System, the three protoplanets to survive more-or-less intact are theasteroidsCeres,Pallas, andVesta.Psyche is likely the survivor of a violent hit-and-run with another object that stripped off the outer, rocky layers of a protoplanet.[8] The asteroidMetis may also have a similar origin history to that of Psyche.[9] The asteroidLutetia also has characteristics that resemble a protoplanet.[10][11]Kuiper-beltdwarf planets have also been referred to as protoplanets.[12] Becauseiron meteorites have been found on Earth, it is deemed likely that there once were other metal-cored protoplanets in theasteroid belt that since have been disrupted and that are the source of these meteorites.[citation needed]
In February 2013 astronomers made the first direct observation of a candidate protoplanet forming in a disk of gas and dust around a distant star,HD 100546.[13][14] Subsequent observations suggest that several protoplanets may be present in the gas disk.[15]
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 starAB 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 theSubaru Telescope and theHubble Space Telescope.[16]
Rings, gaps, spirals, dust concentrations and shadows inprotoplanetary disks 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.[17] One new emerging way to study the effect of protoplanets on the disk aremolecular line observations of protoplanetary disks in the form of gas velocity maps.[17]HD 97048 b is the first protoplanet detected by diskkinematics in the form of a kink in the gas velocity map.[18]
List of confirmed protoplanets (described as "protoplanets" in literature)
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.[25] A number of unconfirmed protoplanet candidates are known and some detections were later questioned.
^McBride, Neil; Iain Gilmour; Philip A. Bland; Elaine A. Moore; Mike Widdowson; Ian Wright (2004).An Introduction to the Solar System. Cambridge:Cambridge University Press. p. 56.ISBN9780521837354.
