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A protoplanet or planetary embryo is a moon-sized astronomical body originated within a protoplanetary disk that has undergone partial melting to produce a differentiated interior, and has the potential to further evolve into a planetary-mass object. Protoplanets are thought to form out of kilometer-sized that gravitationally perturb each other's orbits and collide, gradually coalescing into larger bodies through a process known as "runaway growth". Once accumulated enough mass, protoplanets will begin to assume a spherical shape due to hydrostatic equilibrium and become , those of which that subsequently succeed in dominating their own orbit will become proper.
An alternative formation pathway of protoplanets is a process called disk fragmentation. Formation by this process, also called gravitational (disk) instability, is favoured for giant planets on wide orbits.
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 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 Chemical element sank to the center, whereas lighter elements rose to the surface. Such a process is known as planetary differentiation. The composition of some show that differentiation took place in some .
According to the giant impact hypothesis, the Moon 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 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 have also been referred to as protoplanets. Because have been found on Earth, it is deemed likely that there once were other metal-cored protoplanets in the asteroid belt that since have been disrupted and that are the source of these meteorites.
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.
Rings, gaps, spirals, dust concentrations and shadows in protoplanetary 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. 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 the form of a kink in the gas velocity map.
| +List of confirmed protoplanets (described as "protoplanets" in literature)
!Star
!Exoplanet
!Mass () !Period (yr) !Separation (AU) !Distance to Earth (Parsec) !Year of Discovery !Detection technique | |||||||
| PDS 70 | PDS 70 b | 119 | 20 ± 2 | 112 | 2018 | Direct Imaging | |
| PDS 70 c | 227 | 34 | 112 | 2019 | Direct Imaging | ||
| HD 97048 | HD 97048 b | 2.5 ± 0.5 | 956 | 130 | 184 | 2019 | Disk Kinematics |
| HD 169142 | HD 169142 b | 3 ± 2 | 167 | 37.2± 1.5 | 114 | 2019/2023 | Direct imaging |
| +List of unconfirmed/disputed/refuted protoplanets
!Star/host
!Exoplanet
!Mass () !Period (yr) !Separation (AU) !Distance to Earth (Parsec) !Year of Discovery !Status !Detection technique | ||||||||
| DH Tauri | DH Tauri b | 8-50 | 330 | 135 | 2005 | unconfirmed planetary mass and formation in disk | Direct imaging | |
| GQ Lupi | GQ Lupi b | 1-36 | 103 | 152 | 2005 | unconfirmed planetary mass and formation in disk | Direct imaging | |
| 2M1207 | 2M1207b | 5-6 | 49.8 | 65 | 2005 | unconfirmed formation in disk | Direct imaging | |
| LkCa 15 | LkCa 15 b | 12.7 | 2012 | refuted in 2019 | Direct imaging | |||
| LkCa 15 c | 18.6 | 2015 | Direct imaging | |||||
| LkCa 15 d | 24.7 | 2015 | Direct imaging | |||||
| HD 100546 | HD 100546 b | 4–13 | 249 | 53 ± 2 | 108 | 2015 | disputed in 2017 | Direct imaging |
| Gomez's Hamburger | GoHam b | 350 ± 50 | 250 | 2015 | unconfirmed candidate | Direct imaging | ||
| AB Aurigae | AB Aur b | 9–20 | 94 ± 49 | 156 | 2022 | disputed in 2023 and 2024 | Direct imaging | |
| IM Lupi | 2–3 | 110 | 2022 | unconfirmed candidate | Disk Kinematics | |||
| HD 163296 | multiple? | 2022 | unconfirmed candidates | Disk Kinematics | ||||
| Elias 2-24 | 2–5 | 52 | 2023 | unconfirmed candidate | Direct imaging + Disk Kinematics |
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