Protoplanet
A protoplanet or planetary embryo is an astronomical body originated within a protoplanetary disk that has undergone internal melting to produce a differentiated interior.<ref>Template:Cite journal</ref>
Protoplanets are thought to form out of kilometer-sized planetesimals that gravitationally perturb each other's orbits and collide, gradually coalescing into larger bodies<ref>Template:Cite journal</ref> through a process known as "runaway growth".<ref>Template:Cite book</ref> Once accumulated enough mass, protoplanets will begin to assume a spherical shape due to hydrostatic equilibrium and become dwarf planets, those of which that subsequently succeed in dominating their own orbit will become planets 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.<ref>Template:Cite journal</ref>
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.<ref name="Cessna">Template:Cite web</ref><ref name="Ahrens">Template:Cite journal</ref>
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 planets proper.<ref>Template:Cite book</ref>
Early protoplanets had more radioactive elements,<ref>Template:Cite web</ref> 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 asteroids.
Evidence in the Solar System - surviving remnant protoplanets
In the case of the Solar System, it is thought that the collisions of planetesimals created a few hundred planetary embryos. Such embryos were similar to Ceres and Pluto with masses of about 1022 to 1023 kg and were a few thousand kilometers in diameter.Template:Fact
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.<ref name="Nace2016">Template:Cite web</ref><ref>Template:Cite journal</ref><ref>Template:Cite web</ref>
In the inner Solar System, the three protoplanets to survive more-or-less intact are the asteroids 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.<ref name=NASA15-196>Template:Cite web</ref> The asteroid Metis may also have a similar origin history to that of Psyche.<ref name="Kelley00">Template:Cite journal</ref> The asteroid Lutetia also has characteristics that resemble a protoplanet.<ref>Template:Cite web</ref><ref>Template:Cite web</ref> Kuiper-belt dwarf planets have also been referred to as protoplanets.<ref name=msnbc>Template:Cite web</ref> Because iron meteorites 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.Template:Fact
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.<ref name=":7" /> Subsequent observations, however, refuted the existence of this candidate.<ref name=":8" />
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.<ref>Template:Cite web</ref><ref name="quanz13">Template:Cite journal</ref> Subsequent observations suggest that several protoplanets may be present in the gas disk.<ref>Template:Cite journal</ref>
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.<ref name="CBC">Template:Cite news</ref>
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.<ref name=":3">Template:Cite journal</ref> 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.<ref name=":3" /> HD 97048 b is the first protoplanet detected by disk kinematics in the form of a kink in the gas velocity map.<ref>Template:Cite journal</ref>
| Star | Exoplanet | Mass (Template:Jupiter mass) |
Period (yr) |
Separation (AU) |
Distance to Earth (Parsec) |
Year of Discovery | Detection technique |
|---|---|---|---|---|---|---|---|
| PDS 70 | PDS 70 b | Template:Val | 119 | 20 ± 2 | 112<ref name=":1">Template:Cite journal</ref> | 2018<ref name=":2">Template:Cite web</ref> | Direct Imaging |
| PDS 70 c | Template:Val | 227<ref name=":0">Template:Cite web</ref> | 34 Template:± | 112 | 2019<ref name=":2" /> | Direct Imaging | |
| HD 97048 | HD 97048 b | 2.5 ± 0.5 | 956<ref name=":0" /> | 130 | 184<ref name=":1" /> | 2019<ref>Template:Cite web</ref> | Disk Kinematics |
| HD 169142 | HD 169142 b | 3 ± 2 | 167<ref name=":0" /> | 37.2± 1.5 | 114 | 2019<ref>Template:Cite journal</ref>/2023<ref name=":5">Template:Cite journal</ref> | Direct imaging |
| TYC 5709-354-1 | WISPIT 2b | 5.3 ± 1.0 | 54 | 133 | 2025<ref name="van Capelleveen2025">Template:Cite journal</ref><ref name="Close2025">Template:Cite journal</ref> | Direct imaging |
Unconfirmed protoplanets
The confident detection of protoplanets is difficult. Protoplanets usually exist in gas-rich protoplanetary disks. Over-densities within these disks can mimic protoplanets. A number of unconfirmed protoplanet candidates are known and some detections were later questioned.
| Star/host | Exoplanet | Mass (Template:Jupiter 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<ref>Template:Cite journal</ref> | unconfirmed planetary mass and formation in disk | Direct imaging | |
| GQ Lupi | GQ Lupi b | 1–36 | 103 | 152 | 2005<ref>Template:Cite journal</ref> | unconfirmed planetary mass and formation in disk | Direct imaging | |
| 2M1207 | 2M1207b | 5–6 | 49.8 | 65 | 2005<ref>Template:Cite journal</ref> | unconfirmed formation in disk | Direct imaging | |
| LkCa 15 | LkCa 15 b | 12.7 | 2012<ref name=":7">Template:Cite journal</ref> | refuted in 2019<ref name=":8">Template:Cite journal</ref> | Direct imaging | |||
| LkCa 15 c | 18.6 | 2015<ref name=":6">Template:Cite journal</ref> | Direct imaging | |||||
| LkCa 15 d | 24.7 | 2015<ref name=":6" /> | Direct imaging | |||||
| HD 100546 | HD 100546 b | 4–13<ref>Template:Cite journal</ref> | 249<ref name=":0" /> | 53 ± 2 | 108<ref name=":1" /> | 2015<ref>Template:Cite web</ref> | disputed in 2017<ref>Template:Cite journal</ref> | Direct imaging |
| Gomez's Hamburger | GoHam b | Template:Nowrap | 350 ± 50 | 250 | 2015<ref>Template:Cite journal</ref> | unconfirmed candidate | Direct imaging | |
| AB Aurigae | AB Aur b | 9–20 | 94 ± 49 | 156<ref name=":1" /> | 2022<ref>Template:Cite web</ref> | disputed in 2023<ref>Template:Cite journal</ref> and 2024<ref>Template:Cite journal</ref> | Direct imaging | |
| IM Lupi | 2–3 | 110 | 2022<ref>Template:Cite journal</ref> | unconfirmed candidate | Disk Kinematics | |||
| HD 163296 | multiple?<ref>Template:Cite journal</ref> | 2022<ref name=":4">Template:Cite journal</ref> | unconfirmed candidates | Disk Kinematics | ||||
| Elias 2-24 | 2–5 | 52 | 2023<ref>Template:Cite journal</ref> | unconfirmed candidate | Direct imaging + Disk Kinematics | |||
| 2MJ1612 | 2MJ1612b | 4 | 23.45 ± 0.29 | 132 | 2025<ref name="Li2025">Template:Cite arXiv</ref> | unconfirmed candidate | Direct imaging (ASDI) |
See also
References
External links
- Thread on the definition of a protoplanet (Minor Planet Mailing List : July 15, 2011)