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Formation of rock worlds

How rocky planets develop closer to a star, in hotter regions

Introduction: The "terra incognita" of rocky planets

Most Sun-like stars – especially those of medium or low mass – have protoplanetary disks, consisting of gas and dustThey include:

  • Internal areas (by about a few astronomical units) remain hotter due to the star's radiation, so most volatile substances (e.g. water ice) sublimate.
  • Rocky/silicate materials predominate in these inner zones, where they form terrestrial planets, similar to Mercury, Venus, Earth, and Mars in our solar system.

When comparing exoplanets, we see a wide range super-Earths and other rocky planets close to their stars, indicating that such rocky worlds formation is a common and very important phenomenon. From how it unfolds formation of a rocky planet, belongs to living environments, questions of chemical composition and the possible origin of life.


2. Preparation: Conditions on the internal drive

2.1 Temperature gradients and the “snow line”

Stars in the protoplanetary disk radiation determines the temperature gradient. Snow line The frost line is the point at which water vapor can condense into ice. This limit is typically a few AU from a Sun-like star, but it can vary depending on the age of the disk, the intensity of radiation, and the environment:

  • Inside snow lines: Water, ammonia and CO2 remains gaseous, so the dust is mostly composed of silicates, iron, and other refractory minerals.
  • Outside Snow lines: Ice is abundant, allowing solid cores to grow faster and gas/ice giants to form.

So inner terrestrial region at first quite dry in terms of water ice, although some water may have been brought later, from planetesimals that arrived beyond the snowline [1], [2].

2.2 Disk mass density and time scales

Stars accretion disk often have enough solid material to form several rocky planets in the inner region, but how many of them will form or how large they will be depends on:

  • Top layer density solid particles: Higher density promotes faster planetesimal collisions and embryo growth.
  • Disk life span: Usually 3-10 million years before the gas disappears, but the process of forming rocky planets (already without a gas environment) can continue for tens of millions of years as protoplanets collide in a gas-free environment.

Physical factors – viscous evolution, magnetic fields, star radiation – shape the structure and evolution of the disk, defining the conditions under which “rocky bodies” come together.


3. Dust coagulation and planetesimal formation

3.1 Growth of rocky particles in the inner disk

In hotter weather internal In this area, small dust grains (silicates, metal oxides, etc.) collide and stick together, forming aggregates – “pebbles”. But here comes the “meter-sized barrier":

  • Radial drift: Objects of the size of meters move rapidly towards the star due to friction, so they risk being lost before they reach sufficient size.
  • Fission collisions: As speed increases, collisions can destroy accumulations.

Possible solutions to overcome these barriers:

  1. Streaming instability: Excess dust locally leads to gravitational collapse into km-scale planetesimals.
  2. Pressure ridges: Disc sub-seals (gaps, rings) can trap dust and reduce drift, allowing for more efficient growth.
  3. "Pebble" accretion: If a nucleus forms somewhere, it will rapidly "collect" mm-cm pebbles [3], [4].

3.2 The origin of planetesimals

After the formation of kilometer-sized planetesimals, gravitational concentration further accelerates mergers. On the internal disk Planetesimals are usually rocky, composed of iron, silicates, and perhaps small amounts of carbon. Over tens or hundreds of thousands of years, these planetesimals may coalesce into protoplanet, which reach tens or hundreds of kilometers.


4. Evolution of protoplanets and growth of terrestrial planets

4.1 Oligarchic growth

In a theory called oligarchic growth:

  1. Several large protoplanets in the region become gravitationally dominant "oligarchs."
  2. Smaller planetesimals are either scattered or attracted.
  3. Ultimately, several competing protoplanets and smaller remnant bodies remain in the zone.

This stage can last several million years until several Mars-sized whether Moon-sized embryos.

4.2 The phase of major impacts and final deployment

After the gas from the disk dissipates (there is no damping effect and friction left), these protoplanets continue to collide in a chaotic environment:

  • Big hits: In the final stage, sufficiently large collisions may occur, partially melting the mantles, similar to the hypothetical impact of the Moon's origin between proto-Earth and Theia.
  • Long-term: Formation of rocky planets in the Solar System may have taken about 50-100 years million years until the Earth's orbit was finally stabilized after impacts from Mars-sized bodies [5].

During these collisions, additional differentiation of iron-silicates occurs, planetary cores are formed, and material may also be ejected to form satellites (e.g., Earth's Moon) or rings.


5. Composition and volatile water delivery

5.1 Inside the rock formation

Because volatiles evaporate in the inner, warm part of the disk, planets that form there tend to accumulate refractory materials – silicates, iron-nickel metals, etc. This explains the high density and rocky nature of Mercury, Venus, Earth, and Mars (although the composition and iron content of each planet varies, depending on local disk conditions and giant impact histories).

5.2 Water and organic matter

Despite forming inside the snow line, terrestrial planets can still have water if:

  1. Late brings: Planetesimals from the outer disk or asteroid belt are scattered inward.
  2. Small bodies of ice: Comets or C-type asteroids can deliver enough volatile compounds if they are dispersed inward.

Geochemical studies suggest that Earth's water may have originated in part from igneous chondrite bodies, explaining how we still have water in our essentially dry interior. [6].

5.3 Impact on viability

Volatile substances are crucial for oceans, atmospheres, and surfaces suitable for life.The combination of late collisions, melting processes in the mantle, and the ingress of external planetesimal material determines whether a terrestrial planet can have conditions suitable for life.


6. Observational data and insights from exoplanets

6.1 Exoplanet Observations: Super-Earths and Lava Worlds

Exoplanet studies (Kepler, TESS, etc.) have revealed many super-Earths whether mini-Neptune, orbiting close to their stars. Some may be purely rocky but larger than Earth, others have thick atmospheres. Still others – “lava worlds” – are so close to their star that their surfaces may be molten. These discoveries highlight:

  • Disc differences: Slight differences in parameters in the disk lead to different results – from Earth analogs to hot super-Earths.
  • Impact of migration: Some rocky super-Earths may have formed further away and then moved closer to the star.

6.2 Debris disks as evidence of a terrestrial "construction" process

Detecting around older stars debris Disks of dust left behind by collisions between planetesimals or failed rocky protoplanets signal that ongoing small-scale collisions are occurring there. The warm dust rings around mature stars detected by Spitzer and Herschel may resemble our solar system's zodiacal dust belt, indicating the existence of rocky remnants in a phase of slow frictional attrition.

6.3 Geochemical equivalents

Spectroscopic measurements of the atmospheres of white dwarfs, which contain material from disintegrated planetary debris, show elemental compositions similar to rocky (chondritic) components, confirming that the formation of rocky planets in the inner regions is a relatively common phenomenon in stellar systems.


7. Timescales and final configurations

7.1 Accretion schedule

  • Formation of planetesimals: Maybe within 0.1-1 million years under the influence of streaming instability or slow collisions.
  • Formation of protoplanets: Within 1–10 million years, larger bodies begin to dominate, "cleaning up" or absorbing smaller planetesimals.
  • The phase of major shocks: Tens of millions of years before a few final rocky planets finally form. The final major Earth impact (Moon formation) is thought to have occurred ~30–50 million years after the formation of the Sun [7].

7.2 Variability and final architecture

Differences in disk density, the presence of migrating giant planets, or early star–disk interactions can dramatically alter orbits and compositions. In some places, one or no large terrestrial planets may form (as around many M dwarfs?), in others, several super-Earths close to the star may form. Each system has a unique "fingerprint" reflecting its initial environment.


8. The Road to a Rocky Planet

  1. Dust growth: Silicate and metal grains clump together into mm–cm “pebbles” aided by partial adhesion.
  2. Formation of planetesimals: Kilometer-scale bodies form rapidly through streaming instability or other mechanisms.
  3. Protoplanetary accretion: Gravitational impacts of planetesimals produce embryos that reach Mars or the Moon.
  4. The stage of major shocks: A small number of large protoplanets collide, forming the final rocky planets over tens of millions of years.
  5. Volatile compounds delivery: Water and organics from outer disk planetesimals or comets could give the planet oceans and possible life.
  6. Orbital cleaning: Recent collisions, resonant interactions, or scattering events determine the stable orbits and placement of terrestrial worlds in many systems.

9. Future research and missions

9.1 ALMA and JWST disk imaging

High-resolution maps of the disk show rings, gaps, and perhaps the seeds of protoplanets. If dust accumulation Whether spirals are found inside the disk helps us understand how rocky planetesimals are formed. JWST Infrared data allows the detection of spectral signatures of silicates and gaps/rings within the disk, indicating ongoing planet formation processes.

9.2 Characterization of exoplanets

Current and future exoplanet transit/radiation velocity surveys PLATO and Roman Space Telescope The projects will discover more small, potentially terrestrial exoplanets, determine their orbits, densities, and possibly atmospheric signatures. This helps test and refine models of how rocky worlds settle into or enter a star's habitable zone.

9.3 Transporting samples from the internal disk remnants

Missions that study small bodies formed in the inner solar system, e.g. NASA Psyche (metallic asteroid) or other asteroid sample return missions provide chemical insights into the initial composition of planetesimals. When combined with meteorite studies, it becomes clearer how planets formed from the solid particles of the protoplanetary disk.


10. Conclusion

Formation of rocky worlds naturally occurring in hot protoplanetary disks When dust particles and small rock grains coalesce into planetesimals, gravitational interactions cause rapid protoplanetary Over tens of millions of years, these protoplanets collide again and again—sometimes gently, sometimes violently—and form a series of stable orbits that leave behind rocky planets. The introduction of water and the development of atmospheres can make such worlds habitable, as Earth's geological and biological history suggests.

Observations – both in our own Solar System (asteroids, meteorites, planetary geology) and in exoplanet studies – suggest that the phenomenon of rocky planet formation is likely widespread among many stars. With improvements in disk imaging, dust evolution models, and theories of planet-disk interactions, astronomers are gaining a deeper understanding of the cosmic “recipe” for how star-fed dust pools form. Earth-like or other rocky worlds in our Galaxy. Such studies not only unlock the origin story of our planet, but also explain how the potential building blocks of life form around the myriad of other stars in the Universe.


References and further reading

  1. Hayashi, C. (1981). "Structure of the Solar Nebula, Growth and Decay of Magnetic Fields and Effects of Magnetic and Turbulent Viscosities on the Nebula." Progress of Theoretical Physics Supplement, 70, 35–53.
  2. Weidenschilling, S. J. (1977). "Aerodynamics of solid bodies in the solar nebula." Monthly Notices of the Royal Astronomical Society, 180, 57–70.
  3. Johansen, A., & Lambrechts, M. (2017). "Forming Planets via Pebble Accretion." Annual Review of Earth and Planetary Sciences, 45, 359–387.
  4. Morbidelli, A., Lunine, JI, O'Brien, DP, Raymond, SN, & Walsh, KJ (2012). "Building Terrestrial Planets." Annual Review of Earth and Planetary Sciences, 40, 251–275.
  5. Chambers, J. E. (2014). "Planetary accretion in the inner Solar System." Icarus, 233, 83–100.
  6. Raymond, SN, & Izidoro, A. (2017). "The empty primordial asteroid belt and the role of Jupiter's growth." Icarus, 297, 134–148.
  7. Kleine, T., et al. (2009). "Hf–W chronology of meteorites and the timing of terrestrial planet formation." Geochimica et Cosmochimica Acta, 73, 5150–5188.
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