How rocky planets develop close to the star, in hotter regions
Introduction: The "terra incognita" of rocky planets
Most Sun-type stars – especially those of medium or low mass – have protoplanetary disks made of gas and dust. Within them:
- Inner regions (roughly within a few astronomical units) remain hotter due to stellar radiation, so most volatile materials (e.g., water ice) sublimate.
- Rocky/silicate materials dominate in these inner zones where terrestrial planets form, similar to Mercury, Venus, Earth, and Mars in our Solar System.
Comparing exoplanets, we see a wide range of super-Earths and other rocky planets close to their stars, indicating that the formation of such rocky worlds is common and very important. The way rocky planet formation unfolds determines questions of habitable environments, chemical composition, and possible origins of life.
2. Preparation: conditions in the inner disk
2.1 Temperature gradients and the "snow line"
In the protoplanetary disk, stellar radiation sets the temperature gradient. The snow line (frost line) is the location where water vapor can condense into ice. Usually, this boundary is a few AU from a Sun-like star, but it can vary depending on disk age, radiation intensity, and environment:
- Inside the snow line: Water, ammonia, and CO2 remain gaseous, so dust mostly consists of silicates, iron, and other refractory minerals.
- Outside the snow line: Ice is abundant, allowing faster growth of solid cores and formation of gas/ice giants.
Thus, the inner terrestrial region is initially quite dry regarding water ice, although some water may be delivered later from planetesimals coming from beyond the snow line [1], [2].
2.2 Disk mass density and timescales
A star's accretion disk often contains enough solid material to form several rocky planets in the inner region, but how many form or their sizes depend on:
- Upper layer density of solid particles: Higher density promotes faster planetesimal collisions and embryo growth.
- Disk lifetime: Typically 3–10 million years until the gas dissipates, but the rocky planet formation process (already without a gas environment) can continue for tens of millions of years, with protoplanets colliding in a gas-free environment.
Physical factors – viscous evolution, magnetic fields, stellar radiation – shape the disk's structure and evolution, defining the conditions under which "rocky bodies" assemble.
3. Dust coagulation and planetesimal formation
3.1 Growth of rocky particles in the inner disk
In the hotter inner region, small dust grains (silicates, metal oxides, etc.) collide and stick, forming aggregates – "pebbles." But here arises the "meter-size barrier":
- Radial drift: Meter-sized objects rapidly move toward the star due to friction, risking loss before reaching sufficient size.
- Collision fragmentation: As speed increases, collisions can break up aggregates.
Possible solutions to overcome these barriers:
- Streaming instability: Local dust excess leads to gravitational collapse into km-scale planetesimals.
- Pressure bumps: Disk pressure maxima (gaps, rings) can trap dust and reduce drift, allowing more efficient growth.
- "Pebble" accretion: If a core forms somewhere, it rapidly "collects" mm–cm pebbles [3], [4].
3.2 Planetesimal Seed
Once kilometer-sized planetesimals form, gravitational focusing further accelerates mergers. In the inner disk, planetesimals are generally rocky, composed of iron, silicates, and possibly minor carbon impurities. Over tens or hundreds of thousands of years, these planetesimals can merge into protoplanets reaching tens or hundreds of kilometers.
4. Protoplanet Evolution and Terrestrial Planet Growth
4.1 Oligarchic Growth
In the theory called oligarchic growth:
- Several large protoplanets in the region become gravitationally dominant “oligarchs.”
- Smaller planetesimals are scattered or accreted.
- Eventually, a few competing protoplanets and smaller leftover bodies remain in the zone.
This stage can last several million years until several Mars-sized or Moon-sized embryos form.
4.2 Giant Impact and Final Arrangement Phase
After the gas disperses from the disk (no more damping effects and friction), these protoplanets continue to collide in a chaotic environment:
- Giant impacts: In the final stage, sufficiently large collisions may occur, partially melting mantles, similar to the hypothetical Moon-forming impact between proto-Earth and Theia.
- Long duration: The formation of rocky planets in the Solar System could have taken about 50–100 million years until after Mars-sized body impacts, Earth's orbit finally stabilized [5].
During these collisions, additional iron-silicate differentiation occurs, planetary cores form, and material can also be ejected to form satellites (e.g., Earth's Moon) or rings.
5. Composition and Delivery of Volatile Water
5.1 Rocky Composition Interior
Since volatile substances evaporate in the inner, warm part of the disk, planets forming there usually 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 vary depending on local disk conditions and histories of massive impacts).
5.2 Water and organic materials
Despite snow lines forming inside, terrestrial planets can still acquire water if:
- Late delivery: Planetesimals from the outer disk or asteroid belt are scattered inward.
- Small icy bodies: Comets or C-type asteroids can deliver enough volatiles if scattered inward.
Geochemical studies suggest Earth's water may have partly originated from carbonaceous chondritic bodies, explaining how we have water despite the essentially dry inner region. [6].
5.3 Impact on habitability
Volatiles are crucial for oceans, atmospheres, and life-supporting surfaces. The combination of late collisions, mantle melting processes, and the influx of external planetesimal material determines whether a terrestrial planet can have life-supporting conditions.
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 or mini-Neptunes orbiting close to stars. Some may be purely rocky but larger than Earth, others have thick atmospheres. Still others – "lava worlds" – are so close to the star that their surfaces may be molten. These discoveries emphasize:
- Disk differences: Minor parameter differences in the disk lead to different outcomes – from Earth analogs to hot super-Earths.
- Migration effects: Some rocky super-Earths may have formed farther out and later moved closer to the star.
6.2 "Debris" disks as evidence of the terrestrial "building" process
Debris disks detected around older stars – dust left from collisions between planetesimals or unsuccessfully formed rocky protoplanets – indicate ongoing small collisions. Warm dust rings detected by Spitzer and Herschel around mature stars may resemble our Solar System's zodiacal dust band, showing existing rocky remnants in a slow frictional grinding phase.
6.3 Geochemical correlations
Spectroscopic measurements of white dwarf atmospheres containing disrupted planetary debris show an elemental composition similar to rocky (chondritic) components. This confirms that the formation of rocky planets in inner regions is a fairly common phenomenon in star systems.
7. Timescales and final configurations
7.1 Accretion chart
- Planetesimal formation: Possibly over 0.1–1 million years due to streaming instability or slow collisions.
- Protoplanet formation: Over 1–10 million years, larger bodies begin to dominate, "clearing" or assimilating smaller planetesimals.
- Giant Impact Phase: Tens of millions of years until only a few final rocky planets form. It is thought that the final large Earth impact (Moon formation) occurred ~30–50 million years after the Sun's formation [7].
7.2 Variability and Final Architecture
Differences in disk density, presence of migrating giant planets, or early star–disk interactions can strongly alter orbits and compositions. Some systems may form one or no large terrestrial planets (as around many M dwarfs?), others several close-in super-Earths. Each system has a unique “fingerprint” reflecting its initial environment.
8. The Path to a Rocky Planet
- Dust Growth: Silicate and metal grains stick together into mm–cm “pebbles,” aided by partial adhesion.
- Planetesimal Formation: Kilometer-scale bodies rapidly form via streaming instability or other mechanisms.
- Protoplanet Accretion: Gravitational impacts of planetesimals grow embryos the size of Mars or the Moon.
- Giant Impact Stage: A small number of large protoplanets collide over tens of millions of years forming the final rocky planets.
- Delivery of Volatiles: Water and organics from outer disk planetesimals or comets can provide a planet with oceans and potential habitability.
- Orbital Clearing: Final collisions, resonant interactions, or scattering events lead to stable orbits and the arrangement of terrestrial worlds in many systems.
9. Future Studies and Missions
9.1 ALMA and JWST Disk Imaging
High-resolution disk maps reveal rings, gaps, and possibly protoplanetary seeds. If dust concentrations or spirals are found inside the disk, they help understand how rocky planetesimals form. JWST infrared data allow detection of silicate spectral features and inner disk gaps/rings indicating ongoing planet formation processes.
9.2 Exoplanet Characterization
Current exoplanet transit/radial velocity surveys and upcoming PLATO and Roman Space Telescope missions 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 are distributed or migrate into the star's habitable zone.
9.3 Sample Return from Inner Disk Remnants
Missions studying small bodies formed in the inner Solar System, such as NASA's Psyche (a metallic asteroid) or other asteroid sample return missions, provide chemical insights into the initial composition of planetesimals. Linking this data with meteorite studies clarifies how planet formation proceeded from the initial disk of solid particles.
10. Conclusion
Formation of rocky worlds naturally occurs in the hot regions of protoplanetary disks. When dust particles and small rocky grains combine into planetesimals, gravitational interaction promotes rapid protoplanet formation. Over tens of millions of years, repeatedly colliding — sometimes gently, sometimes violently — these protoplanets form several stable orbits where the remaining rocky planets reside. The delivery of water and the development of atmospheres can make such worlds suitable for life, as Earth's geological and biological history shows.
Observations — both in our Solar System (asteroids, meteorites, planetary geology) and in exoplanet studies — show that the formation of rocky planets is likely common among many stars. By improving disk imaging, dust evolution models, and planet-disk interaction theories, astronomers are gaining deeper understanding of the cosmic “recipe” by which dust clouds fed by stars give rise to Earth-like or other rocky worlds in our Galaxy. Such research not only reveals the history of our planet's origin but also explains how potential building blocks of life form around countless other stars in the Universe.
Links and further reading
- 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.
- Weidenschilling, S. J. (1977). “Aerodynamics of solid bodies in the solar nebula.” Monthly Notices of the Royal Astronomical Society, 180, 57–70.
- Johansen, A., & Lambrechts, M. (2017). “Forming Planets via Pebble Accretion.” Annual Review of Earth and Planetary Sciences, 45, 359–387.
- Morbidelli, A., Lunine, J. I., O’Brien, D. P., Raymond, S. N., & Walsh, K. J. (2012). “Building Terrestrial Planets.” Annual Review of Earth and Planetary Sciences, 40, 251–275.
- Chambers, J. E. (2014). “Planetary accretion in the inner Solar System.” Icarus, 233, 83–100.
- Raymond, S. N., & Izidoro, A. (2017). “The empty primordial asteroid belt and the role of Jupiter's growth.” Icarus, 297, 134–148.
- Kleine, T., et al. (2009). “Hf–W chronology of meteorites and the timing of terrestrial planet formation.” Geochimica et Cosmochimica Acta, 73, 5150–5188.