pebble accretion Archives - Astrobiology

Transition between different accretion regimes as shown by the shading and top labels. From left to right the panels show central AGN masses of 109, 108 and 107 𝑀⊙. The top row shows 𝑣frag = 1m s−1 and the bottom row shows 𝑣frag = 10 m−1 . The symbol 𝑚p gives the characteristic planetesimal mass, 𝑀HB is the transition mass between the Hill and the Bondi accretion regime, 𝑀BG is the transition mass between the Bondi and the geometric accretion regimes, and 𝑀p,iso is the pebble isolation mass. The transition between 2D and 3D accretion is also shown. In general, the objects will accrete in the 3D regime right after formation, except in the 1–10 pc range in the case 𝑣frag = 10 m/s, 𝑀SMBH = 109 𝑀⊙ (lower left). Accretion is also more efficient in the inner disk than in the outer disk, with a progression from Hill, Hill/Bondi, Bondi, and Bondi/Geometric as the central mass object decreases. The horizontal dotted black line is the hydrogen burning limit. -- astro-ph.EP

Astrogeology, Astronomy & Telescopes, Exoplanets, -moons, -comets, Status Report

Active Galactic Nucleus Tori: Potential Birthplace to Millions of Planets

by Keith CowingMay 23, 2026May 23, 2026

The outer regions of AGN disks have temperatures similar to those of circumstellar disks, permitting dust condensation. Therefore, planet formation and growth could be active in these dust tori through […]

HR diagram track of the default case (final mass of 2.2 MJ ; see Table 1), showing its surface temperature (Eqs. 12, 19 or 21, depending on the phase) and total luminosity (Eq. 6). Highlighted key moments show the start point (•), detachment (⋆), the point of maximum luminosity (▼), disk dispersal (■), and the age of Jupiter (+). Table 2 lists the corresponding values for time, mass and radius. In the ascending branch small black triangles (▲) show linearly spaced time indicators every Myr. The black dots (•) represent post-detachment intervals equally spaced in log(time). Grey reference lines show two trends: the predicted slope derived in Sect. A, evaluated with the median late-attached-phase surface density of Σpla = 1.7 g cm−2 (labelled with L ∝ T 8 ), and a constant-radius track for Rp = 1 RJ , which approximates a cooling planet in the evolutionary phase before stellar irradiation becomes relevant (last downturn). -- astro-ph.EP

Astronomy & Telescopes, Exoplanets, -moons, -comets, Imaging & Spectroscopy, Status Report, Stellar Cartography

Planetary Formation Tracks On The Hertzsprung-Russell Diagram: Visualising The Processes Of Giant Planet Growth

by Keith CowingMay 21, 2026May 21, 2026

The Hertzsprung-Russell diagram (HRD) is central to stellar astrophysics but has rarely been used to interpret planet formation.

Sum of water mass accreted by planets in the habitable zone of their respective stars (color lines) and disk mass fraction along the x-axis. Circle size represents average protoplanet mass among each habitable zone containing 6 to 8 protoplanets. The highest mass protoplanets are also the most water-rich and occur in the middle-mass architecture, while the high-mass architecture is unable to deliver water to protoplanets in the habitable zone (truncated from the plot are models where total water mass < 10−8M⊕). The low-mass architecture features a mix of water and protoplanet mass

Astrochemistry, Astrogeology, Biogeochemical Cycles & Geobiology, Exoplanets, -moons, -comets, Habitable Zones, Icy Worlds, Imaging & Spectroscopy, Status Report

Diversity In Planetary Architectures From Pebble Accretion: Water Delivery To The Habitable Zone With Pebble Snow

by Keith CowingSeptember 18, 2025September 18, 2025

“Pebble snow” describes a planet formation mechanism where icy pebbles in the outer disk reach inner planet embryos as the water ice line evolves inward.

Images of dust continuum emission of the HL Tau disk. An arcsinh stretch is applied to the color scale. -- astro-ph.EP

Astrochemistry, Astrogeology, Astronomy & Telescopes, Biochemistry & Organic Chemistry, Exoplanets, -moons, -comets, Imaging & Spectroscopy, Interstellar, Press Release

Multi-Wavelength Dust Characterization of the HL Tau Disk and Implications for Planet Formation

by Keith CowingJuly 25, 2025July 25, 2025

We present a comprehensive analysis of the HL Tau dust disk by modeling its intensity profiles across six wavelengths (0.45 to 7.9 mm) with a resolution of 0.05 arcsec (∼7 […]

Final size of protoplanets after 2 My of Hill regime pebble accretion, for different Stokes numbers of silicate and metal pebbles. The plot shows protoplanet mass relative to Earth (white contours and shading) and protoplanet metal mass fraction (black contours) as functions of the Stokes numbers of metal and silicate pebbles. The red dots represent chondrule and metal grain Stokes numbers based on measured size and density in the Kelly (K), Bjurbole (B), and Hammond Down (H) chondrites, respectively. The turquoise box indicates the implied range of pebble Stokes numbers, including individual metal grains and metal grain aggregates. The red contour corresponds to an Earth-like (Fe+Ni)/Si mass ratio of 2.06 and defines our preferred models with protoplanetary masses between 0.35 and 0.7 Earth masses. -- astro-ph.EP

Astrochemistry, Astrogeology, Biogeochemical Cycles & Geobiology, Exoplanets, -moons, -comets, Status Report

Building Earth With Pebbles Made Of Chondritic Components

by Keith CowingJuly 23, 2025July 23, 2025

Pebble accretion provides new insights into Earth’s building blocks and early protoplanetary disk conditions.

Left: Planet-to-star mass ratio as a function of the planet semi-major axis for all planets orbiting M-dwarf stars. Darker coloured symbols correspond to planetary masses above 0.2 MJup. Horizontal dotted lines indicate the Jupiter-to-Sun and Saturn-to-Sun mass ratios for reference. Right: Host star metallicity as a function of the insolation at the exoplanet semi-major axis. For multi-planetary systems, only the innermost planet is depicted. Vertical dotted lines indicate current insolation values of Jupiter and Saturn for reference. In both panels, different detection techniques are shown as labelled. Star symbols depict planets in the CARMENES sample (filled symbols, this work, and empty symbols, Trifonov et al. 2018, 2020a; Quirrenbach et al. 2022). -- astro-ph.EP

Astrogeology, Astronomy & Telescopes, Exoplanets, -moons, -comets, Gas Giants, Imaging & Spectroscopy, Status Report

The CARMENES Search For Exoplanets Around M Dwarfs. Revisiting The GJ 317, GJ 463, and GJ 3512 Systems And Two Newly Discovered Planets Orbiting GJ 9773 And GJ 508.2

by Keith CowingJuly 22, 2025July 22, 2025

Surveys for exoplanets indicate that the occurrence rate of gas giant planets orbiting late-type stars in orbits with periods shorter than 1000 days is lower than in the case of […]

Planets emerge from the dense disk of gas and dust encircling young stars. -- NASA

Astrochemistry, Astrogeology, Exoplanets, -moons, -comets, Status Report, Stellar Cartography

Can Close-In Exoplanets Form By Pebble Accretion?

by Keith CowingMay 3, 2025

Pebble accretion is the leading theory for the formation of exoplanets more massive than the Earth.

Snapshots show planetary seeds’ growth and migration from two rings of material. This simulation comes from our M6T2 scenario. The top-left panel shows the starting time of the simulation. The time of each snapshot is shown on the bottom-left of each panel. The color of each dot corresponds to the water mass fraction of each planetary seed. We plot planetesimal and pebble isolation masses with dotted and dashed lines, respectively. The planetesimal isolation mass is obtained by extrapolating the initial surface density of the inner planetesimal disk. The vertical blue dashed line shows the location of the water condensation line. The blue solid line is the boundary of the inward and outward migration where the migration torque is zero. -- astro-ph.EP

Exoplanets, -moons, -comets, Habitable Zones, Icy Worlds, Status Report, Water/Hycean Worlds & Oceanography

Formation of Super-Earths and Mini-Neptunes From Rings of Planetesimals

by Keith CowingJanuary 8, 2025

The solar system planetary architecture has been proposed to be consistent with the terrestrial and giant planets forming from material rings at ~1 au and ~5 au, respectively.

Time after which planets reach the pebble isolation mass tiso ” t(M=Miso), as a function of orbital radius and initial mass, using IGM16. White indicates the isolation mass has not been reached. The contours showing the final mass are the same as in Fig. 4. Around 0.49, 0.70 and 1.00 M⊙ stars, the first planets reach Miso within the first 50,000 to 100,000 years of the simulation, which could halt pebble accretion for planets close to the star prematurely. -- astro-ph.EP

Astrogeology, Exoplanets, -moons, -comets, Status Report, TRAPPIST-1

Forming Earth-like and Low-Mass Rocky Exoplanets Through Pebble and Planetesimal Accretion

by Keith CowingDecember 6, 2024

The theory of pebble accretion (PA) has become a popular planet formation theory during the past decade. However, PA studies generally rely on large planetary embryos, much larger than those […]

Planetary formation -- Grok via Astrobiology.com

Astrogeology, Exoplanets, -moons, -comets, Status Report

Did The Terrestrial Planets Of The Solar System Form By Pebble Accretion?

by Keith CowingNovember 15, 2024November 15, 2024

The dominant accretion process leading to the formation of the terrestrial planets of the Solar System is a subject of intense scientific debate. Two radically different scenarios have been proposed.