About

I am a Nevada Center for Astrophysics Postdoctoral Fellow at the University of Nevada, Las Vegas. I am a theorist who runs large 3D simulations to find out how planetesimals, planets and brown dwarfs form in the disks of gas and dust around young stars.

I completed my Ph.D. in astrophysics at Iowa State University in 2026, advised by Prof. Jacob B. Simon, with support from a NASA FINESST fellowship. Before that, I earned my B.S. and M.S. in astronomy at Chungnam National University in Daejeon, South Korea, where I grew up and where my M.S. work with Prof. Jungyeon Cho was on compressible and magnetized turbulence.

The big picture

Every planetary system, including our own, began as a disk of gas and dust around a young star. Building a planet from that dust means growing by more than ten orders of magnitude in size, from micron-sized grains to bodies thousands of kilometers across. Two steps along the way remain poorly understood.

The first is the planetesimal barrier. Dust sticks into millimeter- to centimeter-sized pebbles, but beyond that size collisions tend to bounce or shatter grains, and gas drag makes pebbles drift into the star faster than they can grow. Something must gather pebbles into dense clouds that collapse under their own gravity into the 1–100 km planetesimals that seed planets. The leading candidate is the streaming instability, in which the drag between dust and gas makes dust pile up spontaneously into dense filaments.

The second is what happens in the youngest, most massive disks. When a disk is massive enough, its own gravity can make it break up into bound clumps. This gravitational instability may form giant planets and brown dwarfs on wide orbits directly, and its spiral arms may also trap dust and let it grow early.

My work connects these processes to what we can measure: images of young disks from ALMA, the planetesimals preserved in our own Kuiper Belt, and the spins of planetary and brown-dwarf companions.

Question 1
Where and when can the streaming instability form planetesimals?

How the answer depends on grain size, dust abundance and turbulence.

Question 2
What observable signatures does dust–gas dynamics leave?

Polarization, optical depth and disk dust masses, planetesimal properties.

Question 3
What forms when young, massive disks fragment?

The masses and spins of the fragments, and how dust grows in their spiral arms.

From dust to planets and brown dwarfs Two routes to planetary bodies. Bottom-up: micron dust grains stick into millimetre-to-centimetre pebbles, a growth barrier stops further sticking, the streaming instability concentrates pebbles until self-gravity forms 1 to 100 km planetesimals, which grow into planets. Top-down: massive young disks become gravitationally unstable and spiral arms collapse into clumps that become giant planets and brown dwarfs. Dust in the spiral arms can also collapse into planetary embryos, which join the planetesimal-to-planet stage. Observational tests come from ALMA images of young disks, the Kuiper Belt, and the spins of planetary and brown-dwarf companions. BOTTOM-UPdust grows into planetesimals, then planets Dust grains ~1 µm sticking Pebbles mm – cm growth barrier: bouncing · fragmentation · radial drift Streaming instability concentrates pebbles until their self-gravity takes over Planetesimals 1 – 100 km collisions, pebble accretion Planets 10³ – 10⁵ km dust in spiral arms can form planetary embryos TOP-DOWNmassive young disks fragment under their own gravity Massive young disk youngest (Class 0/I) systems Gravitational instability spiral arms collapse into bound, spinning clumps gaseous clumps contract Brown dwarfs & giant planets what sets their mass and spin? OBSERVATIONAL TESTS Young disks (ALMA) rings, polarization, grain sizes, turbulence, dust masses Solar System (Kuiper Belt) binary planetesimals, Arrokoth, size distribution Exoplanets & companions spins of planetary and brown-dwarf companions
Two routes to planetary bodies and the observations that test them. My published work (teal) asks when the streaming instability can overcome the growth barrier; my current work at UNLV (amber) follows gravitationally unstable young disks.

Research

Conditions for planetesimal formation by the streaming instability Published

The streaming instability can only form planetesimals if it concentrates dust strongly enough for self-gravity to take over (“strong clumping”). Whether it does depends on the size of the grains, how much dust there is relative to gas, and how turbulent the disk is. In a series of 3D simulations I mapped these conditions across a wide range of disk environments: in turbulent disks (Lim et al. 2024), for small grains and low dust abundance (Lim et al. 2025), and in laminar disks (Lim et al. 2026).

The figure summarizes the resulting criteria. In laminar disks, filled points mark simulations that formed planetesimals; in turbulent disks, planetesimals form above the line for each turbulence strength α. The dust abundance needed rises steeply for the smallest grains and as turbulence gets stronger.

Critical dust-to-gas ratio for strong clumping versus grain Stokes number, for laminar and turbulent disks
Dust abundance needed for planetesimal formation (dust-to-gas column density ratio Z) versus grain size (dimensionless stopping time τs), for laminar (points) and turbulent (lines) disks.

Streaming-instability dynamics in unstratified and stratified disks Published

Besides forming planetesimals, the streaming instability drives the coupled motion of dust and gas near the disk midplane. Much of what we know comes from “unstratified” simulations that leave out the star’s vertical gravity, which are cheaper and can be run at higher resolution. In Lim et al. (2025) I compared unstratified and vertically stratified simulations systematically across a range of radial pressure gradients.

Within the parameters explored, the instability behaves in the same way in unstratified disks and in the midplane of stratified disks: both form comparable dust filaments, with the stratified runs more confined vertically. This justifies using high-resolution local models of the midplane to predict observables.

Dust density in unstratified (top) and stratified (bottom) streaming-instability simulations
Dust density in unstratified (top) and stratified (bottom) simulations.

Gravitational instability: fragment masses and spins In progress

At UNLV I am studying how massive young disks fragment, using local 3D simulations with self-gravity, magnetic fields (including ambipolar diffusion) and adaptive mesh refinement that follow the clumps as they collapse. The goal is to measure the masses and spins of the fragments and how magnetic fields and cooling change them.

Dust growth in self-gravitating disks In progress

The spiral arms of a gravitationally unstable disk are pressure maxima that can trap dust. I am adding dust particles, their growth and their self-gravity to these simulations to ask how large grains can grow in the arms, and whether the dust can collapse into planetary embryos while the disk is still young and massive.

Publications

Full and up-to-date lists are on SciX and Google Scholar.

First-author papers

  1. The Streaming Instability in 3D: Conditions for Strong Clumping
    J. Lim, J. B. Simon, R. Li, O. Brouillette, D. G. Rea, W. Lyra · ApJ 1000, 156 (2026)
    JournalarXiv
  2. Bridging Unstratified and Stratified Simulations of the Streaming Instability for τs = 0.1 Grains
    J. Lim, S. A. Baronett, J. B. Simon, C.-C. Yang, D. Sengupta, O. M. Umurhan, W. Lyra · ApJ 993, 12 (2025)
    Journal
  3. Probing Conditions for Strong Clumping by the Streaming Instability: Small Dust Grains and Low Dust-to-gas Density Ratio
    J. Lim, J. B. Simon, R. Li, D. Carrera, S. A. Baronett, A. N. Youdin, W. Lyra, C.-C. Yang · ApJ 981, 160 (2025)
    Journal
  4. Streaming Instability and Turbulence: Conditions for Planetesimal Formation
    J. Lim, J. B. Simon, R. Li, P. J. Armitage, D. Carrera, W. Lyra, D. G. Rea, C.-C. Yang, A. N. Youdin · ApJ 969, 130 (2024)
    Journal
  5. Decay of Turbulence in Fluids with Polytropic Equations of State
    J. Lim, J. Cho · JKAS 53, 49 (2020)
    Journal
  6. Generation of Solenoidal Modes and Magnetic Fields in Turbulence Driven by Compressive Driving
    J. Lim, J. Cho, H. Yoon · ApJ 893, 75 (2020)
    Journal

Co-authored papers

  1. Closing the Gap: A Test of the Streaming Instability Using Simulations and Observations of the Cold-Classical Kuiper Belt Size Distribution
    W. Hall, J. B. Simon, J. Lim, D. Carrera, M. Small · arXiv:2610.04114 (2026)
    arXiv
  2. A Comparison between Particle and Fluid Approaches for Streaming Instability in Global Patch Simulations
    D. Hetzel Ostertag, M. Flock, P. Rosendahl, A. Mignone, J. Lim, J. B. Simon · arXiv:2610.09575 (2026)
    arXiv
  3. Revisiting Planetesimal Formation from mm-sized Grains in Pressure Bumps
    T. S. Tanvir, J. B. Simon, D. Carrera, J. Lim · arXiv:2609.27025 (2026)
    arXiv
  4. Azimuthal Dust Polarization from Aerodynamically Aligned Grains as Evidence for the Streaming Instability in Protoplanetary Disks
    Z.-Y. D. Lin, J. Lim, J. B. Simon, Z.-Y. Li, et al. · ApJ 1006, 95 (2026)
    Journal
  5. A Comparative Study of the Streaming Instability: Unstratified Models with Marginally Coupled Grains
    S. A. Baronett, W. Lyra, et al. (incl. J. Lim) · arXiv:2603.04558 (2026)
    arXiv
  6. On the Mass Budget Problem of Protoplanetary Disks: Streaming Instability and Optically Thick Emission
    D. Godines, W. Lyra, L. Ricci, C.-C. Yang, J. B. Simon, J. Lim, D. Carrera · ApJ 997, 192 (2026)
    Journal
  7. Positive Feedback II: How Dust Coagulation inside Vortices Can Form Planetesimals at Low Metallicity
    D. Carrera, L. E. J. Eriksson, J. Lim, W. Lyra, J. B. Simon · A&A 701, L1 (2025)
    Journal
  8. Positive Feedback: How a Synergy between the Streaming Instability and Dust Coagulation Forms Planetesimals
    D. Carrera, J. Lim, L. E. J. Eriksson, W. Lyra, J. B. Simon · A&A 696, L23 (2025)
    Journal

Talks

Teaching, mentoring and service