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Research

This page is currently being revamped. Follow the publications tab for my full list of papers. Check also this link for the work of our extended collaboration PFITS+: Planet Formation in the Southwest

At Stampede's dedication, Austin TX, April 2013. Each of these towers has 3,000 processors.

My research focuses on planet formation. You can find here brief explanations of the research highlights, and links to the respective publications.

Review

The dynamical state of the protoplanetary disk is the fundamental canvas where the planet formation narrative is etched. For decades, the mechanism of accretion has been elusive. The magnetorotational instability, thought to be responsible for accretion in magnetized disks, is rendered mostly inoperative in protoplanetary disks, as these disks are dense and cold, lacking sufficient ionization. Hence, the search for hydrodynamical sources of turbulence continues. The problem of accretion is intimately connected to planet formation as well, since planet formation starts with mechanical aerodynamical concentration of pebbles. The decade of the 2010s saw the emergence of three possible mechanisms for generating turbulence: the Vertical Shear Instability, the Convective Overstability, and the subcritical Resonant Buoyant Instability (nee "zombie" vortex instability), which operate in well-separated regimes of opacity. In this review, we summarize the properties of these processes, identify their limitations, and discuss where and under what conditions these processes are active in protoplanetary disk.

Publication: Lyra & Umurhan (2019), PASP, 131, 2001. The Initial Conditions for Planet Formation: Turbulence Driven by Hydrodynamical Instabilities in Disks around Young Stars [ADS]

Kuiper belt objects

The Kuiper belt is a gold mine of information of planetesimal formation. In fact, it is the only place in the Solar System where a population of pristine planetesimals, the cold classicals, remains extant to this day. The flyby of Arrokoth in 2019 by the New Horizons gave us detailed constraints on what to expect from the process. Surprisingly, Arrokoth turned out to be a contact binary planetesimal. I produced a model that explains the orbital evolution of the individual lobes into contact with a combination of Kozai-Lidov oscillations and gas drag. The code I developed to solve the combined action of Kozai, induced and permanent tides, and drag (KTJD), is available for download here.  

 
The figure above shows the evolution of the lobes of Arrokoth up to 10 Myr, for simulations with different dynamical terms. The simulation with only the solar terms (green) leads to regular Kozai oscillations at constant semimajor axis. Including the induced quadrupole (blue) has little effect. Inclusion of the permanent quadrupole (orange) leads to irregular Kozai cycles with erratic excursions in inclination and eccentricity. Finally, including the orbital drag leads to a fast decay of semimajor axis, and eventual contact shortly after 2 Myr.

Publication: Lyra, Youdin, & Johansen (2021), Icarus, 356, 113831. Evolution of MU69 from a binary planetesimal into contact by Kozai-Lidov oscillations and nebular drag [ADS]

Another problem I've been concerned about is what information on the planet formation process we can derive from the density of Kuiper belt objects. Density has been measured for a number of objects, for which dynamical mass can be measured (because the objects are binaries), and radii are measured via occultation or thermal radiometry. The pattern that emerges is a bimodal population of low-density low-mass objects, with density gradually increasing toward high-mass objects. I've worked out a solution for this pattern via 1) ice photodesorption off small grains maintains a compositional gradient with radii for the pebbles, with the larger pebbles being shielded and keeping the ice, whereas the small grains preferentially lose ice as they are lofted in the disk atmosphere and get exposed to stellar UV. 2) streaming instability forms planetesimals from the largest pebbles, primarily icy. Finally 3) at this mass range, pebble accretion will preferentially accrete the small pebbles, which are ice-poor in comparison. This solution also avoids the timing problem that ensues if the objects form with too rocky a composition from the outset (the abundance of 26Al would melt the planetesimals).

Applying the polydisperse pebble accretion model with the bimodal distribution of pebbles at 20 AU. The density trends well and reaches Pluto mass in about 5-6 million years.  

 
Publication: Canas & Lyra, et al. (2024), PSJ, 5, 55. A Solution for the Density Dichotomy Problem of Kuiper Belt Objects with Multispecies Streaming Instability and Pebble Accretion [ADS]

Finally, why is there a mass gap between the low-mass and high-mass objects? This gap is really telling us something profound about planet formation. The low-mass side is consistent with the outcome of planetesimal formation, quenched at 400 km. The objects are wide binaries of equal brightness, thus easy to find. The high-mass objects, on the other hand, have small satellites in tight orbits. While the gap itself is probably an observational bias, this hints at an architectural dichotomy imprinted by the pebble accretion process.  

 
 

Publication: Lyra (2025), Icarus, 442, 116737. Where are the missing Kuiper Belt binaries? [ADS]

Pebble Accretion

An Analytical Theory for the Growth from Planetesimals to Planets by Polydisperse Pebble Accretion
https://ui.adsabs.harvard.edu/abs/2023ApJ...946...60L/abstract

Streaming Instability

On the Mass Budget Problem of Protoplanetary Disks: Streaming Instability and Optically Thick Emission
https://ui.adsabs.harvard.edu/abs/2025arXiv250610435G/abstract

Vortex Trapping

Rapid Protoplanet Formation in Vortices: Three-dimensional Local Simulations with Self-gravity
https://ui.adsabs.harvard.edu/abs/2024ApJ...970L..19L/abstract

Vortex solution in elliptic coordinates
https://ui.adsabs.harvard.edu/abs/2021RNAAS...5..180L/abstract

Pebble Trapping in Vortices: Three-dimensional Simulations
https://ui.adsabs.harvard.edu/abs/2021ApJ...913...92R/abstract

Pebble-trapping Backreaction Does Not Destroy Vortices
https://ui.adsabs.harvard.edu/abs/2018RNAAS...2..195L/abstract

Steady State Dust Distributions in Disk Vortices: Observational Predictions and Applications to Transitional Disks
https://ui.adsabs.harvard.edu/abs/2013ApJ...775...17L/abstract

Planet formation bursts at the borders of the dead zone in 2D numerical simulations of circumstellar disks
https://ui.adsabs.harvard.edu/abs/2009A%26A...497..869L/abstract

Standing on the shoulders of giants. Trojan Earths and vortex trapping in low mass self-gravitating protoplanetary disks of gas and solids
https://ui.adsabs.harvard.edu/abs/2009A%26A...493.1125L/abstract

Embryos grown in the dead zone. Assembling the first protoplanetary cores in low mass self-gravitating circumstellar disks of gas and solids
https://ui.adsabs.harvard.edu/abs/2008A%26A...491L..41L/abstract

Cyclogenesis (Vortex Formation)

Rossby wave instability does not require sharp resistivity gradients
https://ui.adsabs.harvard.edu/abs/2015A%26A...574A..10L/abstract


Convective Overstability in Accretion Disks: Three-dimensional Linear Analysis and Nonlinear Saturation
https://ui.adsabs.harvard.edu/abs/2014ApJ...789...77L/abstract

Elliptic and magneto-elliptic instabilities
https://ui.adsabs.harvard.edu/abs/2013EPJWC..4604003L/abstract

A Parameter Study for Baroclinic Vortex Amplification
https://ui.adsabs.harvard.edu/abs/2013ApJ...765..115R/abstract

Rossby Wave Instability at Dead Zone Boundaries in Three-dimensional Resistive Magnetohydrodynamical Global Models of Protoplanetary Disks
https://ui.adsabs.harvard.edu/abs/2012ApJ...756...62L/abstract

On the connection between the magneto-elliptic and magneto-rotational instabilities
https://ui.adsabs.harvard.edu/abs/2012JFM...698..358M/abstract

The baroclinic instability in the context of layered accretion. Self-sustained vortices and their magnetic stability in local compressible unstratified models of protoplanetary disks
https://ui.adsabs.harvard.edu/abs/2011A%26A...527A.138L/abstract

Photoelectric instability

Disentangling Planets from Photoelectric Instability in Gas-rich Optically Thin Dusty Disks
https://ui.adsabs.harvard.edu/abs/2019ApJ...887....6C/abstract


The Interplay between Radiation Pressure and the Photoelectric Instability in Optically Thin Disks of Gas and Dust
https://ui.adsabs.harvard.edu/abs/2018ApJ...856...41R/abstract

Formation of sharp eccentric rings in debris disks with gas but without planets
https://ui.adsabs.harvard.edu/abs/2013Natur.499..184L/abstract

Planet migration

Orbital Migration of Interacting Low-mass Planets in Evolutionary Radiative Turbulent Models
https://ui.adsabs.harvard.edu/abs/2012ApJ...750...34H/abstract


Formation of Planetary Cores at Type I Migration Traps
https://ui.adsabs.harvard.edu/abs/2011ApJ...728L...9S/abstract

Orbital Migration of Low-mass Planets in Evolutionary Radiative Models: Avoiding Catastrophic Infall
https://ui.adsabs.harvard.edu/abs/2010ApJ...715L..68L/abstract

Planetary shocks

On Shocks Driven by High-mass Planets in Radiatively Inefficient Disks. III. Observational Signatures in Thermal Emission and Scattered Light
https://ui.adsabs.harvard.edu/abs/2017ApJ...849..164H/abstract


On Shocks Driven by High-mass Planets in Radiatively Inefficient Disks. II. Three-dimensional Global Disk Simulations
https://ui.adsabs.harvard.edu/abs/2016ApJ...817..102L/abstract

On Shocks Driven by High-mass Planets in Radiatively Inefficient Disks. I. Two-dimensional Global Disk Simulations
https://ui.adsabs.harvard.edu/abs/2015ApJ...804...95R/abstract

Code development

Stability Analysis for General Order Central Finite-difference Hyperdiffusivity with Time Integrators of Arbitrary Accuracy
https://ui.adsabs.harvard.edu/abs/2023RNAAS...7...69L/abstract


Orbital Advection with Magnetohydrodynamics and Vector Potential
https://ui.adsabs.harvard.edu/abs/2017AJ....154..146L/abstract

A Well-posed Kelvin-Helmholtz Instability Test and Comparison
https://ui.adsabs.harvard.edu/abs/2012ApJS..201...18M/abstract

Shallow water
https://ui.adsabs.harvard.edu/abs/2022PSJ.....3..166H/abstract

Ice shell convection
https://ui.adsabs.harvard.edu/abs/2018DPS....5041504C/abstract
https://ui.adsabs.harvard.edu/abs/2017LPICo2048.7021S/abstract
https://ui.adsabs.harvard.edu/abs/2016DPS....4821306U/abstract

MRI

Meridional circulation in turbulent protoplanetary disks
https://ui.adsabs.harvard.edu/abs/2011A%26A...534A.107F/abstract


Global magnetohydrodynamical models of turbulence in protoplanetary disks. I. A cylindrical potential on a Cartesian grid and transport of solids
https://ui.adsabs.harvard.edu/abs/2008A%26A...479..883L/abstract

Observational

The Alpha Centauri binary system. Atmospheric parameters and element abundances
https://ui.adsabs.harvard.edu/abs/2008A%26A...488..653P/abstract


On the difference between nuclear and contraction ages
https://ui.adsabs.harvard.edu/abs/2006A%26A...453..101L/abstract

Fine structure of the chromospheric activity in Solar-type stars - The Hα line
https://ui.adsabs.harvard.edu/abs/2005A%26A...431..329L/abstract

Positional Astronomy

On the Angle of Sunset
https://ui.adsabs.harvard.edu/abs/2022RNAAS...6..257L/abstract

Educational

A historical method approach to teaching Kepler's 2nd law
https://ui.adsabs.harvard.edu/abs/2021AEdJ....1....4L/abstract

Outreach

Ad Astra Academy: Using Space Exploration to Promote Student Learning and Motivation in the City of God, Rio de Janeiro, Brazil
https://ui.adsabs.harvard.edu/abs/2020CAPJ...27....5L/abstract