arXiv subjects

Kangrou Guo

Publications and source records attributed to Kangrou Guo.

12 recordsLinked to original sources

arXiv2026

Impact of Cold Jupiter Scattering on the Mean-Motion Resonance of Inner Small Planets

A key feature of close-in, multiple super-Earth (SE) systems is the tendency for adjacent planet pairs to lie just wide of low-order mean-motion resonances (MMR). This period ratio distribution has motivated numerous theoretical studies, particularly those invoking post-disk processes that perturb initially resonant architectures. We investigate whether orbital instability among cold Jupiters (CJs) can perturb inner SE systems initially in MMR. We show that a single pericenter passage of a highly eccentric CJ can disrupt inner resonances once a critical perturbation strength is exceeded, increasing the libration amplitude of the resonant angles. However, N-body simulations show that deep penetration of CJs into the inner system is uncommon, with $\lesssim 10-20\%$ of cases reaching $\lesssim 10\%$ of the initial semi-major axis of the innermost CJ. Motivated by these results, we use secular perturbation theory to quantify the impact of time-dependent forcing from scattering CJs on the eccentricity and resonant-angle evolution of inner SEs. We find that for typical systems (e.g., with SEs at $\sim 0.1$ au and CJs at a few au), such forcing can efficiently disrupt resonances, driving resonance-angle circulation in most systems ($\gtrsim 60\%$ for 2:1 and $\sim 85\%$ for 3:2 configurations). Thus, even when the "final" CJ has little effect on the "current" SEs, its earlier scattering history can leave significant imprints on the system architecture. This mechanism, and similar ones involving more abundant cold Neptunes, provide a natural source of dynamical "kicks" and offer a pathway for producing the observed trough-peak structure in the period ratio distribution of Kepler multi-planet systems.

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arXiv2026

A universal brown dwarf desert formed between planets and stars

Giant planets and brown dwarfs play a crucial role in star and planet formation, as they are situated at the boundary between planets and stars with uncertain formation mechanisms. Previous observational searches for the formation boundary were hampered by the lack of large unified samples of wide-orbit giant planets and substellar companions. A combined analysis of radial velocity and astrometry mitigates this problem and has significantly enlarged the sample. Here we present a rigorous statistical analysis of the sample of 55 giant planets, brown dwarfs and low-mass stellar companions orbiting FGK stars. We quantitatively analyze the occurrence rates of brown dwarfs and identify a distinct brown dwarf desert at approximately $30\,M_\mathrm{J}$, with no evidence of disappearance up to 20 au. Unlike previous studies that predicted a declining planet occurrence rate beyond the water-ice line, we identify a new population of giant planets and low-mass brown dwarfs in this region. The metallicity and eccentricity trends in our sample suggest that these are the consequences of two different formation scenarios. Our combined population synthesis model successfully accounts for the observed brown dwarf desert, supporting the dual formation hypothesis.

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arXiv2026

Long-Term Evolution of Close-in Sub-Neptunes and Outer Planetary Embryos: Atmospheric Mass Loss and Origin of Planets Inside and Outside the Radius Gap

As a byproduct of sub-Neptune formation, planetary embryos with high eccentricity can remain in outer orbits, near 1 au from the star. In this work, we investigate the long-term evolution of systems consisting of close-in sub-Neptunes (SNs) and outer high-eccentricity embryos. Our analysis focuses on collisions between SNs and embryos, particularly their atmospheric mass loss. We performed N-body simulations for various initial eccentricities and numbers of embryos. We analyzed the impact-induced atmospheric loss using post-processing methods, finding that the embryos and SNs collide at high speeds on timescales of several million years, leading to the loss of the SNs' atmospheres. Depending on the embryos' eccentricity and the orbital radius of the SNs, the impact velocity can be quite high, ranging from 2 to 5 times the escape velocity. On average, about 15%-30% of the atmosphere is dissipated per collision, so after 3-6 collisions, the atmospheric mass of an SN is reduced to about 1/3 of its initial value. Collisions between SNs and embryos can thus explain the presence of planets within the radius gap. Depending upon the initial eccentricity and the number of remaining embryos, additional collisions can occur, potentially accounting for the formation of the radius gap. This study also indicates that collisions between remaining embryos and SNs may help to explain the observed rarity of SNs with atmospheric mass fractions greater than 10%, commonly termed the "radius cliff."

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arXiv2025

Formation of Free-Floating Planets via Ejection: Population Synthesis with a Realistic IMF and Comparison to Microlensing Observations

Microlensing observations suggest that the mass distribution of free-floating planets (FFPs) follows a declining power-law with increasing mass. The origin of such distribution is unclear. Using a population synthesis framework, we investigate the formation channel and properties of FFPs, and compare the predicted mass function with observations. Assuming FFPs originate from planet-planet scattering and ejection in single star systems, we model their mass function using a Monte Carlo based planet population synthesis model combined with N-body simulations. We adopt a realistic stellar initial mass function, which naturally results in a large fraction of planetary systems orbiting low-mass stars. The predicted FFP mass function is broadly consistent with observation: it follows the observed power-law at higher masses ($10 \lesssim m/M_\oplus < 10^4$), while at lower masses ($0.1 < m/M_\oplus \lesssim 10$) it flattens, remaining marginally consistent with the lower bound of the observational uncertainties. Low-mass, close-in planets tend to remain bound, while Neptune-like planets at wide orbits dominate the ejected population due to their large Hill radii and shallow gravitational binding. We also compare the mass distribution of bound planets with microlensing observations and find reasonably good agreement with both surveys. Our model predicts $\simeq 1.20$ ejected planets per star in the mass range of $0.33 < m/M_\oplus < 6660$, with a total FFP mass of $\simeq 17.98~M_\oplus$ per star. Upcoming surveys will be crucial in testing these predictions and constraining the true nature of FFP populations.

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arXiv2025

A Population Synthesis Study on the Formation of Cold Jupiters from Truncated Planetesimal Disks

The occurrence rate of giant planets increases with orbital period and turns over at a location that roughly corresponds to the snow line of solar-type stars. Further, the density distribution of cold Jupiters (CJs) on the semi-major axis - mass diagram shows a relatively steep inner boundary, shaping the desert of warm Jupiters. The eccentricities of CJs show a broad distribution with a decreasing number density towards the larger end. Previous planet formation models fail to reproduce all these features at the same time. We use a planet population synthesis (PPS) model with truncated initial planetesimal distribution and compare the mass and orbital distribution of the simulated planets with the observation. We show that the occurrence of CJs with respect to the orbital period, the slope of the inner boundary of CJs on the semi-major axis - mass diagram, and the eccentricity distribution of CJs agree reasonably well with observation, if CJs form from truncated planetesimal disks of 10 au or wider with suppressed migration. While PPS simulations generally overestimate the fraction of giants with eccentricity below 0.2, $N$-body simulations produce a more consistent eccentricity distribution with observation. While the fraction of high-eccentricity planets can be increased by widening the planetesimal disk or reducing the migration speed, a deficit of giants with eccentricity between 0.2-0.4 exists regardless of the choices of parameters. Our results indicate that CJs are more likely born in truncated disks near the snow line than in classical uniform disks.

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arXiv2024

Probing orbits of stellar mass objects deep in galactic nuclei with quasi-periodic eruptions

Quasi-periodic eruptions (QPEs) are intense repeating soft X-ray bursts with recurrence times about a few to ten hours from nearby galactic nuclei. The origin of QPEs is still unclear. In this work, we investigated the extreme mass ratio inspiral (EMRI) + accretion disk model, where the disk is formed from a previous tidal disruption event (TDE). In this EMRI+TDE disk model, the QPEs are the result of collisions between a TDE disk and a stellar mass object (a stellar mass black hole or a main sequence star) orbiting around a supermassive black hole (SMBH) in galactic nuclei. If this interpretation is correct, QPEs will be invaluable in probing the orbits of stellar mass objects in the vicinity of SMBHs, and further inferring the formation of EMRIs which are one of the primary targets of spaceborne gravitational wave missions. Taking GSN 069 as an example, we find the EMRI wherein is of low eccentricity ($e<0.1$ at 3-$σ$ confidence level) and semi-major axis about $O(10^2)$ gravitational radii of the central SMBH, which is consistent with the prediction of the wet EMRI formation channel, while incompatible with alternatives.

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arXiv2024

Relative Occurrence Rate Between Hot and Cold Jupiters as an Indicator to Probe Planet Migration

We propose a second-order statistic parameter $\varepsilon$, the relative occurrence rate between hot and cold Jupiters ($\varepsilon=η_{\rm HJ}/η_{\rm CJ}$), to probe the migration of gas giants. Since the planet occurrence rate is the combined outcome of the formation and migration processes, a joint analysis of hot and cold Jupiter frequency may shed light on the dynamical evolution of giant planet systems. We first investigate the behavior of $\varepsilon$ as the stellar mass changes observationally. Based on the occurrence rate measurements of hot Jupiters ($η_{\rm HJ}$) from the TESS survey and cold Jupiters ($η_{\rm CJ}$) from the CLS survey, we find a tentative trend (97% confidence) that $\varepsilon$ drops when the stellar mass rises from $0.8$ to $1.4\ M_\odot$, which can be explained by different giant planet growth and disk migration timescales around different stars. We carry out planetesimal and pebble accretion simulations, both of which could reproduce the results of $η_{\rm HJ}$, $η_{\rm CJ}$ and $\varepsilon$. Our findings indicate that the classical core accretion + disk migration model can explain the observed decreasing trend of $\varepsilon$. We propose two ways to increase the significance of the trend and verify the anti-correlation. Future works are required to better constrain $\varepsilon$, especially for M dwarfs and for more massive stars.

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arXiv2024

Tianyu: search for the second solar system and explore the dynamic universe

Giant planets like Jupiter and Saturn, play important roles in the formation and habitability of Earth-like planets. The detection of solar system analogs that have multiple cold giant planets is essential for our understanding of planet habitability and planet formation. Although transit surveys such as Kepler and TESS have discovered thousands of exoplanets, these missions are not sensitive to long period planets due to their limited observation baseline. The Tianyu project, comprising two 1-meter telescopes (Tianyu-I and II), is designed to detect transiting cold giant planets in order to find solar system analogs. Featuring a large field of view and equipped with a high-speed CMOS camera, Tianyu-I will perform a high-precision photometric survey of about 100 million stars, measuring light curves at hour-long cadence. The candidates found by Tianyu-I will be confirmed by Tianyu-II and other surveys and follow-up facilities through multi-band photometry, spectroscopy, and high resolution imaging. Tianyu telescopes will be situated at an elevation about 4000 meters in Lenghu, China. With a photometric precision of 1% for stars with V < 18 mag, Tianyu is expected to find more than 300 transiting exoplanets, including about 12 cold giant planets, over five years. A five-year survey of Tianyu would discover 1-2 solar system analogs. Moreover, Tianyu is also designed for non-exoplanetary exploration, incorporating multiple survey modes covering timescales from sub-seconds to months, with a particular emphasis on events occurring within the sub-second to hour range. It excels in observing areas such as infant supernovae, rare variable stars and binaries, tidal disruption events, Be stars, cometary activities, and interstellar objects. These discoveries not only enhance our comprehension of the universe but also offer compelling opportunities for public engagement in scientific exploration.

astro-ph.IM↗

arXiv2023

The stability of unevenly spaced planetary systems

Studying the orbital stability of multi-planet systems is essential to understand planet formation, estimate the stable time of an observed planetary system, and advance population synthesis models. Although previous studies have primarily focused on ideal systems characterized by uniform orbital separations, in reality a diverse range of orbital separations exists among planets within the same system. This study focuses on investigating the dynamical stability of systems with non-uniform separation. We considered a system with 10 planets with masses of $10^{-7}$ solar masses around a central star with a mass of $1$ solar mass. We performed more than 100,000 runs of N-body simulations with different parameters. Results demonstrate that reducing merely one pair of planetary spacing leads to an order of magnitude shorter orbital crossing times that could be formulated based on the Keplerian periods of the closest separation pair. Furthermore, the first collisions are found to be closely associated with the first encounter pair that is likely to be the closest separation pair initially. We conclude that when estimating the orbital crossing time and colliding pairs in a realistic situation, updating the formula derived for evenly spaced systems would be necessary.

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arXiv2023

Formation of inner planets in the presence of a Cold Jupiter: orbital evolution and relative velocities of planetesimals

We investigate the orbital evolution of planetesimals in the inner disk in the presence of nebula gas and a (proto-) cold Jupiter. By varying the mass, eccentricity, and semi-major axis of the planet, we study the dependence of the relative velocities of the planetesimals on these parameters. For classic small planetesimals ($10^{16}-10^{20} $g) whose mutual gravitational interaction is negligible, gas drag introduces a size-dependent alignment of orbits and keeps the relative velocity low for similar-size bodies, while preventing orbital alignment for different-size planetesimals. Regardless of the location and the mass ratio of the planetesimals, increasing the mass and eccentricity or decreasing the orbital distance of the planet always leads to higher relative velocities of planetesimals. However, for massive planetesimals, the interplay of viscous stirring, gas damping, and secular perturbation results in lower velocity dispersion of equal-size planetesimals when the planet is more massive or when it is located on a closer or more eccentric orbit. The random velocities of such planetesimals remain almost unperturbed when the planet is located beyond Jupiter's current orbit, or when it is less massive or less eccentric than Jupiter. Unlike small planetesimals, such large planetesimals can grow in a runaway fashion as in the unperturbed case. Our results imply that the presence of a cold Jupiter does not impede the formation of inner rocky planets through planetesimal accretion, provided that the planetesimals are initially large.

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arXiv2022

Planetesimal Dynamics in the Presence of a Giant Planet II: Dependence on Planet Mass and Eccentricity

The presence of an early-formed giant planet in the protoplanetary disk has mixed influence on the growth of other planetary embryos. Gravitational perturbation from the planet can increase the relative velocities of planetesimals at the mean motion resonances to very high values and impede accretion at those locations. However, gas drag can also align the orbital pericenters of equal-size planetesimals in certain disk locations and make them dynamically quiet and "accretion-friendly" locations for planetesimals of similar sizes. Following the previous paper, where we investigated the effect of a Jupiter-like planet on an external planetesimal disk, we generalize our findings to extrasolar planetary systems by varying the planet parameters. In particular, we focus on the dependence of the planetesimal relative velocities on the mass and eccentricity of the existing planet. We found that the velocity dispersion of identical-mass particles increases monotonically with increasing planet mass. Meanwhile, the dependence of the relative velocity between different-mass planetesimals on their mass ratio becomes weaker as the planet mass increases. While the relative velocities generally increases with increasing planet eccentricity, the velocity dispersion of smaller-mass particles ($m \lesssim 10^{18}~\rm{g}$) is almost independent of planet eccentricity owing to their strong coupling to gas. We find that the erosion limits are met for a wider range of parameters (planet mass/eccentricity, planetesimal mass ratio) when the planetesimal size decreases. Our results could provide some clues for the formation of Saturn's core as well as the architecture of some exoplanetary systems with multiple cold giant planets.

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arXiv2021

Planetesimal Dynamics in the Presence of a Giant Planet

Standard models of planet formation explain how planets form in axisymmetric, unperturbed disks in single star systems. However, it is possible that giant planets could have already formed when other planetary embryos start to grow. We investigate the dynamics of planetesimals under the perturbation of a giant planet in a gaseous disk. Our aim is to understand the effect of the planet's perturbation on the formation of giant planet cores outside the orbit of the planet. We calculate the orbital evolution of planetesimals ranging from $10^{13}$ to $10^{20}$g, with a Jupiter-mass planet located at 5.2 au. We find orbital alignment of planetesimals distributed in $\simeq 9$-15 au, except for the mean motion resonance (MMR) locations. The degree of alignment increases with increasing distance from the planet and decreasing planetesimal mass. Aligned orbits lead to low encounter velocity and thus faster growth. The typical velocity dispersion for identical-mass planetesimals is $\sim \mathcal{O}(10)$ $\rm{m\text{ } s}^{-1}$ except for the MMR locations. The relative velocity decreases with increasing distance from the planet and decreasing mass ratio of planetesimals. When the eccentricity vectors of planetesimals reach equilibrium under the gas drag and secular perturbation, the relative velocity becomes lower when the masses of two planetesimals are both on the larger end of the mass spectrum. Our results show that with a giant planet embedded in the disk, the growth of another planetary core outside the planet orbit might be accelerated in certain locations.

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