arXiv subjects

Sarah Millholland

Publications and source records attributed to Sarah Millholland.

At least 19 recordsLinked to original sources

arXiv2026

Coupled Orbital and Interior Evolution of Sub-Neptunes

Recent observations have yielded the first measurements of young exoplanet demographics. Close-in sub-Neptune occurrence rates appear to rise from young ($10-100$ Myr) to intermediate (100 Myr $-$ 1 Gyr) ages and then decline sharply in the old ($\gtrsim 1$ Gyr) Kepler field population. In this paper, we test whether these observations can be explained by the effects of planetary cooling, atmospheric mass loss, and tidal orbital migration, which we model through a fully coupled evolution framework. The orbital evolution is assumed to operate exclusively through high-eccentricity migration, in effort to estimate the maximum possible contribution from this migration channel. In reality, only a subset of planetary systems are expected to undergo HEM. We find that high-eccentricity migration rapidly populates the close-in sub-Neptune bin, producing a sharp rise in occurrence within the first $\sim 15$ Myr. After this early phase, the occurrence evolves only weakly. While the modeled young to intermediate-age evolution is broadly consistent with the observed trend within the uncertainty limits, it does not support the idea of a sustained young-to-intermediate rise driven by tidal migration. It also fails to reproduce the sharp decline in occurrence seen from intermediate to old ages, motivating additional physics or formation channels to be included in future models.

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arXiv2026

Shaving the Outskirts of Planetary Systems Probed by Roman via Stellar Flybys

The recently launched Nancy Grace Roman Space Telescope (\textit{Roman}) will soon begin observations that promise to revolutionize our understanding of exoplanet demographics. \textit{Roman}'s Galactic Bulge Time-Domain Survey (GBTDS) will observe the Galactic Bulge via transit and microlensing techniques, providing a unique opportunity to study hot Jupiters as well as long-period/free-floating exoplanets in a dense and kinematically hot stellar environment. In this Letter, we quantify how repeated, impulsive stellar flybys truncate the outskirts of planetary systems in the Bulge. We find that flybys result in ejections on $\sim$Gyr timescales for wide orbits (${\gtrsim} 250\ \text{au}$) around low-mass hosts. Provided these wide orbits are populated, these ejections likely contribute to the population of free-floating planets probed by \textit{Roman}. Surviving planets outside of ${\sim} 10$ au are placed on eccentric and misaligned orbits. We show that, though systems suffer many close encounters, the evolution is dominated by the single strongest perturbation; planetary orbits (even those that evade ejection) undergo superdiffusion (i.e. a Lévy flight). We comment on the implications of our results for the origin of hot Jupiters amenable to $\textit{Roman}$'s transit search. Our work serves as a quantitative baseline from which future dynamical studies can build to better understand the evolution of planetary systems in the Galactic Bulge.

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arXiv2026

Atmospheric Escape Rates of Planets in Stellar Tidal Fields from 3-D Hydrodynamic Simulations

Thermally driven atmospheric escape, including photo-evaporation and core-powered mass-loss, plays a key role in shaping the evolution of close-in exoplanets, yet most current models rely on simplified one-dimensional descriptions of atmospheric escape. In this work, we perform 3D hydrodynamic simulations of atmospheric outflows from a Jupiter-sized planet embedded in the gravitational potential of a solar-type host star, and compare these results with 1D models to identify the regimes where they perform well and where they break down. We explore a range of configurations by varying the degree of Roche-lobe filling and the thermal state of the outflow. We find that systems with weak tidal influence and high-temperature winds produce nearly spherical and isotropic outflows, whereas more Roche-lobe-filling and cooler winds develop strong anisotropy and form two-tailed structures. We show that the commonly used 1D Parker wind model performs well only in the weak-tides regime, while including tidal corrections yields reasonable estimates of mass-loss rates and captures the mean radial density profile across all regimes, but fails to reproduce the intrinsically three-dimensional, angle-dependent nature of the flow as the outflow transitions from spherical to tidally structured tails. Motivated by these results, we develop a physically informed Mixture Model, calibrated using our 3D simulations, that accurately predicts mass-loss rates across the parameter space explored and outperforms 1D model with tidal corrections.

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arXiv2026

Case Study of a Super-eccentric Warm Jupiter Migrating via Equilibrium and Dynamical Tides

A leading theory for hot Jupiter formation is high-eccentricity migration, in which planets are born at large separations and excited to near-unity eccentricities, creating extreme tidal dissipation that shrinks and circularizes their orbits. The most direct evidence for this scenario is the detection of highly eccentric planets caught in the act of migrating. Although such planets are rare, the recent discovery of TIC 241249530 b - a super-eccentric ($e = 0.94$) and retrograde hot Jupiter progenitor - presents an ideal case study to explore high-eccentricity migration and the accompanying tidal physics. In this paper, we examine the migration history of TIC 241249530 b and test two different models of tidal dissipation: equilibrium tides and chaotic dynamical tides. We show that TIC 241249530 b's properties can be explained by high-eccentricity migration triggered by von Zeipel-Lidov-Kozai oscillations induced by the observed distant binary star in the system, but only if the dominant tidal dissipation takes the form of equilibrium tides. Chaotic dynamical tides fail to explain the system because they require the planet to have migrated substantially closer to its star.

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arXiv2026

Revealing the Origin of Desert Dwellers via Stellar Obliquities

Observations suggest that the hot Neptune desert contains the remnants of destroyed gas giants. Recent theoretical work has shown that gas giant destruction via Roche lobe overflow (RLO) can indeed populate the desert with remnant planets, but only if mass transfer removes most of the planet's orbital angular momentum ("lossy" RLO). Motivated by the fact that stellar accretion naturally gives rise to such lossy RLO, in this Letter we examine how planet-to-star mass and angular momentum transfer manifests in the distribution of stellar obliquities. We find that RLO tilts host stars into spin/orbit alignment (within a few ${\sim}$tens of degrees) regardless of initial conditions. Obliquity damping by RLO can only be reversed by the presence of misaligned companion planets within ${\lesssim}$2 au. While tides and mass transfer usually produce stellar spin up, host stars can also emerge from RLO slowly rotating if systems begin strongly retrograde; retrograde RLO reconciles theory with the anomalously slow rotation of the desert dweller host, LTT 9779. Predicted spin/orbit alignment may differentiate RLO from alternative giant planet destruction mechanisms, in particular hot Jupiter disruption during high eccentricity migration (which tends to produce broadly distributed stellar obliquities). We summarize other population-level predictions that can further distinguish RLO from high eccentricity migration. Our work suggests that follow-up obliquity measurements may reveal the formation pathways of desert dwellers, and potentially open a window into gas giants' exposed interiors.

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arXiv2026

Uncovering the Rapidly Evolving Orbits of the Dynamic TOI-201 System

Studying planetary interactions in exoplanet systems informs theories of planet formation and evolution, providing essential context for understanding our own solar system. We combine spectroscopy, transit photometry, transit timing variations, and astrometry to characterize the TOI-201 system. The co-transiting system consists of a super-Earth, warm Jupiter, and massive companion at 5.8, 53, and 2900 day orbital periods, respectively. We perform dynamical simulations to study the past and future of the system. von-Zeipel-Kozai-Lidov oscillations emerge as the most plausible scenario to explain the outer companion's high orbital eccentricity, with planet-planet scattering a possible but less likely contender. Due to non-zero mutual inclinations between the planets, the system is visibly evolving on very short timescales, with the current co-transiting configuration ending in 200 years.

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arXiv2026

Stability of Multiplanet Systems Through Hot Jupiter Destruction

Recent observational and theoretical work suggests that the sub-Jovian desert (periods ${\lesssim}3$ days, masses ${\sim}10{-}100 \ M_{\oplus}$) hosts the remains of destroyed hot Jupiters (``desert dwellers"). In this work, we explore how differing hot Jupiter destruction mechanisms -- Roche lobe overflow (RLO) vs. tidal disruption during high eccentricity migration (HEM) -- may be discerned observationally based on the presence of companion planets to desert dwellers. We show that gas giant destruction via RLO clears out the desert of any companions inside orbital periods ${\lesssim}$4 days; desert dwellers should sit alone in the desert if they form through this mechanism. Numerically mapping the instability threshold in planet mass and orbital distance, we find that the majority of observed companions to desert dwellers are safely in the stability region. RLO therefore does not preclude the existence of nearby companions beyond the desert, in contrast to gas giant tidal disruption during HEM. Further characterization of desert dweller systems may therefore elucidate the fates of hot Jupiters.

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arXiv2025

Are We There Yet? Challenges in Quantifying the Frequency of Earth Analogs in the Habitable Zone

Searching for life elsewhere in the universe is one of the most highly prioritized pursuits in astronomy today. However, the ability to observe evidence of Earth-like life through biosignatures is limited by the number of planets in the solar neighborhood with conditions similar to Earth. The occurrence rate of Earth-like planets in the habitable zones of Sun-like stars, $η_{\oplus}$, is therefore crucial for addressing the apparent lack of consensus on its value in the literature. Here we present a review of the current understanding of $η_{\oplus}$. We first provide definitions for parameters that contribute to $η_{\oplus}$. Then, we discuss the previous and current estimated parameter values and the context of the limitations on the analyses that produced these estimates. We compile an extensive list of the factors that go into any calculation of $η_{\oplus}$, and how detection techniques and surveys differ in their sensitivity and ability to accurately constrain $η_{\oplus}$. Understanding and refining the value of $η_{\oplus}$ is crucial for upcoming missions and telescopes, such as the planned Habitable Worlds Observatory and the Large Interferometer for Exoplanets, which aim to search for biosignatures on exoplanets in the solar neighborhood.

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arXiv2025

Shedding Light on Desert Dwellers

The ``sub-Jovian desert" ($2{\lesssim}R_{\rm p}{\lesssim}10 \ R_{\oplus}$, periods $\lesssim$3 days) is sparsely populated but no longer empty. Recent surveys have revealed that planets residing in the desert are dense ($ρ{\gtrsim}1$ g/cm$^{3}$), massive ($\sim$10${-}$50 $M_{\oplus}$), and orbit metal-rich stars that are indistinguishable from those hosting hot Jupiters. However, their origins remain mysterious. In this work we adopt and test the hypothesis that tidal destruction of hot Jupiters can populate the sub-Jovian desert with stripped remnant planets. We first show that stars hosting desert dwellers exhibit Galactic kinematics indicative of an older population descended from those hosting hot Jupiters. We highlight that tidally-driven Roche lobe overflow (RLO) can indeed populate the desert with planets similar to those observed, but only if angular momentum transfer during RLO is inefficient (``lossy" RLO). The entire width of the sub-Jovian desert can be backfilled with the remnants of hot Jupiters that possessed their empirically inferred spread in entropy. In this picture, current desert dwellers such as LTT 9779b should be tidally decaying at an observationally testable rate of ${\sim}0.5$ ms/yr. Our theory also predicts that desert dweller host stars may rotate up to an order of magnitude more rapidly than field stars; rotation period differences may persist ${\sim}$Gyr after RLO. Lossy RLO may also manifest as a burst of IR excess that could outshine the host star for up to ${\sim}10^{3}$ yr. If these predictions are confirmed by observations, our theory indicates that desert dwellers can be leveraged to study the interiors of giant planets in exquisite detail.

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arXiv2025

Coupled Planetary Interior and Tidal Evolution

We present a new planetary structure/thermal evolution model, designed for use in problems that couple orbital dynamics with planetary structure. We first benchmark our structural/thermal evolution calculations against the \texttt{MESA} stellar evolution code, finding excellent agreement across a wide range of planet mass, equilibrium temperature, entropy, and extra heating deposited at various depths in the planet. We then apply our method to study the tidal migration histories of Neptunes in the recently identified ``ridge" (periods ${\sim}3{-}6$ days), a feature that has been suggested to be populated via high eccentricity migration (HEM) of more distant Neptunes. We find that it is difficult to form a circularized Neptune in the ridge without instigating runaway tidal inflation and likely atmospheric destruction; low eccentricity Neptunes in the ridge can only be emplaced by HEM if they are metal-rich and exhibit finely-tuned tidal quality factors. If follow-up observations confirm that low eccentricity Neptunes in the ridge did arrive via HEM and are not strongly enriched in metals, our calculations indicate that their tidal heating mechanism must operate in the upper reaches of the planet to avoid runaway inflation. Gravity modes excited in upper radiative layers are a possible candidate mechanism, while friction in the core or turbulent dissipation in convective zones could be ruled out.

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arXiv2025

An Eccentric Sub-Neptune Moving Into the Evaporation Desert

Though missions such as Kepler, K2, and TESS have discovered $>$2,000 sub-Neptune and Neptunian planets, there is a dearth of such planets at close-in (P$\lesssim$3 days) orbits. This feature, called the Neptune desert or the evaporation desert, is believed to be primarily shaped by planetary migration and photoevaporation. However, this region is not completely devoid of planets--a small number of very hot Neptunes reside within the desert. These planets provide an opportunity to directly probe the effects of migration and photoevaporation. We present confirmation of TOI-5800 b, an eccentric sub-Neptune on a $\approx$2.6 day period that is likely actively undergoing tidal migration. We use radial velocity measurements from the Carnegie Planet Finder Spectrograph (PFS) to constrain TOI-5800 b's mass and eccentricity. We find that it has an unusually high eccentricity (0.39$\pm$0.07) for its short orbit. TOI-5800 is therefore currently experiencing high levels of tidal heating as it moves into the desert. Ranked as a top candidate for transmission and emission spectroscopy within its temperature and radius regime, TOI-5800 b is a prime target for atmospheric characterization with JWST. TOI-5800 b presents a unique opportunity to study the atmosphere of a planet undergoing tidal heating and to probe the composition of sub-Neptune planets.

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arXiv2025

Tidal Inflation is Stronger for Misaligned Neptune-Sized Planets Than Aligned Ones

Recent observations have revealed an intriguing abundance of polar-orbiting Neptune-sized planets, many of which exhibit unusually inflated radii. While such misaligned orbits point to a complex dynamical history, the connection between their orbital orientations and planetary structures remains poorly understood. In this study, we analyze a sample of 12 misaligned and 12 aligned planets using structure models that incorporate tidal heating. We use various statistical tests to demonstrate with at least $90\%$ confidence that misaligned planets experience more tidally-induced radius inflation compared to aligned planets. This inflation likely stems from their dynamically active histories, which often place them in close-in, eccentric, and highly inclined orbits. We further present a case study of WASP-107~b, an exceptionally inflated polar Neptune, and model its history using a simple coupled orbital and radius evolution approach. Our results place constraints on the planet's tidal quality factor that agree with recent JWST observations.

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arXiv2025

JWST Coronagraphic Images of 14 Her c: a Cold Giant Planet in a Dynamically Hot, Multi-planet System

Most observed multi-planet systems are coplanar, in a dynamically "cold" configuration of concentric orbits like our own Solar System. With the James Webb Space Telescope (JWST) we have detected 14 Her c, the first mature and cold exoplanet directly imaged in a dynamically "hot", multi-planet system. With large eccentricities and a nonzero mutual inclination, the present-day architecture of this system points to a turbulent past and ongoing angular momentum exchange between the planetary orbits of 14 Her b and c. The temperature of 14 Her c rivals both the coldest imaged exoplanet and the coldest known brown dwarf. Moreover, its photometry at 4.4 mu is consistent with the presence of carbon disequilibrium chemistry and water ice clouds in its atmosphere. 14 Her c presents a unique laboratory to study giant planet formation, dynamical evolution of multi-planet system architectures, and atmospheric composition and dynamics in extremely cold worlds.

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arXiv2025

The TOI-1117 Multi-planetary System: 3 sub-Neptunes, 1 in both the Neptunian Desert and Radius Valley

We present the discovery of three sub-Neptune planets around TOI-1117, a Sun-like star with mass $0.97\pm0.02M_{\odot}$, radius $1.05\pm0.03R_{\odot}$, age $4.42\pm1.50$ Gyr and effective temperature $5635\pm62$ K. Light curves from TESS and LCOGT show a transiting sub-Neptune with a $2.23$ day period, mass $M_b=8.90_{-0.96}^{+0.95}M_{\oplus}$ and radius $R_b=2.46_{-0.12}^{+0.13}R_{\oplus}$. This is a rare 'hot Neptune' that falls within the parameter spaces known as the 'Neptunian Desert' and the 'Radius Valley'. Two more planetary signals are detected in HARPS radial velocities, revealing two non-transiting planets with minimum masses $M_c=7.46_{-1.62}^{+1.43}M_{\oplus}$ and $M_d=9.06_{-1.78}^{+2.07}M_{\oplus}$, and periods of $4.579\pm0.004$ and $8.67\pm0.01$ days. The eccentricities were poorly constrained by the HARPS data, with upper limits $e_b=0.11$, $e_c=0.29$, and $e_d=0.24$. However, dynamical simulations of the TOI-1117 system, suggest that the orbits must be nearly circular to be stable. The simulations also show that TOI-1117b and c are likely to be in a near 2:1 resonance. The multi-planet nature of TOI-1117 makes it a more complex case for formation theories of the Neptunian Desert and Radius Valley, as current theories such as high-eccentricity migration are too turbulent to produce a stable, non-eccentric, multi-planet system. Moreover, analysis of TOI-1117b's photoevaporation history found rocky core and H/He atmosphere models to be inconsistent with observations, whilst water-rich scenarios were favoured.

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arXiv2024

Two Earth-size Planets and an Earth-size Candidate Transiting the Nearby Star HD 101581

We report the validation of multiple planets transiting the nearby ($d = 12.8$ pc) K5V dwarf HD 101581 (GJ 435, TOI-6276, TIC 397362481). The system consists of at least two Earth-size planets whose orbits are near a mutual 4:3 mean-motion resonance, HD 101581 b ($R_{p} = 0.956_{-0.061}^{+0.063}~R_{\oplus}$, $P = 4.47$ days) and HD 101581 c ($R_{p} = 0.990_{-0.070}^{+0.070}~R_{\oplus}$, $P = 6.21$ days). Both planets were discovered in Sectors 63 and 64 TESS observations and statistically validated with supporting ground-based follow-up. We also identify a signal that probably originates from a third transiting planet, TOI-6276.03 ($R_{p} = 0.982_{-0.098}^{+0.114}~R_{\oplus}$, $P = 7.87$ days). These planets are remarkably uniform in size and their orbits are evenly spaced, representing a prime example of the "peas-in-a-pod" architecture seen in other compact multi-planet systems. At $V = 7.77$, HD 101581 is the brightest star known to host multiple transiting planets smaller than $1.5~R_{\oplus}$. HD 101581 is a promising system for atmospheric characterization and comparative planetology of small planets.

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arXiv2024

Polar Neptunes are Stable to Tides

There is an intriguing and growing population of Neptune-sized planets with stellar obliquities near $\sim90^{\circ}$. One previously proposed formation pathway is a disk-driven resonance, which can take place at the end stages of planet formation in a system containing an inner Neptune, outer cold Jupiter, and protoplanetary disk. This mechanism occurs within the first $\sim10$ Myr, but most of the polar Neptunes we see today are $\sim$Gyrs old. Up until now, there has not been an extensive analysis of whether the polar orbits are stable over $\sim$Gyr timescales. Tidal realignment mechanisms are known to operate in other systems, and if they are active here, this would cause theoretical tension with a primordial misalignment story. In this paper, we explore the effects of tidal evolution on the disk-driven resonance theory. We use both $N$-body and secular simulations to study tidal effects on both the initial resonant encounter and long-term evolution. We find that the polar orbits are remarkably stable on $\sim$Gyr timescales. Inclination damping does not occur for the polar cases, although we do identify sub-polar cases where it is important. We consider two case study polar Neptunes, WASP-107 b and HAT-P-11 b, and study them in the context of this theory, finding consistency with present-day properties if their tidal quality factors are $Q \gtrsim 10^4$ and $Q \gtrsim 10^5$, respectively.

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arXiv2024

Formation of Close-in Neptunes Around Low-Mass Stars Through Breaking Resonant Chains

Conventional planet formation theories predict a paucity of massive planets around small stars, especially very low-mass ($0.1 - 0.3 \ M_{\odot}$) mid-to-late M dwarfs. Such tiny stars are expected to form planets of terrestrial sizes, but not much bigger. However, this expectation is challenged by the recent discovery of LHS 3154 b, a planet with period of 3.7 days and minimum mass of $13.2 \ M_{\oplus}$ orbiting a $0.11 \ M_{\odot}$ star. Here, we propose that close-in Neptune-mass planets like LHS 3154 b formed through an anomalous series of mergers from a primordial compact system of super-Earths. We perform simulations within the context of the "breaking the chains" scenario, in which super-Earths initially form in tightly-spaced chains of mean-motion resonances before experiencing dynamical instabilities and collisions. Planets as massive and close-in as LHS 3154 b ($M_p \sim 12 - 20 \ M_{\oplus}$, $P < 7$ days) are produced in $\sim$1% of simulated systems, in broad agreement with their low observed occurrence. These results suggest that such planets do not require particularly unusual formation conditions but rather are an occasional byproduct of a process that is already theorized to explain compact multi-planet systems. Interestingly, our simulated systems with LHS 3154 b-like planets also contain smaller planets at around $\sim 30$ days, offering a possible test of this hypothesis.

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arXiv2024

Resonant sub-Neptunes are puffier

A systematic, population-level discrepancy exists between the densities of exoplanets whose masses have been measured with transit timing variations (TTVs) versus those measured with radial velocities (RVs). Since the TTV planets are predominantly nearly resonant, it is still unclear whether the discrepancy is attributed to detection biases or to astrophysical differences between the nearly resonant and non resonant planet populations. We defined a controlled, unbiased sample of 36 sub-Neptunes characterised by Kepler, TESS, HARPS, and ESPRESSO. We found that their density depends mostly on the resonant state of the system, with a low probability (of $0.002_{-0.001}^{+0.010}$) that the mass of (nearly) resonant planets is drawn from the same underlying population as the bulk of sub-Neptunes. Increasing the sample to 133 sub-Neptunes reveals finer details: the densities of resonant planets are similar and lower than non-resonant planets, and both the mean and spread in density increase for planets that are away from resonance. This trend is also present in RV-characterised planets alone. In addition, TTVs and RVs have consistent density distributions for a given distance to resonance. We also show that systems closer to resonances tend to be more co-planar than their spread-out counterparts. These observational trends are also found in synthetic populations, where planets that survived in their original resonant configuration retain a lower density; whereas less compact systems have undergone post-disc giant collisions that increased the planet's density, while expanding their orbits. Our findings reinforce the claim that resonant systems are archetypes of planetary systems at their birth.

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