arXiv subjects

Brad M. S. Hansen

Publications and source records attributed to Brad M. S. Hansen.

At least 19 recordsLinked to original sources

arXiv2026

Generating eccentricity from envelope stripping in the Radius Valley

We demonstrate that the stripping of planetary envelopes can result in the excitation of planetary orbital eccentricity if the stripped gas remains in the system long enough to interact gravitationally and absorb angular momentum from the planetary orbit. We estimate that envelope mass fractions of a few percent can excite eccentricities of order 0.1 for single planets. This process can potentially explain the elevated eccentricities observed for planets whose radii lie in, or near, the radius valley. In multiple planet systems, the stripped gas can mediate angular momentum exchange between neighbouring planets, causing the separation to expand. Using data from the Kepler satellite, we find that planet pairs, whose members straddle the radius valley, do appear to be systematically wider than those in which both members are sub-Neptunes. These results support the idea that planets in, or just below, the radius valley are the stripped cores of planets that have lost mass, because the elevated eccentricities represent evidence that the gas remained in the system long enough to absorb additional angular momentum from the planet.

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arXiv2025

Dynamical Excitation as a probe of planetary origins

We present a set of numerical simulations of the dynamical evolution of compact planetary systems migrating in a protoplanetary disk whose inner edge is sculpted by the interaction with the stellar magnetic field, as described in Yu et al. (2023). We demonstrate that the resulting final distribution of neighbouring planet period ratios contains only a small surviving fraction of resonant systems, in accordance with observations. The resulting planetary architectures are largely in place by the end of the protoplanetary disk phase (within a few Myr), and do not require significant later dynamical evolution. The divergence of planetary pairs during gas disk dispersal also leads to the excitation of eccentricities when pairs cross mean motion resonances in a divergent fashion. The resulting distribution of remnant free eccentricities is consistent with the values inferred from the observation of transit durations and transit timing variations. We furthermore demonstrate that this conclusion is not significantly altered by tides, assuming standard values for tidal dissipation in Earth or Neptune-class planets. These results demonstrate that the observed spacing and residual dynamical excitation of compact planetary systems can be reproduced by migration through a protoplanetary disk, as long as the inner disk boundary is modelled as a gradual rollover, instead of a sharp transition. Such an effect can be achieved when the model accounts for the diffusion of the stellar magnetic field into the disk. The resulting divergence of planetary pairs during the magnetospheric rebound phase breaks the resonant chains, resulting in a better match to observations than disk models with more traditional inner boundaries.

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arXiv2025

Planet Masses, Radii, and Orbits from NASA's K2 Mission

We report the masses, sizes, and orbital properties of 86 planets orbiting 55 stars observed by NASA's K2 Mission with follow-up Doppler measurements by the HIRES spectrometer at the W. M. Keck Observatory and the Automated Planet Finder at Lick Observatory. Eighty-one of the planets were discovered from their transits in the K2 photometry, while five were found based on subsequent Doppler measurements of transiting planet host stars. The sizes of the transiting planets range from Earth-size to larger than Jupiter (1-3 REarth is typical), while the orbital periods range from less than a day to a few months. For 32 of the planets, the Doppler signal was detected with significance greater than 5-sigma (51 were detected with >3-sigma significance). An important characteristic of this catalog is the use of uniform analysis procedures to determine stellar and planetary properties. This includes the transit search and fitting procedures applied to the K2 photometry, the Doppler fitting techniques applied to the radial velocities, and the spectral modeling to determine bulk stellar parameters. Such a uniform treatment will make the catalog useful for statistical studies of the masses, densities, and system architectures of exoplanetary systems. This work also serves as a data release for all previously unpublished RVs and associated stellar activity indicators obtained by our team for these systems, along with derived stellar and planet parameters.

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arXiv2024

Widespread disruption of resonant chains during protoplanetary disk dispersal

We describe the evolution of low mass planets in a dispersing protoplanetary disk around a Solar mass star. The disk model is based on the results of Yu, Hansen & Hasegawa (2023), which describes a region of the inner disk where the direction of the migration torque is outwards due to the diffusion of the stellar magnetic field into the disk and the resultant gradual increase in surface density outwards. We demonstrate that the magnetospheric rebound phase in such a disk leads to diverging orbits for double and triple planet systems, and the disruption of a high fraction of the initial resonant chains. We present simulations of three planet systems with masses based on the observed triple planet systems observed by the Kepler satellite within the context of this model. The final distribution of nearest neighbour period ratios provides an excellent fit to the observations, provided that the initial resonant configurations are compact. The occurrence rate of planets as a function of orbital period also provides a good match to the observations, for final orbital periods P<20 days. These results suggest that the period and period ratio distributions of low mass planets are primarily set in place during the disk dispersal epoch, and may not require significant dynamical evolution thereafter.

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arXiv2024

Bound circumplanetary orbits under the influence of radiation pressure: Application to dust in directly imaged exoplanet systems

We examine the population of simply periodic orbits in the Hill problem with radiation pressure included, in order to understand the distribution of gravitationally bound dust in orbit around a planet. We study a wide range of radiation pressure strengths, which requires the inclusion of additional terms beyond those discussed in previous analyses of this problem. In particular, our solutions reveal two distinct populations of stable wide, retrograde, orbits, as opposed to the single family that exists in the purely gravitational problem. We use the result of these calculations to study the observational shape of dust populations bound to extrasolar planets, that might be observable in scattered or reradiated light. In particular, we find that such dusty clouds should be elongated along the star--planet axis, and that the elongation of the bound population increases with $β$, a measure of the strength of the radiation pressure. As an application of this model, we consider the properties of the Fomalhaut system. The unusual orbital properties of the object Fomalhaut~b can be explained if the observed light was scattered by dust that was released from an object in a quasi-satellite orbit about a planet located in, or near, the observed debris ring. Within the context of the model of Hayakawa \& Hansen (2023), we find that the dust cloud around such a planet is still approximately an order of magnitude fainter than the limits set by current JWST data.

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arXiv2019

The Chaotic Nature of TRAPPIST-1 Planetary Spin States

The TRAPPIST-1 system has 7 known terrestrial planets arranged compactly in a mean motion resonant chain around an ultra-cool central star, some within the estimated habitable zone. Given their short orbital periods of just a few days, it is often presumed that the planets are tidally locked such that the spin rate is equal to that of the orbital mean motion. However, the compact, and resonant, nature of the system implies that there can be significant variations in the mean motion of these planets due to their mutual interactions. We show that such fluctuations can then have significant effects on the spin states of these planets. In this paper, we analyze, using detailed numerical simulations, the mean motion histories of the three planets that are thought to lie within or close to the habitable zone of the system: planets d, e, and f. We demonstrate that, depending on the strength of the mutual interactions within the system, these planets can be pushed into spin states which are effectively non-synchronous. We find that it can produce significant libration of the spin state, if not complete circulation in the frame co-rotating with the orbit. We also show that these spin states are likely to be unable to sustain long-term stability, with many of our simulations suggesting that the spin evolves, under the influence of tidal synchronization forces, into quasi-stable attractor states, which last on timescales of thousands of years.

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arXiv2017

On the Spin States of Habitable Zone Exoplanets Around M Dwarfs: The Effect of a Near-Resonant Companion

One longstanding problem for the potential habitability of planets within M dwarf systems is their likelihood to be tidally locked in a synchronously rotating spin state. This problem thus far has largely been addressed only by considering two objects: the star and the planet itself. However, many systems have been found to harbor multiple planets, with some in or very near to mean-motion resonances. The presence of a planetary companion near a mean-motion resonance can induce oscillatory variations in the mean-motion of the planet, which we demonstrate can have significant effects on the spin-state of an otherwise synchronously rotating planet. In particular, we find that a planetary companion near a mean-motion resonance can excite the spin states of planets in the habitable zone of small, cool stars, pushing otherwise synchronously rotating planets into higher amplitude librations of the spin state, or even complete circulation resulting in effective stellar days with full surface coverage on the order of years or decades. This increase in illuminated area can have potentially dramatic influences on climate, and thus on habitability. We also find that the resultant spin state can be very sensitive to initial conditions due to the chaotic nature of the spin state at early times within certain regimes. We apply our model to two hypothetical planetary systems inspired by the K00255 and TRAPPIST-1 systems, which both have Earth-sized planets in mean-motion resonances orbiting cool stars.

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arXiv2017

Three's Company: An additional non-transiting super-Earth in the bright HD 3167 system, and masses for all three planets

HD 3167 is a bright (V = 8.9), nearby K0 star observed by the NASA K2 mission (EPIC 220383386), hosting two small, short-period transiting planets. Here we present the results of a multi-site, multi-instrument radial velocity campaign to characterize the HD 3167 system. The masses of the transiting planets are 5.02+/-0.38 MEarth for HD 3167 b, a hot super-Earth with a likely rocky composition (rho_b = 5.60+2.15-1.43 g/cm^3), and 9.80+1.30-1.24 MEarth for HD 3167 c, a warm sub-Neptune with a likely substantial volatile complement (rho_c = 1.97+0.94-0.59 g/cm^3). We explore the possibility of atmospheric composition analysis and determine that planet c is amenable to transmission spectroscopy measurements, and planet b is a potential thermal emission target. We detect a third, non-transiting planet, HD 3167 d, with a period of 8.509+/-0.045 d (between planets b and c) and a minimum mass of 6.90+/-0.71 MEarth. We are able to constrain the mutual inclination of planet d with planets b and c: we rule out mutual inclinations below 1.3 degrees as we do not observe transits of planet d. From 1.3-40 degrees, there are viewing geometries invoking special nodal configurations which result in planet d not transiting some fraction of the time. From 40-60 degrees, Kozai-Lidov oscillations increase the system's instability, but it can remain stable for up to 100Myr. Above 60 degrees, the system is unstable. HD 3167 promises to be a fruitful system for further study and a preview of the many exciting systems expected from the upcoming NASA TESS mission.

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arXiv2017

K2-66b and K2-106b: Two extremely hot sub-Neptune-size planets with high densities

We report precise mass and density measurements of two extremely hot sub-Neptune-size planets from the K2 mission using radial velocities, K2 photometry, and adaptive optics imaging. K2-66 harbors a close-in sub-Neptune-sized (2.49$^{+0.34}_{-0.24} R_\oplus$) planet (K2-66b) with a mass of 21.3 $\pm$ 3.6 $M_\oplus$. Because the star is evolving up the sub-giant branch, K2-66b receives a high level of irradiation, roughly twice the main sequence value. K2-66b may reside within the so-called "photoevaporation desert", a domain of planet size and incident flux that is almost completely devoid of planets. Its mass and radius imply that K2-66b has, at most, a meager envelope fraction (< 5%) and perhaps no envelope at all, making it one of the largest planets without a significant envelope. K2-106 hosts an ultra-short-period planet ($P$ = 13.7 hrs) that is one of the hottest sub-Neptune-size planets discovered to date. Its radius (1.82$^{+0.20}_{-0.14} R_\oplus$) and mass (9.0 $\pm$ 1.6 $M_\oplus$) are consistent with a rocky composition, as are all other small ultra-short-period planets with well-measured masses. K2-106 also hosts a larger, longer-period planet (Rp = 2.77$^{+0.37}_{-0.23} R_\oplus$, $P$ = 13.3 days) with a mass less than 24.4 $M_\oplus$ at 99.7% confidence. K2-66b and K2-106b probe planetary physics in extreme radiation environments. Their high densities reflect the challenge of retaining a substantial gas envelope in such extreme environments.

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arXiv2016

Mass Constraints of the WASP-47 Planetary System from Radial Velocities

We report precise radial velocity (RV) measurements of WASP-47, a G star that hosts three transiting planets in close proximity (a hot Jupiter, a super-Earth and a Neptune-sized planet) and a non-transiting planet at 1.4 AU. Through a joint analysis of previously published RVs and our own Keck-HIRES RVs, we significantly improve the planet mass and bulk density measurements. For the super-Earth WASP-47e ($P$ = 0.79 days), we measure a mass of 9.11 $\pm$ 1.17 $M_\oplus$, and a bulk density of 7.63 $\pm$ 1.90 g cm$^{-3}$, consistent with a rocky composition. For the hot Jupiter WASP-47b ($P$ = 4.2 days), we measure a mass of 356 $\pm$ 12 $M_\oplus$ (1.12 $\pm$ 0.04 $M_\rm{Jup}$) and constrain its eccentricity to $<0.021$ at 3-$σ$ confidence. For the Neptune-size planet WASP-47d ($P$ = 9.0 days), we measure a mass of 12.75 $\pm$ 2.70 $M_\oplus$, and a bulk density of 1.36 $\pm$ 0.42 g cm$^{-3}$, suggesting it has a thick H/He envelope. For the outer non-transiting planet, we measure a minimum mass of 411 $\pm$ 18 $M_\oplus$ (1.29 $\pm$ 0.06 $M_\rm{Jup}$), an orbital period of 595.7 $\pm$ 5.0 days, and an orbital eccentricity of 0.27 $\pm$ 0.04. Our new measurements are consistent with but 2$-$4$\times$ more precise than previous mass measurements.

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arXiv2016

197 Candidates and 104 Validated Planets in K2's First Five Fields

We present 197 planet candidates discovered using data from the first year of the NASA K2 mission (Campaigns 0-4), along with the results of an intensive program of photometric analyses, stellar spectroscopy, high-resolution imaging, and statistical validation. We distill these candidates into sets of 104 validated planets (57 in multi-planet systems), 30 false positives, and 63 remaining candidates. Our validated systems span a range of properties, with median values of R_P = 2.3 R_E, P=8.6 d, Tef = 5300 K, and Kp=12.7 mag. Stellar spectroscopy provides precise stellar and planetary parameters for most of these systems. We show that K2 has increased by 30% the number of small planets known to orbit moderately bright stars (1-4 R_E, Kp=9-13 mag). Of particular interest are 37 planets smaller than 2 R_E, 15 orbiting stars brighter than Kp=11.5, five receiving Earth-like irradiation levels, and several multi-planet systems -- including four planets orbiting the M dwarf K2-72 near mean-motion resonances. By quantifying the likelihood that each candidate is a planet we demonstrate that our candidate sample has an overall false positive rate of 15-30%, with rates substantially lower for small candidates (< 2 R_E) and larger for candidates with radii > 8 R_E and/or with P < 3 d. Extrapolation of the current planetary yield suggests that K2 will discover between 500-1000 planets in its planned four-year mission -- assuming sufficient follow-up resources are available. Efficient observing and analysis, together with an organized and coherent follow-up strategy, is essential to maximize the efficacy of planet-validation efforts for K2, TESS, and future large-scale surveys.

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arXiv2016

Eleven Multi-planet Systems from K2 Campaigns 1 & 2 and the Masses of Two Hot Super-Earths

We present a catalog of 11 multi-planet systems from Campaigns 1 and 2 of the K2 mission. We report the sizes and orbits of 26 planets split between seven 2-planet systems and four 3-planet systems. These planets stem from a systematic search of the K2 photometry for all dwarf stars observed by K2 in these fields. We precisely characterized the host stars with adaptive optics imaging and analysis of high-resolution optical spectra from Keck/HIRES and medium-resolution spectra from IRTF/SpeX. We confirm two planet candidates by mass detection and validate the remaining 24 candidates to $>99\%$ confidence. Thirteen planets were previously validated or confirmed by other studies and 24 were previously identified as planet candidates. The planets are mostly smaller than Neptune (21/26 planets) as in the Kepler mission and all have short periods ($P < 50$ d) due to the duration of the K2 photometry. The host stars are relatively bright (most have $Kp < 12.5$ mag) and are amenable to follow-up characterization. For K2-38, we measured precise radial velocities using Keck/HIRES and provide initial estimates of the planet masses. K2-38b is a short-period super-Earth with a radius of $1.55 \pm 0.16~R_\oplus$, a mass of $12.0 \pm 2.9~M_\oplus$, and a high density consistent with an iron-rich composition. The outer planet K2-38c is a lower density sub-Neptune-size planet with a radius of $2.42 \pm 0.29~R_\oplus$ and a mass of $9.9 \pm 4.6~M_\oplus$ that likely has a substantial envelope. This new planet sample demonstrates the capability of K2 to discover numerous planetary systems around bright stars.

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arXiv2016

Two Transiting Low Density Sub-Saturns from K2

We report the discovery and confirmation of two sub-Saturn planets orbiting a bright (V = 11.3), metal-rich ([Fe/H] = 0.42 $\pm$ 0.04 dex) G3 dwarf in the K2 Campaign 2 field. The planets are 5.68 $\pm$ 0.56 Earth-radii and 7.82 $\pm$ 0.72 Earth-radii and have orbital periods of 20.8851 $\pm$ 0.0003 d and 42.3633$\pm$0.0006 d, near to the 2:1 mean-motion resonance. We obtained 32 radial velocities (RVs) with Keck/HIRES and detected the reflex motion due to EPIC-203771098b and c. These planets have masses of 21.0 $\pm$ 5.4 Earth-masses and 27.0 $\pm$ 6.9 Earth-masses, respectively. With low densities of 0.63 $\pm$ 0.25 g/cc and 0.31 $\pm$ 0.12 g/cc, respectively, the planets require thick envelopes of H/He to explain their large sizes and low masses. Interior structure models predict that the planets have fairly massive cores of 17.6 $\pm$ 4.3 Earth-masses and 16.1 $\pm$ 4.2 Earth-masses, respectively. They may have formed exterior to their present locations, accreted their H/He envelopes at large orbital distances, and migrated in as a resonant pair. The proximity to resonance, large transit depths, and host star brightness offer rich opportunities for TTV follow-up. Finally, the low surface gravities of the EPIC-203771098 planets make them favorable targets for transmission spectroscopy by HST, Spitzer, and JWST.

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arXiv2015

Two Transiting Earth-size Planets Near Resonance Orbiting a Nearby Cool Star

Discoveries from the prime Kepler mission demonstrated that small planets (< 3 Earth-radii) are common outcomes of planet formation. While Kepler detected many such planets, all but a handful orbit faint, distant stars and are not amenable to precise follow up measurements. Here, we report the discovery of two small planets transiting K2-21, a bright (K = 9.4) M0 dwarf located 65$\pm$6 pc from Earth. We detected the transiting planets in photometry collected during Campaign 3 of NASA's K2 mission. Analysis of transit light curves reveals that the planets have small radii compared to their host star, 2.60 $\pm$ 0.14% and 3.15 $\pm$ 0.20%, respectively. We obtained follow up NIR spectroscopy of K2-21 to constrain host star properties, which imply planet sizes of 1.59 $\pm$ 0.43 Earth-radii and 1.92 $\pm$ 0.53 Earth-radii, respectively, straddling the boundary between high-density, rocky planets and low-density planets with thick gaseous envelopes. The planets have orbital periods of 9.32414 days and 15.50120 days, respectively, and have a period ratio of 1.6624, very near to the 5:3 mean motion resonance, which may be a record of the system's formation history. Transit timing variations (TTVs) due to gravitational interactions between the planets may be detectable using ground-based telescopes. Finally, this system offers a convenient laboratory for studying the bulk composition and atmospheric properties of small planets with low equilibrium temperatures.

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arXiv2015

A nearby M star with three transiting super-Earths discovered by K2

Small, cool planets represent the typical end-products of planetary formation. Studying the archi- tectures of these systems, measuring planet masses and radii, and observing these planets' atmospheres during transit directly informs theories of planet assembly, migration, and evolution. Here we report the discovery of three small planets orbiting a bright (Ks = 8.6 mag) M0 dwarf using data collected as part of K2, the new transit survey using the re-purposed Kepler spacecraft. Stellar spectroscopy and K2 photometry indicate that the system hosts three transiting planets with radii 1.5-2.1 R_Earth, straddling the transition region between rocky and increasingly volatile-dominated compositions. With orbital periods of 10-45 days the planets receive just 1.5-10x the flux incident on Earth, making these some of the coolest small planets known orbiting a nearby star; planet d is located near the inner edge of the system's habitable zone. The bright, low-mass star makes this system an excellent laboratory to determine the planets' masses via Doppler spectroscopy and to constrain their atmospheric compositions via transit spectroscopy. This discovery demonstrates the ability of K2 and future space-based transit searches to find many fascinating objects of interest.

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arXiv2014

In Situ Models for Planet Assembly around Cool stars

We present a model for the in situ assembly of planetary systems around a 0.5~$M_{\odot}$ star, and compare the resulting statistics to the observed sample of cool Kepler planet candidates from Dressing & Charbonneau (2013). We are able to reproduce the distribution of planetary periods and period ratios, although we once again find an underabundance of single transit systems relative to the observations. We also demonstrate that almost every planetary system assembled in this fashion contains at least one planet in the habitable zone, and that water delivery to these planets can potentially produce a water content comparable to that of Earth. Our results broadly support the notion that habitable planets are plentiful around M~dwarfs in the solar neighbourhood.

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arXiv2014

Eccentric Planets and Stellar Evolution as a Cause of Polluted White Dwarfs

A significant fraction of white dwarfs (WDs) are observed to be polluted with metals despite high surface gravities and short settling times. The current theoretical model for this pollution is accretion of rocky bodies delivered to the WD through perturbations by orbiting planets. Using N-body simulations, we examine the possibility of a single planet as the source of pollution. We determine the stability of test particles on circular orbits in systems with a single planet located at 4 au for a range of masses and eccentricities, comparing the fractions that are ejected and accreted. In particular, we compare the instabilities that develop before and after the star loses mass to form a WD, a process which causes orbiting bodies to migrate outward. We determine that a planet must be eccentric (e > 0.02) to deliver significant (> 0.5 per cent) amounts of material to the host and that the amount increases with the planetary eccentricity. This result is robust with respect to the initial eccentricities of the particles for planetary eccentricity above ~0.4 and for randomly-distributed particle long. of pericentre. We also find that the efficiency of pollution is enhanced as planetary mass is reduced. We demonstrate that a 0.03 M_Jup planet with substantial eccentricity (e > 0.4) can account for the observed levels of pollution for initial disc masses of order 1 M_Earth. Such discs are within the range estimated for initial planetesimals discs and below that estimated for the solar system. However, their survival to the WD stage is uncertain as estimates for the collisional evolution of planetesimal discs suggest they should be ground down below the required levels on Gyr timescales. Thus, planetary scattering by eccentric, sub-Jovian planets can explain the observed levels of WD pollution, but only if current estimates of the collisional erosion of planetesimal discs are in error.

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arXiv2013

Warm Ice Giant GJ 3470b. I. A Flat Transmission Spectrum Indicates a Hazy, Low-methane, and/or Metal-rich Atmosphere

We report our spectroscopic investigation of the transiting ice giant GJ 3470b's atmospheric transmission, and the first results of extrasolar planet observations from the new Keck/MOSFIRE spectrograph. We measure a planet/star radius ratio of Rp/Rs = 0.0789 +/- 0.0020 in a bandpass from 2.09-2.36 micron and in six narrower bands across this wavelength range. When combined with existing broadband photometry, these measurements rule out cloud-free atmospheres in chemical equilibrium assuming either solar abundances (5.4 sigma confidence) or a moderate level of metal enrichment (50x solar abundances, 3.8 sigma), confirming previous results that such models are not representative for cool, low-mass, externally irradiated extrasolar planets. Current measurements are consistent with a flat transmission spectrum, which suggests that the atmosphere is explained by high-altitude clouds and haze, disequilibrium chemistry, unexpected abundance patterns, or the atmosphere is extremely metal-rich (>200x solar). Because GJ 3470b's low bulk density sets an upper limit on the planet's atmospheric enrichment of <300x solar, the atmospheric mean molecular weight must be <9. Thus, if the atmosphere is cloud-free its spectral features should be detectable with future observations. Transit observations at shorter wavelengths will provide the best opportunity to discriminate between plausible scenarios. We obtained optical spectroscopy with the GMOS spectrograph, but these observations exhibit large systematic uncertainties owing to thin, persistent cirrus conditions. Finally, we also provide the first detailed look at the steps necessary for well-calibrated MOSFIRE observations, and provide advice for future observations with this instrument.

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