High-eccentricity Migration Research Articles

BOWIE-ALIGN: how formation and migration histories of giant planets impact atmospheric compositions

ABSTRACT Hot Jupiters present a unique opportunity for measuring how planet formation history shapes present-day atmospheric composition. However, due to the myriad pathways influencing composition, a well-constructed sample of planets is needed to determine whether formation history can be accurately traced back from atmospheric composition. To this end, the BOWIE-ALIGN survey (A spectral Light Investigation into hot gas Giant origiNs by the collaboration of Bristol, Oxford, Warwick, Imperial, Exeter, +) will compare the compositions of eight hot Jupiters around F stars, four with orbits aligned with the stellar rotation axis, and four misaligned. Using the alignment as an indicator for planets that underwent disc migration or high-eccentricity migration, one can determine whether migration history produces notable differences in composition between the two samples of planets. This paper describes the planet formation model that motivates our observing programme. Our model traces the accretion of chemical components from the gas and dust in the disc over a broad parameter space to create a full, unbiased model sample from which we can estimate the range of final atmospheric compositions. For high metallicity atmospheres ($\mathrm{ O}\mathrm{ /H}\ge 10 \times$ solar), the C/O ratios of aligned and misaligned planets diverge, with aligned planets having lower C/O ($\lt 0.25$) due to the accretion of oxygen-rich silicates from the inner disc. However, silicates may rain out instead of releasing their oxygen into the atmosphere. This would significantly increase the C/O of aligned planets (C/O $\gt 0.6$), inverting the trend between the aligned and misaligned planets. Nevertheless, by comparing statistically significant samples of aligned and misaligned planets, we expect atmospheric composition to constrain how planets form.

The OATMEAL Survey. I. Low Stellar Obliquity in the Transiting Brown Dwarf System GPX-1

We introduce the OATMEAL survey, an effort to measure the obliquities of stars with transiting brown dwarf companions. We observed a transit of the close-in (P orb = 1.74 days) brown dwarf GPX-1 b using the Keck Planet Finder spectrograph to measure the sky-projected angle between its orbital axis and the spin axis of its early F-type host star (λ). We measured λ = 6.°9 ± 10.°0, suggesting an orbit that is prograde and well aligned with the stellar equator. Hot Jupiters around early F stars are frequently found to have highly misaligned orbits, with polar and retrograde orbits being commonplace. It has been theorized that these misalignments stem from dynamical interactions, such as von Zeipel–Kozai–Lidov cycles, and are retained over long timescales due to weak tidal dissipation in stars with radiative envelopes. By comparing GPX-1 to similar systems under the frameworks of different tidal evolution theories, we argued that the rate of tidal dissipation is too slow to have re-aligned the system. This suggests that GPX-1 may have arrived at its close-in orbit via coplanar high-eccentricity migration or migration through an aligned protoplanetary disk. Our result for GPX-1 is one of few measurements of the obliquity of a star with a transiting brown dwarf. By enlarging the number of such measurements and comparing them with hot-Jupiter systems, we will more clearly discern the differences between the mechanisms that dictate the formation and evolution of both classes of objects.

KPF Confirms a Polar Orbit for KELT-18 b

We present the first spectroscopic transit results from the newly commissioned Keck Planet Finder on the Keck-I telescope at W. M. Keck Observatory. We observed a transit of KELT-18 b, an inflated ultrahot Jupiter orbiting a hot star (T eff = 6670 K) with a binary stellar companion. By modeling the perturbation to the measured cross-correlation functions using the Reloaded Rossiter–McLaughlin technique, we derived a sky-projected obliquity of λ = − 94.°8 ± 0.°7 ( ψ=93.8−1.8+1.6° for isotropic i ⋆). The data are consistent with an extreme stellar differential rotation (α = 0.9), though a more likely explanation is moderate center-to-limb variations of the emergent stellar spectrum. We see additional evidence for the latter from line widths increasing toward the limb. Using loose constraints on the stellar rotation period from observed variability in the available TESS photometry, we were able to constrain the stellar inclination and thus the true 3D stellar obliquity to ψ=91.7−1.8+2.2° . KELT-18 b could have obtained its polar orbit through high-eccentricity migration initiated by Kozai–Lidov oscillations induced by the binary stellar companion KELT-18 B, as the two likely have a large mutual inclination as evidenced by Gaia astrometry. KELT-18 b adds another data point to the growing population of close-in polar planets, particularly around hot stars.

HATS-38 b and WASP-139 b Join a Growing Group of Hot Neptunes on Polar Orbits* * Based on observations made with the ESO telescopes at the La Silla Paranal Observatory under program ID 111.24VT.001, 111.24VT.002, and 112.25W1.001.

We constrain the sky-projected obliquities of two low-density hot Neptune planets, HATS-38 b and WASP-139 b, orbiting nearby G and K stars using Rossiter–McLaughlin (RM) observations with VLT/ESPRESSO, yielding λ=−108−16+11 deg and −85.6−4.2+7.7 deg, respectively. To model the RM effect, we use a new publicly available code, ironman, which is capable of jointly fitting transit photometry, Keplerian radial velocities, and RM effects. WASP-139 b has a residual eccentricity e=0.103−0.041+0.050 while HATS-38 b has an eccentricity of e=0.112−0.070+0.072 , which is compatible with a circular orbit given our data. Using the obliquity constraints, we show that they join a growing group of hot and low-density Neptunes on polar orbits. We use long-term radial velocities to rule out companions with masses ∼0.3–50 M J within ∼10 au. We show that the orbital architectures of the two Neptunes can be explained with high-eccentricity migration from ≳2 au driven by an unseen distant companion. If HATS-38 b has no residual eccentricity, its polar and circular orbit can also be consistent with a primordial misalignment. Finally, we perform a hierarchical Bayesian modeling of the true obliquity distribution of Neptunes and find suggestive evidence for a higher preponderance of polar orbits for hot Neptunes compared to Jupiters. However, we note that the exact distribution is sensitive to the choice of priors, highlighting the need for additional obliquity measurements of Neptunes to robustly compare the hot Neptune obliquity distribution to that of Jupiters.

Evidence for Primordial Alignment: Insights from Stellar Obliquity Measurements for Compact Sub-Saturn Systems

Despite decades of effort, the mechanisms by which the spin axis of a star and the orbital axes of its planets become misaligned remain elusive. In particular, it is of great interest whether the large spin–orbit misalignments observed are driven primarily by high-eccentricity migration—expected to have occurred for short-period, isolated planets—or reflect a more universal process that operates across systems with a variety of present-day architectures. Compact multiplanet systems offer a unique opportunity to differentiate between these competing hypotheses, as their tightly packed configurations preclude violent dynamical histories, including high-eccentricity migration, allowing them to trace the primordial disk plane. In this context, we report measurements of the sky-projected stellar obliquity (λ) via the Rossiter–McLaughlin effect for two sub-Saturns in multiple-transiting systems: TOI-5126 b (λ = 1 ± 48°) and TOI-5398 b ( λ=−8.1−6.3+5.3° ). Both are spin–orbit aligned, joining a fast-growing group of just three other compact sub-Saturn systems, all of which exhibit spin–orbit alignment. In aggregate with archival data, our results strongly suggest that sub-Saturn systems are primordially aligned and become misaligned largely in the postdisk phase, as appears to be the case increasingly for other exoplanet populations.

Obliquity Constraints for the Extremely Eccentric Sub-Saturn Kepler-1656 b

The orbits of close-in exoplanets provide clues to their formation and evolutionary history. Many close-in exoplanets likely formed far out in their protoplanetary disks and migrated to their current orbits, perhaps via high-eccentricity migration (HEM), a process that can also excite obliquities. A handful of known exoplanets are perhaps caught in the act of HEM, as they are observed on highly eccentric orbits with tidal circularization timescales shorter than their ages. One such exoplanet is Kepler-1656 b, which is also the only known nongiant exoplanet (<100 M ⊕) with an extreme eccentricity (e = 0.84). We measured the sky-projected obliquity of Kepler-1656 b by observing the Rossiter–McLaughlin effect during a transit with the Keck Planet Finder. Our data are consistent with an aligned orbit but are also consistent with moderate misalignment with λ < 50° at 95% confidence, with the most likely solution of 35.0−21.6+14.9 deg. A low obliquity would be an unlikely outcome of most eccentricity-exciting scenarios, but we show that the properties of the outer companion in the system are consistent with the coplanar HEM mechanism. Alternatively, if the system is not relatively coplanar (≲20° mutual inclination), Kepler-1656 b may be presently at a rare snapshot of long-lived eccentricity oscillations that do not induce migration. Kepler-1656 b is only the fourth exoplanet with e > 0.8 to have its obliquity constrained; expanding this population will help establish the degree to which orbital misalignment accompanies migration. Future work that constrains the mutual inclinations of outer perturbers will be key for distinguishing plausible mechanisms.

Tracking Advanced Planetary Systems (TAPAS) with HARPS-N

Aims. The star HD 118203, classified as a K0 subgiant, was known to harbour a transiting hot Jupiter planet on a 6.1-day eccentric orbit. Previous studies also revealed a linear trend in the radial velocity (RV) domain, indicative of a companion on a wide orbit. Such a hierarchical orbital architecture could be helpful in studies of the origins of hot Jupiters. Methods. We acquired precise RV measurements over 17 yr using the 9.2 m Hobby-Eberly Telescope and the 3.6 m Telescopio Nazionale Galileo. Combining these observations with space-born photometric time series from the Transiting Exoplanet Survey Satellite, we constructed a two-planetary model for the system. Astrometric observations from HIPPARCOS and Gaia were used to constrain the orbital inclination of the wide-orbit companion and its mass. Numerical simulations were used to investigate the dynamics of the system. The photometric data were searched for additional transit-like flux drops. Results. We found that the additional companion is an 11-Jupiter mass planet orbiting HD 118203 on a 14-yr moderately eccentric orbit, constituting a hierarchical planetary system with the hot Jupiter. Both planets were found to be dynamically decoupled mainly due to the general relativistic apsidal precession of the inner planet, marginalising secular interactions. The orbits of both planets might have a relatively low mutual inclination unless the longitudes of the ascending node differ substantially. This configuration favours the coplanar high-eccentricity migration as a path to the present-day orbital configuration. No other transiting planets with radii down to 2 Earth radii and orbital periods less than 100 days were found in the system.

TOI–757 b: an eccentric transiting mini–Neptune on a 17.5–d orbit

ABSTRACT We report the spectroscopic confirmation and fundamental properties of TOI$-$757 b, a mini$-$Neptune on a 17.5$-$d orbit transiting a bright star ($V\, =\, 9.7$ mag) discovered by the TESS mission. We acquired high$-$precision radial velocity measurements with the HARPS, ESPRESSO, and PFS spectrographs to confirm the planet detection and determine its mass. We also acquired space$-$borne transit photometry with the CHEOPS space telescope to place stronger constraints on the planet radius, supported with ground$-$based LCOGT photometry. WASP and KELT photometry were used to help constrain the stellar rotation period. We also determined the fundamental parameters of the host star. We find that TOI$-$757 b has a radius of $R_{\mathrm{p}} = 2.5 \pm 0.1 R_{\oplus }$ and a mass of $M_{\mathrm{p}} = 10.5^{+2.2}_{-2.1} M_{\oplus }$, implying a bulk density of $\rho _{\text{p}} = 3.6 \pm 0.8$ g cm$^{-3}$. Our internal composition modelling was unable to constrain the composition of TOI$-$757 b, highlighting the importance of atmospheric observations for the system. We also find the planet to be highly eccentric with e = 0.39$^{+0.08}_{-0.07}$, making it one of the very few highly eccentric planets among precisely characterized mini$-$Neptunes. Based on comparisons to other similar eccentric systems, we find a likely scenario for TOI$-$757 b’s formation to be high eccentricity migration due to a distant outer companion. We additionally propose the possibility of a more intrinsic explanation for the high eccentricity due to star$-$star interactions during the earlier epoch of the Galactic disc formation, given the low metallicity and older age of TOI$-$757.

Host-star Properties of Hot, Warm, and Cold Jupiters in the Solar Neighborhood from Gaia Data Release 3: Clues to Formation Pathways

Giant planets exhibit diverse orbital properties, hinting at their distinct formation and dynamic histories. In this paper, using Gaia Data Release 3 (DR3), we investigate if and how the orbital properties of Jupiters are linked to their host star properties, particularly their metallicity and age. We obtain metallicities for main-sequence stars of spectral type F, G, and K, hosting hot, warm, and cold Jupiters with varying eccentricities. We compute the velocity dispersions of the host stars of these three groups using kinematic information from Gaia DR3 and obtain average ages using a velocity dispersion–age relation. We find that the host stars of hot Jupiters are relatively metal rich ([Fe/H] = 0.18 ± 0.13) and young (median age of 3.97 ± 0.51 Gyr) compared to the host stars of cold Jupiters in nearly circular orbits, which are relatively metal poor (0.03 ± 0.18) and older (median age of 6.07 ± 0.79 Gyr). The host stars of cold Jupiters in high-eccentricity orbits, on the other hand, show metallicities similar to those of the hosts of hot Jupiters, but are older, on average (median age of 6.25 ± 0.92 Gyr). The similarity in metallicity between the hosts of hot Jupiters and the hosts of cold Jupiters in high-eccentricity orbits supports high-eccentricity migration as the potential origin of hot Jupiters, with the latter serving as the progenitors of hot Jupiters. However, the average age difference between them suggests that the older hot Jupiters may have been engulfed by their host star over timescales ∼ 6 Gyr. This allows us to estimate the value of stellar tidal quality factor, Q*′∼106±1 .

The Aligned Orbit of the Eccentric Proto Hot Jupiter TOI-3362b* *Based on observations made with ESO Telescopes at the La Silla Paranal Observatory under program ID 110.23Y8.004.

High-eccentricity tidal migration predicts the existence of highly eccentric proto hot Jupiters on the “tidal circularization track,” meaning that they might eventually become hot Jupiters, but that their migratory journey remains incomplete. Having experienced moderate amounts of tidal evolution of their orbital elements, proto hot Jupiter systems can be powerful test beds for the underlying mechanisms of eccentricity growth. Notably, they may be used for discriminating between variants of high-eccentricity migration, each predicting a distinct evolution of misalignment between the star and the planet’s orbit. We constrain the spin–orbit misalignment of the proto hot Jupiter TOI-3362b with high-precision radial-velocity observations using ESPRESSO at Very Large Telescope. The observations reveal a sky-projected obliquity ° and constrain the orbital eccentricity to e = 0.720 ± 0.016, making it one of the most eccentric gas giants for which the obliquity has been measured. Although the large eccentricity and the striking orbit alignment of the planet are puzzling, we suggest that ongoing coplanar high-eccentricity migration driven by a distant companion is a possible explanation for the system's architecture. This distant companion would need to reside beyond 5 au at 95% confidence to be compatible with the available radial-velocity observations.

Making hot Jupiters in stellar clusters – II. Efficient formation in binary systems

ABSTRACT Observations suggested that the occurrence rate of hot Jupiters (HJs) in open clusters is largely consistent with the field ($\sim 1{{\ \rm per\ cent}}$) but in the binary-rich cluster M67, the rate is $\sim 5{{\ \rm per\ cent}}$. How does the cluster environment boost HJ formation via the high-eccentricity tidal migration initiated by the extreme-amplitude von Zeipel–Lidov–Kozai (XZKL) mechanism forced by a companion star? Our analytical treatment shows that the cluster’s collective gravitational potential alters the companion’s orbit slowly, which may render the star–planet–companion configuration XZKL-favourable. We have also performed direct Gyr N-body simulations of the star cluster evolution and XZKL of planets’ orbit around member stars. We find that an initially single star may acquire a companion star via stellar scattering and the companion may enable XZKL in the planets’ orbit. Planets around an initially binary star may also be XZKL-activated by the companion. In both scenarios, the companion’s orbit has likely been significantly changed by stellar scattering and the cluster potential before XZKL occurs. Across different cluster models, 0.8–3 per cent of the planets orbiting initially single stars have experienced XZKL while the fraction is 2–26 per cent for initially binary stars. Around a star that is binary at 1 Gyr, 13–32 per cent of its planets have undergone XZKL, and combined with single stars, the overall XZKL fraction is 3–21 per cent, most affected by the cluster binarity. If 10 per cent of the stars in M67 host a giant planet, our model predicts an HJ occurrence rate of $\sim 1{{\ \rm per\ cent}}$. We suggest that HJ surveys target old, high-binarity, not-too-dense open clusters and prioritize wide binaries to maximize HJ yield.

Hot Jupiters Have Giant Companions: Evidence for Coplanar High-eccentricity Migration

This study considers the characteristics of planetary systems with giant planets based on a population-level analysis of the California Legacy Survey planet catalog. We identified three characteristics common to hot Jupiters (HJs). First, while not all HJs have a detected outer giant planet companion (), such companions are ubiquitous when survey completeness corrections are applied for orbital periods out to 40,000 days. Giant-harboring systems without an HJ also host at least one outer giant planet companion per system. Second, the mass distributions of HJs and other giant planets are indistinguishable. However, within a planetary system that includes an HJ, the outer giant planet companions are at least 3× more massive than the inner HJs. Third, the eccentricity distribution of the outer companions in HJ systems (with an average model eccentricity of 〈e〉 = 0.34 ± 0.05) is different from the corresponding outer planets in planetary systems without HJs (〈e〉 = 0.19 ± 0.02). We conclude that the existence of two gas giants, where the outermost planet has an eccentricity ≥0.2 and is 3× more massive, are key factors in the production of an HJ. Our simple model based on these factors predicts that ∼10% of warm and cold Jupiter systems will by chance meet these assembly criteria, which is consistent with our measurement of a 16% ± 6% relative occurrence of HJ systems to all giant-harboring systems. We find that these three features favor coplanar high-eccentricity migration as the dominant mechanism for HJ formation.

TOI-332 b: a super dense Neptune found deep within the Neptunian desert

ABSTRACT To date, thousands of planets have been discovered, but there are regions of the orbital parameter space that are still bare. An example is the short period and intermediate mass/radius space known as the ‘Neptunian desert’, where planets should be easy to find but discoveries remain few. This suggests unusual formation and evolution processes are responsible for the planets residing here. We present the discovery of TOI-332 b, a planet with an ultra-short period of 0.78 d that sits firmly within the desert. It orbits a K0 dwarf with an effective temperature of 5251 ± 71 K. TOI-332 b has a radius of $3.20^{+0.16}_{-0.12}$ R⊕, smaller than that of Neptune, but an unusually large mass of 57.2 ± 1.6 M⊕. It has one of the highest densities of any Neptune-sized planet discovered thus far at $9.6^{+1.1}_{-1.3}$ g cm−3. A 4-layer internal structure model indicates it likely has a negligible hydrogen-helium envelope, something only found for a small handful of planets this massive, and so TOI-332 b presents an interesting challenge to planetary formation theories. We find that photoevaporation cannot account for the mass-loss required to strip this planet of the Jupiter-like envelope it would have been expected to accrete. We need to look towards other scenarios, such as high-eccentricity migration, giant impacts, or gap opening in the protoplanetary disc, to try and explain this unusual discovery.

Doppler confirmation of TESS planet candidate TOI−1408.01: grazing transit and likely eccentric orbit

ABSTRACT We report an independent Doppler confirmation of the TESS planet candidate orbiting an F-type main-sequence star TOI−1408 located 140 pc away. We present a set of radial velocities obtained with a high-resolution fibre-optic spectrograph FFOREST mounted at the SAO RAS 6-m telescope (BTA-6). Our self-consistent analysis of these Doppler data and TESS photometry suggests a grazing transit such that the planet obscures its host star by only a portion of the visible disc. Because of this degeneracy, the radius of TOI−1408.01 appears ill-determined with lower limit about ∼1 RJup, significantly larger than in the current TESS solution. We also derive the planet mass of 1.69 ± 0.20 MJup and the orbital period ∼4.425 d, thus making this object a typical hot Jupiter, but with a significant orbital eccentricity of 0.259 ± 0.026. Our solution may suggest that the planet is likely to experience a high tidal eccentricity migration at the stage of intense orbital rounding, or may indicate possible presence of other unseen companions in the system, yet to be detected.

TESS spots a mini-neptune interior to a hot saturn in the TOI-2000 system

ABSTRACT Hot jupiters (P &amp;lt; 10 d, M &amp;gt; 60 M⊕) are almost always found alone around their stars, but four out of hundreds known have inner companion planets. These rare companions allow us to constrain the hot jupiter’s formation history by ruling out high-eccentricity tidal migration. Less is known about inner companions to hot Saturn-mass planets. We report here the discovery of the TOI-2000 system, which features a hot Saturn-mass planet with a smaller inner companion. The mini-neptune TOI-2000 b (2.70 ± 0.15 R⊕, 11.0 ± 2.4 M⊕) is in a 3.10-d orbit, and the hot saturn TOI-2000 c ($8.14_{-0.30}^{+0.31}$ R⊕ , $81.7_{-4.6}^{+4.7}$ M⊕) is in a 9.13-d orbit. Both planets transit their host star TOI-2000 (TIC 371188886, V = 10.98, TESS magnitude = 10.36), a metal-rich ([Fe/H] = 0.439 $_{-0.043}^{+0.041}$) G dwarf 173 pc away. TESS observed the two planets in sectors 9–11 and 36–38, and we followed up with ground-based photometry, spectroscopy, and speckle imaging. Radial velocities from CHIRON, FEROS, and HARPS allowed us to confirm both planets by direct mass measurement. In addition, we demonstrate constraining planetary and stellar parameters with MIST stellar evolutionary tracks through Hamiltonian Monte Carlo under the PyMC framework, achieving higher sampling efficiency and shorter run time compared to traditional Markov chain Monte Carlo. Having the brightest host star in the V band among similar systems, TOI-2000 b and c are superb candidates for atmospheric characterization by the JWST, which can potentially distinguish whether they formed together or TOI-2000 c swept along material during migration to form TOI-2000 b.

A High-Eccentricity Warm Jupiter Orbiting TOI-4127

We report the discovery of TOI-4127 b, which is a transiting, Jupiter-sized exoplanet on a long-period ( days) and a high-eccentricity orbit around a late F-type dwarf star. This warm Jupiter was first detected and identified as a promising candidate from a search for single-transit signals in TESS Sector 20 data, and was later characterized as a planet following two subsequent transits (TESS Sectors 26 and 53) and follow-up ground-based RV observations with the NEID and SOPHIE spectrographs. We jointly fit the transit and RV data to constrain the physical (, ) and orbital parameters of the exoplanet. Given its high orbital eccentricity (), TOI-4127 b is a compelling candidate for studies of warm Jupiter populations and of hot Jupiter formation pathways. We show that the present periastron separation of TOI-4127 b is too large for high-eccentricity tidal migration to circularize its orbit, and that TOI-4127 b is unlikely to be a hot Jupiter progenitor unless it is undergoing angular momentum exchange with an undetected outer companion. Although we find no evidence for an external companion, the available observational data are insufficient to rule out the presence of a perturber that can excite eccentricity oscillations and facilitate tidal migration.

Discovery of a massive giant planet with extreme density around the sub-giant star TOI-4603

We present the discovery of a transiting massive giant planet around TOI-4603, a sub-giant F-type star from NASA’s Transiting Exoplanet Survey Satellite (TESS). The newly discovered planet has a radius of 1.042−0.035+0.038 RJand an orbital period of 7.24599−0.00021+0.00022days. Using radial velocity measurements with the PARAS and TRES spectrographs, we determined the planet’s mass to be 12.89−0.57+0.58 MJ, resulting in a bulk density of 14.1−1.6+1.7g cm−3. This makes it one of the few known massive giant planets with an extreme density. It lies in the transition mass region of massive giant planets and low-mass brown dwarfs, an important addition to the population of fewer than five known objects in this mass range. The eccentricity of 0.325 ± 0.020 and an orbital separation of 0.0888 ± 0.0010 AU from its host star suggest that the planet is likely undergoing high-eccentricity tidal migration. We find a fraction of heavy elements of 0.13−0.06+0.05and metal enrichment of the planet (ZP/Zstar) of 4.2−2.0+1.6. Detection of such systems will enable us to gain valuable insights into the governing mechanisms of massive planets and improve our understanding of their dominant formation and migration mechanisms.

Evidence for Hidden Nearby Companions to Hot Jupiters

The first discovered extrasolar worlds—giant, “hot Jupiter” planets on short-period orbits—came as a surprise to solar system–centric models of planet formation, prompting the development of new theories for planetary system evolution. The near absence of observed nearby planetary companions to hot Jupiters has been widely quoted as evidence in support of high-eccentricity tidal migration, a framework in which hot Jupiters form further out in their natal protoplanetary disks before being thrown inward with extremely high eccentricities, stripping systems of any close-in planetary companions. In this work, we present new results from a search for transit timing variations across the full 4 yr Kepler data set, demonstrating that at least 12% ± 6% of hot Jupiters have a nearby planetary companion. This subset of hot Jupiters is expected to have a quiescent dynamical history such that the systems could retain their nearby companions. We also demonstrate a ubiquity of nearby planetary companions to warm Jupiters (≥70% ± 16%), indicating that warm Jupiters typically form quiescently. We conclude by combining our results with existing observational constraints to propose an “eccentric migration” framework for the formation of short-period giant planets through postdisk dynamical sculpting in compact multiplanet systems. Our framework suggests that hot Jupiters constitute the natural end stage for giant planets spanning a wide range of eccentricities, with orbits that reach small enough periapses—either from their final orbital configurations in the disk phase or from eccentricity excitation in the postdisk phase—to trigger efficient tidal circularization.

TOI-4562b: A Highly Eccentric Temperate Jupiter Analog Orbiting a Young Field Star

We report the discovery of TOI-4562b (TIC-349576261), a Jovian planet orbiting a young F7V-type star, younger than the Praesepe/Hyades clusters (<700 Myr). This planet stands out because of its unusually long orbital period for transiting planets with known masses (P orb = 225.11781 days) and because it has a substantial eccentricity (e = 0.76). The location of TOI-4562 near the southern continuous viewing zone of TESS allowed observations throughout 25 sectors, enabling an unambiguous period measurement from TESS alone. Alongside the four available TESS transits, we performed follow-up photometry using the South African Astronomical Observatory node of the Las Cumbres Observatory and spectroscopy with the CHIRON spectrograph on the 1.5 m SMARTS telescope. We measure a radius of R J and a mass of 2.30 M J for TOI-4562b. The radius of the planet is consistent with contraction models describing the early evolution of the size of giant planets. We detect tentative transit timing variations at the ∼20 minutes level from five transit events, favoring the presence of a companion that could explain the dynamical history of this system if confirmed by future follow-up observations. With its current orbital configuration, tidal timescales are too long for TOI-4562b to become a hot Jupiter via high-eccentricity migration though it is not excluded that interactions with the possible companion could modify TOI-4562b’s eccentricity and trigger circularization. The characterization of more such young systems is essential to set constraints on models describing giant-planet evolution.

Statistical Analysis of the Dearth of Super-eccentric Jupiters in the Kepler Sample

Hot Jupiters may have formed in situ, or been delivered to their observed short periods through one of two categories of migration mechanisms: disk migration or high-eccentricity migration. If hot Jupiters were delivered by high-eccentricity migration, we would expect to observe some “super-eccentric” Jupiters in the process of migrating. We update a prediction for the number of super-eccentric Jupiters we would expect to observe in the Kepler sample if all hot Jupiters migrated through high-eccentricity migration and estimate the true number observed by Kepler. We find that the observations fail to match the prediction from high-eccentricity migration with 94.3% confidence and show that high-eccentricity migration can account for at most ∼62% of the hot Jupiters discovered by Kepler.