Signatures of Obliquity in Thermal Phase Curves of Hot Jupiters

Adams, Arthur D.; Millholland, Sarah; Laughlin, Gregory

doi:10.3847/1538-3881/ab2b35

Cited by 19 publications

(16 citation statements)

References 122 publications

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“…Further modeling work is necessary to investigate this in detail. Finally, signatures of nonzero obliquities, which can induce obliquity tides, can be probed by phase curve modeling (Adams et al 2019) and secondary eclipse mapping (Rauscher 2017).…”

Section: Predictions and Observational Signaturesmentioning

confidence: 99%

Tidal Inflation Reconciles Low-density Sub-Saturns with Core Accretion

Millholland

Petigura

Batygin

2020

ApJ

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While the solar system contains no planets between the sizes of Uranus and Saturn, our current exoplanet census includes several dozen such planets with well-measured masses and radii. These sub-Saturns exhibit a diversity of bulk densities, ranging from ∼0.1 to 3 g cm −3 . When modeled simply as hydrogen/helium envelopes atop rocky cores, this diversity in densities translates to a diversity in planetary envelope fractions, f env =M env /M p , ranging from ∼10% to ∼50%. Planets with f env ≈50% pose a challenge to traditional models of giant planet formation by core-nucleated accretion, which predict the onset of runaway gas accretion when M env ∼M core . Here, we show that many of these apparent f env ≈50% planets are less envelope-rich than they seem, after accounting for tidal heating. We present a new framework for modeling sub-Saturn interiors that incorporates envelope inflation due to tides, which are driven by the observed nonzero eccentricities, as well as potential obliquities. Consequently, when we apply our models to known sub-Saturns, we infer lower f env than tides-free estimates. We present a case study of K2-19 b, a moderately eccentric sub-Saturn. Neglecting tides, K2-19 b appears to have f env ≈50%, poised precariously near the runaway threshold; by including tides, however, we find f env ≈10%, resolving the tension. Through a systematic analysis of 4-8 R ⊕ planets, we find that most (but not all) of the similarly envelope-rich planets have more modest envelopes of f env ≈10%-20%. Thus, many sub-Saturns may be understood as sub-Neptunes that have undergone significant radius inflation, rather than a separate class of objects. Tidally induced radius inflation likely plays an important role in other size classes of planets including ultra-low-density Jupitersize planets like WASP-107 b.Unified Astronomy Thesaurus concepts: Exoplanets (498); Extrasolar gas giants (509); Exoplanet atmospheres (487); Exoplanet structure (495); Exoplanet tides (497) & Fortney (2014) constructed model planets consisting of Earthcomposition cores surrounded by low-density envelopes of H/He. Considering a grid of M core , M env , age, and incident stellar flux, they computed the planetary radius evolution in response to various sources of envelope heating and cooling. 5 Critically, given these assumptions, one may invert these models and translate the masses and radii of observed planets into constraints on their core and envelope masses.Using this approach, there has emerged a subset of lowdensity sub-Saturns with atmospheric envelopes comprising ∼50% of their total mass. (That is, their envelope mass fractions are f env ≡M env /M p ≈50%.) Examples of such planets, which are perplexing for reasons we will describe below, include the recently discovered K2-24 c (Petigura et al. 2018a) and K2-19 b (Petigura et al. 2020). Both planets are found near or in mean-motion resonances (MMRs) with neighboring companions and have nonzero eccentricities, e∼0.08 for K2-24 c and e∼0.2 for K2-19 b. Using the Lope...

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Section: Predictions and Observational Signaturesmentioning

confidence: 99%

Tidal Inflation Reconciles Low-density Sub-Saturns with Core Accretion

Millholland

Petigura

Batygin

2020

ApJ

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“…Gaps and rings in the disk, which we now know are quite common (e.g. ALMA Partnership et al 2015;Andrews et al 2016;Isella et al 2016;Avenhaus et al 2018;Andrews et al 2018), may also impact the resonant encounters and therefore lend themselves as excellent topics for future study.…”

Section: Suppression Due To Giant Planet Perturbationsmentioning

confidence: 99%

“…For close-in planets, features in full-phase, optical or infrared light curve observations may point toward nonzero obliquities (Schwartz et al 2016;Rauscher 2017;Ohno & Zhang 2019a,b;Adams et al 2019). Similarly, the detection of seasonal variability from infrared photometry or direct imaging has also been proposed as a means of constraining obliquity (Gaidos & Williams 2004;Kawahara 2016;Kane & Torres 2017).…”

Section: Introductionmentioning

confidence: 99%

Excitation of Planetary Obliquities through Planet–Disk Interactions

Millholland

Batygin

2019

ApJ

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The tilt of a planet's spin axis off its orbital axis ("obliquity") is a basic physical characteristic that plays a central role in determining the planet's global circulation and energy redistribution. Moreover, recent studies have also highlighted the importance of obliquities in sculpting not only the physical features of exoplanets but also their orbital architectures. It is therefore of key importance to identify and characterize the dominant processes of excitation of non-zero axial tilts. Here we highlight a simple mechanism that operates early on and is likely fundamental for many extrasolar planets and perhaps even Solar System planets. While planets are still forming in the protoplanetary disk, the gravitational potential of the disk induces nodal recession of the orbits. The frequency of this recession decreases as the disk dissipates, and when it crosses the frequency of a planet's spin axis precession, large planetary obliquities may be excited through capture into a secular spin-orbit resonance. We study the conditions for encountering this resonance and calculate the resulting obliquity excitation over a wide range of parameter space. Planets with semi-major axes in the range 0.3 AU a 2 AU are the most readily affected, but large-a planets can also be impacted. We present a case study of Uranus and Neptune and show that this mechanism likely cannot help explain their high obliquities. While it could have played a role if finely tuned and envisioned to operate in isolation, large-scale obliquity excitation was likely inhibited by gravitational planet-planet perturbations.

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“…Nevertheless, there are prospects for better constraints on exoplanetary obliquities in the coming years (7-9). Substantial obliquities are of great theoretical interest for their proposed role in explaining peculiar thermal phase curves of transiting planets (10,11) and various other open dynamical puzzles (12)(13)(14)(15).The obliquity of a planet reflects its dynamical history. Some obliquities could be generated in the earlest phase of planet formation, when/if proto-planetary disks are turbulent and twisted (16,17).…”

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confidence: 99%

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Non-Trivial Oblique Spin Equilibria of Super-Earths in Multi-planetary Systems

Su,

Lai

2021

Preprint

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Many Sun-like stars are observed to host close-in super-Earths (SEs) as part of a multiplanetary system. In such a system, the spin of the SE evolves due to spin-orbit resonances and tidal dissipation. In the absence of tides, the planet's obliquity can evolve chaotically to large values. However, for close-in SEs, tidal dissipation is significant and suppresses the chaos, instead driving the spin into various steady states. We find that the attracting steady states of the SE's spin are more numerous than previously thought, due to the discovery of a new class of "mixed-mode" high-obliquity equilibria. These new equilibria arise due to subharmonic responses of the parametrically-driven planetary spin, an unusual phenomenon that arises in nonlinear systems. Many SEs should therefore have significant obliquities, with potentially large impacts on the physical conditions of their surfaces and atmospheres.Introduction. The obliquity of a planet, the angle between the spin and orbital axes, plays an important role in the atmospheric dynamics, climate, and potential habitability of the planet. For instance, the atmospheric circulation of a planet changes dramatically as the obliquity increases beyond 54 • , as the averaged insolation at the poles becomes greater than that at the equator (1, 2). Variations in insolation over long timescales can also affect the habitability of an exoplanet (3, 4). In the Solar System, planetary obliquities range from nearly zero for Mercury and 3.1 • for Jupiter, to 23 • for Earth and 26.7 • for Saturn, to 98 • for Uranus. The obliquities of exoplanets are challenging to measure, and so far only loose constraints have been obtained for the obliquities of faraway planetary-mass companions (5, 6). Nevertheless, there are prospects for better constraints on exoplanetary obliquities in the coming years (7-9). Substantial obliquities are of great theoretical interest for their proposed role in explaining peculiar thermal phase curves of transiting planets (10,11) and various other open dynamical puzzles (12)(13)(14)(15).The obliquity of a planet reflects its dynamical history. Some obliquities could be generated in the earlest phase of planet formation, when/if proto-planetary disks are turbulent and twisted (16,17). Large obliquities are commonly attributed to giant impacts or planet collisions as a result of dynamical instabilities of planetary orbits (18-24). Repeated planet-planet scatterings could also lead to modest obliquities (25,26). Substantial obliquity excitation can be achieved over long (secular) timescales via spin-orbit resonances, when the spin precession and orbital (nodal) precession frequencies of the planet

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Signatures of Obliquity in Thermal Phase Curves of Hot Jupiters

Cited by 19 publications

References 122 publications

Tidal Inflation Reconciles Low-density Sub-Saturns with Core Accretion

Tidal Inflation Reconciles Low-density Sub-Saturns with Core Accretion

Excitation of Planetary Obliquities through Planet–Disk Interactions

Non-Trivial Oblique Spin Equilibria of Super-Earths in Multi-planetary Systems

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