Origins of Hot Jupiters Dawson, Rebekah I; Johnson, John Asher
Annual review of astronomy and astrophysics,
09/2018, Letnik:
56, Številka:
1
Journal Article
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Hot Jupiters were the first exoplanets to be discovered around main sequence stars and astonished us with their close-in orbits. They are a prime example of how exoplanets have challenged our ...textbook, solar-system inspired story of how planetary systems form and evolve. More than twenty years after the discovery of the first hot Jupiter, there is no consensus on their predominant origin channel. Three classes of hot Jupiter creation hypotheses have been proposed: in situ formation, disk migration, and high-eccentricity tidal migration. Although no origin channel alone satisfactorily explains all the evidence, two major origin channels together plausibly account for properties of hot Jupiters themselves and their connections to other exoplanet populations.
Stellar Obliquities in Exoplanetary Systems Albrecht, Simon H.; Dawson, Rebekah I.; Winn, Joshua N.
Publications of the Astronomical Society of the Pacific,
08/2022, Letnik:
134, Številka:
1038
Journal Article
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Abstract
The rotation of a star and the revolutions of its planets are not necessarily aligned. This article reviews the measurement techniques, key findings, and theoretical interpretations related ...to the obliquities (spin–orbit angles) of planet-hosting stars. The best measurements are for stars with short-period giant planets, which have been found on prograde, polar, and retrograde orbits. It seems likely that dynamical processes such as planet–planet scattering and secular perturbations are responsible for tilting the orbits of close-in giant planets, just as those processes are implicated in exciting orbital eccentricities. The observed dependence of the obliquity on orbital separation, planet mass, and stellar structure suggests that in some cases, tidal dissipation damps a star’s obliquity within its main-sequence lifetime. The situation is not as clear for stars with smaller or wider-orbiting planets. Although the earliest measurements of such systems tended to find low obliquities, some glaring exceptions are now known in which the star’s rotation is misaligned with respect to the coplanar orbits of multiple planets. In addition, statistical analyses based on projected rotation velocities and photometric variability have found a broad range of obliquities for F-type stars hosting compact multiple-planet systems. The results suggest it is unsafe to assume that stars and their protoplanetary disks are aligned. Primordial misalignments might be produced by neighboring stars or more complex events that occur during the epoch of planet formation.
It is debated whether the two hot Jupiter populations-those on orbits misaligned from their host star's spin axis and those well-aligned-result from two migration channels or from two tidal ...realignment regimes. Here I demonstrate that equilibrium tides raised by a planet on its star can account for three observed spin-orbit alignment trends: the aligned orbits of hot Jupiters orbiting cool stars, the planetary mass cut-off for retrograde planets, and the stratification by planet mass of cool host stars' rotation frequencies. The first trend can be caused by strong versus weak magnetic braking (the Kraft break), rather than realignment of the star's convective envelope versus the entire star. The second trend can result from a small effective stellar moment of inertia participating in the tidal realignment in hot stars, enabling massive retrograde planets to partially realign to become prograde. The third trend is attributable to higher-mass planets more effectively counteracting braking to spin up their stars. Both hot and cool stars require a small effective stellar moment of inertia participating in the tidal realignment, e.g., an outer layer weakly coupled to the interior. I demonstrate via Monte Carlo that this model can match the observed trends and distributions of sky-projected misalignments and stellar rotation frequencies. I discuss implications for inferring hot Jupiter migration mechanisms from obliquities, emphasizing that even hot stars do not constitute a pristine sample.
ABSTRACT The giant impact phase of terrestrial planet formation establishes connections between super-Earths' orbital properties (semimajor axis spacings, eccentricities, mutual inclinations) and ...interior compositions (the presence or absence of gaseous envelopes). Using N-body simulations and analytic arguments, we show that spacings derive not only from eccentricities, but also from inclinations. Flatter systems attain tighter spacings, a consequence of an eccentricity equilibrium between gravitational scatterings, which increase eccentricities, and mergers, which damp them. Dynamical friction by residual disk gas plays a critical role in regulating mergers and in damping inclinations and eccentricities. Systems with moderate gas damping and high solid surface density spawn gas-enveloped super-Earths with tight spacings, small eccentricities, and small inclinations. Systems in which super-Earths coagulate without as much ambient gas, in disks with low solid surface density, produce rocky planets with wider spacings, larger eccentricities, and larger mutual inclinations. A combination of both populations can reproduce the observed distributions of spacings, period ratios, transiting planet multiplicities, and transit duration ratios exhibited by Kepler super-Earths. The two populations, both formed in situ, also help to explain observed trends of eccentricity versus planet size, and bulk density versus method of mass measurement (radial velocities versus transit timing variations). Simplifications made in this study-including the limited time span of the simulations, and the approximate treatments of gas dynamical friction and gas depletion history-should be improved on in future work to enable a detailed quantitative comparison to the observations.
A question driving many studies is whether the thousands of exoplanets known today typically formed where we observe them or formed further out in the disk and migrated in. Early discoveries of giant ...exoplanets orbiting near their host stars and exoplanets in or near mean motion resonances were interpreted as evidence for migration and its crucial role in the beginnings of planetary systems. Long-scale migration has been invoked to explain systems of planets in mean motion resonant chains consisting of three or more planets linked by integer period ratios. However, recent studies have reproduced specific resonant chains in systems via short-scale migration, and eccentricity damping has been shown to capture planets into resonant chains. We investigate whether the observed resonant chains in Kepler-80, Kepler-223, Kepler-60, and TRAPPIST-1 can be established through long-scale migration, short-scale migration, and/or only eccentricity damping by running suites of N-body simulations. We find that, for each system, all three mechanisms are able to reproduce the observed resonant chains. Long-scale migration is not the only plausible explanation for resonant chains in these systems, and resonant chains are potentially compatible with in situ formation.
Exoplanet orbital eccentricities offer valuable clues about the history of planetary systems. Eccentric, Jupiter-sized planets are particularly interesting: they may link the "cold" Jupiters beyond ...the ice line to close-in hot Jupiters, which are unlikely to have formed in situ. To date, eccentricities of individual transiting planets primarily come from radial-velocity measurements. Kepler has discovered hundreds of transiting Jupiters spanning a range of periods, but the faintness of the host stars precludes radial-velocity follow-up of most. Here, we demonstrate a Bayesian method of measuring an individual planet's eccentricity solely from its transit light curve using prior knowledge of its host star's density. We show that eccentric Jupiters are readily identified by their short ingress/egress/total transit durations-part of the "photoeccentric" light curve signature of a planet's eccentricity-even with long-cadence Kepler photometry and loosely constrained stellar parameters. A Markov Chain Monte Carlo exploration of parameter posteriors naturally marginalizes over the periapse angle and automatically accounts for the transit probability. To demonstrate, we use three published transit light curves of HD 17156 b to measure an eccentricity of e = (ProQuest: Formulae and/or non-USASCII text omitted), in good agreement with the discovery value e = 0.67 + or - 0.08 based on 33 radial-velocity measurements. We present two additional tests using Kepler data. In each case, the technique proves to be a viable method of measuring exoplanet eccentricities and their confidence intervals. Finally, we argue that this method is the most efficient, effective means of identifying the extremely eccentric, proto-hot Jupiters predicted by Socrates et al.
Torques from a mutually inclined perturber can change a transiting planet's impact parameter, resulting in variations in the transit shape and duration. Detection of and upper limits on changes in ...impact parameter yield valuable constraints on a planetary system's three-dimensional architecture. Constraints for warm Jupiters are particularly interesting because they allow us to test origins theories that invoke a mutually inclined perturber. Because of warm Jupiters' high signal-to-noise ratio transits, it is feasible to detect changes in impact parameter. However, here we show that allowing the impact parameter to vary uniformly and independently from transit to transit leads to incorrect inferences about the change, propagating to incorrect inferences about the perturber. We demonstrate that an appropriate prior on the change in impact parameter mitigates this problem. We apply our approach to eight systems from the literature and find evidence for changes in impact parameter for warm Jupiter Kepler-46b. We conclude with our recommendations for light-curve fitting, including when to fit impact parameters versus transit durations.
The orbits of giant extrasolar planets often have surprisingly small semimajor axes, large eccentricities, or severe misalignments between their orbit normals and their host stars' spin axes. In some ...formation scenarios invoking Kozai-Lidov oscillations, an external planetary companion drives a planet onto an orbit having these properties. The mutual inclinations for Kozai-Lidov oscillations can be large and have not been confirmed observationally. Here we present evidence that observed eccentric warm Jupiters with eccentric giant companions have mutual inclinations that oscillate between 35° and 65°. Our inference is based on the pairs’ observed apsidal separations, which cluster near 90°. The near-orthogonality of periapse directions is effected by the outer companion's quadrupolar and octupolar potentials. These systems may be undergoing a stalled version of tidal migration that produces warm Jupiters over hot Jupiters, and they provide evidence for a population of multiplanet systems that are not flat and have been sculpted by Kozai-Lidov oscillations.
We determine the orbital eccentricities of individual small Kepler planets, through a combination of asteroseismology and transit light-curve analysis. We are able to constrain the eccentricities of ...51 systems with a single transiting planet, which supplement our previous measurements of 66 planets in multi-planet systems. Through a Bayesian hierarchical analysis, we find evidence that systems with only one detected transiting planet have a different eccentricity distribution than systems with multiple detected transiting planets. The eccentricity distribution of the single-transiting systems is well described by the positive half of a zero-mean Gaussian distribution with a dispersion e = 0.32 0.06, while the multiple-transit systems are consistent with . A mixture model suggests a fraction of of single-transiting systems have a moderate eccentricity, represented by a Rayleigh distribution that peaks at . This finding may reflect differences in the formation pathways of systems with different numbers of transiting planets. We investigate the possibility that eccentricities are self-excited in closely packed planetary systems, as well as the influence of long-period giant companion planets. We find that both mechanisms can qualitatively explain the observations. We do not find any evidence for a correlation between eccentricity and stellar metallicity, as has been seen for giant planets. Neither do we find any evidence that orbital eccentricity is linked to the detection of a companion star. Along with this paper, we make available all of the parameters and uncertainties in the eccentricity distributions, as well as the properties of individual systems, for use in future studies.
A Preponderance of Perpendicular Planets Albrecht, Simon H.; Marcussen, Marcus L.; Winn, Joshua N. ...
Astrophysical journal. Letters,
07/2021, Letnik:
916, Številka:
1
Journal Article
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Abstract
Observing the Rossiter–McLaughlin effect during a planetary transit allows the determination of the angle
λ
between the sky projections of the star’s spin axis and the planet’s orbital axis. ...Such observations have revealed a large population of well-aligned systems and a smaller population of misaligned systems, with values of
λ
ranging up to 180°. For a subset of 57 systems, we can now go beyond the sky projection and determine the 3D obliquity
ψ
by combining the Rossiter–McLaughlin data with constraints on the line-of-sight inclination of the spin axis. Here we show that the misaligned systems do not span the full range of obliquities; they show a preference for nearly perpendicular orbits (
ψ
= 80°–125°) that seems unlikely to be a statistical fluke. If confirmed by further observations, this pile-up of polar orbits is a clue about the unknown processes of obliquity excitation and evolution.
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