11:15 AM - 11:30 AM
[PEM11-08] The flipped orbit of KELT-19Ab inferred from the symmetric TESS transit light curves
Keywords:Orbital evolution, Transit method, Hot Jupiter
Introduction:
The spin axis of the Sun and the orbital axis of all planets in the solar system are aligned within 10°. However, many exoplanets are known to exhibit spin-orbit misalignments, with the current distribution of around 60 samples clustered around 0° and 90° (Albrecht et al. 2021). Additionally, no planets have yet been discovered with a misalignment (or obliquity) larger than 125° or an orbit that is flipped. The existence of such counter-orbiting planets remains uncertain.
One mechanism known to excite significant misalignment is the Kozai-Lidov mechanism (Kozai 1962; Lidov 1962), where a planet is acted on by secular torques from inclined companions and eccentricity and obliquity are exited (e.g. Fabrycky & Tremaine 2007; Anderson et al. 2016). However, much of current literature suggests that while misalignments are common, flipped orbits are not.
Method:
A crucial tool for measuring obliquity, ψ, is the analysis of anomalies in transit light curves caused by stellar gravity darkening. Gravity darkening is the latitude dependent brightness inhomogeneity in fast-rotating stars due to the stronger centrifugal force around the equator, causing the denser poles to be hotter and hence brighter than the equator. As planets transit gravity darkened stars, the inhomogeneity will be imprinted as unique symmetries or asymmetries in the shape of light curves (Barnes 2009). The apt targets for gravity darkening analysis are therefore transiting planets around fast-rotating hosts with high equilibrium temperature.
The method has primarily been used to find perpendicular planets with asymmetric transits crossing both the stellar poles and the equator. In this work, the method is used for the first time to explore the possibility of a flipped orbit, rather than searching for perpendicular planets with transit asymmetries. To rule out the possibility that symmetric transits are caused by a mere lack of gravity darkening and not counter-orbiting planets passing through the gravity darkened equator, we provide the model with prior knowledge of host star’s rotational velocity to constrain the degree of gravity darkening.
Result:
We apply the gravity darkening analysis to hot Jupiter KELT-19Ab around an Am star KELT-19A, with the rotational velocity vsini* = 84.2 km s-1 and effective temperature Teff = 7500K. Using TESS 2-minute cadence data, we find that the obliquity of KELT-19Ab exceeds 125° with 86% confidence, strongly favoring a flipped orbit.
Discussion:
KELT-19Ab is likely the first known planet to have a flipped orbit, and such conjecture can be strengthened with further observations. Although most literature suggest that the generation of flipped orbits are improbable, recent numerical simulations have shown that such counter-orbiting systems can arise from Kozai-Lidov mechanism if planets formed in primordially misaligned disks (Vick et al. 2022).
The possibility of a misaligned protoplanetary disk is explored theoretically in numerous works (e.g. Bate et al. 2010; Batygin & Adams 2013) and also bolstered by the observations of misaligned coplanar multi-planet systems or misaligned inner disks (e.g. Huber et al. 2013; Marino et al. 2015). The calculated timescale of Kozai-Lidov mechanism for KELT-19Ab (< 0.2 Gyr) is also well below the system age (1.1 Gyr), consistent with the hypothesis that Kozai-Lidov mechanism on a primordially misaligned KELT-19Ab played a role in its orbital evolution
The spin axis of the Sun and the orbital axis of all planets in the solar system are aligned within 10°. However, many exoplanets are known to exhibit spin-orbit misalignments, with the current distribution of around 60 samples clustered around 0° and 90° (Albrecht et al. 2021). Additionally, no planets have yet been discovered with a misalignment (or obliquity) larger than 125° or an orbit that is flipped. The existence of such counter-orbiting planets remains uncertain.
One mechanism known to excite significant misalignment is the Kozai-Lidov mechanism (Kozai 1962; Lidov 1962), where a planet is acted on by secular torques from inclined companions and eccentricity and obliquity are exited (e.g. Fabrycky & Tremaine 2007; Anderson et al. 2016). However, much of current literature suggests that while misalignments are common, flipped orbits are not.
Method:
A crucial tool for measuring obliquity, ψ, is the analysis of anomalies in transit light curves caused by stellar gravity darkening. Gravity darkening is the latitude dependent brightness inhomogeneity in fast-rotating stars due to the stronger centrifugal force around the equator, causing the denser poles to be hotter and hence brighter than the equator. As planets transit gravity darkened stars, the inhomogeneity will be imprinted as unique symmetries or asymmetries in the shape of light curves (Barnes 2009). The apt targets for gravity darkening analysis are therefore transiting planets around fast-rotating hosts with high equilibrium temperature.
The method has primarily been used to find perpendicular planets with asymmetric transits crossing both the stellar poles and the equator. In this work, the method is used for the first time to explore the possibility of a flipped orbit, rather than searching for perpendicular planets with transit asymmetries. To rule out the possibility that symmetric transits are caused by a mere lack of gravity darkening and not counter-orbiting planets passing through the gravity darkened equator, we provide the model with prior knowledge of host star’s rotational velocity to constrain the degree of gravity darkening.
Result:
We apply the gravity darkening analysis to hot Jupiter KELT-19Ab around an Am star KELT-19A, with the rotational velocity vsini* = 84.2 km s-1 and effective temperature Teff = 7500K. Using TESS 2-minute cadence data, we find that the obliquity of KELT-19Ab exceeds 125° with 86% confidence, strongly favoring a flipped orbit.
Discussion:
KELT-19Ab is likely the first known planet to have a flipped orbit, and such conjecture can be strengthened with further observations. Although most literature suggest that the generation of flipped orbits are improbable, recent numerical simulations have shown that such counter-orbiting systems can arise from Kozai-Lidov mechanism if planets formed in primordially misaligned disks (Vick et al. 2022).
The possibility of a misaligned protoplanetary disk is explored theoretically in numerous works (e.g. Bate et al. 2010; Batygin & Adams 2013) and also bolstered by the observations of misaligned coplanar multi-planet systems or misaligned inner disks (e.g. Huber et al. 2013; Marino et al. 2015). The calculated timescale of Kozai-Lidov mechanism for KELT-19Ab (< 0.2 Gyr) is also well below the system age (1.1 Gyr), consistent with the hypothesis that Kozai-Lidov mechanism on a primordially misaligned KELT-19Ab played a role in its orbital evolution