The stellar obliquity of a planetary system is often used to help constrain the system's formation and evolution. One of the mechanisms to reorient the stellar spin involves a secular resonance crossing due to the dissipation of the protoplanetary disk when the system also has an inclined, distant ($\sim 300\;\mathrm{AU}$) binary companion. This mechanism is likely to operate broadly due to the $\sim 50\%$ binary fraction of FGK dwarfs and can play an important role in setting the initial stellar obliquities prior to any dynamical evolution. In this work, we revisit this mechanism analytically for idealized, homologously evolving disk models and show that the resulting stellar obliquities are broadly distributed between $60^\circ$ and $180^\circ$ for most warm and cold planets. We further show that non-homologus disk dissipation, such as the development of a photoevaporatively-opened gap at $\sim 2\;\mathrm{AU}$, can help maintain orbital alignment of warm planets, in agreement with observations. Our results represent the proper primordial obliquities for planetary systems with distant binary companions. They also represent the obliquities of stars with no present-day binary companions if these companions are dynamically unbound during the birth cluster phase of evolution, a process that occurs on a comparable timescale as the disk-driven obliquity excitation.
Changes in planetary obliquity, or axial tilt, influence the climates on Earth-like planets. In the solar system, the Earth's obliquity is stabilized due to interactions with our moon and the resulting {small amplitude variations ($\sim$2.4\degree)} are beneficial for advanced life. Most Sun-like stars have at least one stellar companion and the habitability of circumstellar exoplanets is shaped by their stellar companion. We show that a stellar companion can dramatically change whether {Earth-like obliquity stability is} possible through planetary orbital precession relative to the binary orb…
Young low-mass stars of equal-mass exhibit a distribution of rotation periods. At the very early phases of stellar evolution, this distribution is set by the star-disc locking mechanism. The primordial disc lifetime and, consequently, the duration of the disc-locking mechanism, can be significantly shortened by the presence of a close companion, making the rotation period distribution of close binaries different from that of either single stars or wide binaries. We use new data to investigate and better constrain the range of ages, the components separation and the mass ratio dependence at whi…
The light curve of an eclipsing system shows anomalies whenever the eclipsing body passes in front of active regions on the eclipsed star. In some cases, the pattern of anomalies can be used to determine the obliquity $Ψ$ of the eclipsed star. Here we present a method for detecting and analyzing these patterns, based on a statistical test for correlations between the anomalies observed in a sequence of eclipses. Compared to previous methods, ours makes fewer assumptions and is easier to automate. We apply it to a sample of 64 stars with transiting planets and 24 eclipsing binaries for which pr…
An Aligned Very-Low-Mass Star Orbiting an M dwarf and Obliquity Patterns Across Giant Planets, Brown Dwarfs, and Binary Stars
Westlake University（西湖大学）；Université de Montréal（蒙特利尔大学）；Observatoire du Mont-Mégantic, Université de Montréal（蒙特利尔大学马格尼克天文台）；Université de Genève（日内瓦大学）；Observatoire de Genève, Département d’Astronomie, Université de Genève（日内瓦大学天文学系日内瓦天文台）
An Earth-analog orbiting within the habitable zone of $α$ Centauri B was shown to undergo large variations in its obliquity, or axial tilt, which affects the planetary climate by altering the radiative flux for a given latitude. We examine the potential implications of these obliquity variations for climate through Milankovitch cycles using an energy balance model with ice sheets. Similar to previous studies, the largest amplitude obliquity variations from spin-orbit resonances induce snowball states within the habitable zone, while moderate variations can allow for persistent ice caps or an i…
The spin-orbit obliquity of a planetary system constraints its formation history. A large obliquity may either indicate a primordial misalignment between the star and its gaseous disk or reflect the effect of different mechanisms tilting planetary systems after formation. Observations and statistical analysis suggest that system of planets with sizes between 1 and 4 R$_{\oplus}$ have a wide range of obliquities ($\sim0-30^{\circ}$), and that single- and multi-planet transiting have statistically indistinguishable obliquity distributions. Here, we revisit the ``breaking the chains'' formation m…
CHIANTE I: Obliquity Measurements of Four High-Priority Ariel Targets in Binaries
Lowell Observatory（洛厄尔天文台）；Yale University（耶鲁大学）；University of Michigan（密歇根大学）；Flatiron Institute, Simons Foundation（平托研究所，西蒙斯基金会）；San Francisco State University（旧金山州立大学）；Northern Arizona University（北亚利桑那大学）；McDonald Observatory, University of Texas at Austin（麦克唐纳天文台，德克萨斯大学奥斯汀分校）
The EBLM Project XVIII. 3D Obliquities of Five Low-Mass Eclipsing Binaries
Tufts University（塔夫茨大学）；University of Birmingham（伯明翰大学）；Massachusetts Institute of Technology（麻省理工学院）；Michigan State University（密歇根州立大学）；University of Leeds（利兹大学）；Keele University（基尔大学）；The Ohio State University（俄亥俄州立大学）；Jet Propulsion Laboratory, California Institute of Technology（加州理工学院喷气推进实验室）；Observatoire Astronomique de l’Université de Genève（日内瓦大学天文台）
Binary systems where the axis of rotation (spin) of one or both components is tilted w.r.t. the axis of revolution are called misaligned binary systems. The angle of misalignment, obliquity, has been measured for a handful of stars and extrasolar planets to date. Here we present a mathematical framework for a complete and rigorous treatment of misalignment and introduce an extension to the public PHOEBE code that implements this framework. We discuss misalignment for the Roche geometry and introduce methods for computing stellar shapes, equilibrium (generalized Lagrange) points of the potentia…
Following its flyby and first imaging the Pluto-Charon binary, the New Horizons spacecraft visited the Kuiper-Belt-Object (KBO) (486958) 2014 MU69 (Arrokoth). Imaging showed MU69 to be a contact-binary, made of two individual lobes connected by a narrow neck, rotating at low spin period (15.92 h), and having high obliquity (~98 deg), similar to other KBO contact-binaries inferred through photometric observations. The origin of such peculiar configurations is puzzling, and all scenarios suggested for the origins of contact-binaries fail to reproduce such properties and their likely high frequen…
An overabundance of single-transiting Kepler planets suggests the existence of a sub-population of intrinsically multi-planet systems possessing large mutual inclinations. However, the origin of these mutual inclinations remains unknown. Recent work has demonstrated that mutual inclinations can be excited soon after protoplanetary disk-dispersal due to the oblateness of the rapidly-rotating host star, provided the star is tilted. Alternatively, distant giant planets, which are common in systems of close-in Kepler planets, could drive up mutual inclinations. The relative importance of each of t…
A dozen short-period detached binaries are known to host transiting circumbinary planets. In all circumbinary systems so far, the planetary and binary orbits are aligned within a couple of degrees. However, the obliquity of the primary star, which is an important tracer of their formation, evolution, and tidal history, has only been measured in one circumbinary system until now. EBLM J0608-59/TOI-1338 is a low-mass eclipsing binary system with a recently discovered circumbinary planet identified by TESS. Here, we perform high-resolution spectroscopy during primary eclipse to measure the projec…
A large fraction of stars are in binary systems, yet the evolution of proto-planetary discs in binaries has been little explored from the theoretical side. In this paper we investigate the evolution of the discs surrounding the primary and secondary components of binary systems on the assumption that this is driven by photoevaporation induced by X-rays from the respective star. We show how for close enough separations (20-30 AU for average X-ray luminosities) the tidal torque of the companion changes the qualitative behaviour of disc dispersal from inside out to outside in. Fewer transition di…
The Rosetta spacecraft observations revealed that the nucleus of comet 67P/Churyumov-Gerasimenko consists of two similarly sized lobes connected by a narrow neck. Here we evaluate the possibility that 67P is a collapsed binary. We assume that the progenitor of 67P was a binary and consider various physical mechanisms that could have brought the binary components together, including small-scale impacts and gravitational encounters with planets. We find that 67P could be a primordial body (i.e., not a collisional fragment) if the outer planetesimal disk lasted <10 Myr before it was dispersed by …
The habitability of planets in binary star systems depends not only on the radiation environment created by the two stars, but also on the perturbations to planetary orbits and rotation produced by the gravitational field of the binary and neighbouring planets. Habitable planets in binaries may therefore experience significant perturbations in orbit and spin. The direct effects of orbital resonances and secular evolution on the climate of binary planets remain largely unconsidered. We present latitudinal energy balance modelling of exoplanet climates with direct coupling to an N Body integrato…
Tidal coplanarization of circumbinary planetary systems through stellar Cassini states
School of Astronomy and Space Science, Nanjing University（南京大学天文与空间科学学院）；Key Laboratory of Modern Astronomy and Astrophysics in Ministry of Education, Nanjing University（南京大学现代天文与天体物理教育部重点实验室）
The recent characterization of transiting close-in planets has revealed an intriguing population of sub-Neptunes with highly tilted and even polar orbits relative to their host star's equator. Any viable theory for the origin of these close-in, polar planets must explain (1) the observed stellar obliquities, (2) the substantial eccentricities, and (3) the existence of Jovian companions with large mutual inclinations. In this work, we propose a theoretical model that satisfies these requirements without invoking tidal dissipation or large primordial inclinations. Instead, tilting is facilitated…
Having a massive moon has been considered as a primary mechanism for stabilized planetary obliquity, an example of which being our Earth. This is, however, not always consistent with the exoplanetary cases. This article details the discovery of an alternative mechanism, namely that planets orbiting around binary stars tend to have low spin-axis variations. This is because the large quadrupole potential of the stellar binary could speed up the planetary orbital precession, and detune the system out of secular spin-orbit resonances. Consequently, habitable zone planets around the stellar binarie…
Stars are commonly formed in binary systems, which provide a natural laboratory for studying planet formation in extreme conditions. In our first paper (Paper I) of a series Xie et al. (2011), we have shown that the intermediate stage - from planetesimals to planetary embryos/cores - of planet formation can proceed even in highly inclined binaries. Following Paper I, here we numerically study the late stage of terrestrial planet formation, i.e., from embryos to full planets, in binary systems of various orbital configurations. We identify an orbital alignment or anti-alignment effect; namely, …
Permanently deformed objects in binary systems can experience complex rotation evolution, arising from the extensively studied effect of spin-orbit coupling as well as more nuanced dynamics arising from spin-spin interactions. The ability of an object to sustain an aspheroidal shape largely determines whether or not it will exhibit non-trivial rotational behavior. In this work, we adopt a simplified model of a gravitationally interacting primary and satellite pair, where each body's quadrupole moment is approximated by two diametrically opposed point masses. After calculating the net gravitati…
One of first the stages of planet formation is the growth of small planetesimals and their accumulation into large planetesimals and planetary embryos. This early stage occurs much before the dispersal of most of the gas from the protoplanetary disk. At this stage gas-planetesimal interactions play a key role in the dynamical evolution of \emph{single} intermediate-mass planetesimals ($m_{p}\sim10^{21}-10^{25}g$) \emph{through gas dynamical friction} (GDF). A significant fraction of all Solar system planetesimals (asteroids and Kuiper-belt objects) are known to be binary planetesimals (BPs). H…
High-resolution imaging of some protoplanetary disks in scattered light reveals presence of the global spiral arms of significant amplitude, likely excited by massive planets or stellar companions. Assuming that these arms are density waves, evolving into spiral shocks, we assess their effect on the thermodynamics, accretion, and global evolution of the disk. We derive analytical expressions for the direct (irreversible) heating, angular momentum transport, and mass accretion rate induced by the disk shocks of arbitrary strength. We find these processes to be very sensitive to the shock amplit…
