Nader Haghighipour

Kepler-47: A Transiting Circumbinary Multiplanet System

A Pair of Planets Around a Pair of Stars Most of the planets we know about orbit a single star; however, most of the stars in our galaxy are not single. Based on data from the Kepler space telescope, Orosz et al. (p. 1511, published online 28 August) report the detection of a pair of planets orbiting a pair of stars. These two planets are the smallest of the known transiting circumbinary planets and have the shortest and longest orbital periods. The outer planet resides in the habitable zone—the “goldilocks” region where the temperatures could allow liquid water to exist. This discovery establishes that, despite the chaotic environment around a close binary star, a system of planets can form and persist. Data from the Kepler space telescope reveal two small planets orbiting a pair of two low-mass stars. We report the detection of Kepler-47, a system consisting of two planets orbiting around an eclipsing pair of stars. The inner and outer planets have radii 3.0 and 4.6 times that of Earth, respectively. The binary star consists of a Sun-like star and a companion roughly one-third its size, orbiting each other every 7.45 days. With an orbital period of 49.5 days, 18 transits of the inner planet have been observed, allowing a detailed characterization of its orbit and those of the stars. The outer planet’s orbital period is 303.2 days, and although the planet is not Earth-like, it resides within the classical “habitable zone,” where liquid water could exist on an Earth-like planet. With its two known planets, Kepler-47 establishes that close binary stars can host complete planetary systems.

The Neptune-sized Circumbinary Planet Kepler-38b

We discuss the discovery and characterization of the circumbinary planet Kepler-38b. The stellar binary is single-lined, with a period of 18.8 days, and consists of a moderately evolved main-sequence star (M_A = 0.949 ± 0.059 M_☉ and R_A = 1.757 ± 0.034 R_☉) paired with a low-mass star (M_B = 0.249 ± 0.010 M_☉ and R_B = 0.2724 ± 0.0053 R_☉) in a mildly eccentric (e = 0.103) orbit. A total of eight transits due to a circumbinary planet crossing the primary star were identified in the Kepler light curve (using Kepler Quarters 1-11), from which a planetary period of 105.595 ± 0.053 days can be established. A photometric dynamical model fit to the radial velocity curve and Kepler light curve yields a planetary radius of 4.35 ± 0.11 R_⊕, or equivalently 1.12 ± 0.03 R_(Nep). Since the planet is not sufficiently massive to observably alter the orbit of the binary from Keplerian motion, we can only place an upper limit on the mass of the planet of 122 M_⊕ (7.11 M_(Nep) or equivalently 0.384 M_(Jup)) at 95% confidence. This upper limit should decrease as more Kepler data become available.

A planetary system around the nearby M dwarf GJ 667C with at least one super-earth in its habitable zone

We re-analyze 4 years of HARPS spectra of the nearby M1.5 dwarf GJ 667C available through the European Southern Observatory public archive. The new radial velocity (RV) measurements were obtained using a new data analysis technique that derives the Doppler measurement and other instrumental effects using a least-squares approach. Combining these new 143 measurements with 41 additional RVs from the Magellan/Planet Finder Spectrograph and Keck/High Resolution Echelle Spectrometer spectrometers reveals three additional signals beyond the previously reported 7.2 day candidate, with periods of 28 days, 75 days, and a secular trend consistent with the presence of a gas giant (period ~10 years). The 28 day signal implies a planet candidate with a minimum mass of 4.5 M ⊕ orbiting well within the canonical definition of the stars liquid water habitable zone (HZ), that is, the region around the star at which an Earth-like planet could sustain liquid water on its surface. Still, the ultimate water supporting capability of this candidate depends on properties that are unknown such as its albedo, atmospheric composition, and interior dynamics. The 75 day signal is less certain, being significantly affected by aliasing interactions among a potential 91 day signal, and the likely rotation period of the star at 105 days detected in two activity indices. GJ 667C is the common proper motion companion to the GJ 667AB binary, which is metal-poor compared to the Sun. The presence of a super-Earth in the HZ of a metal-poor M dwarf in a triple star system supports the evidence that such worlds should be ubiquitous in the Galaxy.

On Pressure Gradients and Rapid Migration of Solids in a Nonuniform Solar Nebula

We study the motions of small solids, ranging from micron-sized dust grains to 100 m objects, in the vicinity of a local density enhancement of an isothermal gaseous solar nebula. Being interested in possible application of the results to the formation of planetesimals in the vicinity of clumps and spiral arms in a circumstellar disk, we numerically integrate the equations of motion of such solids and study their migrations for different values of their sizes and masses and also for different physical properties of the gas, such as its density and temperature. We show that, considering the drag force of the gas, it is possible for solids, within a certain range of size and mass, to migrate rapidly (i.e., within ~1000 yr) toward the location of a local maximum density, where collisions and coagulation may result in an accelerated rate of planetesimal formation.

Irregular Satellites of the Planets: Products of Capture in the Early Solar System

All four giant planets in the Solar system possess irregular satellites, characterized by large, highly eccentric and/or inclined orbits that are distinct from the nearly circular, uninclined orbits of the regular satellites. This difference can be traced directly to different modes of formation. Whereas the regular satellites grew by accretion within circumplanetary disks the irregular satellites were captured from initially heliocentric orbits at an early epoch. Recently, powerful survey observations have greatly increased the number of known irregular satellites, permitting a fresh look at the group properties of these objects and motivating a re-examination of the mechanisms of capture. None of the suggested mechanisms, including gas-drag, pull-down, and three-body capture, convincingly fit the group characteristics of the irregular satellites. The sources of the satellites also remain unidentified.

On Gas Drag-Induced Rapid Migration of Solids in a Nonuniform Solar Nebula

We study the motions of small solids, ranging from micron-sized dust grains to meter-sized objects, in the vicinity of local pressure enhancements of a gaseous nebula. Integrating numerically, we show that as a result of the combined effect of gas drag and pressure gradients, solids tend to accumulate at the locations where the pressure of the gas maximizes. The rate of migration of solids varies with their sizes and densities and also with the physical properties of the gas. The results of our numerical simulations indicate that such migrations are most rapid for meter-sized objects. The applicability of the results to the enhancement of the collision and coagulation of solids and also to the growth rate of planetesimals is discussed.

Rapid Formation of Ice Giant Planets

Abstract The existence of Uranus and Neptune presents severe difficulties for the core accretion model for the formation of ice giant planets. We suggest an alternative mechanism, namely disk instability leading to the formation of gas giant protoplanets, coagulation and settling of dust grains to form ice–rock cores at their centers, and photoevaporation of their gaseous envelopes by a nearby OB star, as a possible means of forming ice giant planets.

Effect of stellar spots on high-precision transit light-curve

Institute for Astronomy and NASA Astrobiology Institute, University of Hawaii-Manoa, 2680 Woodlawn Drive, Honolulu, HI96822,USAReceived XXX; accepted XXXABSTRACTStellar-activity features such as spots can complicate the determination of planetary parameters through spectroscopic andphotometric observations. The overlap of a transiting planet and a stellar spot, for instance, can produce anomalies in thetransit light-curves that may lead to an inaccurate estimation of the transit duration, depth, and timing. These inaccuraciescan for instance affect the precise derivation of the planet radius. We present the results of a quantitative study on the effectsof stellar spots on high-precision transit light-curves. We show that spot anomalies can lead to an estimate of a planet radiusthat is 4% smaller than the real value. Likewise, the transit duration may be estimated about 4%, longer or shorter. Dependingon the size and distribution of spots, anomalies can also produce transit-timing variations (TTVs) with significant amplitudes.For instance, TTVs with signal amplitudes of 200 seconds can be produced when the spot is completely dark and has the sizeof the largest Sun spot. Our study also indicates that the smallest size of a stellar spot that still has detectable affects on ahigh-precision transit light-curve is around 0.03 time the stellar radius for typical Kepler telescope precision. We also show thatthe strategy of including more free parameters (such as transit depth and duration) in the fitting procedure to measure thetransit time of each individual transit will not produce accurate results for active stars.Key words. methods: numerical- planetary system- techniques: photometry, Stellar activity

Kepler 453 b: the 10th Kepler Transiting Circumbinary Planet

We present the discovery of Kepler-453 b, a 6.2 R⊕ planet in a low-eccentricity, 240.5 day orbit about an eclipsing binary. The binary itself consists of a 0.94 and 0.195 M⊙ pair of stars with an orbital period of 27.32 days. The plane of the planets orbit is rapidly precessing, and its inclination only becomes sufficiently aligned with the primary star in the latter portion of the Kepler data. Thus three transits are present in the second half of the light curve, but none of the three conjunctions that occurred during the first half of the light curve produced observable transits. The precession period is ~103 years, and during that cycle, transits are visible only ~8.9% of the time. This has the important implication that for every system like Kepler-453 that we detect, there are ~11.5 circumbinary systems that exist but are not currently exhibiting transits. The planets mass is too small to noticeably perturb the binary, and consequently its mass is not measurable with these data; however, our photodynamical model places a 1σ upper limit of 16 M⊕. With a period 8.8 times that of the binary, the planet is well outside the dynamical instability zone. It does, however, lie within the habitable zone of the binary, making it the third of 10 Kepler circumbinary planets to do so.

Habitable Planet Formation in Binary Planetary Systems

Recent radial velocity observations have indicated that Jovian-type planets can exist in moderately close binary star systems. Numerical simulations of the dynamical stability of terrestrial-class planets in such environments have shown that, in addition to their giant planets, these systems can also harbor Earth-like objects. In this paper we study the late stage of terrestrial planet formation in such binary planetary systems, and present the results of the simulations of the formation of Earth-like bodies in their habitable zones. We consider a circumprimary disk of Moon- to Mars-sized objects and numerically integrate the orbits of these bodies at the presence of the Jovian-type planet of the system and for different values of the mass, semimajor axis, and orbital eccentricity of the secondary star. Results indicate that Earth-like objects, with substantial amounts of water, can form in the habitable zone of the primary star. Simulations also indicate that by transferring angular momentum from the secondary star to protoplanetary objects, the giant planet of the system plays a key role in the radial mixing of these bodies and the water contents of the final terrestrial planets. We will discuss the results of our simulation and show that the formation of habitable planets in binary planetary systems is more probable in binaries with moderate to large perihelia.