In the last year, Haghighipour completed a systematic study of the formation of habitable planets in the habitable zone of binary-planetary systems. The motivation behind this study comes from the fact that among more than 270 extrasolar planets discovered to-date, approximately 25% are within binary star systems. Among these planetary systems, the two binaries of Gamma Cephei and GJ 86 are unique in the sense that their separations are smaller than 20 AU, and their primary stars host giant planets. Haghighipour’s study focused on understanding whether such binary-planetary system can harbor habitable planets. In collaboration with Sean Raymond from NAI/Colorado, Haghighipour simulated the interactions of several hundred Moon- to Mars-sized objects, in a region between the giant planet and the primary of a binary system, and for different values of the mass and orbital parameters of the binary. Results indicate that, binary-planetary systems can form and harbor Earth-like objects in their habitable zones, however, the efficiency of these processes are strongly affected by the eccentricity of the binary. In binaries with high orbital eccentricities, the interaction of the secondary star with planet-forming material is strong and results in the ejection of water-carrying objects. As a result, in such systems, final terrestrial planets are mostly dry and un-habitable. However, binaries with lower eccentricity and separations between 20 to 40 AU show a more hospitable environment for the formation and long-term stability of Earth-like planets around their primary stars. Results of simulations by Haghighipour and Raymond show that binary systems in which the giant planet maintains its orbit for a long time are more favorable to formation of habitable planets.
2) Formation of Planetesimals
The formation of planetesimals is one of the main unresolved issues of planetary science. Traditionally it is assumed that km-sized objects are formed through collision and growth of cm-sized bodies. However, numerical simulations have shown that the relative velocities of cm-sized objects are so large that their collisions may result in fragmentation. To overcome this difficulty, an alternative mechanism has recently been proposed by Youdin and Shu (2002) in which planetesimals are formed through fragmentation of a gravitationally unstable disk of cm-sized objects at the midplane of the nebula. While small particles settle on the midplane, they form a rich layer of particulate material, which subsequently collapses and breaks into km-sized bodies.
The main issue with this scenario is that because of the differences between the rotational velocities of gas and solid objects, a layer of cm-sized particles will undergo shear-induced turbulence prior to reaching the critical density required for its gravitational instability. In his research, Haghighipour has shown that in a dynamically evolving disk, regions may appear where the density of the gas is locally enhanced, and shear-induced turbulence is non-existence. The presence of pressure gradients on both sides of these structures causes solid objects to undergo inward and outward migrations, and accumulate at the location of maximum gas density. At these locations, pressure gradient vanishes and particles and gas rotate at the same Keplerian velocity. As a result, no shear-induced turbulence is created, and solid objects can settles on the midplane and increase its density. Haghighipour has shown that this mechanism will facilitate planetesimals formation through gravitational instability, and can help km-sized objects to form in a short time.
3) Origin and dynamics of irregular satellites
Despite the differences in their composition, structure, and the mechanisms of formation, the giant planets of our solar system have one common feature: they all host irregular satellites. Marked by their highly eccentric orbits, and/or high orbital inclinations, irregular satellites revolve around their host planets at large distances. The dynamics of these objects is affected by perturbation from the Sun, and their precessions are controlled by solar tugs.
Although several of the dynamical characteristics of irregular satellites have already been studied, there is one interesting feature that has recently been observed in the distribution of Jovain irregulars and has not yet been accounted for. The region extending from the orbit of Callisto, the outermost Galilean satellite, to approximately 80 Jupiter-radii is void of irregulars. To understand the reason for this phenomenon, in collaboration with David Jewitt from NAI/Hawaii, Haghighipour completed an extensive numerical study of the orbital stability of small bodies in the region between 30 and 80 Jupiter-radii. Haghighipour and Jewitt simulated the dynamics of a large battery of small objects in this region and mapped their parameter-space for different values of their semimajor axes, eccentricities, and orbital inclinations. Their numerical simulations show that, except at large distances from the outer boundaries of the influence zones of Ganymede and Callisto, the lack of irregular satellites between Callisto and Themisto can be attributed to the instability of test particles caused by their interactions with the two outermost Galilean satellites. At larger distances (e.g., between 40 and 80 Jupiter-radii), however, the perturbations of Galilean satellites do not seem to be able to account for the instability of small bodies. A viable explanation for the lack of irregular satellites at such distances is to assume that their instability has been the result of their interactions with Jovian satellitesimals and protosatellites, during the formation of Jupiter's regular moons.
4) Detection of Close-in Extrasolar Terrestrial Planets
Despite many observational techniques that are currently used in detecting extrasolar planets, the problems associated with the detection of terrestrial-sized objects are still unresolved. The residual effects, due to a possible terrestrial-class planet in an extrasolar planetary system, are so small that they fall outside the range of the sensitivity of most detection techniques. In many cases, the dynamical characteristics of an extrasolar planetary system, or its unexpected assembly of bodies (e.g., hot Jupiters around single stars, or Jupiter-like planets in dual- and triple-star systems) have made the matter more complicated. For instance, in a system where a star is host to a close-in Jovian-type planet, the strong perturbations from the planetary companion cause the residual effects of a possible terrestrial planet to have so much uncertainty that they become unreliable. In collaboration with E. Agol from the Astrobiology team of the University of Washington in Seattle, Haghighipour has initiated a new project that aims to overcome this difficulty by measuring the transit time of an extrasolar planet and analyzing its variations due to the presence of another planetary object. Using variations in the periodicity of the time of the transit of a giant planet in a single-planet extrasolar planetary system, the TTV (Transit Timing Variation) method is capable of identifying an additional planet, and also constraining its mass and orbital elements. In general, TTV is capable of constraining the mass of a planetary companion approximately one order of magnitude smaller than those detected by a 5 m/s precision radial velocity technique. Haghighipour has completed a study of the dynamics of terrestrial planets in resonant orbits in system with hot-Jupiters to determine the variation in their transit timing due to perturbations from the giant body.
Gaidos, E., N. Haghighipour, E. Agol, D. Latham, S. N. Raymond, and J. Rayner (2007). New Worlds on the Horizon: Earth-Size Planets Close to Other Stars, Science 318, 210-213.
Haghighipour, N., and S. N. Raymond (2007). Habitable Planet Formation in Binary- Planetary Systems, Astrophys. J. 666, 436-446.
Jewittn D., and N. Haghighipour (2007). Irregular Satellites of the Planets: Products of Capture in the Early Solar System, A.R.A. & A. 45, 261-295.
Rivera E. J., and N. Haghighipour (2007). On the Stability of Test Particles in Extrasolar Multiple Planet Systems, M.N.R.A.S. 374, 599-613.
Haghighipour, N., and E. Scott (2007). Numerical Modeling to Test Meteorite Constraints on the Early Stages of Planetary Growth in Inner Solar System. B.A.A.S. 39, 474.
Haynes, J., and N. Haghighipour (2007). Dynamical Stability of Terrestrial and Giant Planets in the HD 155358 Planetary System, B.A.A.S. 39, 451.
Haghighipour, N. (2007). Formation of Planetesimals via Gravitational Instability and the Effect of Gas-Density Enhancements. B.A.A.S. 39, #6.03.
Haghighipour, N. (2007). Models of Water-Delivery to Earth. B.A.A.S. 39, 233.
Castro, J., and N. Haghighipour (2007). Earth-like Planets Around GJ 876. B.A.A.S. 39, 203.
Rugheimer, S., and N. Haghighipour (2007). Habitable Planets in the Planetary System of HD 69830. B.A.A.S. 39, 203.
Haghighipour, N. (2007). Habitable planet formation; A review of current status. B.A.A.S.38, 1071.
Castro, J., and N. Haghighipour (2007). Is the binary-planetary system of Gamma Cephei dynamically full? B.A.A.S. 38, 1007.