Showing papers on "Hill sphere published in 2005"

Formation of Kuiper-belt binaries through multiple chaotic scattering encounters with low-mass intruders

Abstract: The discovery that many trans-Neptunian objects exist in pairs, or binaries, is proving invaluable for shedding light on the formation, evolution and structure of the outer Solar system. Based on recent systematic searches it has been estimated that up to 10 per cent of Kuiper-belt objects might be binaries. However, all examples discovered to date are unusual, as compared with near-Earth and main-belt asteroid binaries, for their mass ratios of the order of unity and their large, eccentric orbits. In this article we propose a common dynamical origin for these compositional and orbital properties based on four-body simulations in the Hill approximation. Our calculations suggest that binaries are produced through the following chain of events. Initially, long-lived quasi-bound binaries form by two bodies getting entangled in thin layers of dynamical chaos produced by solar tides within the Hill sphere. Next, energy transfer through gravitational scattering with a low-mass intruder nudges the binary into a nearby non-chaotic, stable zone of phase space. Finally, the binary hardens (loses energy) through a series of relatively gentle gravitational scattering encounters with further intruders. This produces binary orbits that are well fitted by Kepler ellipses. Dynamically, the overall process is strongly favoured if the original quasi-bound binary contains comparable masses. We propose a simplified model of chaotic scattering to explain these results. Our findings suggest that the observed preference for roughly equal-mass ratio binaries is probably a real effect; that is, it is not primarily due to an observational bias for widely separated, comparably bright objects. Nevertheless, we predict that a sizeable population of very unequal-mass Kuiper-belt binaries is probably awaiting discovery.

Prospects for Habitable “Earths” in Known Exoplanetary Systems

Abstract: We have examined whether putative Earth-mass planets could remain confined to the habitable zones (HZs) of the 111 exoplanetary systems confirmed by 2004 August. We find that in about half of these systems there could be confinement for at least the past 1000 Myr, though in some cases only in variously restricted regions of the HZ. The HZ migrates outward during the main-sequence lifetime, and we find that in about two-thirds of the systems an Earth-mass planet could be confined to the HZ for at least 1000 Myr sometime during the main-sequence lifetime. Clearly, these systems should be high on the target list for exploration for terrestrial planets. We have reached our conclusions by detailed investigations of seven systems, which has resulted in an estimate of the distance from the giant planet within which orbital stability is unlikely for an Earth-mass planet. This distance is given by nRH, where RH is the Hill radius of the giant planet and n is a multiplier that depends on the giant's orbital eccentricity and on whether the Earth-mass planet is interior or exterior to the giant planet. We have estimated n for each of the seven systems by launching Earth-mass planets in various orbits and following their fate with a hybrid orbital integrator. We have then evaluated the habitability of the other exoplanetary systems using nRH derived from the giant's orbital eccentricity without carrying out time-consuming orbital integrations. A stellar evolution model has been used to obtain the HZs throughout the main-sequence lifetime.

An ultradeep survey for irregular satellites of uranus: limits to completeness

Abstract: We present a deep optical survey of Uranus's Hill sphere for small satellites. The 8 m Subaru Telescope was used to survey about 3.5 square degrees with a 50% detection efficiency at limiting red magnitude mR = 26.1. This magnitude corresponds to objects that are about 7 km in radius (assuming an albedo of 0.04). We detected (without prior knowledge of their positions) all previously known outer satellites and discovered two new irregular satellites (S/2001 U2 and S/2003 U3). The two inner satellites Titania and Oberon were also detected. One of the newly discovered bodies (S/2003 U3) is the first known irregular prograde satellite of the planet. The population, size distribution, and orbital parameters of Uranus's irregular satellites are remarkably similar to those of the irregular satellites of gas giant Jupiter. Both have shallow size distributions (power-law indices q ~ 2 for radii larger than 7 km) with no correlation between the sizes of the satellites and their orbital parameters. However, unlike those of Jupiter, Uranus's irregular satellites do not appear to occupy tight, distinct dynamical groups in semimajor-axis versus inclination phase space. Two groupings in semimajor-axis versus eccentricity phase space appear to be statistically significant.

The dependence of protoplanet migration rates on co-orbital torques

Abstract: We investigate the migration rates of high-mass protoplanets embedded in accretion discs via two- and three-dimensional hydrodynamical simulations. The simulations follow the planet's radial motion and employ a nested-grid code that allows for high resolution close to the planet. We concentrate on the possible role of the co-orbital torques in affecting migration rates. We analyse two cases: (a) a Jupiter-mass planet in a low-mass disc; and (b) a Saturn-mass planet in a high-mass disc. The gap in case (a) is much cleaner than in case (b). Planet migration in case (b) is much more susceptible to co-orbital torques than in case (a). We find that, for both cases, the co-orbital torques do not depend sensitively on whether the planet is allowed to migrate through the disc or is held on a fixed orbit. We also examine the dependence of the planet's migration rate on the numerical resolution near the planet. For case (a), numerical convergence is relatively easy to obtain, even when including torques arising from deep within the planet's Hill sphere, since the gas mass contained within the Hill sphere is considerably less than the planet's mass. The migration rate in this case is numerically of the order of the Type II migration rate and much smaller than the Type I rate, if the disc has 0.01 solar masses inside 26 au. Torques from within the Hill sphere provide a substantial opposing contribution to the migration rate. In case (b), the gas mass within the Hill sphere is greater than the planet's mass and convergence is more difficult to obtain. Torques arising from within the Hill sphere are strong, but nearly cancel. Any inaccuracies in the calculation of the torques introduced by grid discretization can introduce spurious torques. If the torques within the Hill sphere are ignored, convergence is more easily achieved but the migration rate is artificially large. At our highest resolution, the migration rate for case (b) is much lower than the Type I rate, but somewhat larger than the Type II rate.