arXiv subjects

Renu Malhotra

Publications and source records attributed to Renu Malhotra.

At least 109 records · Page 6Linked to original sources

arXiv2007

The Formation and Evolution of Planetary Systems: Placing Our Solar System in Context with Spitzer

We provide an overview of the Spitzer Legacy Program ``Formation and Evolution of Planetary Systems" (FEPS) which was proposed in 2000, begun in 2001, and executed aboard the Spitzer Space Telescope between 2003 and 2006. This program exploits the sensitivity of Spitzer to carry out mid-infrared spectrophotometric observations of solar-type stars. With a sample of ~ 328 stars ranging in age from ~ 3 Myr to ~ 3 Gyr, we trace the evolution of circumstellar gas and dust from primordial planet-building stages in young circumstellar disks through to older collisionally generated debris disks. When completed, our program will help define the time scales over which terrestrial and gas giant planets are built, constrain the frequency of planetesimal collisions as a function of time, and establish the diversity of mature planetary architectures. In addition to the observational program, we have coordinated a concomitant theoretical effort aimed at understanding the dynamics of circumstellar dust with and without the effects of embedded planets, dust spectral energy distributions, and atomic and molecular gas line emission. Together with the observations, these efforts will provide astronomical context for understanding whether our Solar System - and its habitable planet - is a common or a rare circumstance. Additional information about the FEPS project can be found on the team website: feps.as.arizona.edu

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arXiv2006

Assessing the massive young Sun hypothesis to solve the warm young Earth puzzle

A moderately massive early Sun has been proposed to resolve the so-called faint early Sun paradox. We calculate the time-evolution of the solar mass that would be required by this hypothesis, using a simple parametrized energy-balance model for Earth's climate. Our calculations show that the solar mass loss rate would need to have been 2-3 orders of magnitude higher than present for a time on the order of ~2 Gy. Such a mass loss history is significantly at variance (both in timescale and in the magnitude of the mass loss rates) with that inferred from astronomical observations of mass loss in younger solar analogues. While suggestive, the astronomical data cannot completely rule out the possibility that the Sun had the required mass loss history; therefore, we also examine the effects of the hypothetical historical solar mass loss on orbital dynamics in the solar system, with a view to identifying additional tests of the hypothesis. Planetary and satellite orbits provide a few tests, but these are weak or non-unique.

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arXiv2006

Are Debris Disks and Massive Planets Correlated?

Using data from the Spitzer Space Telescope Legacy Science Program ``Formation and Evolution of Planetary Systems'' (FEPS), we have searched for debris disks around 9 FGK stars (2-10 Gyr), known from radial velocity (RV) studies to have one or more massive planets. Only one of the sources, HD 38529, has excess emission above the stellar photosphere; at 70 micron the signal-to-noise ratio in the excess is 4.7 while at wavelengths < 30 micron there is no evidence of excess. The remaining sources show no excesses at any Spitzer wavelengths. Applying survival tests to the FEPS sample and the results for the FGK survey published in Bryden et al. (2006), we do not find a significant correlation between the frequency and properties of debris disks and the presence of close-in planets. We discuss possible reasons for the lack of a correlation.

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arXiv2006

Dynamical transport of asteroid fragments from the nu6 resonance

A large disruption in the main asteroid belt can cause a large flux, an "asteroid shower", on the terrestrial planets. We quantitatively examine the hypothesis that such an event was the cause of the lunar late heavy bombardment (LHB). We performed numerical integrations of about 20000 test particles starting in the vicinity of the nu6 secular resonance in the main asteroid belt. The purpose of these integrations is to calculate, for each of the terrestrial planets, the collision probability of asteroids coming from an asteroid break-up event in the inner part of the main belt. Compared with previous studies, we simulate nearly two orders of magnitude larger number of particles, and we include the orbital effects of the eight planets, Mercury to Neptune. We also examined in detail the orbital evolution of asteroid fragments once they enter the Earth's activity sphere, including the effect of the Earth-Moon orbit. We obtained the collision probability, the distributions of impact velocities, impact positions, and impact angles of asteroid fragments on the Moon. The collision probability with the Moon (~0.1%) suggests that a fairly large parent body, 1000-1500 km in diameter, is required if the LHB event is to be ascribed to a single asteroid disruption. An even larger parent body is required for less favorable initial conditions than we investigated here. We conclude that an asteroid disruption event is not a viable explanation for the LHB.

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arXiv2005

The origin of planetary impactors in the inner solar system

New insights into the history of the inner solar system are derived from the impact cratering record of the Moon, Mars, Venus and Mercury, and from the size distributions of asteroid populations. Old craters from a unique period of heavy bombardment that ended $\sim$3.8 billion years ago were made by asteroids that were dynamically ejected from the main asteroid belt, possibly due to the orbital migration of the giant planets. The impactors of the past $\sim$3.8 billion years have a size distribution quite different from the main belt asteroids, but very similar to the population of near-Earth asteroids.

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arXiv2005

Neptune's Migration into a Stirred-Up Kuiper Belt: A Detailed Comparison of Simulations to Observations

Nbody simulations are used to examine the consequences of Neptune's outward migration into the Kuiper Belt, with the simulated endstates being compared rigorously and quantitatively to the observations. These simulations confirm the findings of Chiang et al. (2003), who showed that Neptune's migration into a previously stirred-up Kuiper Belt can account for the Kuiper Belt Objects (KBOs) known to librate at Neptune's 5:2 resonance. We also find that capture is possible at many other weak, high-order mean motion resonances, such as the 11:6, 13:7, 13:6, 9:4, 7:3, 12:5, 8:3, 3:1, 7:2, and the 4:1. The more distant of these resonances, such as the 9:4, 7:3, 5:2, and the 3:1, can also capture particles in stable, eccentric orbits beyond 50 AU, in the region of phase space conventionally known as the Scattered Disk. Indeed, 90% of the simulated particles that persist over the age of the Solar System in the so-called Scattered Disk zone never had a close encounter with Neptune, but instead were promoted into these eccentric orbits by Neptune's resonances during the migration epoch. This indicates that the observed Scattered Disk might not be so scattered. This model also produced only a handful of Centaurs, all of which originated at Neptune's mean motion resonances in the Kuiper Belt. We also report estimates of the abundances and masses of the Belt's various subpopulations (e.g., the resonant KBOs, the Main Belt, and the so-called Scattered Disk), and also provide upper limits on the abundance of Centaurs and Neptune's Trojans, as well as upper limits on the sizes and abundances of hypothetical KBOs that might inhabit the a>50 AU zone.

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arXiv2005

Dynamical models of Kuiper Belt dust in the inner and outer Solar System

We report several results related to the dynamical evolution of dust produced in the Kuiper Belt (KB). We show that its particle size frequency distribution in space is greatly changed from the distribution at production, as a results of the combined effects of radiation forces and the perturbations of the planets. We estimate the contribution of KB dust to the zodiacal cloud by calculating the radial profile of its number density near the ecliptic. We also study the contribution of KB dust to the population of interplanetary dust particles (IDPs) collected at Earth, by calculating geocentric encounter velocities and capture rates. Our models show, in contrast with previous studies, that KB dust grains on Earth-crossing orbits have high eccentricities and inclinations and, therefore, their encounter velocities are similar to those of cometary grains and not to asteroidal grains. We estimate that at most 25% in number of captured IDPs have cometary or KB origin; the KB contribution may be as low as 1%-2%. We present the velocity field of KB dust throughout the solar system; this, together with the number density radial profile, is potentially useful for planning spacecraft missions to the outer solar system.

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arXiv2005

Formation and Evolution of Planetary Systems: Cold Outer Disks Associated with Sun-like stars

We present the discovery of debris systems around three solar mass stars based upon observations performed with the Spitzer Space Telescope as part of a Legacy Science Program, ``the Formation and Evolution of Planetary Systems'' (FEPS). We also confirm the presence of debris around two other stars. All the stars exhibit infrared emission in excess of the expected photospheres in the 70 micron band, but are consistent with photospheric emission at <= 33 micron. This restricts the maximum temperature of debris in equilibrium with the stellar radiation to T < 70 K. We find that these sources are relatively old in the FEPS sample, in the age range 0.7 - 3 Gyr. Based on models of the spectral energy distributions, we suggest that these debris systems represent materials generated by collisions of planetesimal belts. We speculate on the nature of these systems through comparisons to our own Kuiper Belt, and on the likely planet(s) responsible for stirring the system and ultimately releasing dust through collisions. We further report observations of a nearby star HD 13974 (d =11 pc) that is indistinguishable from a bare photosphere at both 24 micron and 70 micron. The observations place strong upper limits on the presence of any cold dust in this nearby system (L_IR/L_* < 10^{-5.2}).

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arXiv2003

The Dynamics of Known Centaurs

We have numerically investigated the long term dynamical behavior of known Centaurs. This class of objects is thought to constitute the transitional population between the Kuiper Belt and the Jupiter-family comets (JFCs). In our study, we find that over their dynamical lifetimes, these objects diffuse into the JFCs and other sinks, and also make excursions into the Scattered Disk, but (not surprisingly) do not diffuse into the parameter space representing the main Kuiper Belt. These Centaurs spend most of their dynamical lifetimes in orbits of eccentricity 0.2-to-0.6 and perihelion distance 12-to-30 AU. Their orbital evolution is characterized by frequent close encounters with the giant planets. Most of these Centaurs will escape from the solar system (or enter the Oort Cloud), while a fraction will enter the JFC population and a few percent will impact a giant planet. Their median dynamical lifetime is 9 Myr, although there is a wide dispersion in lifetimes, ranging from less than 1 Myr to more than 100 Myr. We find the dynamical evolution of this sample of Centaurs to be less orderly than the planet-to-planet "hand-off" described in previous investigations. We discuss the implications of our study for the spatial distribution of the Centaurs as a whole.

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arXiv2003

Survival of Trojan-Type Companions of Neptune During Primordial Planet Migration

We investigate the survivability of Trojan-type companions of Neptune during primordial radial migration of the giant planets Jupiter, Saturn, Uranus, and Neptune. Loss of Neptune Trojans during planetary migration is not a random diffusion process. Rather, losses occur almost exclusively during discrete episodes when Trojan particles are swept by secondary resonances associated with mean-motion commensurabilities of Uranus with Neptune. The single greatest episode of loss ejects nearly 75% of existing Neptune Trojans and occurs just prior to Neptune reaching its final orbit.

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arXiv2002

Observational Limits on a Cold Distant Kuiper Belt

Almost all of the $>600$ known Kuiper belt objects (KBOs) have been discovered within 50 AU of the Sun. One possible explanation for the observed lack of KBOs beyond 50 AU is that the distant Kuiper belt is dynamically very cold, and thus thin enough on the sky to have slipped between previous deep survey fields. We have completed a survey designed to search for a dynamically cold distant Kuiper belt near the invariable plane of the Solar system. In $2.3 {\rm deg}^2$ we have discovered a total of 33 KBOs and 1 Centaur, but no objects in circular orbits beyond 50 AU. We find that we can exclude at 95% CL the existence of a distant disk inclined by $i\le1\arcdeg$ to the invariable plane and containing more than 1.2 times as many $D>185$ km KBOs between 50 and 60 AU as the observed inner Kuiper belt, if the distant disk is thinner than $σ=1\fdg75$.

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arXiv2002

A Study of the Dynamics of Dust from the Kuiper Belt: Spatial Distribution and Spectral Energy Distribution

The dust produced in the Kuiper Belt (KB) spreads throughout the Solar System forming a dust disk. We numerically model the orbital evolution of KB dust and estimate its equilibrium spatial distribution and its brightness and spectral energy distributions (SED), assuming greybody absorption and emission by the dust grains. We show that the planets modify the KB disk SED, so potentially we can infer the presence of planets in spatially unresolved debris disks by studying the shape of their SEDs. We point out that there are inherent uncertainties in the prediction of structure in the dust disk, owing to the chaotic dynamics of dust orbital evolution imposed by resonant gravitational perturbations of the planets.

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arXiv2002

A dynamical mechanism for establishing apsidal resonance

We show that in a system of two planets initially in nearly circular orbits, an impulse perturbation that imparts a finite eccentricity to one planet's orbit causes the other planet's orbit to become eccentric as well, and also naturally results in a libration of their relative apsidal longitudes for a wide range of initial conditions. We suggest that such a mechanism may explain orbital eccentricities and apsidal resonance in some exo-planetary systems. The eccentricity impulse could be caused by the ejection of a planet from these systems, or by torques from a primordial gas disk. The amplitude of secular variations provides an observational constraint on the dynamical history of such systems.

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arXiv2000

The Edge of the Solar System

The population of the Kuiper Belt within 50 AU of the Sun has likely been severely depleted by gravitational perturbations from the giant planets, particularly Neptune. The density of Kuiper Belt objects is expected to be two orders of magnitude higher just beyond 50 AU, where planetary perturbations are insignificant. In 1998 and 1999, we surveyed for Kuiper Belt Objects (KBOs) in 6 fields of the ecliptic (total sky area 1.5 deg^2) to limiting magnitudes between R=24.9 and R=25.9. This is deep enough to detect KBOs of diameter >~ 160 km at a distance of 65 AU. We detected 24 objects. None of these objects, however, is beyond 53 AU. Our survey places a 95% CL upper limit of Sigma<5 deg^-2 on the surface density of KBOs larger than ~160 km beyond 55 AU. This can be compared to the surface density of ~6 deg^-2 of 160 km KBOs at distances 30--50 AU determined from this survey and previous shallower surveys. The mean volume density of D>160 km KBOs in the 55--65 AU region is, at >95% confidence, less than the mean density in the 30--50 AU region, and at most twothirds of the mean density from 40--50 AU. Thus, a substantial density increase beyond 50 AU is excluded in this model-independent estimate, implying that some process or event in the history of the Solar System has truncated the distribution of 160-km planetesimals at ~50 AU. A dense primordial disk could be present beyond 50 AU if it contains only smaller objects, or is sufficiently thin and inclined to have escaped detection in our 6 survey fields.

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arXiv2000

The Upsilon Andromedae System: Models and Stability

Radial velocity observations of the F8 V star Upsilon Andromedae taken at Lick and at Whipple Observatories have revealed evidence of three periodicities in the line-of-sight velocity of the star. These periodicities have been interpreted as evidence for at least three low mass companions (LMCs) revolving around Upsilon Andromedae. The mass and orbital parameters inferred for these companions raise questions about the dynamical stability of the system. We report here results from our independent analysis of the published radial velocity data as well as new unpublished data taken at Lick Observatory. Our results confirm the finding of three periods in the data. Our best fits to the data, on the assumption that these periods arise from the gravitational perturbations of companions in keplerian orbits, is also generally in agreement, but with some differences, from the earlier findings. We find that the available data do not constrain well the orbital eccentricity of the middle companion in a three-companion model of the data. We also find that in order for our best-fit model to the Lick data to be dynamically stable over the lifetime of the star (~2 billion years), the system must have a mean inclination to the plane of the sky greater than 13 degrees. The corresponding minimum inclination for the best fit to the Whipple data set is 19 degrees. These values imply that the maximum mass for the outer companion can be no greater than about 20 Jupiter masses. Our analysis of the stability of the putative systems also places constraints on the relative inclinations of the orbital planes of the companions. We comment on global versus local (i.e., method of steepest descent) means of finding best-fit orbits from radial velocity data sets.

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arXiv1999

Orbital Evolution of Planets Embedded in a Planetesimal Disk

The existence of the Oort Comet Cloud, the Kuiper Belt, and plausible inefficiencies in planetary core formation, all suggest that there was once a residual planetesimal disk of mass 10-100 Earth-masses in the vicinity of the giant planets following their formation. Since removal of this disk requires an exchange of orbital energy and angular momentum with the planets, significant planetary migration can ensue. The planet migration phenomenon is examined numerically by evolving the orbits of the giant planets while they are embedded in a planetesimal disk having a mass of M_d=10 to 200 Earth-masses. We find that Saturn, Uranus, and Neptune evolve radially outwards as they scatter the planetesimals, while Jupiter's orbit shrinks as it ejects mass. Higher-mass disks result in more rapid and extensive planet migration. If orbit expansion and resonance trapping by Neptune is invoked to explain the eccentricities of Pluto and its cohort of Kuiper Belt Objects at Neptune's 3:2 mean-motion resonance, then our simulations suggest that a disk mass of order M_d~50 Earth-masses is required to expand Neptune's orbit by ~7 AU in order to pump up Plutino eccentricities to e~0.3. Such planet migration implies that the initial Solar System was more compact in the past, with the Jupiter-Neptune separation having been smaller by about 30%. The planetesimal disk is also the source of the Oort Cloud of comets. Using the results of our simulations together with a simple treatment of Oort Cloud dynamics, we estimate that ~12 Earth-masses of disk material was initially deposited in the Oort Cloud, of which ~4 Earth-masses will persist over the age of the Solar System. The majority of these comets originated from the Saturn-Neptune region of the solar nebula.

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arXiv1995

The Phase Space Structure Near Neptune Resonances in the Kuiper Belt

The Solar system beyond Neptune is believed to house a population of small primordial bodies left over from the planet formation process. The region up to heliocentric distance $\sim 50 AU$ (a.k.a. the Kuiper Belt) may be the source of the observed short period comets. In this region, the phase space structure near orbital resonances with Neptune is of special interest for the long term stability of orbits. There is reason to believe that a significant fraction (perhaps most) of the Kuiper Belt objects reside preferentially in these resonance locations. This paper describes the dynamics of small objects near the major orbital resonances with Neptune. Estimates of the widths of stable resonance zones as well as the properties of resonant orbits are obtained from the circular, planar restricted three-body model. Although this model does not contain the full complexity of the long term orbital dynamics of Kuiper Belt objects subject to the full N-body perturbations of all the planets, it does provide a baseline for the phase space structure and properties of resonant orbits in the trans-Neptunian Solar system.

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arXiv1995

THE ORIGIN OF PLUTO'S ORBIT: IMPLICATIONS FOR THE SOLAR SYSTEM BEYOND NEPTUNE

The origin of the highly eccentric, inclined, and resonance-locked orbit of Pluto has long been a puzzle. A possible explanation has been proposed recently [Malhotra, R., {\it Nature} 365:819-21 (1993)] which suggests that these extraordinary orbital properties may be a natural consequence of the formation and early dynamical evolution of the outer Solar system. A resonance capture mechanism is possible during the clearing of the residual planetesimal debris and the formation of the Oort Cloud of comets by planetesimal mass loss from the vicinity of the giant planets. If this mechanism were in operation during the early history of the planetary system, the entire region between the orbit of Neptune and approximately 50 AU would have been swept by first order mean motion resonances. Thus, resonance capture could occur not only for Pluto, but quite generally for other trans-Neptunian small bodies. Some consequences of this evolution for the present-day dynamical structure of the trans-Neptunian region are: (i) most of the objects in the region beyond Neptune and up to $\sim\!50$ AU exist in very narrow zones located at orbital resonances with Neptune (particularly the 3:2 and the 2:1 resonances), and (ii) these resonant objects would have significantly large eccentricities. The distribution of objects in the Kuiper Belt as predicted by this theory is presented here. Keywords{solar system: origins, dynamics --- Pluto --- comets --- Kuiper Belt}

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