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Project Progress

A. Project Description

To understand the origin of life on Earth, we must first understand the formation of our own planetary system. In particular, we must develop a comprehensive knowledge of the formation, and dynamical transport of the organic materials from which life evolved. Planetary systems form by collapse of dense interstellar cloud cores. Some stages in this evolution can be directly observed when stellar nurseries are imaged, while other stages remain cloaked behind an impenetrable veil of dust and gas.

Dense cloud cores are very cold (10-50 K), and their dust grains are coated with ices comprised of water and organic compounds. Many of these organics have potential relevance to the origin or early evolution of life, if delivered to planets. The ultimate delivery of these primitive organics to young planets and their moons also evolves with time, as the bodies grow in size and as the nebula clears.

Perhaps one of the best methods for probing how this process took place within the Solar System is to study the chemical composition of comets. Thus, the goal of Theme 1 of our investigation is to study volatile organics in icy planetesimals by astronomical sensing of comets, and establish their taxonomic classes based on their organic composition. This taxonomy can only achieve its full significance, however, if the chemical composition of a particular object is linked to its formation location in the solar nebula. If this can be accomplished with enough objects, we will have powerful information concerning the structure of the disk of material from which the planets formed.

The standard picture is that most comets were formed in the outer region of the solar nebula in a zone from ∼ 5 to ∼ 40 AU. As a result of this wide range of heliocentric distances, it is reasonable to suppose that a chemical gradient would appear among comets, with the objects forming near the orbit of Jupiter having compositions consistent with formation at higher temperatures than their more distant brethren. In the current paradigm, Oort cloud comets originated in the region between Jupiter and Neptune and were later scattered outward by close approaches with these giant planets. At the same time according to the standard picture, the scattered disk, which is probably the source of Jupiter-family comets was constructed from objects scattered outward by Neptune and thus which formed in the region beyond roughly 30 AU. The role of PI Levison in this program is to reevaluate these ideas in the context of modern theories of planet formation and Solar System evolution.

B. Accomplishments to Date

Unfortunately, detailed modeling of the dynamical evolution of the planets fail to reproduce all the available observational constraints. Of particular note is a problem concerning the relationship between the scattered disk and the Oort cloud. The Oort cloud currently contains at least ∼4 × 10¹¹ objects with radii® greater than 1 km. Our models of Oort cloud and scattered disk formation predict the ratio between the number of comets in the scattered disk and the number in the Oort cloud should be ∼ 0.1. This implies a population of at least 4 × 10¹⁰ objects with R > 1 km in the scattered disk. However, the observations suggest this number is actually 2 × 10⁸. Although there are significant uncertainties in both of these numbers, we must conclude that they disagree with one another by at least an order of magnitude.

The simplest interpretation of the above information leads to the conclusion that the Oort cloud is at least an order of magnitude more massive than our theories would predict. Thus, we decided to look for a new source of Oort cloud comets. In particular, we explored that idea that Oort cloud comets formed around other stars and were gravitationally captured by the Sun when it was still in its birth cluster. We studied this idea through a set of numerical simulations of a large number of comets and stars embedded in a spherically symmetric distribution of gas.

We found that comets can be captured with high enough efficiency to explain the mass of the Oort cloud. Indeed, we found two important capture mechanisms. Although the comets are initially in orbit about the stars most of the comets are stripped away due to close stellar encounters. These comets then orbit freely within the cluster. Thus, when the cluster disperses, most of the stars have a large population of comets that are sufficiently close to be potential Oort cloud members. A fraction of these, by chance, will have velocities similar to one of these stars. Thus, as the cluster expands, these comets will head in the same direction as that star and thus will become bound to it.

Direct exchange of comets between two stars involved in a close encounter is the second important process of Oort cloud formation that we observe in our simulations. In this encounter 19% of the comets initially in the scattered disk of a 0.32 M⊙ star are transferred to orbits about a 1 M⊙ star.

Our simulations show that these capture mechanisms could, in principal, explain the discrepancies described above. Unfortunately, it is impossible to predict from first principles how much material was captured in the Oort cloud because it depends on unknowns like the typical structure of extra-solar planetary systems. However, if true than roughly 90% of the Oort cloud is composed of objects that formed in the proto-planetary disks of other stars. This includes some of the most famous comets in history, including 1P/Halley and C/1995 O1 (Hale-Bopp). We have published a paper in SCIENCE explaining this work (Levison, et al. 2010, SCIENCE 329, 187.). PI Levison has given 10 talks on this subject, including one at Caltech, Yale, RPI, and Boston University. This work was ranked #23 in a list of the most important scientific developments of 2010 by DISCOVER magazine.

Another significant discrepancy between the expectations of our models and observations has to do with the so-called Halley-type comets (HTCs: roughly speaking, these can be thought of as objects with orbital periods between 20 and 200 years). The standard idea is that these objects are from the Oort cloud. However, there is a significant problem with this idea. The real HTCs have a median inclination of only 55◦. Models of direct capture from an isotropic Oort cloud, however, predict that retrograde HTCs should slightly outnumber the prograde HTCs. Levison et al. (2006, ICARUS 184, 619) supplied us with an innovative solution to this problem. In that paper we argued that the HTCs are derived from bodies that have recently leaked off the outer edge of the scattered disk. That is, they are objects that have spent the last ∼ 4 Gyr in the scattered disk and have just recently evolved to semi-major axes large enough for the effects of the Galaxy to become important. In their case the Galactic tides forced their perihelion distances (Q) to decrease, driving them back into the planetary region and becoming HTCs. Thus, we argued that some of the observed HTCs are coming from the scattered disk rather than the Oort cloud. However, our simulations show that some of these objects immediately become visible LPCs before becoming HTCs. It is still not clear whether this result is consistent with observations or the other models of LPC formation.

So, in the last few years, several innovative ideas have been put forward to explain known problems in the origin of comets. However, with these solutions have come new problems. For example, what fraction of the Oort cloud is captured from other stars? Is our new model for the origin of HTCs consistent with the observed population of LPCs. The only way to address these issues is to construct a comprehensive, end-to-end model of the origin and evolution of the comet reservoirs. This model, which is on the way, follows the evolution of planetesimals from their origin in the proto-planetary disk, into their current reservoirs, and back to visible comets. It includes the effects of the Sun’s birth cluster, the early migration of Jupiter and Saturn, and the Nice model.