2000 Annual Science Report
Carnegie Institution of Washington Reporting | JUL 1999 – JUN 2000
Studies in Planetary Formation and Evolution
Task 1. Detection and Characterization of Extrasolar Planets (Butler). Over the last year our collaboration at Lick and Keck, (Geoff Marcy-UC Berkeley, Steve Vogt – UC Santa Cruz, Debra Fischer – UC Berkeley) has produced more than 1 planet a month, including the only known multiple-planet system orbiting a Sun-like star (upsilon Andromedae), the only known transit planet (HD 209458), and the first two sub-Saturn-mass planets (79 Ceti and HD 46375). These discoveries have helped push the total number of known extrasolar planets over forty. We are currently focused on improving measurement precision so that we will be able to planets down to the 1 Neptune-mass range. Along with Steve Shectman (Carnegie Observatories), we are putting together a southern hemisphere planet search on the 6.5-m Magellan I telescope, to commence in 2001. Together with our current 200-star survey on the 3.9-m Anglo-Australian Telescope (AAT), the Magellan survey will extend our sourthern hemisphere search to 800 stars. Along with our surveys on the Lick 3-m and the Keck 10-m, the AAT and Magellan projects will allow us to observe the 2000 nearest Sun-like stars. Our long term goals include the discovery of Jupiter and Saturn-like planets beyond 4 AU to compare with our own Solar System, providing targets for follow up with new techniques such as interferometry and IR imaging, and finding enough planets to generate statistically meaningful distributions of planet mass, orbital distance, eccentricity, and metallicity of planet-bearing stars. These distributions will be required to constrain theories of planet formation and evolution.
Task 2. Formation of Gas-Giant Planets (Boss). Gas giant planets have been detected in orbit around an increasing number of nearby stars. Two theories have been advanced for the formation of such planets, core accretion and disk instability. Core accretion, the generally accepted mechanism, requires several million years or more to form a gas giant planet in a protoplanetary disk like the solar nebula. Disk instability, on the other hand, can form a gas-giant protoplanet in a few hundred years. However, disk instability has previously been thought to be important only in relatively massive disks. Co-I Boss has calculated a series of new three-dimensional hydrodynamical models which show that a disk instability could form Jupiter-mass clumps, even in a disk with a mass (0.091 solar masses within 20 AU) low enough to be in the range inferred for the solar nebula. The clumps form with initially eccentric orbits, and their survival will depend on their ability to contract to higher densities before they can be tidally disrupted at successive periastrons. Because the disk mass in these models is comparable to that apparently required for the core accretion mechanism to operate, Boss’s models imply that disk instability could obviate the core accretion mechanism, in the solar nebula and elsewhere, possibly with important consequences for the formation of terrestrial planets in these systems.
Task 3. Formation of Earth-like Planets (Wetherill).
The long-term goal of our research is to contribute to the understanding of the extent to which globally habitable planets like Earth are abundant or rare. Ultimately, the answer to this question will require superior observations of other planetary systems. Our work seeks to contribute to providing a theoretical framework for the planning and interpretation of these observations. Our immediate goal is to help solve some vexing problems that now stand in the way of achieving this.
These problems have recently come to light while trying to understand formation of our own planetary system. This difficulty could be a consequence of such systems being so rare that very special circumstances would be required to form them, circumstances that would be considered ad hoc if they were suggested. Our more optimistic assumption is that our theoretical tools have as yet advanced only far enough to display previously unrecognized difficulties, but not far enough to resolve them.
These difficulties arise as a consequence of the powerful gravitational effects of the giant planets Jupiter and Saturn. After these planets become present, they play a dominant role in controlling the growth of all other bodies in the Solar System. Until recently, it had been assumed that the growth of Jupiter could be delayed by as much as 10 million years, long enough to permit inner Solar System bodies to grow from dust to bodies about the size of Mars without undo interference by the giant planets. After they reach this size, the effects of the giant planets would not be sufficient to keep the Earth and its partners from continuing on to their present size.
Now, on both observational and theoretical grounds, it appears likely that if Jupiter (and Saturn) formed in the conventional manner, these giant planets would rapidly drift into the Sun during their formation. Our colleague Alan Boss has proposed and quantitatively developed a very promising alternative way to form the giant planets. In this theory, the giant planets grow far more rapidly, as a result of massive gravitational instabilities in the pre-solar disk of dust and gas. An important feature of this model is that the giant planets could finish growing before they drifted away. Then, if the gas were removed soon enough, possibly after about 1 million years, these planets might drift only to their present distance. As is the case for our own theoretical work, the details of this drifting phenomenon are still under development by qualified scientists.
The conventional theoretical techniques for calculating the growth of terrestrial planets from very small planetesimals do not work in the presence of giant planet perturbations that would occur in this new model. During the past year we have developed the theory and computer algorithms necessary to make such calculations. We find that for possible assumptions regarding the drift process, that the Earth and other terrestrial planets are likely to grow large enough to withstand the gravitational effects of the giant planets as quickly as 10000 to 100000 years. This is not the consequence of the well established process of runaway growth. Rather, it is a new phenomenon, involving different physical processes that have a similar result, and on the same time scale, despite fundamental differences in the gravitational processes responsible. At present we are examining the consequences of this mechanism for the formation of the asteroids, and continuing this represents our present plan for the coming year. Because of their greater distance from the Sun, as well as their greater proximity to Jupiter, asteroids will rather certainly not be able to grow as large as the terrestrial planets. But on the other hand, they did not grow as large as the terrestrial planets. We hope that for some combination of other people’s drift models, and our planetesimal models, it will prove possible to fit them into this picture. If not, we or our successors will have to find some other way to resolve these problems, the solution to which is essential if the goals of this Institute are to be achieved.
Task 4. Evolution of Planetary Water (Solomon)
One of the long-term goals of this NAI project is to assess the likelihood, timing, and physical and chemical environments of hydrothermal systems on Solar System objects other than Earth. Our efforts on this project during the past year have focused on Mars, primarily because of the influx of important new data from the Mars Global Surveyor (MGS) mission. As a result of measurements by the Mars Orbital Laser Altimeter (MOLA), we now have much-improved estimates for the volume of water ice at the martian poles. We have also explored the conditions (local heat flux, mix of sediment and ice in polar deposits) under which basal melting might occur beneath the polar ice. New gravity measurements by MGS have revealed negative gravity anomalies in the northern plains of Mars. Because these gravity anomalies have no expression in MOLA topography, subsurface mass deficiencies are implied. On the basis of the linear form of the anomalies and their location downslope from channel features a higher elevations, we have proposed that the anomalies represent ancient buried channels now filled with sediment of lower density that average crustal material. An implication of this hypothesis is that the volume of water transported over Martian history from southern highlands to northern lowlands has been much greater than heretofore appreciated.
PROJECT MEMBERS:Alan Boss
R. Paul Butler
RELATED OBJECTIVES:Objective 8.0
Search for evidence of ancient climates, extinct life and potential habitats for extant life on Mars.
Determine the presence of life's chemical precursors and potential habitats for life in the outer solar system.
Determine (theoretically and empirically) the ultimate outcome of the planet-forming process around other stars, especially the habitable ones.
Define climatological and geological effects upon the limits of habitable zones around the Sun and other stars to help define the frequency of habitable planets in the universe.