2001 Annual Science Report
Carnegie Institution of Washington Reporting | JUL 2000 – JUN 2001
Studies in Planetary Formation and Evolution
Task 1. Detection and Characterization of Extrasolar Planets (dm)
Our group is surveying the nearest 1,200 Sun-like stars (of spectral type F8-M5) with the precision Doppler technique at the Lick 3-m, Keck 10-m, and Anglo-Australian 3.9-m telescopes to search for extra-solar planets. Over the past 6 years, these three surveys have led to the discovery of two-thirds of the known extrasolar planets. All 12 extrasolar planet discoveries published in peer-reviewed journals over the last year were found from these three surveys, including two new multiple planet systems. All of our Doppler surveys have achieved state of the art velocity precision of 3 m/s. These are the only active surveys sufficient to detect Solar System type planets (with masses similar to Jupiter and Saturn).
Beginning in early 2002 we will expand our program with the Magellan 6.5-m telescope at Las Campanas in Chile. By adding 800 stars with this telescope, we will complete a volume-limited survey of G dwarfs (solar type stars) out to 50 parsecs, K dwarfs out to 30 parsecs, and M dwarfs out to 10 parsecs. This first reconnaissance of all nearby dwarf stars will provide the target list for more intensive follow-up observations from ground and space-based interferometers, transit observations, reflected light searches, transit photometry, and ultimately direct imaging and spectroscopy with the NASA Terrestrial Planet Finder.
Task 2. Formation of Gas-Giant Planets (dm)
Two theories are competing to explain the formation of the gas-giant planets discovered in orbit around nearby stars. Core accretion, developed to explain our Solar System and currently the generally accepted mechanism, requires several million years or more to form a gas-giant planet in a protoplanetary disk. However, recent observations have shown that in most cases, protoplanetary disks lose their gas in several million years or less, implying that if core accretion is the only means of forming gas-giant planets, these planets should be rare. The ongoing census of extrasolar gas-giant planets strongly suggests otherwise. The alternative mechanism, disk instability, can form a gas-giant protoplanet rapidly, in about a thousand years. Last year we used three-dimensional hydrodynamical models to show that disk instability could form Jupiter-mass clumps even in a disk with a mass comparable to that inferred for the solar nebula and similar to that needed to form gas-giant planets by the core accretion mechanism. This year we have extended these models to include a complete thermodynamical description of the disk instability process, including a solution of the three-dimensional energy equation with radiative transfer in the diffusion approximation, and with detailed equations of state and dust grain opacities. Because the disk cooling times are comparable to the local orbital period, disk instabilities proceed in much the same manner as in the previous “locally isothermal’' models, though rising temperatures in the clumps do restrict their growth. The models imply that the disk instability mechanism 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 (dm)
A fundamental goal of astrobiological research is to understand the extent to which planets favorable to life are frequent or rare. Although it may be that chemical and physical processes different from those on Earth may provide habitable planets, Earth is today the only planet we know of that contains a living system of any kind. Therefore, understanding the frequency of Earth-like planets is an essential goal for this Institute. Our planetary system provides a unique body of firm facts concerning a system in which not only life, but cognitive life actually did occur. Furthermore, we have abundant knowledge of the present physical state of our Solar System. For example, we know for an absolute fact that our gas giant planets did not drift near the Sun, despite the observational data that clearly shows that this does occur in other planetary systems. At present, available instrumentation does not permit observation of Jupiter-size planets at the distance of Jupiter from their stars, but the time will soon come when this is possible. Before that time, it is essential that a body of theoretical work exists that can be compared with new observational data in order to provide the necessary balance between theory and observation. We are trying to contribute to this need by devising a model that can include both the observation of our Solar System and the presently observed extrasolar planets.
At this time most workers make use of a “standard model” for the formation of our Solar System, at least out to the distance of Saturn. According to this model, in the terrestrial planet region, planetesimals grow from ~1014g bodies to ~1026g bodies in about 105 years by a characteristic instability termed “runaway growth.” This has the consequence that Mars-size “planetary embryos” form at ~1 AU in about 105 years and grow to Earth size in about 108 years even in the presence of giant planets that form after only ~107 years, according to this standard model. The problem is that if the limited observational data of extrasolar planets is typical of planetary systems in general, then the incoming giant plants should drift into the central star and also disturb the terrestrial planets.
As discussed last year, we developed a quantitative alternative to the standard model that is based on a new theory for giant planet formation. In this model, Jupiter grows on a very short timescale (~105 years). This work shows that in the presence of early-formed gas giant planets and nebular gas, Mars-size planetary embryos in the terrestrial planet region are formed despite the very strong gravitational perturbations by the giant planets. This is the consequence of synchronism in the orbital elements of the planetesimals, permitting sufficiently low relative velocities despite strong perturbations by the giant planets.
To a first approximation, this model also predicts that the asteroid belt will be very strongly depleted, as it actually is. However, in the extreme case of Jupiter growth before formation of any planetesimals at all, this goes too far, and growth of the asteroids approaching the size of those in the present asteroid belt will be precluded. However, disk evolution is not sufficiently well understood to rule out the likelihood of a delay of ~105 years in the development of the gravitational disk instabilities that led to the formation of Jupiter and Saturn. If this delay occurs, an asteroid belt with its largest bodies the size of Ceres can be formed. The detailed orbital and fragmentation evolution of such a system remains to be studied.
In a separate project, we have studied the formation of Jupiter and Saturn in the framework of the standard model, but including the effects of fragmentation and loss of planetesimals by the high-velocity perturbations caused by growth of the massive giant planet cores. Fragmentation reduces the quantity of material available to form a Jupiter core considerably. As a consequence, the usual choice of disk grain opacity makes it unlikely that the gravitational instability necessary to capture the gas mantle of Jupiter can form. However, if the grain opacity is reduced by a factor of ~100, a Jupiter mass planet can form within ~107 years. While not out of the question, recent work supports lower timescales than this for the formation of Sun-like stars. In addition, there remains the major problem of a planet in a gaseous environment avoiding drift into the inner Solar System on a timescale as long as 107 years. For these reasons, we believe that the alternative to the conventional model of planet formation should be considered seriously.
Task 4. Evolution of Planetary Water (dm)
One of the long-term goals of this astrobiology 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. The Tharsis region of Mars has long been recognized as a major center of magmatism and deformation that has been active throughout martian history as a source of heat and atmospheric volatiles. Recent work on defining the stages of deformation in the region has demonstrated that much of the tectonic activity was concentrated in the Noachian epoch of martian history, the period prior to the end of heavy bombardment of the inner solar system about 3.8 Ga ago. On the basis of the global topography and gravity anomaly fields determined by MGS, we have shown that much of the long-wavelength aspects of those fields can be explained as the response of the martian interior to loading by the volcanic materials of the Tharsis rise. Of particular importance to the history of martian water, we have also shown that the downhill directions of late Noachian valley networks on Mars appear to have been influenced by the long-wavelength slopes produced by this global response to Tharsis loading. There are two implications of this result. The first is that the Tharsis rise must have been substantially in place prior to the late Noachian, a conclusion indicating that the martian volcanic flux early in the planet’s history was much higher than during any subsequent period. The second implication is that volatiles released from magmas erupted during the construction of Tharsis may have contributed substantially to the inventories of water and other species in the atmosphere-surface system, perhaps modifying climate and influencing the formation of valley networks.
PROJECT MEMBERS:Sean Solomon
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.