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Project Progress

1. From Molecular Clouds to Habitable Planetary Systems



1. Formation of Habitable Planetary Systems

Chambers developed a new semi-analytic model for the oligarchic growth stage of planet formation. The model simulates the orbital and collisional evolution of solid bodies in a protoplanetary nebula during an important stage of planetary growth that saw bodies grow from the size of asteroids to the size of Mars in the inner Solar System and the size of giant-planet cores in the outer Solar System. Rapid growth is achieved when the largest objects acquire essentially circular and coplanar orbits due to dynamical friction. Collisional fragmentation of smaller objects also promotes growth as fragments acquire circular, coplanar orbits due to gas drag. In the late stages of oligarchic growth, accretion rates are enhanced when large bodies acquire massive atmospheres of gas captured from the protoplanetary nebula. Calculations made using the model demonstrate that Mars-sized bodies would have formed in the inner solar nebula within 0.1 million years, while 10-Earth-mass bodies formed in the outer nebula in 1 million years, for reasonable nebula parameters. These results are consistent with the formation of Earth-mass terrestrial planets in the habitable zone, and gaseous giant planets in the outer regions, of a Sun-like star.

2. Protoplanetary and Debris Disks

Weinberger continued her studies of young circumstellar disks as the sites of planet formation and evolution. In collaboration with Inseok Song of Gemini Observatory and Eric Becklin and Ben Zuckerman of UCLA, she studied a unique 300-Myr-old star, HIP 8920, which has a massive, warm debris disk. The dust exhibits a very strong silicate feature with broad peaks at 10 and 11 μm. Fits of the spectra show that small amorphous and crystalline silicate grains, < 2.5 μm, are present in the disk. While the interstellar medium, and therefore presumably proto-planetary disks, are dominated by small grains, this star is too old for the dust to be primordial. The very high line-to-continuum ratio, produced by the abundant small grains, is much larger than for even Solar System comets. Perhaps a massive asteroid collision recently pulverized a 200-km object.

With Postdoctoral Fellow Aki Roberge, Weinberger completed work on the visual spectrum of the disk around TW Hydrae. The team discovered an asymmetry in the inner part of the disk that indicates the inner disk is warped with respect to the outer part, perhaps because of perturbation by a massive planet. In addition, they showed that the disk must be dominated by relatively large grains, but collisions may be shattering these grains in the inner part of the disk.

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Weinberger and Roberge probed the disk around the nearby M-type star AU Microscopii with far-ultraviolet spectroscopy. They showed that the disk’s primordial gas has largely been dissipated in less than about 12 Myr, whereas some dust remains. It is extremely difficult to form a giant planet around AU Mic with current core accretion scenarios in such a short time. The dissipation timescale of the gas was just as short for this low-luminosity star’s disk as for the much larger luminosity, coeval star Beta Pictoris. This result constrains the dissipation mechanism to be largely independent of luminosity.

3. Searching for Extrasolar Habitable Planetary Systems

Butler continued work on three precision Doppler surveys. The first program is a survey of the nearest 1,800 Sun-like stars (late F, G, K, and M dwarfs) using the Keck 10-m and Lick 3-m telescopes in the north and the Magellan 6.5-m telescope and 4-m Anglo-Australian Telescope (AAT) in the south. This survey represents the first reconnaissance of the nearest stars within 50 parsecs, with a limiting measurement precision of 3 m/s. These surveys have produced two-thirds of the known extrasolar planets, including all nearby planetary systems. Butler and Geoff Marcy (UC Berkeley) lead this program. An important recent discovery by the group was the detection of a 7-Earth-mass planet that may be the first rocky analogue of the terrestrial planets in orbit around another normal star, the M dwarf GJ 876.

The second program on which Butler is working is the N2K survey of metal-rich stars, led by Debra Fischer (San Francisco State University). The Keck 10-m, Subaru 8-m, and Magellan 6.5-m telescopes are surveying the nearest 2,000 metal-rich stars within 100 parsecs for short period planets. Short-period planets have a greatly enhanced chance of transiting their host stars. This program has produced four new short-period planets, including only the second nearby bright transit planet orbiting HD 149026. Remarkably this is a Saturn-mass planet with a significantly smaller radius and higher density than Saturn, implying a massive (70 Earth-mass) core.

Over the last year Butler’s team has begun a new initiative to achieve 1 m/s precision on the nearest stars. They are currently surveying the nearest 200 stars with the Keck and AAT as part of this program and hope to expand this program to include the nearest 1,000 Sun-like stars within 30 parsecs, which will require a dedicated 8-m class telescope. Such a program would be the dominant planet-search program for the next 30 years.

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Boss’s contribution to Carnegie’s NAI effort in the last year centered on his leadership of a new ground-based astrometric planet detection effort, to be conducted with the 2.5-m du Pont telescope at Carnegie’s Las Campanas Observatory in Chile. Last summer Boss and his team were awarded funds by the National Science Foundation to build the Carnegie Astrometric Planet Search (CAPS) camera, a specialized camera that should yield astrometric accuracies of 0.25 millarcsec per epoch. This accuracy is sufficient to detect planets with masses as low as 1/10 the mass of Jupiter on 12-year orbits around nearby late M dwarf stars, with a signal-to-noise ratio of four. Boss placed the order for the $235K Rockwell Hawaii-2RG focal plane array in October 2004 and for the Barr Associates lambda/30 combination filter/window in March 2005. The camera will be completed by the fall of 2005 and will be mounted on the du Pont telescope during the December 2005 observing run. Boss observed for five nights on the du Pont in May 2005 in preparation for beginning work with the CAPS camera.

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4. Characterizing Extrasolar Habitable Planetary Systems

Seager and colleague’s extrasolar planet atmosphere work led to the detection of thermal emission from the extrasolar planet HD209458b with the Spitzer Space Telescope. This landmark study marked the first time that photons were detected from an extrasolar planet. Atmospheric models consistent with the dayside flux measurements at thermal and near-infrared wavelengths constrain the H₂O absorption band depths and the general character of atmospheric circulation. The models point to future wavelength- and phase-dependent observations that can discriminate among current models.

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5. Habitable Environments in the Solar System

One of the long-term goals of CIW NAI team is to assess the likelihood, timing, and physical and chemical characteristics of potential habitable environments on Solar System objects other than Earth. Solomon’s efforts on this project during the past year have focused on hydrothermal systems on Mars as such candidate environments, primarily because of the influx of important new data from the Mars Global Surveyor (MGS), Mars Odyssey, Mars Express, and Mars Exploration Rover (MER) missions.

A first step in that evaluation was a synthesis of spacecraft observations and inferences from Martian meteorites regarding the role of water in the early geophysical, geochemical, and geochemical evolution of the planet. For its first billion years, the planet Mars was at its most active stage, and water linked interior and surface processes. Global differentiation segregated core, mantle, and crust within a few tens of millions of years of solar system formation; a magnetic dynamo in a convecting fluid core magnetized large areas of the ancient crust; and the Tharsis province became a focus for volcanism, deformation, and outgassing of water and carbon dioxide, possibly in quantities sufficient to induce episodes of global climate warming. A substantial early water budget contributed to widespread erosion, sediment transport, and chemical alteration of crustal material. A more massive early atmosphere was shielded against solar wind stripping by the global magnetic field. Deep hydrothermal circulation of water in the Martian crust acted to accelerate crustal cooling and preserve variations in crustal thickness. Such circulation chemically altered the carriers of crustal magnetization, likely rendering any residual crustal magnetization beneath the lowest areas of major drainage basins undetectable from orbit, an inference that permits a Martian dynamo to have persisted for as long as one billion years. Cessation of the dynamo, widespread reduction in the crustal field, and waning of interior outgassing allowed the early atmosphere to dissipate and the planet’s surface to cool to conditions near those of the modern era, but substantial quantities of subsurface liquid water may have persisted to the present.