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Project Progress

1. Formation of Habitable Planetary Systems

The work this year of Co-Investigator Chambers involved the development of a model for the oligarchic growth stage of planet formation, a key stage that determines many of the final characteristics of a planetary system. This stage begins with solid bodies no larger than asteroids and ends with the cores of the giant planets and the precursors of the terrestrial planets. The new model finds that growth occurs more rapidly than in some previous models due to several processes: disequilibrium between the processes that determine the velocity distribution of small bodies called planetesimals; substantial collisional fragmentation of these small bodies; and efficient capture of fragments by primordial atmospheres of the protoplanets. Calculations show that planets like those in the Solar System can form on timescales shorter than the lifetime of most protoplanetary disks if the Sun’s disk was a few times more massive than the minimum-mass solar nebula. Typically, precursors of the terrestrial planets form in a few hundred thousand years, while giant-planet cores form in about one million years.

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2. Protoplanetary and Debris Disks

Co-Investigator Weinberger studied circumstellar disks to measure their compositions in as many direct and indirect ways as possible.

With former Postdoctoral Fellow Aki Roberge, she studied the gas around the prototypical debris disk around Beta Pictoris using archival spectra from two NASA missions — the Far Ultraviolet Explorer and Hubble Space Telescope. A startling result emerged — the Beta Pictoris disk is extraordinarily carbon rich, with a carbon-to-oxygen ratio 18 times solar. Perhaps the large planetesimals orbiting Beta Pictoris are outgassing methane while keeping their water and silicates trapped deep within, much as Titan is today. Perhaps carbon-rich planetesimals are delivering their contents to nascent planets within the disk. The excess carbon in the disk helps stabilize the circumstellar gas from rapid ejection by stellar radiation pressure. The new observations thus resolve a long-standing puzzle about why Beta Pictoris has a disk at all.

Not only young stars, but also the remnants of planetary systems can provide information on the composition of planetesimals. With colleagues from the NAI team at UCLA, Weinberger studied the dust around the white dwarf GD 362. They found what is only the second known disk around one of these old, post-main sequence stellar remnants. The disk is probably rich in silicate dust, much like young systems, and was formed by the tidal disruption of a large asteroid. The same team investigated the incredibly dusty star BD +20 307 and found that its dust was composed mostly of small silicate grains. This project is continuing with an approved program on the Spitzer Space Telescope to study the detailed mineralogy and temperature of the silicates.

With Postdoctoral Fellow John Debes, Weinberger is executing a program with the Hubble Space Telescope to study the near-infrared colors of disks around 1-10 Myr old stars. Work on one disk, around HR 4796A, shows that the extremely red color is an excellent match to long organic chains seen on outer Solar System bodies. Thus, we may be seeing another disk in which organics are available for delivery to newborn planets. This project will also search for evidence of water and methane ices around six other stars.

3. Searching for Extrasolar Habitable Planetary Systems

Ten years after the discovery of the first confirmed extrasolar planet, nearly 200 exoplanets are known. About 95% of these, including all those orbiting nearby stars, have been found by precision Doppler surveys, primarily by Co-Investigator Butler’s group and the Geneva group. Over this past year both groups have worked to improve long-term Doppler precision to 1 m/s. These surveys are now sensitive to terrestrial-mass planets in small orbits and Saturn-mass planets orbiting beyond 5 AU. The struggle is to get sufficient telescope time to survey the nearest 1,000 stars at 1 m/s precision. In addition to long-term surveys, Butler’s group is building a Planet Hunting Spectrometer for Magellan, a 2.4-m robotic planet finding telescope at Lick, and two 32-inch robotic photometry telescopes at Las Campanas. The Carnegie-NSF Planet Hunting Spectrometer will allow us to reach 1 m/s on the nearest 200 southern hemisphere stars. The Lick Robotic Telescope will allow us to follow interesting low-amplitude, terrestrial-planet-mass signals every night. The robotic photometry telescopes will allow Butler’s group to assess the photometric stability of their target stars. With on-going development and funding delays for
NASA’s SIM and TPF space missions, precision Doppler surveys remain the primary means of discovering and characterizing nearby planetary systems.

The contribution of Co-Investigator Boss to Carnegie’s NAI effort in the last year centered on his leadership of a new ground-based astrometric planet detection effort being conducted with the 2.5-m du Pont telescope at Carnegie’s Las Campanas Observatory in Chile. In 2004 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. The $235K Rockwell Hawaii-2RG focal plane arrays (engineering and science grade) were delivered in mid-2005, along with the Barr Associates lambda/30 combination filter/window. The CAPS camera is nearing completion and will be mounted on the du Pont telescope during the December 2006 observing run. A Ronchi-ruling-type calibrator for the CAPSCam has been built by George Gatewood of the Allegheny Observatory. Boss observed for 18 nights on the du Pont in 2005-06 in preparation for beginning work with the CAPS camera.

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4. Characterizing Extrasolar Habitable Planetary Systems

Co-Investigator Seager continued working on atmosphere models for hot Jupiter extrasolar giant planets. She published models that were the first to combine several different data sets on the exoplanet HD209458b and present consistent interpretations. The models point to future wavelength- and phase-dependent observations that can discriminate among current models. Seager and colleagues measured the thermal emission from a new transiting planet around a bright star, HD189733b. Seager also worked on the topic of Earth as an exoplanet with summer interns Sonali Shukla and Jennifer Ortega. The interns used General Circulation Models and Seager’s radiative transfer code to consider Earth during an ice age, during a glacial period in the Neoproterozoic, and with different obliquities than present-day Earth.