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Project Progress

1. Formation of Habitable Planetary Systems

Building on results described in last year’s report, Co-investigator John Chambers’s work this year has focused on models for the growth of planets in the presence of planetary migration. Inward migration of planetary orbits is widely believed to be an important process, but it is neglected in most studies of planet formation due to doubts about whether planets would survive. Chambers’s existing semi-analytic model for the oligarchic-growth stage of planet formation has now been extended to include migration and the acquisition of gaseous envelopes by solid planetary cores, leading to the formation of gas-giant planets like Jupiter and Saturn. This expanded model shows that planets can form and survive in the presence of migration, in some cases even under the most extreme migration rates considered likely. It also appears that planets are most likely to form and survive in low-mass protoplanetary disks, a somewhat counterintuitive result, since migration is limited in these cases. The model shows that a wide variety of outcomes is possible, suggesting that the planets in the Solar System are likely to be atypical in some respects.

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Postdoctoral Fellow Hannah Jang-Condell has been working on theoretical modeling of observable signatures of giant planet formation. A Jupiter-mass planet forming in a disk will create a perturbation much larger in size than the planet itself, resulting in a promising way to look for planets around young stars. A goal of this research is to determine differences between signatures of core accretion and disk instability in order to determine which process is occurring in observed disks, which can help resolve the debate over which mechanism is the correct paradigm for planet formation. This research will also provide motivation for and drive observational campaigns at future high-resolution telescopes, such as the Giant Magellan Telescope (GMT) and the Atacama Large Millimeter Array (ALMA).

2. Protoplanetary and Debris Disks

Co-Investigator Alycia Weinberger studied circumstellar disks to measure their compositions in as many direct and indirect ways as possible. With Postdoctoral Researcher John Debes, she studied the color of light reflected from such disks. For disks composed of optically thin dust, the amount of starlight scattered by the dust grains, as a function of wavelength, is determined by the dust composition and particle size. By using the Hubble Space Telescope Near Infrared Camera (NICMOS) to image disks at wavelengths from the visible to 2.2 microns, a spectrum of the disk is constructed. It appears common that disks are red, whereas the common materials of the interstellar medium, such as silicates, should scatter neutrally. A detailed study of the disk around HR 4796 is underway. Its dust is successfully modeled as composed of organic-rich materials, such as the “tholins” postdulated in the atmosphere of Titan and reported in the spectra of Kuiper belt objects. This is the first detection of organics in a debris disk; their presence implies that the basic building blocks of life are common in the later stages of planet formation.

Weinberger has also been investigating the interaction of very young disks with the jets driven by disk accretion. Imaging in the near-infrared 2.12-micron line of molecular hydrogen penetrates the dusty environs of young stars and allows the stellar outflow structure to be measured. The first result from this project to study protostars in the Ophiuchus star-forming region with Carnegie’s Magellan telescopes showed that a precessing jet around Elias 29 may be responsible for generating a wide-angle outflow cavity.

3. Searching for Extrasolar Habitable Planetary Systems

Co-Investigator Alan 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, 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 (NSF) to build the Carnegie Astrometric Planet Search (CAPS) Camera, a specialized camera that should yield astrometric accuracies better than 1 millarcsec per epoch. The $235K Teledyne 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 CAPSCam was completed in early 2007 and was mounted on the du Pont telescope during the March 2007 observing run. Engineering tests were run, and plans were made for developing a second-generation electronics board, as well as the installation of a Uniblitz electronic shutter. Boss observed for a total of 22 nights on the du Pont in 2006-07 for the CAPSCam project.

Of the more than 200 exoplanets known, about 95% – including all those orbiting nearby stars – have been found by precision Doppler surveys, mostly by Co-Investigator Paul Butler and his colleagues and by an independent group in Geneva. Butler and his collaborators are conducting long-term precision Doppler surveys with the Keck 10-m, Magellan 6.5-m, Lick 3-m, and Anglo-Australian 3.9-m telescopes. These surveys have found about 140 planets over the past 12 years. Over this past year Butler’s team published discovery observations for 24 of the 40 new planets announced, as well as the first “catalog” of extrasolar planets. They are nearing completion of a Planet Hunting Spectrometer for Magellan, a 2.4-m robotic planet finding telescope at Lick, and two 80-cm robotic photometry telescopes at Las Campanas. The Carnegie-NSF Planet Hunting Spectrometer will allow them to reach 1 m/s on the nearest 200 southern hemisphere stars. The Lick Robotic Telescope will allow them to follow interesting low-amplitude, terrestrial-planet signals every night. The robotic photometry telescopes will allow them to assess the photometric stability of their target stars.

With the delay of the Space Interferometry Mission (SIM) and the Terrestrial Planet Finder (TPF), the study of extrasolar planets around nearby stars will continue to be dominated by precision Doppler surveys over the next decade. Over the past two years both Butler’s group and the Geneva group have pushed toward achieving a precision of 1 m/s, and both groups have found Neptune-mass (~15 Earth-mass) and terrestrial-mass (<10 Earth-mass) planets. With a measurement precision of 1 m/s and long telescope runs, precision Doppler’s sensitivity to terrestrial mass planets is comparable to, or better than, the hoped-for astrometric precision of SIM for relatively short-period orbits. With observing runs of 2 to 6 months, Butler and his group can find terrestrial-mass planets in the habitable zone of nearby M dwarf stars now.

Although ground-based planet searches are inexpensive compared with multi-billion dollar space missions such as SIM and TPF, they do require significant new support. Purchasing 2 months of time on a 4-m class telescope costs roughly $1M. Such a program can survey about 20 stars. Building two dedicated 8-m telescopes capable of surveying all 1,000 of the nearest stars over the next 20 years would cost about $150M. Such a survey would find all the Saturn-mass planets orbiting within 9 AU, all Neptune-mass planets orbiting within 1 AU, and the first large trove of terrestrial-mass planets in smaller orbits. For K and M dwarfs, these planets would be within the habitable zone.

Along with Co-I Butler, Postdoctoral Fellow Mercedes LÓpez-Morales continued working on the characterization of fundamental parameters of very low-mass stars, the most likely candidates to host Earth-like planets, on the construction of instrumentation for photometric extrasolar planet searches (the two new robotic telescopes at Carnegie’s Las Campanas Observatory), and on precision photometry observational campaigns in search of extrasolar planet transits and transit timing.

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4. Characterizing Extrasolar Habitable Planetary Systems

Co-Investigator (now Collaborator) Sara Seager has interpreted the observations of three transiting extrasolar planets based on observations of extrasolar planets with the Spitzer Space Telescope. The first spectrum of an extrasolar planet (8 to 13 microns) showed that no water vapor absorption features were present but that a possible silicate emission feature was present. Photometry revealed the hottest planet, indicating a planet with a stratosphere, and the first thermal emission from a Neptune-mass exoplanet.

Seager also completed a study on the mass-radius relationships for solid exoplanets, with Carnegie Institution of Washington Research Scientist Burkhard Militzer and former Postdoctoral Fellow Catherine Hier-Majumder. This work will help to identify transiting exoplanets that are potentially habitable.

5. Habitable Environments in the Solar System

One of the long-term goals of the 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. PI Sean Solomon’s efforts in the past year continued to focus on hydrothermal systems on Mars as such candidate environments, in large part because of the continuing influx of important new data from the Mars Odyssey, Mars Express, and Mars Exploration Rover missions.

A synthesis of spacecraft observations and inferences from Martian meteorites regarding the role of water on early Mars (the Noachian epoch prior to 3.7 Ga) has revealed that interior and surface processes involving water were strongly interlinked. On the basis of short- and long-lived radionuclides in the source regions of the Martian meteorites, global differentiation of core, mantle, and crust on Mars occurred within a few tens of millions of years of solar system formation. Impact structures buried beneath the northern plains of Mars and discernible from radar sounding and topography support the view that the northern hemisphere crust, and by implication the crustal thickness dichotomy, are as ancient as the crust in the southern hemisphere, although the late heavy bombardment complicates the assignment of crater retention ages in the Noachian. The large areas of coherently magnetized Noachian crust imply the presence of a magnetic dynamo, presumably generated in an actively convecting fluid core. By Late Noachian the Tharsis province had become a major 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 planetary magnetic field. Deep hydrothermal circulation of water in the Martian crust likely accelerated crustal cooling and the preservation of variations in crustal thickness. Such circulation could have chemically altered the carriers of earliest crustal magnetization, rendering any residual crustal magnetization beneath the northern plains and the lowest areas of other major drainage basins undetectable from orbit, an inference that permits a Martian dynamo to have persisted to the end of the Noachian. 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.

Postdoctoral Fellow Isamu Matsuyama developed models for spin-axis variations driven by internal and surface processes on planetary bodies. He developed deformational models that can account for topographic variations along postulated ancient shorelines on the surface of Mars, reviving the idea that there was a Martian ocean. Matsuyama analyzed in detail the idea that the active south polar region of Saturn’s moon Enceladus may owe its polar location to the presence of a rising plume, and he also studied the reorientation of icy satellites by impact basin formation.