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Project Progress

During the course of the last five years, the Department of Terrestrial Magnetism (DTM) astronomy group has become perhaps the leading group worldwide studying the detection, formation, and evolution of planetary systems, with a special emphasis on systems containing habitable planets. The addition of staff members Paul Butler, Alycia Weinberger, and Sara Seager during this period, with the anticipated arrival of new staff member John Chambers in early 2004, coupled with a strong contingent of postdoctoral fellows, has buttressed the ongoing theoretical efforts of George Wetherill and Alan Boss to understand the formation of the Solar System in particular and of habitable planets in general. The DTM astronomy group continues to make major contributions to Carnegie’s astrobiology research efforts.

Task 1. Detection and Characterization of Extrasolar Planets (Boss, Butler, Haghighipour, McCarthy, Rivera, Seager, von Braun, Weinberger)

Over the last year Butler, postdoctoral fellow Chris McCarthy, and their colleagues have discovered 10 planets from the Lick, Keck, and Anglo-Australian Planet Search programs, which rely on high-precision radial velocity measurements to detect planets indirectly through the wobble of the host star about the center of mass of the star-planet system. These discoveries include two new multiple planet systems, two new sub-Saturn-mass planets, and a planet orbiting at 3.3 AU in a circular orbit. This last system is the closest analogue to the Solar System found to date. Giant planets in circular orbits beyond 3 AU appear to be rare, this being only the second one known.

Butler’s goal is to survey the nearest 2,000 solar-type and lower mass stars within 50 parsecs in preparation for the next generation of space-based missions designed to detect and study extrasolar planets, such as NASA’s James Webb Space Telescope (JWST), Space Interferometry Mission (SIM), and Terrestrial Planet Finder (TPF), and the European Space Agency’s (ESA’s) Global Atmospheric Integration for Astrobiology (GAIA) and Darwin missions. Toward this goal, Butler inaugurated the Magellan Planet Search in December 2002 at Carnegie’s Las Campanas Observatory (LCO), adding 500 new Southern Hemisphere stars to his program. Butler and McCarthy are currently surveying a total of 1,700 stars from Lick-Keck in the north and AAT-Magellan in the Southern Hemisphere.

Seager is co-leading a search for short-period giant planets by the transit technique, where the planet’s presence is inferred by the dimming of the host star’s light when the planet passes in front of the star. The radius of each transiting planet can be determined from the known radius of the star and the amount of its dimming; the radius (together with the mass) is a key constraint on planetary evolution models. Seager’s group has three planet candidates for which follow-up radial velocity observations were made during the reporting period. Data analysis is in progress. In addition, Seager and postdoctoral fellow Kaspar von Braun have started a new planet transit search in open star clusters in the southern hemisphere using the Carnegie Swope telescope at the LCO.

Seager has also been working on considering what the Earth would look like if viewed as an extrasolar planet, by interpreting data taken of Earthshine reflected off the dark side of the Moon. These measurements capture the Earth’s spatially unresolved spectrum. This work is important for NASA’s TPF mission, which will observe similarly spatially unresolved planetary spectra.

Boss and Weinberger have begun a long-term astrometric planet search on LCO’s DuPont telescope, starting with a six-night run in May. The goal is to be able to detect long-period Jupiter-mass planets around nearby solar-type and lower mass stars by looking for the positional wobble of the host star as it orbits around the system’s center of mass. Astrometric detections are complementary to Butler’s radial velocity detections, as astrometry is preferentially sensitive to long-period planets and is able to determine the true mass of the planet, rather than a lower limit on the mass. By determining which of the nearest stars are orbited by Jupiter-like planets in Jupiter-like orbits, this astrometric planet search is intended to help refine the target list for NASA’s TPF mission, simplifying the search for Solar System analogues.

Postdoctoral fellow Eugenio Rivera has been performing dynamical fits to the radial velocity data of a few stars with more than one planetary companion. Most of Rivera’s work has been on the GJ 876 system, for which the inclusion of the mutual planetary perturbations in the fits to the radial velocity data results in a significantly improved solution. Eventually, it is hoped that these types of fits will strongly constrain the orbital inclinations (and thus the masses) of the planets in this system. Most recently, Rivera was able to put a constraint on the relative inclination between GJ 876’s two known planets under the assumption that the recent astrometrically determined inclination of the outer planet was correct.

In collaboration with Rivera, postdoctoral fellow Nader Haghighipour has studied the long-term stability of Earth-like planets in the known extrasolar planetary systems of 47 UMa and 55 Cnc. They found that although 47 UMa’s planetary configuration resembles a three-body system similar to that of the Sun, Jupiter, and Saturn, an Earth-like planet could not have a stable orbit in this system since it would be in an unstable (1:3) resonance with the innermost giant planet. Their work is a part of an ongoing, larger study of the possibility of habitable planets in known extrasolar planetary systems.

In a collaborative research project with members of the NAI team at UCLA, Haghighipour searched for stable, periodic orbits at the location of different orbital resonances with other planets in the system. The idea is to apply the results to extrasolar planetary systems in order to search for regions where habitable planets could have stable orbits for long times (e.g., for the age of the Solar System). The initial focus of this work was on searching for stable orbits in a system with two giant planets in a 1:2 orbital resonance, since the results would then be applicable to the planetary system of GJ 876.

Task 2. Formation of Planetary Systems (Boss, Haghighipour, Roberge, Weinberger, Wetherill)

Weinberger pursued several searches for circumstellar disks around young stars using the W. M. Keck Observatory and the new 6.5-m Magellan Telescopes at LCO. She is using deep 10- and 20-micron imaging to detect reprocessed stellar energy as thermal emission from dusty disks (Figure 1). Weinberger has now surveyed over 50 stars younger than 100 Myr and within 100 pc of Earth. Disks with substantial material in the terrestrial planet region around stars older than 5 Myr seem to be quite rare. This means either that terrestrial planet formation will not take place or that it has already progressed to the point where collisions that generate substantial dust are rare.

In the coming months Weinberger will complete her mid-infrared surveys of the nearby Beta Pictoris and TW Hydrae associations. In July, with postdoctoral fellow Aki Roberge, she will undertake a search for Herbig-Haro objects associated with rather old circumstellar disks. Whether these older stars have outflows will be important to interpreting data on molecular hydrogen to be returned by NASA’s Space Infrared Telescope Facility – does this emission arise from an outflow or from a potentially planet-forming disk?

The formation mechanism of Uranus and Neptune is uncertain. A new hypothesis was presented in the last year by Boss, Wetherill, and Haghighipour, envisioning rapid formation of several gaseous protoplanets in a marginally gravitationally unstable protoplanetary disk, coagulation and settling of dust grains within the protoplanets to form rock and ice cores, followed by loss of the outer disk gas and the gaseous envelopes of the protoplanets through extreme ultraviolet (EUV) photoevaporation driven by nearby OB stars. In the past year Boss demonstrated that the first part of this new scenario for ice giant planet formation is feasible in a disk with a gas density similar to that believed to be necessary to form the gas giant planets. A three-dimensional gravitational hydrodynamics code, including a full treatment of thermodynamics and radiative transfer in the diffusion approximation, was used to show that a disk is likely to form two or more gravitationally bound clumps, with masses on the order of 2 Jupiter masses, between 20 AU and 30 AU from a solar-mass star (Figure 2). Such protoplanets could be sufficiently massive to explain the production of the ice giant planets Uranus and Neptune, following photoevaporation of most of their gaseous envelopes. This new scenario implies that planetary systems similar to our own could form even in seemingly hostile regions of high-mass star formation. This scenario, if correct, has important implications for the frequency of habitable planetary systems, as it raises the likelihood of finding such systems by roughly a factor of five, because most stars are believed to form in regions of high-mass star formation.

Haghighipour has been studying the combined effect of gas drag and pressure gradients in expediting the formation of planetesimals in a marginally gravitationally unstable protoplanetary disk. Because the disk gas is partially supported against gravity by the gas pressure gradient, the disk gas orbits faster than Keplerian for orbits inside that of a local pressure maximum in the disk and slower than Keplerian outside a local pressure maximum, producing gas drag forces on solids that lead to tailwinds and headwinds, respectively. As a result, solids migrate toward local pressure maxima, where they can collide and grow larger. Haghighipour has generalized this work to three dimensions by considering a non-negligible height for the disk and including the fully three-dimensional motions of solids. The results indicate an increase in the rate of the vertical and radial migration of solids toward the location of local density enhancements in the midplane of a non-uniform disk. The immediate implication is a shorter time for the formation of planetesimals and thus somewhat hastened formation of habitable planets similar to the Earth.

Task 3. 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. Solomon’s efforts on this project during the past year have focused on Mars, primarily because of the influx of new data from the Mars Global Surveyor (MGS) and Mars Odyssey missions. An important contribution has been the development of the suggestion that the pattern of paleomagnetic anomalies on Mars may have been strongly influenced by hydrothermal alteration of magnetic carriers, as a result of processes similar to those that occur in very young oceanic crust on Earth. In particular, his suggestion is that the paucity of magnetic anomalies discernible by orbiting spacecraft in the northern lowlands and young impact basins on Mars may be the result of chemical alteration and consequent lowered magnetization enabled by deep hydrothermal circulation beneath prominent drainage basins on Mars. Beyond its potential importance for constraining the history of the Martian crust and dynamo, this hypothesis may provide a basis for assessing the depth of penetration of hydrothermal activity in the Martian crust and the locus and timing of areas of such activity.

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