Science

Remote Reconnaissance of Exosolar Systems and Comparative Planetary Science

Before 1995, objects intermediate in mass between planets and stars, were a purely theoretical notion, after numerous surveys had only turned up one borderline object that remained controversial and inexplicable until brown dwarf science matured. In addition, exoplanets were relegated primarily to the realm of science fiction.

Copyright © 2010 Rebecca Oppenheimer

In 1995 that all changed, with the near simultaneous announcements at the Cool Stars IX meeting in Italy of both a bona-fide brown dwarf companion of a nearby star and a peculiar Jupiter-sized planet orbiting a Sun-like star. At present hundreds of astronomers around the world are working on substellar companions of nearby stars and brown dwarfs. Some 700 brown dwarfs have been identified and many studied spectroscopically. A small fraction of these were found as companions of stars or other brown dwarfs. Also, more than 1000 planets outside our solar system have been identified. 

These two populations of objects, which may be intrinsically related, offer a vast diversity of salient properites. This challenges the concept in astronomy that most celestial bodies can be fundamentally understood by measuring only a few basic parameters, as suggested by the Vogt-Russell theorem, whereby knowing the mass and metallicity of a star reveals the entire nature of that star, including its evolutionary path and all other fundamental parameters. Such a simplification has less and less utility and meaning as one proceeds to lower and lower masses along the stellar main sequence.  For example, in the brown dwarf regime (below about 7.5% the mass of the Sun), a chemistry, far more complex than what exists in any stellar atmosphere, has tremendous effects on the emergent spectral energy density and affects the dynamics and physics of the objects themselves.  In the planet-mass regime (commonly defined as objects below roughly 13 times the mass of Jupiter), one need only take a very superficial look at the objects in our solar system to see a vast diversity. Indeed, the giant planets of our solar system are all roughly of the same radius, of nearly the same metallicity and presumably of the same age. Yet the spectra and general appearances of Jupiter, Uranus and Neptune are all quite different. An inventory of well-studied moons of the solar system as well as the terrestrial planets, again, reveals that a few simple parameters are insufficient to understand these objects’s physical and chemical structures and processes in the context of their observable features.  More than that, a comprehensive theory of planet formation, evolution and constitution cannot be derived without spectroscopic and astrometric study of hundreds, or, one might hope, thousands of these objects. Unfortunately, very few of the planets outside our solar system (exoplanets) have been directly seen or studied spectroscopically.

This is the goal of Project 1640: to acquire spectral and orbital information for as many exoplanets and brown dwarfs orbiting nearby stars as possible, to aid in the development of a comprehensive theory of planet evolution and diversity. Some of the questions we seek to answer are:  

1. How common are planets around stars?
2. Is there such a thing as “solar system architecture?” 
3. What types of planets exist? 
4. How do planets and planetary systems form, evolve and die?
5. Are brown dwarfs part of this picture or not? 

Ultimately, as the technology to conduct remote reconnaissance of exosolar systems improves, astronomical instruments should be able to address the question "Are there other planets capable of sustaining life as we know it?" This will be done via spectroscopy of a planet to reveal a thermochemical imbalance that can only be explained by biological activity on a distant planet. Further details on the history of this subject can be found here and a comprehensive treatment is contained in Oppenheimer and Hinkley (2009) and references therein.

Circumstellar disks

Understanding the connection between circumstellar disks and planets is crucial for a complete picture of planet formation. The first few million years of planet formation involves a competition between planetary accretion and gradual clearing of the disk, as gas-rich protoplanetary disks transition into full-fledged planetary systems with gas-poor disks. Does this allow enough time for the accretion of planetary cores and subsequent rapid accumulation of massive gas envelopes, which is the process generally thought to build gas giants? Or does gas giant formation proceed from gravitational disk instability, which seems to require only a few hundred years? Observing a statistically significant number of systems having both young gas giants and circumstellar disks could tell us whether core accretion or disk instability, or some combination of the two processes is the more probable scenario.

Young planetary systems are likely to harbor not only planets but also left over material from early stages of planet formation, like asteroid belts and comet reservoirs, which can be traced by the dust produced in collisions between their rocky constituents. Many such circumstellar debris disks have been detected by IRAS and Spitzer from observing mid- and far-infrared emission of heated dust grains, and follow up observations of nearby disks by the Hubble Space Telescope have spatially resolved some of these disks in scattered optical and near-infrared light. Because Project 1640 has a sensitivity much higher than Hubble's NICMOS instrument, it will be able to image faint debris disks, mapping morphological features like spirals, warps, offsets, gaps, or other asymmetries that could imply interaction with unseen planets. It will also allow us to obtain spatially resolved, low resolution near-infrared spectra of bright nearby debris disks, giving us information on grain composition and size distributions of particles, as well as how water is distributed at radial distances from a few to 100 AU. In addition, the age of observed systems covers the important period in the evolution of our Solar System known as the late-heavy bombardment, and could have implications for the possible emergence of life.

Evolved stars

Although not currently included in the sample of stars selected for our survey, evolved stars and their circumstellar envelopes could be successfully observed with a high-contrast imaging instrument like Project 1640. As stars enter the last stages of evolution before they decay into stellar remnants, dust and gas are spewed into the interstellar medium. Exactly how stars do this remains poorly understood. Project 1640 can study this process in detail, estimating mass-loss rates and looking for arcs, elongated and bipolar structures, spirals, clumps, etc. Imaging detached shells around evolved stars could possibly give ages and formation time-scales which could be compared to various theories.