One of the major news stories of the year was the detection of multiple planets around a Sun-like star (Upsilon Andromedae). Aside from being a "first" detection, this discovery was very interesting because the arrangement of the three planets in the Upsilon Andromedae system is drastically different from the arrangement of our own system. The two outer Upsilon Andromedae planets are considerably more massive than Jupiter, and they have orbits that are much more eccentric than those of the major planets in our system. However, the radial velocity observations used to make the discovery can determine only a lower limit for the planetary masses. Furthermore, there were several different data sets compiled by competing teams of observers. Two important questions thus remained, both of which were addressed by Ames-based theoretical research:
- What is the true mass of the planets? and
- Which set of published orbital parameters best represents the true configuration of the system?
Work in FY99 focused extensively on these questions, and examined other aspects of the general problem of planetary orbital stability. By performing over ten billion years worth of numerical integrations covering many different configurations that are compatible with the observed data from the Upsilon Andromedae system, the Ames research effort significantly narrowed the possible orbital parameters of the system. It was proved that in order for the system to survive over the 2-3-billion-year age of the parent star, the orbital planes of the planets are being viewed close to edge-on. This finding indicates that the companion masses are close to their minimal, nominal values, and are hence true planets. It was also shown that the observations of the team from the University of California at Berkeley were likely to be the most accurate. The Ames effort has now been confirmed by several other teams of researchers.
In a related line of research, large-scale numerical experiments have shown how the effects of the close passage of a binary pair of stars can disrupt an otherwise orderly system of planets (see figure 1). This effect is now understood to be important in the dense open clusters that are the birthplace of many stars. In the FY99 research, the simulations were extended to study the ramifications of this process for the history and future of our own solar system. The research showed that the solar system has existed more or less in isolation since its birth. The nearly perfect circular orbit of Neptune indicates that the Sun has never suffered a significant encounter with another star, suggesting that low-density regions of star formation such as the Taurus Molecular Cloud are the most promising nurseries for planets that eventually develop Earth-like environments. One interesting auxiliary result was the calculation of the odds of Earth being ejected or captured from the solar system by another star prior to the Sun's red giant phase: a scant one part in one hundred thousand!
Point of Contact: G. Laughlin
(650) 604-0125
gpl@acetylene.arc.nasa.gov
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Fig. 1. The computer simulation of figure 1 shows the outcome of a close encounter between a red dwarf binary pair and the Sun-Earth system. The red dwarf pair approaches the Sun from a direction perpendicular to the figure plane. Earth is almost immediately handed off to the smaller star and stays with that star for three long, looping excursions. After slightly more than 1000 years, Earth is recaptured by the Sun, and remains in a solar orbit for the next 6500 years, as the Sun suffers many complicated close encounters with the other stars. After 7500 years, Earth is captured into orbit around the larger red dwarf star, and soon thereafter this star escapes with the Earth in tow. This particular simulation is one of several million performed in order to understand how planetary systems are affected by encounters with the other stars.
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