Science
Nugget—Exosolar Planetesimals The impressive progress in discovering exoplanets, now numbering over 200, has resulted quite remarkably in our beginning to view these as complex "systems" with many interacting components. The several stars that have multiple planets clearly fall into this category, with enough information in some cases (e.g., GJ876; Fischer et al. 2003) to indicate, for example, resonant interactions among them. But a planetary system consists of more than the star and its planetary entourage. Gas, dust, asteroids, and comets percolate around and among the planets, and they have a fascinating story to tell. The evolving configuration of planetesimals so intimate to the formation of the planets determines, in non-trivial ways that are apparently heavily stochastic (Gomes et al. 2007), key planetary properties such as water content (and therefore life) in the habitable zone and the occurrence of global catastrophic events like the Solar System's Late Heavy Bombardment. Determining the nature and distribution of planetesimals in a young system is therefore a key constraint guiding our assessment of the multiple possible outcomes of the planet formation process. Particularly at mid-IR (3-30 µm) wavelengths, TMT will have an enormous impact on the detailed study of these early asteroid and Kuiper-Belt analogs around other stars, as illustrated by the recent resolution, the first ever, of a probable asteroid-belt analog orbiting the 230 million year-old A star Zeta Leporis located 21 pc away (Moerchen et al. 2007). The excess IR thermal emission at 18 μm from Zeta Lep arises in warm starlight-heated dust particles apparently generated by grinding collisions among a population of planetesimals located about 3 AU from the star, a distribution comparable in size to our asteroid belt (see Figure 1). Simple modeling of the azimuthally averaged brightness profile (Figure 2) suggests that there are two dust annuli, the outer perhaps associated with the smaller-sized, "higher-ß" particles, which have periastrons within the inner annulus and apastons in the outer one. But this source is barely resolved at 18 μm with an 8-m telescope, so this model is highly speculative. The nearly four-times better angular resolution of TMT (Figure 3) would reveal not only the radial structure of the emitting particles around Ζ Lep, but also the azimuthal structure, which holds clues to the actual origin of the particles. Their azimuthal distribution (and therefore that of their parent bodies) might be uniform and indicative of steady grinding of a vast ring full of planetesimals the dynamical stability of which reflects an intimate balancing act with an ensemble of planets. Such appears to be the case for the unresolved asteroidal dust ring (Beichman et al. 2005) orbiting at about 1 AU in the triple Neptune system associated with the K0 V star HD69830 (Lovis et al 2006). But the "ring" may, instead, be clumpy. Clumpiness could result, for example, from resonant trapping of the planetesimals, as is the case for the Trojan asteroids at Jupiter's L4 and L5 points, or it might hint at a single, more catastrophic event, as may have been observed in the thermal IR in the central part of the Kuiper Belt surrounding the 10-20 million-year-old A star Beta Pictoris (Telesco et al. 2005). If the emitting region is highly structured, with many unresolved features, rather than uniform, the mid-IR integration time needed to probe this texture will be nearly 200 times less with TMT than with the largest groundbased telescopes now in operation. These stunning gains will open the door dramatically to the exploration of exosolar planetesimals.
Figure 1. Azimuthally averaged profiles at 18.3 µm made at Gemini South of Ζ Lep (red dots) and a PSF star (black dots). Ζ Lep is wider, and the excess thermal IR emission appears associated with dust particles orbiting at 3.0 ± 0.3 AU (Moerchen et al. 2007)
Figure 2.
Figure 3. References Beichman C. A., et al. 2005, ApJ, 626, 1061. |
