Free Astronomy Magazine January-February 2015

SPACE CHRONICLES kedly different than found in our solar system. "We are now seeing a similar diversity in the architecture of accompanying debris systems," Schneider said. "How are the plan- ets affecting the disks, and how are the disks affecting the planets? There is some sort of interdepen- dence between a planet and the ac- companying debris that might affect the evolution of these exo- planetary debris systems." From this small sample, the most important message to take away is one of diversity, Schneider said. He added that astronomers really need to understand the internal and ex- ternal influences on these systems, such as stellar winds and interac- tions with clouds of interstellar ma- terial, and how they are influenced by the mass and age of the parent star, and the abundance of heavier elements needed to build planets. Though astronomers have found nearly 4,000 exoplanet candidates since 1995, mostly by indirect detec- tion methods, only about two doz- en light-scattering, circumstellar debris systems have been imaged over that same time period. That's because the disks are typically 100,000 times fainter than, and often very close to, their bright par- ent stars. The majority have been seen because of Hubble's ability to perform high-contrast imaging, in which the overwhelming light from the star is blocked to reveal the faint disk that surrounds the star. The new imaging survey also yields insight into how our solar system formed and evolved 4.6 billion years ago. In particular, the suspected plan- et collision seen in the disk around HD 181327 may be similar to how the Earth-Moon system formed, as well as the Pluto-Charon system over 4 billion years ago. In those cases, collisions between planet-sized bod- ies cast debris that then coalesced into a companion moon. T his is a set of images from a NASA Hubble Space Telescope survey of the ar- chitecture of debris systems around young stars. Ten previously discovered circumstellar debris systems, plus MP Mus (a mature protoplanetary disk of age comparable to the youngest of the debris disks), were studied. Hubble's sharp view uncovers an unexpected diversity and complexity in the structures. As the accompanying scale shows, the disk-like structures are vast, many times larger than the planetary distribution in our solar system, gauged by the diameter of Neptune's orbit. Some disks are tilted edge-on to our view, others nearly face-on. Asymmetries and warping in the disks might be caused by the host star's passage though interstellar space. Alternatively, the disks may be affected by the action of unseen planets. In particular, the asymmetry in HD 181327 looks like a spray of material that is very distant from its host star. It might be the aftermath of a colli- sion between two small bodies, suggesting that the unseen planetary system may be chaotic. The stars surveyed may be as young as 10 million years old and as mature as more than 1 billion years old. The visible-light survey was done with the Space Telescope Imaging Spectrograph (STIS). The STIS coronagraph blocks out the light from the host star so that the very faint reflected light from the dust structures can be seen. The images have been artificially colored to enhance de- tail. [NASA, ESA, and Z. Levay (STScI), and G. Schneider (University of Arizona)] n