Free Astronomy Magazine January-February 2015

SPACE CHRONICLES A stronomers using NASA's Hub- ble Space Telescope have com- pleted the largest and most sensitive visible-light imaging sur- vey of dusty debris disks around other stars. These dusty disks, likely created by collisions between left- over objects from planet formation, were imaged around stars as young as 10 million years old and as ma- ture as more than 1 billion years old. "It's like looking back in time to see the kinds of destructive events that once routinely happened in our solar system after the planets formed," said survey leader Glenn Schneider of the University of Arizona's Steward Observatory. Once thought to be simply pancake-like structures, the unexpected diversity and complexity of these dusty debris structures strongly suggest they are being grav- itationally affected by un- seen planets orbiting the star. Alternatively, these ef- fects could result from the stars' passing through inter- stellar space. The researchers discovered that no two "disks" of mate- rial surrounding stars look the same. "We find that the systems are not simply flat with uniform surfaces," Schneider said. "These are actually pretty com- plicated three-dimensional debris sy- stems, often with embedded smaller structures. Some of the substructu- res could be signposts of unseen pla- nets." The astronomers used Hubble's Space Telescope Imaging Spectrograph to study 10 previously discovered circumstellar debris sy- stems, plus MP Mus, a mature proto- planetary disk of age comparable to the youngest of the debris disks. Irregularities observed in one ring- like system in particular, around a star called HD 181327, resemble the ejection of a huge spray of debris into the outer part of the system from the recent collision of two bod- ies. "This spray of material is fairly di- stant from its host star — roughly twice the distance that Pluto is from the Sun," said co-investigator Chris- topher Stark of NASA's Goddard Space Flight Center, Greenbelt, Mary- land. "Catastrophically destroying an object that massive at such a large distance is diffi- cult to explain, and it should be very rare. If we are in fact seeing the recent aftermath of a massive collision, the unseen planetary system may be quite chaotic." Another interpretation for the irregularities is that the disk has been mysteriously warped by the star's passage through interstellar space, directly interacting with un- seen interstellar material. "Either way, the answer is ex- citing," Schneider said. "Our team is currently analyzing follow-up observations that will help reveal the true cause of the irregularity." Over the past few years as- tronomers have found an in- credible diversity in the ar- chitecture of exoplanetary systems — planets are arran- ged in orbits that are mar- Hubble surveys debris- strewn exoplanetary construction yards T his NASA/ESA Hubble Space Telescope image shows the bright ring of dusty debris surrounding the star HD 181327. [NASA, ESA, and Z. Levay (STScI), and G. Schneider (University of Arizona)] by NASA/ESA/STScI