Free Astronomy Magazine March-April 2016

45 MARCH-APRIL 2016 SPACE CHRONICLES a disc dust grain temperature of only −266 degrees Celsius (only 7 degrees above absolute zero, or 7 Kelvin) at a distance of about 15 bil- lion kilometres from the central star. This is the first direct measurement of the temperature of large grains (with sizes of about one millimetre) in such objects. This temperature is much lower than the −258 to −253 degrees Cel- sius (15 to 20 Kelvin) that most cur- rent models predict. To resolve the discrepancy, the large dust grains must have different properties than those currently assumed, to allow them to cool down to such low tem- peratures. “To work out the impact of this discovery on disc structure, we have to find what plausible dust properties can result in such low temperatures. We have a few ideas — for example the temperature may depend on grain size, with the bigger grains cooler than the smal- ler ones. But it is too early to be sure,” adds co-author Emmanuel di Folco (Laboratoire d'Astrophysique de Bordeaux). If these low dust temperatures are found to be a normal feature of pro- toplanetary discs this may have many consequences for understand- ing how they form and evolve. For example, different dust properties will affect what happens when these particles collide, and thus their role in providing the seeds for planet formation. Whether the re- quired change in dust properties is significant or not in this respect can- not yet be assessed. Low dust temperatures can also have a major impact for the smaller dusty discs that are known to exist. If these discs are composed of most- ly larger, but cooler, grains than is currently supposed, this would mean that these compact discs can be arbitrarily massive, so could still form giant planets comparatively close to the central star. Further observations are needed, but it seems that the cooler dust found by ALMA may have signifi- cant consequences for the under- standing of protoplanetary discs. T he young star 2MASS J16281370 -2431391 lies in the spectacular Rho Ophiuchi star formation re- gion, about 400 light-years from Earth. It is surrounded by a disc of gas and dust — such discs are cal- led protoplanetary discs as they are the early stages in the creation of planetary systems. This particular disc is seen nearly edge-on, and its appearance in visible light pictures has led to its being nicknamed the Flying Saucer. The main image shows part of the Rho Ophiuchi re- gion and a much enlarged close-up infrared view of the Flying Saucer from the NASA/ESA Hubble Space Telescope is shown as an insert. [Digitized Sky Survey 2/NASA/ESA] n