Free Astronomy Magazine March-April 2016

44 MARCH-APRIL 2016 SPACE CHRONICLES 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 astronomers used the Atacama Large Millimeter/sub- millimeter Array (ALMA) to observe the glow coming from carbon mon- oxide molecules in the 2MASS J16281370-2431391 disc. They were able to create very sharp images and found something strange — in some cases they saw a negative sig- nal! Normally a negative signal is physically impossible, but in this case there is an explanation, which leads to a surprising conclusion. Lead author Guilloteau takes up the story: “This disc is not observed against a black and empty night sky. Instead it’s seen in silhouette in front of the glow of the Rho Ophiuchi Nebula. This diffuse glow is too extended to be detected by ALMA, but the disc absorbs it. The resulting negative signal means that parts of the disc are colder than the background. The Earth is quite literally in the shadow of the Flying Saucer!” The team combined the ALMA mea- surements of the disc with observa- tions of the background glow made with the IRAM 30-metre telescope in Spain (the IRAM measurements were needed as ALMA itself was not sensitive to the extended signal from the background). They derived ALMA finds cold grains in planet-forming disc by ESO A n international team led by Stéphane Guilloteau, at the Laboratoire d'Astrophysique de Bordeaux, France, measured the temperature of large dust grains around the young star 2MASS J16281370-2431391 in the spectacu- lar Rho Ophiuchi star formation re- gion, about 400 light-years from Earth. This star is surrounded by a disc of gas and dust — such discs are called protoplanetary discs as they are the early stages in the creation