Free Astronomy Magazine January-February 2015

PLANETOLOGY fact produced what astronomers consider the best-ever image obtained in the milli- metric domain (1.3 mm, or 230 GHz). The target on which researchers pointed the array of antennas is a young solar-type star, formed less than 1 million years ago in the Taurus Molecular Cloud, about 450 light- years away from Earth. This is a star of mag- nitude 15 in visible light (approximately 9 in infrared light), around which nearly 40 years ago was discovered an excess of in- frared radiation, that could be attributed to a protoplanetary disk. Ten years later, this hypothesis was confirmed by observa- tions in the millimetric domain, indicating a gaseous disk structure extending about 2,000 astronomical units (AU), with a mass equal to 1/10 that of the Sun. Using carbon monoxide (CO) as “tracer”, researchers were able to confirm that the disk was rotating around the star. There were, therefore, all the precondi- tions for ALMA to unveil something new and interesting around HL Tauri. The more recent observations with interval exposures were carried out between 24 and 31 Octo- ber 2014, for a total in- tegration time of 4.5 hours (much like saying that astronomers took a long-exposure shot but a little at a time). The resulting image was immediately de- fined revolutionary by the same ALMA’s man- agers and scientists, since what it shows challenges at least a part of the main math- ematical model that describes the formation T op left, the turbulent neb- ulosity envelop- ing the HL Tauri system, shown enlarged in the inset box with clearly visible the spectacular con- centric rings of the planetary disk. Above, the higher density rings differenti- ated from the orbital regions swept clear by the planets’ ac- cretion. The smal- lest discernible details are about 5 AU wide. In the left video, a “dive” into the Milky Way, up to the protoplane- tary disk of HL Tauri. [ALMA (ESO/NAOJ/NRAO) ESA/Hubble and NASA/N. Risinger]