Free Astronomy Magazine September-October 2023

30 SEPTEMBER-OCTOBER 2023 ASTRO PUBLISHING A spectrum of the protoplanetary disk of PDS 70, obtained with Webb’s MIRI (Mid-Infrared Instrument), displays a number of emission lines from water vapor. Scientists determined that the water is in the system’s inner disk, at dis- tances of less than 100 million miles from the star – the region where rocky, ter- restrial planets may be forming. [NASA, ESA, CSA, Joseph Olmsted (STScI)] inner disk of PDS 70. However, they do see the raw materials for build- ing rocky worlds in the form of sili- cates. The detection of water vapor implies that if rocky planets are forming there, they will have water available to them from the begin- ning. “We find a relatively high amount of small dust grains. Com- bined with our detection of water vapor, the inner disk is a very excit- ing place,” said co-author Rens Wa- ters of Radboud University in The Netherlands. The discovery raises the question of where the water came from. The MINDS team considered two differ- ent scenarios to explain their find- ing. One possibility is that water molecules are forming in place, where we detect them, as hydrogen and oxygen atoms combine. A sec- ond possibility is that ice-coated dust particles are being transported from the cool outer disk to the hot inner disk, where the water ice sub- limates and turns into vapor. Such a transport system would be surpris- ing, since the dust would have to cross the large gap carved out by the two giant planets. Another question raised by the dis- covery is how water could survive so close to the star, when the star’s ul- traviolet light should break apart any water molecules. Most likely, surrounding material such as dust and other water molecules serves as a protective shield. As a result, the water detected in the inner disk of PDS 70 could survive destruction. Ultimately, the team will use two more of Webb’s instruments, NIR- Cam (Near-Infrared Camera) and NIRSpec (Near-Infrared Spectro- graph) to study the PDS 70 system in an effort to glean an even greater understanding. !