Free Astronomy Magazine September-October 2023

29 SEPTEMBER-OCTOBER 2023 ASTRO PUBLISHING W ater is essential for life as we know it. However, scientists debate how it reached the Earth and whether the same processes could seed rocky ex- oplanets orbiting distant stars. New insights may come from the plane- tary system PDS 70, located 370 light-years away. The star hosts both an inner disk and outer disk of gas and dust, separated by a 5 billion- mile-wide (8 billion kilometer) gap, and within that gap are two known gas-giant planets. New measurements by NASA’s James Webb Space Telescope’s MIRI (Mid-Infrared Instrument) have de- tected water vapor in the system’s inner disk, at distances of less than 100 million miles (160 million kilo- meters) from the star – the region where rocky, terrestrial planets may be forming. (The Earth orbits 93 mil- lion miles from our Sun.) This is the first detection of water in the ter- restrial region of a disk already known to host two or more proto- planets. “We’ve seen water in other disks, but not so close in and in a system where planets are currently assem- bling. We couldn’t make this type of measurement before Webb,” said lead author Giulia Perotti of the Max Planck Institute for Astronomy (MPIA) in Heidelberg, Germany. “This discovery is extremely excit- ing, as it probes the region where rocky planets similar to Earth typi- cally form,” added MPIA director Thomas Henning, a co-author on the paper. Henning is co-principal investigator of Webb’s MIRI (Mid-In- frared Instrument), which made the detection, and the principal investi- gator of the MINDS (MIRI Mid-In- T his artist concept portrays the star PDS 70 and its inner protoplanetary disk. New measurements by NASA’s James Webb Space Telescope have detected water vapor at distances of less than 100 million miles from the star – the region where rocky, terrestrial planets may be forming. This is the first detection of water in the terrestrial region of a disk already known to host two or more protoplanets, one of which is shown at upper right. [NASA, ESA, CSA, Joseph Olmsted (STScI)] frared Disk Survey) program that took the data. PDS 70 is a K-type star, cooler than our Sun, and is estimated to be 5.4 million years old. This is relatively old in terms of stars with planet- forming disks, which made the dis- covery of water vapor surprising. Over time, the gas and dust content of planet-forming disks declines. Ei- ther the central star’s radiation and winds blow out such material, or the dust grows into larger objects that eventually form planets. As previous studies failed to detect water in the central regions of sim- ilarly aged disks, astronomers sus- pected it might not survive the harsh stellar radiation, leading to a dry environment for the formation of any rocky planets. Astronomers haven’t yet detected any planets forming within the