A team of Japanese astronomers has recently observed a protoplanetary disk covered in ice, which could lead to a better understanding of water’s role in the formation of planets.
The ice-rich circumstellar disk orbits HD 100546, a hot young star with 2.4 times the mass of our Sun and 36 times its luminosity. It’s located 320 light-years from Earth and it is estimated to be 10 million years old. The star is orbited by a large gas planet 17.5 times the mass of Jupiter at 6.5 AU (1 astronomical unit, AU, is the average Earth-Sun distance).
HD 100546 has a surface temperature of over 10,000 Kelvin (9,700ºC /17,540ºF) and emits a lot of ultraviolet light, which should disintegrate ice crystals, so it’s surprising to see ice crystals so close to the star. The discovery has important implications as scientists believe ice grains to be the precursors to the pebbles that evolve into fully-fledged planets.
The disk extends from 0.2 AU to 4 AU and from 13 AU to several hundred AU. The disk is very turbulent with swirls and complex spiral patterns of unknown origin. It is also fairly flat, which suggests it’s in an advanced evolutionary state.
The team used the Near-Infrared Coronagraphic Imager (NICI) installed on the Gemini South Telescope in Chile to investigate the distribution of water ice molecules within the disk, hoping to find the system’s snowline – the area where ice crystal are not thawed by the strong UV radiation.
"The ice sublimation/condensation front called snowline is considered to be the boundary of the forming regions of the terrestrial and Jovian planets. Snowline is also suggested as a possible forming site of the planetesimals," the researchers wrote in the paper, which is available in arXiv.
Surprisingly, the water ice is present not only beyond the frost line but throughout the protoplanetary disk. The team believes there’s a balance between evaporation and condensation of the ice crystals around HD 100546.
Water ice is believed to help with the formation of the core of massive planets, by enhancing the density of solid materials condensing into those objects. This discovery raises more questions on what happens in the early years of planetary systems.