July 13, 2016 – New observations with the Atacama Large Millimeter/submillimeter Array (ALMA) have produced the first image of a water snow line within a protoplanetary disk. This line marks where the temperature in the disk surrounding a young star drops sufficiently low for snow to form.
Young stars are often surrounded by dense, rotating discs of gas and dust, known as protoplanetary discs, from which planets are born. The heat from a typical young solar-type star means that the water within a protoplanetary disc is gaseous up to distances of around 3 au from the star — less than 3 times the average distance between the Earth and the Sun — or around 450 million kilometers.
Further out, due to the extremely low pressure, the water molecules transition directly from a gaseous state to form a patina of ice on dust grains and other particles. The region in the protoplanetary disc where water transitions between the gas and solid phases is known as the water snow line.
But the star V883 Orionis is unusual. A dramatic increase in the brightness of the young star flash heated the inner portion of the disk, pushing the water snow line out to a distance of around 40 au (about 6 billion kilometers or roughly the size of the orbit of the dwarf planet Pluto in our Solar System). This huge increase, combined with the resolution of ALMA at long baselines, has allowed a team led by Lucas Cieza (Millennium ALMA Disk Nucleus and Universidad Diego Portales, Santiago, Chile) to make the first ever resolved observations of a water snow line in a protoplanetary disc.
The sudden brightening that V883 Orionis experienced is an example of what occurs when large amounts of material from the disc surrounding a young star fall onto its surface. V883 Orionis is only 30% more massive than the Sun, but thanks to the outburst it is experiencing, it is currently a staggering 400 times more luminous — and much hotter.
Lead author Lucas Cieza explains: “The ALMA observations came as a surprise to us. Our observations were designed to look for disc fragmentation leading to planet formation. We saw none of that; instead, we found what looks like a ring at 40 au. This illustrates well the transformational power of ALMA, which delivers exciting results even if they are not the ones we were looking for.”
“The distribution of water ice around a young star is fundamental to planet formation and even the development of life on Earth. ALMA’s observation sheds important light on how and where this happens in protoplanetary disks when young planets are still forming,” said Zhaohuan Zhu, an astronomer at Princeton University, New Jersey, and co-author on the paper. “We now have direct evidence that a frosty region conducive to planet formation exists around other stars.”
The discovery that these outbursts may blast the water snow line to about 10 times its typical radius is very significant for the development of good planetary formation models. Such outbursts are believed to be a stage in the evolution of most planetary systems, so this may be the first observation of a common occurrence. In that case, this observation from ALMA could contribute significantly to a better understanding of how planets throughout the Universe formed and evolved.
“Since water ice is more abundant than dust itself beyond the snowline, planets can aggregate more solid material and form bigger and faster there. In this way, giant planets like Jupiter and Saturn can form before the protoplanetary disk is gone,” noted Zhu.
This study will be published in the journal Nature on July 14, 2016.