Water Snow Line Around Young Star Spotted by ALMA

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2068
CREDIT: A. Angelich (NRAO/AUI/NSF)/ALMA (ESO/NAOJ/NRAO)

Astronomers have, for the first time, managed to make the first ever resolved observations of a water snow line in a protoplanetary disc around a young star called V883 Orionis.

The observations were made using ALMA by a team led by Lucas Cieza of Universidad Diego Portales, Santiago, Chile who says that the observations were surprising for they were looking for disc fragmentation leading to planet formation and instead observed what looks like a ring at 40 au – the water snow line – which is otherwise mostly seen at distances of around 3 au and few million kilometres after that from the star.

As it goes, heat from a typical young solar-type star converts water within a protoplanetary disc into gaseous form 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 kilometres. 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 and this region is called the water snow line.

The star V883 Orionis is however different because the dramatic increase in its brightness has pushed the water snow line out to a distance of around 40 au. The bizarre idea of snow orbiting in space is fundamental to planet formation. The presence of water ice regulates the efficiency of the coagulation of dust grains — the first step in planet formation. Within the snow line, where water is vapourised, smaller, rocky planets like our own are believed to form. Outside the water snow line, the presence of water ice allows the rapid formation of cosmic snowballs, which eventually go on to form massive gaseous planets such as Jupiter.

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.

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