A team of researchers has imaged the water snow line in a forming exosolar system for the first time. The system in question, V883 Ori, is only about half a million years old and is in an early stage of development, with planet formation probably not yet started. The observations, taken in radio wavelengths by the Atacama Large millimeter/submillimeter Array (ALMA), identify where in the system it gets cool enough for water to freeze out into a solid, influencing the formation and composition of bodies within the exosolar system.

Liquid water can’t exist in space, so ice that gets close enough to its star or protostar will heat up enough to sublimate—going from its solid form straight to gas. The snow line is the distance from the protostar where this transition takes place. Other substances, like carbon dioxide and carbon monoxide, have their own snow lines at varying distances from the protostar; the one discussed here is for water.

It has been difficult to image snow lines previously because they’re so close to their protostar, usually within about five astronomical units (AU) of it. For comparison, the Earth is only one AU from the Sun.

The snow line plays a big role in planet formation. Outside of the snow line, ice-covered grains can easily form and clump together. These can form the basis for comets, planetesimals (dwarf planets that can roam early exosolar systems) or even contribute to the cores of gas giants like Jupiter. Inside the snow line, ice sublimates, leaving behind dust grains that can break up easily. The results of this difference can be seen in our own Solar System: the rocky planets all formed inside the snow line, while the gas giants formed outside of it.

“Since water ice is more abundant than dust itself beyond the snow line, 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,” said Zhaohuan Zhu of Princeton University, one of the paper’s authors.

The ALMA observation was taken at an interesting time for V883 Ori: it’s undergoing an outburst of brightness. That’s because the protostar, which is constantly drawing in new material, began a temporary period of consuming more rapidly. As material spirals inward, it heats up, producing the extra light. V883 Ori is thirty percent more massive than the Sun but, during its current outburst, it’s 400 times as bright.

Not surprisingly, that has an effect on the snow line. Because the protostar was producing so much more light, the region around the star became hotter. Consequently, the snow line moved outward to about 42 AU. This greater distance from V883 Ori allowed ALMA to resolve its location despite V883 Ori’s distance of 1,350 light years from Earth.

"The ALMA observations came as a surprise to us," said Lucas Cieza of Diego Portales University, Santiago, Chile, the paper’s lead author. "Our observations were designed to image disk fragmentation, which is one of the proposed mechanisms for the formation of giant planets. We saw none of that, as the disk is probably too warm to fragment despite its very large mass. 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."

This shifting of the snow line hasn’t been taken into account in previous models of planet formation. The snow line was known to move as the reservoir of material around the star is depleted and the star matures. But this is well after the stage of development imaged here. Outbursts like the one here could allow it to shift while planets are forming. It’s not currently clear how common these sorts of outbursts are, nor what their effect could be on planet formation.

Nature, 2015. DOI: doi:10.1038/nature18612 (About DOIs)