Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile have observed a water snow line within a protoplanetary disk for the first time – a breakthrough that they detailed in research scheduled for publication in the journal Nature.

Protoplanetary disks are rotating circumstellar rings of dense gas and dust that surround newly formed young stars, and it is from this material that planets originate, researchers affiliated with the European Southern Observatory (ESO) involved with the study said in a statement.

Located within each of these disks is a boundary marking where its internal temperature drops sufficiently enough for snow to form – the water snow line – and now, a stellar outburst from a young star named V883 Orionis has enabled lead author Lucas A. Cieza of Universidad Diego Portales in Santiago, Chile, to make the first ever resolved observations of this feature.

“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,” Cieza said. “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.”

Findings could improve planetary formation models

According to the study authors, the inner portion of the disk surrounding V883 Orionis had been flash heated due to a dramatic increase in the star’s brightness, causing the water snow line to be pushed outward to a greater-than-normal distance and making it observable for the first time.

Typically, the heat from a young solar-type star means that the water inside a protoplanetary disk is gaseous up to distances of 3 au from the star, or less than three times the average distance from the Earth to the Sun (approximately 450 million km), the researchers explained. Due to the lower pressure further out, water molecules transition directly from gas to ice on dust grains.

The area where this gas-to-ice transition takes place is the water snow line, and the one located in the protoplanetary disk of V883 Orionis is somewhat unusual because, as Cieza noted earlier, the dramatic increase in brightness experienced by the star has caused the boundary to be pushed out to a distance of about 40 au (6 billion km, or close to the size of Pluto’s orbit around the Sun).

Cieza and his colleagues explained that the sudden increase in brightness experienced by V883 Orionis occurs when large amounts of material from the protoplanetary disk fall onto the surface of the star. Although this young solar-time star is just 30% more massive than the sun, it is now 400 times more luminous and far hotter because of this ongoing stellar outburst.

“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,” the ESO said. “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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Image credit: ESO

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