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Stellar snapshot: Is there snow in space?

Strange as it may seem, great disks of snow and ice often orbit young stars, playing a crucial role in the formation of planets. For the first time, astronomers have caught one on camera.

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    This image of the planet-forming disc around the young star V883 Orionis was obtained by ALMA in long-baseline mode. This star is currently in outburst, which has pushed the water snow line further from the star and allowed it to be detected for the first time. The dark ring midway through the disc is the water snow line, the point from the star where the temperature and pressure dip low enough for water ice to form.
    L. Cieza/ALMA (ESO/NAOJ/NRAO)
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For the first time, scientists have observed what they call a “water snow line,” the point at which temperatures in the disk circling a young star drop low enough to allow the formation of snow.

Using the Atacama Large Millimeter/submillimeter Array (ALMA), a state-of-the-art telescope perched high in the Chilean Andes, astronomers were observing the star V883 Orionis, seeking to study a process known as disk fragmentation. Instead, they found the water snow line, where water vapor freezes into ice without ever transitioning through a liquid phase.

So, odd as it may sound, there actually is snow in space. Moreover, this icy orbit swirling around newborn stars plays a crucial role in the formation of planets and the kind of characteristics they exhibit.

“While the fact that V883 Ori might reveal some of the very early steps towards planet formation is fascinating in itself,” write the researchers in the journal Nature, “the outward movement of the water snow-line ... has far-reaching consequences for our understanding of disk evolution and planet formation in general.”

This "outward movement" is key in understanding how the astronomers were able to detect a snow water line in space for the first time.

Young stars often find themselves surrounded by dense orbiting clouds of gas and dust, called protoplanetary disks, from which new planets evolve. Because of the heat generated by the star itself, any water within those disks is usually in gaseous form up to a distance of about 280 million miles away from the star.

Beyond that, because of the negligible amounts of pressure, the water vapor skips its liquid form and solidifies straight into ice, forming a frost patina on dust grains and other particles. This region, where water becomes ice, is the snow water line, and it is usually a fixed distance from the star.

Our friend V883 is unusual in this regard. For some reason, large amounts of material from its orbiting disc have fallen into the star, causing a brilliant flare of energy that boosts its brightness to such an extent that it is now 400 times as luminous as our own sun, while exhibiting a mass only 30 percent greater.

The consequence of this surge of energy, and the accompanying jump in temperature, is that the snow water line was shoved far deeper into space, to a distance of 3.7 billion miles. Being that much further out allowed ALMA to detect it.

“The ALMA observations came as a surprise to us,” said lead author Lucas Cieza, an astronomer from Universidad Diego Portales in Santiago, Chile. “This illustrates well the transformational power of ALMA, which delivers exciting results even if they are not the ones we were looking for.”

Snow water lines are important because they delineate something of a boundary between different kinds of planets. Those forming inside that border, where water swirls through space as a vapor, are likely to be the small rocky kind, like Earth.

But outside, the presence of ice allows the formation of cosmic snowballs, which in turn can form the basis of giant gaseous planets like Jupiter.

Armed with ALMA’s new discovery – that water snow lines can be pushed 10 times deeper into space by outbursts of stellar energy – scientists should be able to produce more intricate models of planetary formation.

“The water snow line is the most elusive of all snow lines, because the sublimation temperature of water is high,” wrote Brenda Matthews of the Herzberg Astronomy and Astrophysics Programs, National Research Council of Canada, in a commentary. “The fact that the location of the snow line can evolve with time has strong implications for planet formation.”

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