ALMA captures young star's water snow line

The line marking the transition in a protoplanetary disc from gas to ice has been observed clearly for the first time, after a young star's sudden burst in temperature caused it to shift dramatically.

An image of the disc around V883 Orionis captured by ALMA. The image shows the position of the shifted water snow line, plus the orbits of Pluto and Neptune for scale.
Credit: ALMA (ESO/NAOJ/NRAO)/L. Cieza

Astronomers have been able to make the first ever resolved observation of the ‘water snow line’ around a young star, after an increase in stellar brightness pushed the snow line far enough to see from Earth.

Young stars are surrounded by a protoplanetary disc of gas and dust out of which planets and, eventually, a planetary system will form. The heat from the star means the materials in the disc remain gaseous up to distances of 3 AU, or 450 million kilometres from it.

However, farther out, low pressure and falling temperatures cause the gaseous water to form ice on dust grains and other particles. The border between these two states is known as the water snow line.

The water snow line is seen as key to planet formation. Within the line, where water is gaseous, small rocky planets are thought to form, while outside of the snow line, water ice causes massive snowballs to form, which eventually become gas giants like Jupiter.

Illustration showing how the sudden brightening of star V883 Orionis caused a shift in the water snow line
Credit: ALMA (ESO/NAOJ/NRAO)/L. Cieza

Using the Atacama Large Millimeter/submillimeter Array (AMLA), astronomers were able to observe the water snow line around star V883 Orionis. This happened because the star went through a sudden increase in brightness that pushed the water snow line out to a distance of around 40 AU, or 6 billion kilometres. This is about the size of the orbit of Pluto.

The sudden brightening of V883 Orionis occurred because of material from the disc falling onto the young star’s surface and heating up, causing it to become 400 times more luminous than it would normally be.

Carousel image: Artist’s impression of the water snow line around V883 Orionis 
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