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Researchers spot the first ‘snow line’ ever seen around young protostar

Researchers working with the Atacama Large Millimeter / sub-millimeter array have spotted the first direct evidence of frozen water in orbit around a young star. This boundary is thought to serve as the demarcation point between rocky inner planets and the gas giants that tend to form farther away from the host star, but we’ve never been able to observe it directly before.

The star in question, V883 Orionis, is a young protostar with an associated protoplanetary disk. In most cases, the water “snow line” is too difficult for researchers to see with current telescopes because it’s too close to the host star (within 5 AU). In this case, however, V883 Orionis is undergoing an outburst in luminosity — meaning the amount of energy pouring off the star has increased. This, in turn, has moved the water snow line from 5 AU back to approximately 42 AU, as shown in the diagram below:

This has significant implications for planetary formation. Our own solar system is composed of rocky planets closer to the sun, gas giants (Jupiter and Saturn) farther away, and ice giants (Neptune, Uranus, and possibly Planet 9) even farther away. So-called ice giants are called this because they’re principally composed of elements other than hydrogen and helium, not because they necessarily contain significant amounts of frozen water.

“The distribution of water ice around a young star is fundamental to planet formation and even the development of life on Earth,” Zhaohuan Zhu, an astronomer at Princeton University and a co-author on the new work, said in the statement. “ALMA’s observation sheds important light on how and where this happens in protoplanetary disks when young planets are still forming.”

“We now have direct evidence that a frosty region conducive to planet formation exists around other stars,” Zhu added.

Water ice is considered important because it changes how efficiently dust and planitesimals combine in early protoplanetary disks. A star that experiences dramatic shifts in luminosity and corresponding changes to its snow line could have a very different set of planets than one in which these characteristics are fairly constant. It’s possible that shifting snow lines could also play a role in the formation of so-called “hot Jupiters.” These are gas giants that orbit close to their host stars (0.015 – 0.5 AU). Many of the planets identified by Kepler are classified as hot Jupiters, but it’s unclear if these planets are as common as they seem to be, or if they simply appear to be common because our current methods of planet detection favor them.

The alternative hypothesis is that hot Jupiters form beyond the frost line but often migrate inwards, forming a stable orbit close to their host stars. Some researchers have argued that Jupiter may have migrated inwards at one point in our solar system’s history before its path was reversed by the formation of Saturn. In this “Grand Tack” hypothesis, Jupiter would’ve cleared away swathes of material that might have otherwise accreted to Mars — explaining why the fourth planet is much smaller than Venus or Earth.

Seeing the snow line in this case confirms what’s been suspected about how water ice is distributed in a forming protoplanetary disc, but we’ll need more than one example to parse out more information on actual planet formation. Hopefully we can find a similarly accommodating star in a slightly later stage of the process.

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