Cosmic “snow lines” reveal a solar system's earliest daysS

This artist's conception shows the present state of TW Hydrae, a star located about 176 light-years away. It's mostly the same as our Sun, with one major difference: its age. While our sun is 5 billion years old, TW Hydrae is just 5 million years old.

That means TW Hydrae is just a baby by cosmic standards, and its proximity to Earth allows us a chance to see up close what our own solar system might have looked like in its infancy. This image is a particularly striking illustration of the latest findings from astronomers at the European Southern Observatory.

Young solar systems have massive discs teeming with all sorts of molecules, including water, carbon dioxide, carbon monoxide, and methane. Those molecules freeze the further they get away from their star's heat, and their different chemical compositions mean the molecules all freeze at different temperatures – and, by extension, different distances from their Sun. This creates what are known as "snow lines," named after the line that, due to difference in altitude and temperature, marks the transition from a mountain's snow-capped peak to its exposed rock below. Those snow lines are important because they mark out where different types of planets are likely to eventually form. ESO astronomers explain:

Starting from the star and moving outwards, water (H2O) is the first to freeze, forming the first snow line. Further out from the star, as temperatures drop, more exotic molecules can freeze and turn to snow, such as carbon dioxide (CO2), methane (CH4), and carbon monoxide (CO). These different snows give the dust grains a sticky outer coating and play an essential role in helping the grains to overcome their usual tendency to break up in collisions, allowing them to become the crucial building blocks of planets and comets. The snow also increases how much solid matter is available and may dramatically speed up the planetary formation process.

Each of these different snow lines — for water, carbon dioxide, methane and carbon monoxide — may be linked to the formation of particular kinds of planets. Around a Sun-like star in a planetary system like our own, the water snow line would correspond to a distance between the orbits of Mars and Jupiter, and the carbon monoxide snow line would correspond to the orbit of Neptune.

In the image up top, the blue part of the disc shows the dust grains covered in water ice, while the green portion shows those grains covered in frozen carbon monoxide. There's more on this story over at the ESO website, as well as the original paper at Science Express.

Image credit: B. Saxton & A. Angelich/NRAO/AUI/NSF/ALMA (ESO/NAOJ/NRAO).