When you look at a snow-capped mountain, the line that separates the snow-less lower slopes and the snowy peak is descriptively known as a “snow line.” It’s above this line where you’d want to go with your snowboard or skis. Now, for the first time, astronomers have discovered the snow line for a particularly frosty star system 175 light-years away — but there’s no word on the quality of the snow for slaloming.
Stellar snow lines work in a similar way to their terrestrial mountain counterparts. On a mountain, there’s a certain altitude where the atmosphere becomes so cold that water will freeze and snow will collect around the frozen mountain peak. Around a star, on the other hand, there’s a certain distance from the hot star where it becomes so cold that it allows water (and other volatiles, such as methane) to freeze as ice particles.
Astronomers using the brand new Atacama Large Millimeter/submillimeter Array (ALMA) in Chile have used a neat little trick to resolve the stellar snow line around the young sun-like star TW Hydrae. And by doing so, they have taken a glimpse at what our solar system may have looked like during the early stages of its evolution.
“ALMA has given us the first real picture of a snow line around a young star, which is extremely exciting because of what it tells us about the very early period in the history of the Solar System,” said Charlie Qi of the Harvard-Smithsonian Center for Astrophysics, one of the two lead investigators of this study. “We can now see previously hidden details about the frozen outer reaches of another planetary system similar to our own.”
An added bonus to observing the snow line around another star is that it can give us a sense as to how planets, comets and even organic molecules may form.
Water (H2O) is the first molecule to condense and freeze when it reaches the necessary distance from the star, forming the first snow line. Moving further away from a star, other molecules start to freeze, such as carbon dioxide (CO2), methane (CH4), and carbon monoxide (CO). As each species of molecule has a different freezing point, the snow line of each can be found at different distances from the star.
In the case of our solar system, the formation of the planets may have been influenced by the different snow lines at different orbital distances from the sun. During the protoplanetary phase of the sun, frozen volatiles at certain snow lines would have given the protoplanetary dust a sticky outer layer, potentially accelerating planet formation through the process of agglomeration. For example, the water snow line for the sun can be found between the orbits of Mars and Jupiter; the CO snow line is at the orbit of Neptune. How does this factor into planetary formation at those distances?
The CO snow line is also interesting, say Qi and his team, for its implication in the formation of methanol, one of the key ingredients for organic chemistry. If comets formed beyond the solar system’s CO snow line, they could have been enriched in organic compounds that were then transported to the inner solar system, depositing the life-giving cocktail on Earth. We could be witnessing this process unfolding around TW Hydrae.
But seeing the CO snow line isn’t easy — CO cannot be observed directly. So, the team hunted down the fragile, yet easily identifiable diazenylium (N2H+) molecule that generates a strong and identifiable millimeter wavelength emission that is perfect for ALMA to detect. N2H+’s chemistry is easily broken down by the presence of CO. So, through a simple astronomical process of elimination, where N2H+ isn’t, CO is. By seeking out the absence of N2H+, they’d created a negative picture of the location of the CO’s snow point around the star.
“For these observations we used only 26 of ALMA’s eventual full complement of 66 antennas. Indications of snow lines around other stars are already showing up in other ALMA observations, and we are convinced that future observations with the full array will reveal many more of these and provide further, exciting insights into the formation and evolution of planets. Just wait and see,” said co-investigator Michiel Hogerheijde of Leiden Observatory, the Netherlands.
Images: Top: Artist’s impression of the TW Hydrae system — the CO snowline is depicted as the protoplanetary disk changes from blue to green. Credit: B. Saxton & A. Angelich/NRAO/AUI/NSF/ALMA (ESO/NAOJ/NRAO). Inset: ALMA observation of the CO snow line around TW Hydrae. Credit: ALMA (ESO/NAOJ/NRAO)