The sight of a snowfall can thrill children, but the first-ever snow line seen around a distant star gives astronomers an even greater thrill because of what it reveals about the formation of planets and our solar system’s history.
Using the new Atacama Large Millimeter/submillimeter Array telescope, or ALMA, in Chile, University of Virginia and Harvard-Smithsonian Center for Astrophysics astronomers have taken the first-ever image of a snow line in an infant solar system. This frosty landmark is thought to play an essential role in the formation and chemical make-up of planets around a young star.
On Earth, snow lines typically form at high elevations where falling temperatures turn atmospheric moisture to snow. In much the same way, snow lines are thought to form around young stars in the distant, colder reaches of the stars’ disks, from which solar systems form. Depending on the distance from the star, however, other more exotic molecules can freeze and turn to snow.
Water ice freezes first, then moving outward in concentric circles other abundant gases like carbon dioxide, methane and carbon monoxide freeze, forming a frost on dust grains, which are the building blocks of planets and comets.
ALMA spotted a never-before-seen carbon monoxide snow line around TW Hydrae, a young star 175 light-years away from Earth. Astronomers believe this nascent solar system has many of the same characteristics that our own solar system had when it was just a few million years old. The results were published July 18 in Science Express .
Snow lines have, until now, only been detected by their spectral signatures; they have never been imaged directly, so their precise location and extent could not be determined.
This is because snow lines form exclusively in the relatively narrow central plane of a protoplanetary disk. Above and below this region, stellar radiation keeps the gases warm, preventing them from forming ice. Only with the insulating effect of the concentrated dust and gas in the central plane of the disk can temperatures drop sufficiently for carbon monoxide and other gases to cool and freeze.
Normally, this outer cocoon of hot gas would prevent astronomers from peering inside the disk where the gas had frozen.
“It would be like trying to find a small, sunny patch hidden within a dense fogbank,” said Karin Oberg, who conducted the research as an assistant professor of chemistry and astronomy in U.Va.’s College of Arts & Sciences before recently taking a position at Harvard-Smithsonian. Her co-investigator was Chunhua “Charlie” Qi, a researcher at Harvard-Smithsonian.
The astronomers were able to pierce the intervening carbon monoxide fog by instead hunting for a different molecule known as diazenylium. This fragile molecule is easily destroyed in the presence of carbon monoxide gas, so would only appear in detectable amounts in regions where carbon monoxide had frozen, and is hence a proxy for carbon monoxide ice.
Diazenylium shines brightly in the millimeter portion of the spectrum, which can be detected from Earth by a radio telescope like ALMA.
ALMA’s unique sensitivity and resolution allowed the astronomers to trace the presence and distribution of diazenylium, finding a clearly defined boundary approximately 30 astronomical units from the star TW Hydrae (one astronomical unit is the average distance from the sun to Earth).
“Using this technique, we were able to create, in effect, a photonegative of the carbon monoxide snow in the disk surrounding TW Hydrae,” Oberg said. “With this we could see the carbon monoxide snow line precisely where theory predicts it should be – the inner rim of the diazenylium ring.”
Snow lines, astronomers believe, serve a vital role in the formation of a solar system. They help dust grains overcome their normal tendency to collide and self-destruct by giving the grains a stickier outer coating. They also increase the amount of solids available and may dramatically speed up the planet formation process. Since there are multiple snow lines, each may be linked to the formation of specific kinds of planets.
Around a Sun-like star, the water snow line would correspond approximately to the orbit of Jupiter and the carbon monoxide snow line would roughly correspond to the orbit of Neptune. The transition to carbon monoxide ice could also mark the starting point where smaller icy bodies like comets and dwarf planets like Pluto would form.
Oberg also points out that the carbon monoxide snow line is particularly interesting since carbon monoxide ice is needed to form methanol, which is a building block of more complex organic molecules that are essential for life. Comets and asteroids could then ferry these molecules to newly forming Earth-like planets, seeding them with the ingredients for life.
The observations were made with only a portion of ALMA’s eventual full complement of 66 antennas. The researchers hope future observations with the full array will reveal other snow lines and provide additional insights into the formation and evolution of planets.
“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 our own solar system,” Harvard-Smithsonian’s Qi said. “We can now see previously hidden details about the frozen outer reaches of another solar system, one that has much in common with our own when it was less than 10 million years old.”