Context:  Recently, Interstellar comet 2I/Borisov is providing a glimpse of another star system’s planetary building blocks, using new observations from NASA’s Hubble Space Telescope.  
Background 

• Comets are condensed samples of gas, ice, and dust that form swirling in the disk around a star during the birth of its planets.

What is an interstellar comet?

• Comets are snowballs of ice, dust and frozen gas. When totally frozen (or “inactive”), they’re approximately the diameter of a small town, but when heated by the Sun their tails can extend for millions of miles.
• 2I/Borisov is about the length of nine football fields, or 0.61 miles (0.98 kilometers), making it relatively small. The new results on the comet’s composition are published in the journal Nature Astronomy. 
• All comets form in the primordial disk of material that encircles a young star, preserving remnants of a planetary system’s ancient past. 
• Comets from our own solar neighborhood reveal the history of materials, including water, that made Earth the planet we know today, as well as our other planetary neighbors. 
• An interstellar comet, on the other hand, is a chemical ambassador from an entirely different star system — containing a treasure trove of clues to worlds too far to reach with modern technology. 

Where did it come from? 

• A high carbon-monoxide-to-water ratio suggests that the comet has traveled from a very cold place — as cold as the area where Pluto is in relation to our Sun, called the Kuiper Belt. 
• The group using Hubble additionally theorizes 2I/Borisov may have originated around the most common type of star in the Milky Way: a red dwarf. 
• Red dwarfs are much smaller and dimmer than the Sun, so the planet-forming material around them would be colder than the building blocks of our solar system. 

About the key Findings

• Borisov is the first known comet to originate from a different star system than our own. Measurements find that it has an unusual abundance of carbon monoxide largely unlike comets belonging to our solar system. 
• Researchers say its unusual composition points to a likely birthplace of a carbon-rich circumstellar disk around a cool red dwarf class of star. These observations are a prime opportunity to sample the chemistry of the material in a primordial disk around another star. 
• Studying comets is important because astronomers are still trying to understand the role they play in the buildup of planets. 
• They can also redistribute organic material among young planets, and may have brought water to the early Earth. These activities are likely happening in other planetary systems, as demonstrated by Borisov’s makeup. 
• The team used Hubble’s unique ultraviolet sensitivity to spectroscopically detect carbon monoxide gas escaping from comet Borisov’s solid comet nucleus. 
• Hubble’s Cosmic Origins Spectrograph observed the comet on four separate occasions, which allowed the researchers to see the object’s chemical composition change quickly, as different ice mixtures, including carbon monoxide, oxygen, and water, sublimated under the warmth of the Sun. 
• The Hubble astronomers were surprised to find that the interstellar comet’s coma, the gas cloud surrounding the nucleus, contains a high amount of carbon monoxide gas, at least 50% more abundant than water vapour.  
• Carbon monoxide ice is very volatile. It doesn’t take much sunlight to heat the ice and convert it to gas that escapes from a comet’s nucleus.  
• The Hubble measurements suggest that some carbon monoxide ice was locked inside the comet’s nucleus, revealed only when the Sun’s heat stripped away layers of water ice. 
• The researchers suggest that the comet may have been ejected from a carbon-rich disk of icy debris around a red dwarf star, the most common type of star in our Milky Way galaxy.