Thursday, April 15, 2021

Interstellar visitor 2I / Borisov is the most pristine comet humans have studied

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We’ve studied enough comets in our solar system to know that they formed in its early days, when a ton of matter swirled around and merged into individual bodies. They are mostly made from ice, but in order to survive they must form at a distance where heat and radiation from the sun will not cause them to melt instantly. Other star systems likely give birth to comets in the same way. The further away they are from the star’s radiation, the more they retain their original composition and chemistry from their formation about 4.5 billion years ago. This “pristine” quality means that comets are like preserved time capsules of star systems in their early days.

Comet dust in particular tells us what the solar system was made of when it gave birth to comets, and the same principle can theoretically apply to interstellar comets. “By studying the composition and structure of dust particles in the 2I / Borisov dust coma, we can make educated guesses about the conditions of dust formation and locations,” says Bin Yang, astronomer at the Observatory. Southern European and lead author of one of the studies.

The first article, edited by Stefano Bagnulo at the Armagh Observatory and Planetarium in the UK, focuses on reflected light. Light is made up of waves, and these waves normally oscillate in many different directions at once. However, when these waves are polarized, they oscillate in a specific direction. If the light is polarized by a comet’s coma (the hazy outer shell of gas and dust expelled when the comet is heated by the sun), studying this light can provide information about the size and composition of dust, which helps us understand how the comet formed – and, by extension, provides insight into the history of its original star system.

The new data, collected by the Chile-based Very Large Telescope, tells us that the light reflected by Borisov and filtered through his coma is more polarized than light from any other object we have studied in the solar system. This is a sign that the coma particles are small and very fine, suggesting that they weren’t much disturbed by a star’s radiation and heat (forces that would otherwise cause the random ejection of large pieces of the surface). The authors conclude that Borisov is perhaps one of the most intact objects ever detected. The only object that comes close to polarization is C / Hale-Bopp, perhaps the brightest comet ever observed, and certainly one of the most studied comets of the 20th century. It is believed that Hale-Bopp approached the sun only once before its last solar flyby in 1997. The authors therefore believe that similar conditions may have given rise to Borisov and Hale-Bopp, in two different star systems.

Meanwhile, the team led by Yang had set out to understand how Borisov had formed, using the VLT as well as Chile’s Atacama Large Millimeter / submillimeter Array (ALMA) to detect the heat of large particles suspended in the Borisov’s coma.

According to these observations, Borisov’s coma consists of compact grains the size of a millimeter – pebbles that are unusually large for a comet. These pebbles, rich in carbon monoxide and water, probably formed first in the inner region of the star system, before being transported outward and gradually mixing with various ice sheets formed at different locations further afield. of the star. This “gravitational agitation”, induced by giant planets, is believed to have occurred in our own solar system (we even think we helped Hale-Bopp to train). Borisov basically came together as an agglomeration of materials from different parts of his star system, before finding a secluded place to feel at home away from his mother star.

Taken together, the results help tell us several things. An abundance of carbon monoxide and water in the dust suggests that the comet resided in low temperature environments (i.e. far from a star), where these compounds could have remained cold and stable, for most of his life. The discovery of “pristine” characteristics reinforces this idea.

The similarities between Borisov and Hale-Bopp, as well as the evidence that the star systems of the two comets underwent gravitational stirring, suggests that the evolution of our solar system may not be as unique as we might have thought. . It would also suggest that the conditions that give rise to a habitable planet like Earth are more common in the galaxy than one might imagine.

Or maybe it’s a red herring, and Borisov’s original star system is actually very exotic. Neil Dello Russo, a Johns Hopkins University astronomer who was not involved in the study, said he was surprised at the height of the carbon monoxide and water values ​​- higher than anything seen in the comets of our solar system.

Other questions also persist. The new findings still can’t tell us exactly when the coma pebbles formed, or even what they’re made of.

The bigger problem could be that the two articles seem to promote two different ideas about the particles that make up Borisov: Yang’s article highlights the discovery of large pebbles in a coma, while Bagnulo’s article suggests that the coma is dominated by small, smoke-like grains. this can cause extreme polarization of light. But Michael Kelley, a comet specialist at the University of Maryland who was not involved in the new studies, thinks it’s probably “just a consequence of the different techniques” – each favoring the detection of a specific type. particle. Future analyzes should be able to compare and combine the two data sets and reconcile them within the framework of Borisov’s evolution.

Borisov is a strange object, but what’s really strange is the idea that it could have come from a star system not too different from ours. This interstellar comet strength be one of the most normal visitors we have ever said hello to.

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