Recent observations from the European Southern Observatory's (ESO) Very Large Telescope (VLT) reveal that interstellar comet 2I/Borisov is among the most pristine comets ever studied, likely having never encountered a star before its close solar approach.
On August 30, 2019, amateur astronomer Gennady Borisov discovered an anomalous comet and alerted the International Astronomical Union's Minor Planet Center. Follow-up observations confirmed its unusual trajectory: this comet originated from outside our Solar System.
Designated 2I/Borisov after its discoverer, it became the second interstellar object cataloged here, following 'Oumuamua.
Unlike the outbound 'Oumuamua—possibly a planetary fragment—Borisov was inbound, offering astronomers a rare chance to study it in detail.
Initial spectra hinted at similarities to Solar System comets, but deeper analysis showed key differences.
Two studies (here and here), drawing on Hubble and ALMA (Atacama Large Millimeter/submillimeter Array) data, highlight Borisov's coma—the gas envelope around its nucleus—as exceptionally rich in carbon monoxide. In contrast, Solar System comets are dominated by water ice.
This points to formation in an ultra-cold region (below -250°C) far from its home star, followed by ejection—the exact mechanism remains a mystery.
After millions or billions of years adrift, 2I/Borisov approached our Sun, marking its fiery debut.
A new study in Nature Communications, led by experts at the Armagh Observatory and Planetarium in Northern Ireland, used the VLT's FORS2 instrument to measure the comet's polarimetric properties (light polarization).
Results show 2I/Borisov differs from most Solar System comets, except the famed Hale-Bopp, which captivated observers during its 1997 perihelion on April 1. Hale-Bopp was deemed one of the most pristine due to minimal prior solar exposure.
Comparative analysis suggests 2I/Borisov surpasses even Hale-Bopp in pristineness, implying it had never before brushed any star ahead of its 2019 solar flyby.
Lead researcher Bagnulo anticipates more such opportunities, with the European Space Agency's (ESA) comet-intercept mission slated for launch in 2028.
