Comet 3I/Atlas, an interstellar iceball that traveled into the solar system from another star, likely formed in a cold and isolated region of the Milky Way long before the solar system took shape, astronomers reported Thursday. The conclusion centers on the chemistry of the comet’s water, which a research team analyzed with observations from the ALMA observatory in Chile’s Atacama Desert.
Scientists said the comet—described as “errant but harmless” ice—was discovered last summer, giving NASA and the European Space Agency time to coordinate follow-up observations as it moved through the inner solar system. Astronomers aimed multiple space telescopes at the comet as it zoomed past Mars in October and later made its closest approach to Earth in December. It is now well past Jupiter on its path out of the solar system, visible mainly to professional astronomers, not the general public.
The researchers said ALMA measurements detected extremely high amounts of deuterium, also called heavy hydrogen, in the comet’s water. Paneque-Carreno of the University of Michigan said in an email that the deuterium finding points to a formation environment that was far colder than the comet’s “own cosmic neighborhood,” and that the comet likely originated in a place considerably colder than the region where this solar system formed.
Paneque-Carreno also said that while the sun may have been surrounded by other newborn stars during its formation, the comet’s home star could have been more of a “loner,” leading to less heating and colder conditions. She added that the study’s results, when “linking all these ‘puzzle pieces together’ may give an idea to how the planet-forming conditions were at these early times,” according to the email.
The work was published in Nature Astronomy. Hubble Space Telescope observations put the comet’s nucleus size somewhere between a quarter-mile and 3.5 miles, or 440 meters and 5.6 kilometers. Scientists also estimated the comet’s age and said Comet 3I/Atlas could be up to 11 billion years old—more than twice as old as the sun—making it potentially among the earliest-building blocks of planetary systems.
Interstellar objects are rare in the solar system. The first known visitor, ‘Oumuamua, was discovered in 2017 with a telescope in Hawaii, and Comet 2I/Borisov followed in 2019, named for an amateur astronomer in Crimea who first spotted it.