Witze reports that astronomers are grappling with the meaning of the first objects entering our Solar System from distant regions. From the tallest peak in Hawaii to a high plateau in the Andes, some of the biggest telescopes on Earth will point towards a faint smudge of light over the next few weeks. The same patch of sky will draw the attention of Gennady Borisov, an amateur astronomer in Crimea, and many other hobbyists who will sacrifice proper sleep and doze through their day jobs rather than miss this golden opportunity.
What they’re looking for is a rare visitor that is about to make its closest approach to the Sun. After that, they have just months to grab as much information as they can from the object before it disappears forever into the blackness of space. This chunk of rock and ice started its journey many light years from Earth, millions of years ago. The object got kicked out of its own neighbourhood by a violent gravitational push — maybe from a nearby planet, maybe from a passing star. Since then, it has been adrift in the space between the stars, eventually heading in our direction.
On 30 August, Borisov spotted the object in the predawn sky — it was glowing dimly, with a broad stubby tail. Later named Comet 2I/Borisov after its discoverer, it captured global attention because it’s only the second object — aside from exotic dust particles — ever known to have entered our Solar System from interstellar space. “This is my eighth comet, and so amazing,” says Borisov, who adds that it was “great luck that I got such a unique object”. It is remarkably different from the first interstellar interloper, which was a small, dark, rocky-looking object named 1I/‘Oumuamua that whizzed past the Sun in 2017. Together, these two interstellar objects are rewriting what researchers know about the icy bodies — estimated to number as many as 1026 — that float unmoored throughout the Milky Way.
Among other things, 1I/‘Oumuamua and 2I/Borisov have provided the first direct glimpse of the physics and chemistry of the squashed debris clouds that surround young stars and serve as the birthing grounds for planets. These samples from other planetary systems are allowing scientists to explore whether the Solar System is unique or whether it shares building blocks with other planetary systems in the Milky Way. Because astronomers spotted 2I/Borisov on its way into the Solar System, they have many months to study it (see ‘Dropping by’) — unlike their fleeting glimpse of ‘Oumuamua, which was discovered on its way out. As a result, they expect to learn much more from 2I/Borisov, such as what chemical compounds make up its icy heart. It is their best look yet at an object known to have formed around another star.
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