Feature 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 plan- ets. These samples from other planetary sys- tems 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. NASA/ESA/D. JEWITT (UCLA) A composite image of interstellar Comet 2I/Borisov, taken by the Hubble Space Telescope. Because astronomers spotted 2I/Borisov on its way into the Solar System, they have many months to study it — unlike their fleet- ing glimpse of ‘Oumuamua, which was discov- ered 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. INTERSTELLAR And as telescopes continue to probe the sky for faint, fast-moving objects, researchers expect that they will spot many more inter- stellar interlopers in coming years. “It’s been so much fun to see this suddenly crack open and watch a new field develop,” says Michele INTRUDERS Bannister, a planetary astronomer at Queen’s Astronomers grapple with the meaning of University Belfast, UK. the first two objects entering our Solar System Dusty origins Interstellar objects probably began their lives from distant regions. By Alexandra Witze when icy grains clumped together in a disk of gas and dust around a young star. These are the same regions where planets grow from rom the tallest peak in Hawaii to a What they’re looking for is a rare visitor small nuclei and then ping-pong into different high plateau in the Andes, some of the that is about to make its closest approach to orbits around the star because of collisions biggest telescopes on Earth will point the Sun. After that, they have just months to and gravitational shoves. towards a faint smudge of light over grab as much information as they can from The planets push through the icy rubble like the next few weeks. The same patch of the object before it disappears forever into a snowplough shouldering its way through a sky will draw the attention of Gennady the blackness of space. pile of hailstones. And modelling results sug- Borisov, an amateur astronomer in This chunk of rock and ice started its journey gest that the planets fling more than 90% of Crimea, and many other hobbyists many light years from Earth, millions of years those ‘hailstones’ out of their star’s sphere of Fwho will sacrifice proper sleep and doze ago. The object got kicked out of its own neigh- influence and into interstellar space. There through their day jobs rather than miss this bourhood by a violent gravitational push — they drift, as lonely scattered objects, until they golden opportunity. maybe from a nearby planet, maybe from a happen to pass close enough to another star 434 | Nature | Vol 575 | 21 November 2019 ©2019 Spri nger Nature Li mited. All ri ghts reserved. ©2019 Spri nger Nature Li mited. All ri ghts reserved. to be attracted by its gravity for a quick visit. Astronomers had expected that the first DROPPING BY Hailing from interstellar space, Comet 2I/Borisov is making a quick pass through interstellar object they saw would look like the Solar System. Its trajectory will carry it closest to the Sun in early December. a typical comet. Most comets in the Solar System hail from the distant realm known as the Oort cloud, a sort of cosmic deep freeze N Uranus eptu that lies roughly 1,000 times farther away ne o rbit from the Sun than Pluto. Occasionally, some- thing perturbs one of these comets and sends it careering towards the Sun; as it gets closer and warms up, its nucleus sprays out dust and Earth gas that form a classic cometary tail. But when the first interstellar visitor showed up, it didn’t look like a conventional comet. Despite its exotic origin, Jupiter observations so far suggest Saturn Unlike them, ‘Oumuamua was tiny — just Comet 2I/Borisov is much like 200 metres or so across — and rocky. Also, it was other comets in the Solar System. shaped like a cigar and tumbling end over end. *Position as of 12 October 2019. That’s about all scientists could work out before ‘Oumuamua headed out of the Solar System1. By contrast, 2I/Borisov looks like an ordinary ADAPTED FROM ILLUSTRATION BY NASA/ESA/J. OLMSTED AND F. SUMMERS (STSCI) NASA/ESA/J. F. AND BY OLMSTED ILLUSTRATION FROM ADAPTED comet — and researchers are taking advantage of their time to study it (see ‘Dropping by’). “We are Florida Space Institute at the University of rapid succession after decades of fruitless keenly interested in seeing what the chemistry of Central Florida in Orlando. searching. “I remain confused and astounded this comet is, to see if it is different from those in Early observations also suggest that that the second object came along so fast,” says the Solar System,” says Karen Meech, an astrobi- 2I/Borisov might contain relatively low Robert Jedicke, an asteroid specialist at the ologist at the University of Hawaii in Honolulu. amounts of carbon-chain molecules such as C2 University of Hawaii, who has worked to cal- 8 2I/Borisov is reddish in colour and is stead- and C3 (ref. 6). About 30% of the comets in the culate the frequency of interstellar visitors . ily spraying out dust particles2,3. Its nucleus Solar System are similarly carbon-depleted. “They’re like buses,” says Alan Fitzsimmons, is relatively small, perhaps just one kilometre They typically come from relatively close to an astronomer at Queen’s University Belfast. across, but that’s not unheard of for Solar the Sun, rather than from the far reaches of “You wait decades for one to come along, and System comets. the Oort cloud. then two come along almost at once.” “After ‘Oumuamua, we had to completely As months pass and astronomers gather Some astronomers are now poring through revise what we thought interstellar objects more observations of 2I/Borisov, they hope to archival data to see whether objects spotted might be like,” says Matthew Knight, a comet be able to understand much more about the years ago were actually interstellar visitors specialist at the University of Maryland in planet-forming disk where it originated. “It’s that researchers did not recognize at the time. College Park. “But now the second one coming going to be really exciting to figure out what And the future rate of discovery is expected to through looks more or less, so far, like what we the building blocks of other systems are going rise — perhaps to one interstellar object a year thought we might see from a comet ejected to look like relative to ours,” says Malena Rice, — when the Large Synoptic Survey Telescope from another star. Now I feel a lot better.” That a graduate student in astronomy at Yale Uni- goes online in Chile in 2022, from where it will suggests that the star systems where other survey the entire visible sky every three nights. worlds form might be much like our own. “They’re like buses. You The European Space Agency has been work- The discoveries are coming fast. Just three ing on a spacecraft concept, known as Comet weeks after 2I/Borisov was first seen, astron- wait decades for one to Interceptor, that could visit future interstellar omers trained the 4.2-metre William Herschel come along, and then two objects as they wing their way past the Sun. Telescope in Spain’s Canary Islands on it and come along almost at once.” Once astronomers have 10 or 20 interstellar spotted molecules of cyanide gas streaming objects under their belts, they should have a off the comet4. It was the first-ever detection much better picture of what these deep-space of gas from an alien visitor to the Solar System. versity in New Haven, Connecticut. wanderers are really like. “Eventually we’ll be On 11 October, another research team used Researchers also hope to start unravelling talking about the Galaxy as something in which a 3.5-metre telescope in New Mexico to detect how interstellar objects might have voyaged we are exchanging the products of planetary oxygen coming off the comet5. The oxygen through deep space before showing up in the systems,” says Bannister. “It will be an entirely probably came from water breaking apart in the Solar System. Estimates suggest the objects different way of doing astronomy.” comet’s nucleus, making this the first time that experience many forces as they orbit the centre researchers have spotted water from another of the Galaxy, including occasional encounters Alexandra Witze writes for Nature from star system entering our own.