ROB WERYK jokingly calls himself the guardian of Earth. Each day, the University of Hawaii postdoc goes into his office amid the palm trees of suburban Honolulu and reviews the night’s information on threats from outer space. He is often the first to see a new, potentially hazardous “near-Earth object” (NEO) in the data streaming from the planet’s early-warning system. The twin detectors of the Panoramic Survey Telescope and Rapid Response System, , which sit atop the 3000-metre Haleakala – a peak on the island of Maui, constantly scan the sky for space rocks straying too close.
At first glance, the object spotted on the night of 19 October 2017 was just another dim trail of light, fast moving against the starry backdrop. A passing comet or asteroid, no doubt. It was only when Weryk looked back into the previous night’s data that he sniffed something amiss. The object was there – but not where he expected it to be. “Something didn’t make sense,” he says. He got on the phone to Marco Micheli, a University of Hawaii graduate now at the European Space Agency, who was able to commandeer a telescope at ESA’s Optical Ground Station on the Spanish island of Tenerife. Separately, they began watching.
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The headlines have since been written. This object wasn’t orbiting the sun, but paying us a visit from far outside our solar system. Such interstellar callers had been expected, but never before seen. Now one has, it opens up a new era in astronomy and raises fundamental questions about how planetary systems, including our own, are made.
It took several nights of observations to fully convince Weryk and Micheli of what their telescopic eyes were seeing. Asteroids or comets orbiting the sun generally follow closed, elliptical paths. This object’s trajectory was an open curve, a hyperbola, meaning it couldn’t be orbiting the sun (see Diagram). It had already made its closest approach, and was racing away at 38 kilometres per second, fast enough to escape back into interstellar space.
Weryk posted the discovery to the NEO confirmation page hosted by the Minor Planet Center at the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, the clearing house for such things. By 25 October, there were enough follow-up observations to officially confirm the object as interstellar, with the designation .
“At first it was just another dim light trail, fast moving against a starry backdrop”
The C for comet fitted with how we expect such interstellar travellers to arise – as a faraway version of those from our own planetary system. When a star forms from a collapsing cloud of molecular gas, some material is left over. This creates a churning disc of dust that slowly smashes and crunches together, gradually accreting into larger objects, and eventually forming planets. Occasionally, debris can be thrown out from these vast construction yards, perhaps by the force of the impacts, by the gravitational interactions of migrating planets or by a nudge from a passing star. The objects most susceptible to ejection are icy bodies on the periphery of their planetary systems, much like the comets from the Kuiper belt and Oort cloud on our solar system’s outskirts (see “Solar wanderers“).
With the object receding, time was of the essence for further observation, so a colleague of Weryk’s, Karen Meech, bagged crucial observing time on the Hubble Space Telescope and two of the biggest ground-based telescopes, the Very Large Telescope and the Gemini South telescope, both in Chile. Her team was looking for three things: cometary activity, a light curve by which to determine the object’s shape, and spectral signatures of any volatiles or minerals on its surface. Shortly after, Michele Bannister, a planetary scientist from Queen’s University Belfast, UK, who normally hunts for Kuiper belt objects, teamed up with Meg Schwamb of the Gemini North telescope in Hawaii to get a sense of the interstellar interloper’s colour.
Although on the small side for a comet, with a maximum length of several hundred metres, the object at first seemed unexceptionable. It was , much as comets do, with an entirely reasonable rotation period of somewhere between 7 and 8 hours. Its colour was a dull red hue familiar from the comet 67P/Churyumov–Gerasimenko, investigated by the Rosetta spacecraft from 2014 to 2016 – a shade shared by around 15 per cent of objects in the Kuiper belt. In the solar system, this colour is produced by 4.6 billion years’ worth of solar ultraviolet light reacting with simple carbon-based molecules such as methane to produce complex, ruddy-coloured organic compounds called tholins.
But that was it for similarities. When comets from our solar system’s recesses approach the sun, some of their ice turns to gas, forming a visible atmosphere called a coma, and often a tail. Meech’s team found no evidence of such activity. The object seemed inert, an asteroid-like rock. Its official designation was changed to A/2017 U1 (A for asteroid), and then to 1I/2017 U1 ‘Oumuamua, the I referring to its interstellar origin. ‘Oumuamua, suggested by Meech’s team, is a Hawaiian phrase meaning “a messenger from afar arriving first”.
‘Oumuamua’s lack of cometary credentials could point to an unexpected origin closer to the centre of the planetary system it formed in. Or perhaps during the object’s long journey through interstellar space, bombardment by cosmic rays, consisting of high-energy particles such as atomic nuclei, drove chemical reactions that formed a thick crust of tholins encasing the object. Bannister’s colleague Alan Fitzsimmons suspects that such a crusty mantle might have and growing a dust tail as it passed the sun – or that the object simply lost its ice through the energy of cosmic-ray impacts. “I think ‘Oumuamua started with a relatively ice-rich composition,” he says.
The apparent lack of ice wasn’t the only surprise. “What is really puzzling people is its shape – how do you get a shape like that?” says Meech. As ‘Oumuamua recedes, it is spinning head over heels. As it does so, its brightness changes when light reflects from sides of different sizes. , the light varies by a factor of 10:1, meaning it is 10 times longer than it is wide.
That would be astonishing: there’s nothing native to our solar system with dimensions like that. Meech argues that the true ratio could be even higher, pointing out that ‘Oumuamua’s axis of rotation may be tilted with respect to our line of sight, so we see it as shorter than it really is. Others are less convinced. David Jewitt at the University of California, Los Angeles, thinks that because we are seeing the object at an angle relative to the sun, light may only be reflecting off part of it, and that this could exaggerate its length. From his observations, he puts the ratio at a more conservative 6:1.
This isn’t unprecedented, but would still mean that our first interstellar visitor has the dimensions of London’s Shard skyscraper. Some have compared it to a smaller version of Rama, the 50-kilometre-long cylindrical alien spacecraft in Arthur C. Clarke’s classic science-fiction story Rendezvous with Rama. The Breakthrough Listen project, which searches for extraterrestrial intelligence, even tried detecting radio signals from it, with no success.
So that’s probably just another example of alien wishful thinking (see “7 things we thought were made by aliens (but weren’t… probably)“), and we’re left seeking natural explanations for this odd interstellar visitor. “If they’re all this shape, that is a bit strange, since they should form just like everything in our solar system,” says Jewitt. Most objects in our neighbourhood are rounded or lumpy, a natural consequence, we think, of the accretion processes that formed them.
‘Oumuamua could simply be a statistical outlier. Or perhaps, Jewitt muses, everything initially forms with an elongated shape, but is ground down over the aeons by collisions, and ‘Oumuamua just happened to be ejected from its home system before it could be eroded. Then again, maybe there’s something more fundamental amiss, and we need to revisit our accretion models to explain how planetary building blocks can adopt such a strange shape.
If we want to better understand the differences between ‘Oumuamua and objects native to our solar system, we need to find more of these interstellar wanderers to better compare and contrast. The good news is that now we’ve seen one, we may not have to wait long for the next one to show up.
Like ships in the fog
In a in February 2017, a team led by Toni Engelhardt at the University of Hawaii and including Fitzsimmons described the likelihood of ever finding an interstellar object in the solar system as “bleak”. The discovery of ‘Oumuamua has flipped that assessment on its head. Pan-STARRS has only really been able to detect such objects since late 2016, when its second telescope saw first light – and yet we have already seen one at less than half the distance of the sun.
Jewitt compares the situation to being on a ship at sea in the middle of dense fog. You might think you are alone, until the ghostly silhouette of another ship sails past. Based on that one sighting, the size of the area in which you have visibility and the time it took to see another ship, you can calculate how many other ships might be sailing the same sea as you, hidden by the fog.
On that basis, Jewitt estimates that 10,000 interstellar objects of similar size to ‘Oumuamua are passing through the solar system within the orbit of Neptune at any one time. Each year, 1000 depart, while another 1000 replace them. “That was the truly jaw-dropping result as far as I’m concerned,” he says. Extrapolating even further suggests there could be as many as 1026 such objects wandering between the Milky Way’s stars. Jewitt thinks we will see another within a year.
Weryk suspects we might even have seen numerous interstellar objects already and not known it. “There’ve been many cases where we’ve seen a near-Earth object candidate once and not been able to find it after that,” he says. That could be because their hyperbolic orbits put them somewhere astronomers didn’t think to look. To improve the odds, Weryk is writing computer code to assist searches for objects on such orbits.
From 2022, when the 8.4-metre Large Synoptic Survey Telescope in Chile will start work, those odds should be much more in our favour. Designed to conduct the most extensive all-sky survey ever undertaken, collecting up to 15 terabytes of data each night, the LSST may well find dozens, if not hundreds, of interstellar objects, and allow us to begin answering the intriguing questions that ‘Oumuamua has posed. Are most interstellar visitors rocky or cometary, and what does that tell us about what’s going on in their home planetary systems? Are their shapes all as wacky, or was this a one-off? What do their colours tell us about the effects of interstellar space travel?
“Now we’ve seen one, we may not have to wait long for the next to show up”
A few months on from its discovery, ‘Oumuamua is already some 600 million kilometres past Earth, departing at an angle of 20 degrees to the ecliptic, the plane containing Earth and the other planets. By May, it will have passed the orbit of Jupiter, heading in the general direction of the constellation Pegasus, perhaps there to perplex the denizens of some other planetary system.
This interstellar interloper’s appearance in our neck of the woods is ultimately reassuring, says Bannister. In the same way a piece of driftwood appearing on a lonely island tells us there are trees growing on distant shores, “‘Oumuamua is a tangible signal that we’re beginning to understand how planetary systems form and evolve,” she says
Solar Wanderers
Interstellar space rock ‘Oumuamua represents the first of a distinct group of minor bodies that can pass close by our planet, but there are two others.
On New Year’s Day 1801, Sicilian astronomer Giuseppe Piazzi discovered Ceres, the first of millions of asteroids whose home is a belt between Mars and Jupiter. Rubble left over from planetary construction, asteroids are occasionally nudged by the gravitational interactions of the planets onto trajectories that can pass closer to us.
In 1992, David Jewitt and Jane Luu discovered the first Kuiper belt object beyond the orbit of Neptune. There are now believed to be over 100 billion of them. The discovery of Eris, a Kuiper belt object as big as Pluto, was one reason why Pluto was reclassified as a dwarf planet in 2006. Because Kuiper belt objects form beyond the “snow line”, where it is cold enough for volatiles such as ammonia and carbon dioxide to freeze, they have a different composition to asteroids. The Kuiper belt, and the even more distant Oort cloud, are the source of the icy comets that periodically also pass by Earth.
As with the asteroids and Kuiper belt objects before them, there could be many more interstellar interlopers such as ‘Oumuamua just waiting to be found.
Where did ‘Oumuamua come from?
Working out the origin of an interstellar wanderer such as ‘Oumuamua would provide a great deal of context about it. While we can’t say anything for definite, by tracking its trajectory back and simulating the motions of stars to see which ones it might have passed close to, we can make some educated guesses.
Fabo Feng and Hugh Jones of the University of Hertfordshire, UK, think ‘Oumuamua came from the Local Association, a group of stars associated with the Pleiades star cluster some 440 light years away. Meanwhile, a team led by Eric Gaidos of the University of Hawaii has run calculations that suggest ‘Oumuamua formed in one of the young stellar clusters found in the constellations of Carina and Columba, located in the southern hemisphere sky.
If either proposition is correct – and that’s a big if – then ‘Oumuamua would be relatively young: the stars of the Local Association are no more than 150 million years old, and the young clusters in Carina and Columba are just 45 million years old.
This article appeared in print under the headline “Interstellar interloper”
