Not from around here

36 | NewScientist | 11 June 2016

Not from around here Some unexplained alien interlopers could disturb our cosy story of the solar system’s origins, says astronomer Simon Portegies Zwart

Renaud Vigourt

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OOK up at the sky on a dark, clear night and you will see the moon, a few planets and many stars. Without a large telescope, you will not notice the asteroid belt, the band of icy rock that girdles the sun between Mars and Jupiter. The clouds of rubble lying far beyond the most distant planet, Neptune, are entirely invisible, except perhaps for the pinprick of light that is Pluto. This is the solar system’s liminal zone, where it peters out into interstellar space. Here, in the Kuiper belt that houses Pluto and even further out in the Oort cloud, which stretches a substantial part of the way to the next star, there may be more bodies than there are stars in the entire Milky Way. Here, too, are the answers to mysteries surrounding how our cosmic neighbourhood came to be. As yet, we know little: our first foray into these chilly climes was the fly-by of Pluto by the New Horizons spacecraft last July. But bit by bit, using more indirect methods, we are building up a picture of what is – and is not –

out there. I believe that what is being revealed requires a fundamental rethink of the solar system’s origins, and even of what a solar system is. Put simply, our solar system might not be entirely ours at all. We have a pretty settled, if rudimentary, picture of how the solar system formed. An outside disturbance – generally thought to be a nearby supernova – caused a cloud of dust and gas to start collapsing in on itself. This cloud began to spin faster, and its centre ignited to form the sun. The leftover material settled into a disc rotating around the new star’s midriff, from which, over time, bigger and bigger clumps of rock condensed. In the inner reaches of the solar system, the result was the rocky planets: Mercury, Venus, Earth, Mars. Further out, colder temperatures meant more material condensed, and the gas giants formed: Jupiter, Saturn, Uranus, Neptune. Further out still, the density of material was low and there was probably no chance of massive planets forming; relatively small lumps of ice and rock known as planetesimals were the limit of the achievable. This material formed the Kuiper belt – more properly the Edgeworth-Kuiper belt, as its existence was independently proposed around 1950 by the Irishman Kenneth Edgeworth and the Dutch-American Gerard Kuiper. It lies outside Neptune’s orbit at 30 AU (1 AU, or astronomical unit, is the distance of Earth from the sun), and extends to perhaps 40 AU. The first object discovered there – besides Pluto and its moon Charon – showed up in 1992, and is still known only as (15760)

1992 QB1. Today, we have charted the orbits of more than 1000 Kuiper belt objects. The standard model of the solar system’s formation suggests no reason for the Kuiper belt to stop where it does, at the “Kuiper cliff” some 40 AU out. Yet only much further away, starting perhaps a few thousand AU out, do things possibly start to become a little more crowded in the Oort cloud. Its existence was hypothesised in 1950 by a predecessor of mine at Leiden Observatory, Jan Oort, although the Estonian Ernst Öpik had vaguely floated a similar idea in 1932. The Oort cloud has never been seen. The justification for it remains Oort’s original one: that “long-period” comets, swinging by Earth and the sun perhaps once every few hundred years, must come from somewhere. Hale-Bopp, the great comet of 1997, is the most prominent recent example.

Eccentric orbits My story really starts, however, not in the Kuiper belt or the Oort cloud, but in that mysterious gap between the two. The longperiod comets are evidence that the orbits of smaller bodies in the Oort cloud are not static. The tug of nearby stars and fluctuations in the galaxy’s gravitational pull disturb them, sometimes slingshotting them towards the inner solar system. Something similar is true of bodies in the Kuiper belt. Typically perturbed by the outermost giant planets, Uranus and Neptune, they adopt inclined, highly elliptical – “eccentric” – orbits that tend to end up in sync with the giants’ orbits. The dwarf planet Eris > 11 June 2016 | NewScientist | 37

Alien interloper The strangely elongated, inclined orbits of Sedna and about a dozen other bodies discovered between the Kuiper belt and Oort cloud since 2003 suggest they might originate outside the solar system

SEDNA Orbital period: 11,400 years Closest distance to the sun: 76 AU Furthest distance from the sun: 940 AU

PLUTO Orbital period: 248 years Closest distance to the sun: 30 AU Furthest distance from the sun: 49 AU

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OORT CLOUD (never directly observed)

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Distance (Astronomical units: 1 AU = Earth-sun distance) Long-period comets occasionally observed in the inner solar system are thought to originate in the Oort cloud

and its tiny moon Dysnomia – the names of a mother and daughter in Greek mythology – are examples of objects with wide, eccentric orbits as a result of being bullied around by Neptune. Such ructions, incidentally, make these bodies rather relevant for life. They may have made Earth habitable by bringing water inwards: when Earth formed, it would have been so hot that any water would boil off. One class of meteorites is rich in nucleobases, a building block of DNA, and may even have fertilised Earth. On the other hand, comets threaten life. Probably one of them did for the dinosaurs 65 million years ago: a crater as large as Chicxulub off the coast of the Yucatán peninsula in Mexico, which dates from that time, is best explained by the impact of a fastmoving object such as a comet. In the zone between the Kuiper belt and Oort cloud, bodies are far enough away from the sun and the giant planets on the one hand,

and the nearby stars on the other, that their orbits would remain unperturbed. Only patient observation with telescopes can reveal anything in this region, and we saw nothing until November 2003. Then, Mike Brown of the California Institute of Technology in Pasadena and his team discovered the dwarf planet Sedna. Sedna is considerably smaller than the moon but hugely more reflective: it would be almost as bright as the full moon if it were the moon’s distance away. Being 30,000 times further away at present, it is very hard to spot, and it moves so slowly that it hardly stands out among the stars. In a sense, finding Sedna was a lucky shot: if its surface were as dark as that of a normal asteroid, it probably would have remained invisible. Sedna’s orbit is curious (see diagram, above). It is very elongated, getting as close as 76 AU from the sun but extending out to over 900 AU at its furthest. We can see it only when

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it is close, and recent estimates have suggested that there could be some 500 Sedna-like objects awaiting detection. We have found about a dozen such bodies  – and with them a mightily strange problem. They all orbit in practically the same plane, but it is not the same as the plane occupied by the solar system’s major planets. What’s more, viewed from the sun, their points of closest approach all lie in roughly the same direction. So they can’t have been booted out of the Kuiper belt in the solar system’s earliest days, because that would have randomised both the inclination and their direction of closest approach. A similar problem means they cannot have come from the Oort cloud.

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Attempts to explain this coincidence have led to the current hype about an unseen “planet IX”, with a gravitational pull that might keep these orbits aligned. My work suggests a different conclusion: Sedna and its family did not originally belong to this solar system at all. I came at this problem from a rather odd angle. Until a few years ago, I was not the least bit interested in the solar system. I was a computational astrophysicist studying the dynamics of black holes and star clusters, and I regarded the sun as a single, unexceptional star in a nondescript corner of the Milky Way. During the Christmas holidays of 2008, a sudden thought made me begin to revise my opinion. Star clusters reveal that stars are not born in isolation, but in litters of perhaps thousands as the shock wave of a supernova shakes up its immediate environment. Eventually, buoyed by the gravitational tides of the Milky Way, these stars bob their separate ways. The sun may be a single star now, but was not when it was born. And the gravitational jostling between the sun’s siblings would have left its mark on the early solar system – something standard models fail to take into account. My colleague Lucie Jílková and I set out to change that. An encounter between two stars is a deterministic process, meaning that one can precisely calculate their trajectories from first principles – in this case Newton’s laws of motion – just as, in forensic science, the trajectory of bullets can be traced back from the point of impact to where the gun was fired. Unfortunately, the sheer number of surrounding planetesimals needed to make a realistic simulation complicates the problem considerably: it becomes more like tracing the bullet trajectories from two machine-gun-

wielding gangsters shooting at each other while running. But take the solar system as it looks today as the desired end point of a computer simulation, and you can begin to characterise what early close encounters might have led to it. Working out the details is still a mammoth undertaking, involving some 16 parameters, such as the stars’ masses and angle of approach, that can vary independently. Solving this requires not just computing power, but algorithms that “learn” which combinations of parameters produce the closest fit to today’s solar system, and use those as the basis for the next stage of the search. We have been working on this problem for the past couple of years, culminating in a calculation that lasted weeks on dozens of workstations crunching along in parallel. What emerges is a close brush between our solar system and that of a star almost twice as massive as the sun. Its disc of rubble extended to beyond 160 AU, and it approached at 4.3 kilometres per second to within 230 AU of the sun. In cosmic terms, that is scarily close, although luckily not close enough to have upset the orbits of the solar system’s main planets. For smaller bodies, the jolt was felt as far in as about 40 AU, ripping out any planetesimals beyond that point into

WALTER PACHOLKA, ASTROPICS/SCIENCE PHOTO LIBRARY

“Sedna and its family did not originally belong to the solar system at all” interstellar space – in other words, producing the Kuiper cliff (Monthly Notices of the Royal Astronomical Society, vol 453, p 3157). Apart from having reshaped the solar system’s outer regions, the simulations show that more than 2000 planetesimals orbiting the other star would have become bound to the young sun, about half ending up in orbits similar to that of Sedna. Most of these bodies were probably considerably smaller and dimmer than Sedna, making them even harder to find now. If this idea is right, Sedna’s name would be strangely appropriate. The Inuit girl after whom it was named was supposedly abducted by a gull-like bird god after her husband abandoned her on a cold, deserted beach – perhaps not so different from the solar system’s frozen outer reaches where the celestial Sedna was found. Around 500 more foreign bodies would have been deposited further out, between

around 1000 AU and 5000 AU in the Oort cloud. The remainder would wind up within Neptune’s orbit. These innermost interlopers would have been scattered by the giant planets – most of them probably out of the solar system once more, but some perhaps our way. It is exciting to speculate that the meteorite collections of the Natural History Museum in London or the Smithsonian Museum in Washington DC, or even, in my neck of the woods, the Teylers Museum in Haarlem, might contain material that originated in another solar system. How the proximity of a more massive star might have affected mineral crystallisation in ways we could conclusively identify is a research project ripe for adoption. How else might we find more proof to back up what the simulations appear to be saying? Many hopes are pinned on the Gaia satellite, launched in December 2013 by the European Space Agency and soon to start mapping a billion stars in our quadrant of the galaxy. Naively, we might expect that stars born in the same cluster will have similar chemical compositions and be moving in similar ways. If so, Gaia may be able to identify our sun’s siblings and pinpoint any that might have disturbed its early development. Gaia’s keen eye will also be able to spy out objects in the inner Oort cloud directly for the first time. Should it or other surveys, such as the US PanSTARRS and Japanese Hyper Suprime-Cam projects, continue to find clumpings of highly anomalous orbits, that could provide

The appearance of comets such as Hale-Bopp suggest the solar system extends far further than we can see

important substantiating evidence. Apart from delivering material into the solar system, the star we brushed up against must have captured something from us. The number of bodies depends on the size of the sun’s planetesimal disc at the time: if it extended out to 90 AU, it would have been an equal swap of around 2000 objects each. It is fanciful to suggest we might spot these objects in orbit around another star given current observational capabilities – but one day, who knows? In any case, these particular hostages have probably long since been released. Being more massive, that other star surely has already burned itself out through a redgiant phase into a white dwarf. In that case any planetesimals in wide orbits would become unbound, free-floating in the dark and cold space between the stars. A similar fate is expected for Sedna when the sun becomes a white dwarf billions of years hence. In Inuit mythology, Sedna eventually drowns in a cold, bottomless ocean. Our Sedna might again become a wanderer between solar systems – and perhaps ultimately an alien intruder in a second solar system. n Simon Portegies Zwart is an astronomer at Leiden Observatory in the Netherlands 11 June 2016 | NewScientist | 39