It was unprecedented. In 2017, astronomers discovered the first known interstellar object in our Solar System: 'Oumuamua, a mysterious cigar-shaped enigma, identified as our first visitor from outer, outer space.
But just because 'Oumuamua was the first detected interstellar object, doesn't mean it was the first ever. Just five years ago, in fact, Earth's atmosphere was struck by something that may have originated far outside our own Solar System – and we never even realised it.
In a new paper, a pair of Harvard researchers propose that a meteor that collided with Earth's atmosphere in January 2014 was actually another interstellar traveller with distant, mysterious origins.
But unlike the hurtling 'Oumuamua – which is on a 20,000-year trajectory that will see it eventually exit our Solar System – this meteor's long journey was fated to be a one-way ticket, ending with a fiery finale five years ago, as the object burnt up in the skies above Papua New Guinea.
While 'Oumuamua is a large object, and was detected very far from Earth, the team behind the meteor hypothesis says much smaller interstellar immigrants could be far more commonplace, and potentially exist a lot closer to home.
"Instead of looking far out into space, and given the fact that there should be a higher abundance of interstellar objects smaller than 'Oumuamua, we thought, 'Why not look locally and find these smaller interstellar objects as they collide with the Earth's atmosphere?'" first author, astronomer Amir Siraj told Newsweek.
Working with Harvard astrophysicist Avi Loeb, Siraj pored through a catalogue of meteor impacts kept by NASA's Centre for NEO Studies (CNEOS).
Hidden in the CNEOS data, there lurked a remarkable outlier: a 2014 fireball that rushed Earthwards at a velocity of around 60 kilometres per second (37 mps) as it passed the Sun.
We can be grateful that this object was quite small – less than a metre across in total – because if it were significantly larger, it could have made for a disastrous impact with Earth's surface, rather than the harmless atmospheric fizzle that eventuated.
But that happy anti-climax isn't the primary takeaway of the meteor's blistering speed.
When Siraj and Loeb calculated the meteor's orbital trajectory based on its velocity, their numbers suggested the object wasn't orbitally bound to the Sun: it was travelling so fast before its fiery end, it slipped straight through the Sun's gravitational pull.
For that to be possible, the researchers suggest, the meteor had to originate from somewhere else, far beyond our Solar System.
Per their calculations, the meteor's speed "implies a possible origin from the deep interior of a planetary system or a star in the thick disk of the Milky Way galaxy".
It's a wild idea, but while the findings remain purely hypothetical for now, Siraj and Loeb's paper – which has not yet been peer-reviewed – has been welcomed by some in the astronomy community.
"I think it is reasonable to conclude that this very high speed impactor came from the population of interstellar objects," theoretical astrophysicist Kat Volk from the University of Arizona, who wasn't involved with the study, told National Geographic.
"I expect interstellar objects to be common enough – both from theoretical considerations and from the implications of 'Oumuamua – that I think an interstellar origin is the simplest explanation for this bolide."
But while 'Oumuamua's discovery has definitely shifted the landscape, helping to make real the previously theoretical underpinnings of interstellar migrations, not everybody is yet convinced that this 2014 bolide held such a fantastic passport.
"The result is interesting, but rests upon measurements for a single event," astronomer Eric Mamajek from NASA's Jet Propulsion Laboratory told Science News.
"Was the event a statistical fluke or an actual interstellar meteor?"
The jury is still out on that one, but the hypothetical possibilities in this area of science are as beguiling as an invitation to trek beyond the stars.
Per Siraj and Loeb's calculations, these interstellar bolide events would have already occurred countless times in Earth's history, and by looking out for future visits, we could learn much about these distant travellers' backgrounds.
"Future meteor surveys could flag incoming objects with excess heliocentric velocities for follow-up pre-impact observations," the researchers write.
"Spectroscopy of gaseous debris from these objects as they burn up in the Earth's atmosphere would reveal their composition… Potentially, interstellar meteors could deliver life from another planetary system and mediate panspermia."
Any way you look at it, amazing things are coming our way.
The findings are available on the pre-print website arXiv, and have been submitted to the Astrophysical Journal Letters.