When `Oumuamua (pronounced: oh-MOO-a-MOO-a) entered our Solar System in 2017, astronomers declared it our very first interstellar visitor.
That's a pretty impressive feat – but was it actually the first?
No. At least, that's the argument put forth by Harvard professor Avi Loeb and undergraduate student Amir Siraj. In a pre-print paper (awaiting peer-review) published on the open access astronomy site arxiv.org, the pair say `Oumuamua was proceeded by another interstellar traveler that flew past Earth on January 6, 2014.
"One would expect a much higher abundance of smaller interstellar objects, with some of them colliding with Earth frequently enough to be noticeable," the researchers wrote.
To try and find out whether or not any of these "smaller interstellar objects" struck past Earth any time recently, the team studied observations logged in the Center for Near-Earth Object Studies (CNEOS) catalog – a record of the geographic coordinates and geocentric velocity components for extra-terrestrial objects detected by US government sensors.
However, in order to escape the gravitational pull of a star and travel from one solar system to another, an object has to travel incredibly quickly. And so, lead author Siraj and Loeb focused on the very fastest objects logged.
The one that caught their eyes was a meteor identified on January 6, 2014, close to Manus Island, Papua New Guinea, which "had an unusually high pre-impact heliocentric velocity". The object was just 0.9 meters (3 feet) wide and was hurtling through the skies at speeds of 216,000 kilometers per hour (134,200 miles per hour), suggesting it was unbound to the Sun.
Using computer simulations, the researchers calculated the meteor's trajectory by tracing its motion back in time. They found "no substantial" gravitational interactions between it and another planet until it reached Earth, implying it has interstellar origins.
"Its size, trajectory, and excess speed exclude the possibility that it was gravitationally scattered within the Solar System prior to impact," the researchers wrote.
Siraj and Loeb noted two other possibly interstellar objects in the data, which covers 30 years' worth of sightings in total. Only one of these had an orbit that appeared gravitationally bound to the Sun and it was hard to actually confirm the interstellar origins of the second.
Still, they continue, assuming that there are indeed three objects with the potential to be interstellar in origin every 30 years, there would be around a million of these objects for every cubic astronomical unit (the distance between Earth and the Sun, roughly 150 million kilometers/93 million miles) in the galaxy. Take this further, they say, and it suggests that each nearby star catapults 60 billion trillion of these objects during its lifetime.
That sounds like a lot. But as NASA points out, scientists think interstellar objects like these pass through the Solar System regularly.
The problem is most are too small and too far away to detect. `Oumuamua was exceptional not just because of its interstellar origins but because of its size, shape and the fact that it was an asteroid and not a comet (as scientists had predicted). Scientists estimate something akin to `Oumuamua crosses Earth's orbit several of times each year. It's just that they have been too faint and so too hard to detect.
As for Siraj and Loeb's paper, it has been submitted to The Astrophysical Journal Letters but is yet to be peer-reviewed.