Maybe 'Oumuamua wasn't the first visitor from another star system after all.
When the mysterious, stadium-size object sped past our sun in 2017 before disappearing from view, scientists believed they were witnessing a rare event. But a new paper suggests that a pint-sized 'Oumuamua-like object came our way in 2014, briefly blazing as a meteor in the skies over Papua New Guinea.
The paper's authors, Harvard astronomer Avi Loeb and Harvard undergraduate Amir Siraj, took data from a worldwide network of U.S. government sensors designed to scan the skies for missiles and instead searched for meteors moving fast enough to have come from outside our solar system. They found a yardstick-sized object that slammed into Earth at 37 miles per second, and by tracing its path concluded that it came from interstellar space.
The object is thought to have disintegrated before reaching the ground, but its existence raises the possibility that interstellar objects could be studied firsthand. Loeb said the government system could be modified to alert scientists when a fast-moving meteor is detected, so they can search for fragments that survived all the way to the ground.
"It's a new way of looking for interstellar objects," said Loeb, who raised eyebrows in 2018 when he said 'Oumuamua could have been an alien spacecraft. "It saves you the trip. You don't need to go to another planetary system. You get material objects that you can potentially examine."
If confirmed, the meteor's discovery means our solar system was visited by two interstellar objects in a span of just three years. Loeb said this implies there should be a least a million more objects that we can't see whizzing through the inner solar system at any given time, and that an interstellar meteor hits Earth every 10 years.
Seeding life on Earth
Astronomers have long hypothesized that asteroids or comets could have carried to ancient Earth the organic molecules that became the building blocks of life. But there's no rule that says they had to have come from our own solar system.
Asteroids are rockier, while comets are icier. 'Oumuamua looked like an asteroid but moved like a comet, spewing gas in its wake. Once one of these objects enters our atmosphere, it becomes a meteor, and any piece that survives all the way to the ground becomes a meteorite.
The possible discovery of an interstellar meteor raises an intriguing possibility: Scientists might eventually be able to examine interstellar meteorites to determine if they could transport life between star systems.
"We have suspected that panspermia, the seeding of life between planets and planetary systems, could help spread life across our galaxy," said Franck Marchis, a senior planetary astronomer at the SETI Institute in Mountain View, California. "It will be great to get a fragment of [an interstellar meteor] to truly understand the conditions of the long trip between stars."
He added that such a trip could last millions of years, and any organic molecules would have to be well-protected to survive.
Just a plain old meteor?
Not everyone is convinced that the meteor identified by Loeb and Siraj originated outside our solar system. The government won't say how accurate its sensors are, so it's impossible to know how closely the object's actual speed and direction matched what was reported.
"I don't think we can rule out the bound [inside our solar system] trajectories based on the available evidence," California Institute of Technology astronomer Quanzhi Ye said on Twitter. Peter Brown, a meteor astronomer at the University of Western Ontario, called the uncertainties a "huge red flag," adding, "It's very difficult to measure the orbits and the speeds with enough accuracy to say definitively, 'this particular one is interstellar.'"
Loeb and Siraj address the uncertainties in their paper, referring to two previous studies that compared government sensor data with results from known, calibrated sensors. One study found that meteor speeds could be off by as much as 28 percent, but Loeb said it would take a 45 percent speed error for the Papua New Guinea meteor to have originated in the solar system.
Lots of objects
Scientists might eventually be able to use telescopes outfitted with special equipment to watch interstellar meteors burn up in the atmosphere and decode their compositions from the trails of burning gas they leave in their wakes. That could mean more opportunities for scientists who study asteroids and comets to learn about the formation of other star systems.
"We want to be able to understand the way the building blocks of planets formed around our sun," said Michele Bannister, a postdoctoral research fellow at Queen's University Belfast in Northern Ireland. "Did that happen the same way around other stars? That's the fundamental process we're trying to dig into here: How did the building blocks of planets form and grow?"
Bannister said she's not yet convinced the Papua New Guinea meteor came from outside the solar system, "but they're asking the right science question," she said of Loeb and Siraj.
In an unpublished paper, Loeb hypothesizes that interstellar meteors could be a way for aliens to communicate with earthlings, though he admits the idea is far-fetched: "Some of them might even represent defunct technological equipment from alien civilizations, which drifted towards Earth by chance, just like a plastic bottle swept ashore on the background of natural seashells."
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