A pair of researchers who previously identified what may be the first known interstellar meteor to impact Earth have now presented evidence of a second object that could have originated beyond the solar system, before it burned up in our planet’s skies and potentially fell to the surface, according to a new study.
Amir Siraj, a student in astrophysics at Harvard University, and astronomer Avi Loeb, who serves as Harvard’s Frank B. Baird Jr. Professor of Science, suggest that a fast-moving meteor that burst into a fireball hundreds of miles off the coast of Portugal on March 9, 2017, is an “additional interstellar object candidate” that they call interstellar meteor 2 (IM2) in a study posted to the preprint server arXiv this week. The paper has not been peer-reviewed.
In addition to their potential origin beyond the solar system, these objects appear to be extraordinarily robust, as they rank as the first- and third-highest meteors in material strength in a NASA catalog that has collected data about hundreds of fireballs.
“We don't have a large enough sample to say how much stronger interstellar objects are than solar system objects, but we can say that they are stronger,” Siraj said in an email. “The odds of randomly drawing two objects in the top 3 out of 273 is 1 in 10 thousand. And when we look at the specific numbers relative to the distribution of objects, we find that the Gaussian odds are more like 1 in a million.”
This makes IM2 “an outlier in material strength,” Loeb added in a follow-up call with Siraj. “To us, it means that the source is different from planetary systems like the solar system.”
Sign up for Motherboard’s daily newsletter for a regular dose of our original reporting, plus behind-the-scenes content about our biggest stories.
Loeb has attracted widespread attention in recent years over his speculation that the first interstellar object ever identified, known as ‘Oumuamua, was an artifact of alien technology. Spotted in 2017, ‘Oumuamua sped through the solar system and was up to a quarter-mile in scale, making it much larger than the interstellar meteor candidates identified by Siraj and Loeb, which are a few feet across. Loeb’s claims of an artificial origin for ‘Oumuamua have provoked substantial pushback from many scientists who do not consider a technological explanation to be likely.
Loeb also thinks these interstellar meteor candidates could be alien artifacts, though he and Siraj present a mind-boggling natural explanation for the strangely robust objects in the study: The meteors may be a kind of interstellar shrapnel produced by the explosions of large stars, called supernovae.
Previous observations have revealed that the extreme environments within dying stars can produce objects loaded with refractory elements, which are heavy metals, that shoot out of supernovae like cosmic “bullets,” according to the study.
“Supernovae have been observed to produce large chunks of iron-rich matter,” Siraj said. “These overdensities of refractory elements could potentially be where objects like IM1 and IM2 form. There is a much larger mass of refractory elements available from supernovae as opposed to from planetary systems.”
Loeb, of course, is keeping his mind open. “We don't say, necessarily, that it is artificial,” Loeb said in the call, referring to the supernovae explanation. But, he added, “obviously, there is a possibility that a spacecraft was designed to sustain such harsh conditions as passing through the Earth's atmosphere, so we should allow for that.”
As with pretty much everything Loeb does, some in the scientific community are skeptical over his latest claim about a second interstellar meteor possibly falling to Earth.
Hadrien Devillepoix, research associate at Curtin University’s School of Earth and Planetary Sciences, told Motherboard over email that the raw data Loeb and Siraj are pulling from is classified and having even high-level bolide (meteor) data from that database is a “blessing” that’s been helpful in some applications but found to be limited in others.
"Does this mean that those sensors are bad at the job they were designed for? No,” he said. “The satellite sensors behind the CNEOS catalog were presumably designed for detecting rocket and missile launches. Since the average meteor comes in at 10 times the speed of a ballistic missile, it is not surprising that in some cases the speed of these fast meteors can be poorly estimated.”
Analyzing meteor trajectories is hard enough using dedicated instruments, he said, and is an active area of research. “So without a team of meteor trajectory experts looking at the raw data, it is not possible to tell if these bolides came from interstellar trajectories,” he said.
This may well be where the source of such classified data—the U.S. government—comes into play. Loeb and Siraj previously identified an interstellar meteor candidate, called IM1, by examining a small object that burst in the skies near Papua New Guinea after impacting at 130,000 miles per hour in heliocentric speed, meaning its motion relative to the Sun, which is also unusually fast for a meteor. IM2, which was about 13 times more massive than IM1, struck Earth’s atmosphere at about 112,000 miles per hour, a speed that likewise suggests it hurtled into the solar system from an unknown location across interstellar space.
“We checked and found that this object actually had a heliocentric speed at impact of around 50 kilometers per second,” Siraj said in the call. “The first interstellar object,” meaning IM1, “was about 60 kilometers per second, when it hit the Earth, relative to the Sun. That's well above the threshold of around 42 kilometers per second that delineates whether something was bound to the Sun or not.”
The U.S. The Department of Defense (DoD) conducted an independent study of IM1 and “confirmed that the velocity estimate reported to NASA is sufficiently accurate to indicate an interstellar trajectory,” according to a statement released in April 2022. Some scientists remain unconvinced that the evidence presents a slam-dunk case for an interstellar origin for reasons similar to Devillepoix’s.
Loeb and Siraj will present their findings about IM1 in a study that is forthcoming from The Astrophysical Journal. They are also awaiting a response about IM2 from DoD, and organizing efforts to search the seafloors for traces of the exploded objects.
“There are a lot of puzzles here that imply that we will learn something new, no matter what,” Loeb said. “I think in a way it is a celebration of science—we need more evidence and the evidence will guide us.”
