31.08.2023

Controversial astrophysicist says metallic spheres are alien, but others say it is “nonsense”

The evidence connecting spherules like this one to an origin outside the Solar System is weak, researchers say.INTERSTELLAR EXPEDITION

In 2014, a rock from space blazed through the atmosphere and exploded off the coast of Papua New Guinea with such ferociousness that some researchers believe the object came from beyond the Solar System. Now, a team of researchers says it has recovered remnants of the meteor from the floor of the Pacific Ocean and claims that a preliminary analysis of their unusual composition points to an origin around another star.

Only in the past few years have astronomers realized that interstellar objects sometimes whizz through the Solar System and might even hit Earth. Finding a lump of rock from another planetary system would be an unbelievable stroke of scientific fortune, one that could shed light on the formation of alien planets and stars.

Avi Loeb, the controversial theoretical physicist at Harvard University who led the team, believes that’s what his high-risk ocean mission has achieved. “If you don’t allow for surprises, you won’t learn something new,” he says. On 29 August, the team released a preprint describing the claims, which it has submitted to the journal Ocean Science.

But others are dismissive of the preprint, which has not been peer reviewed. Although the geochemical analysis of the debris is solid, the conclusions that Loeb and his colleagues hang on them are “nonsense,” says Martin Schiller, a cosmochemist at the University of Copenhagen. “I’m surprised anyone would take it seriously.” Larry Nittler, a cosmochemist at Arizona State University (ASU), calls it “very weak sauce.”

Loeb has gained notoriety in recent years for his view that ‘Oumuamua, the first interstellar object discovered in 2017, might be an alien spacecraft. In 2021, he launched the Galileo Project, a privately funded effort to use scientific methods to search for evidence of alien technology on or near Earth.

The origin of his latest project lies in his discovery in 2019 that the U.S. Department of Defense’s Space Command keeps a catalog of meteors detected by its surveillance satellitesas they explode on entry into Earth’s atmosphere. Loeb asked his postdoc at the time, Amir Siraj, to study the 272 events in the catalog to gauge whether any of the meteors could have been on a trajectory from outside the Solar System. One seemed to fit the bill. It had plummeted into the atmosphere just north of Manus Island in Papua New Guinea on 8 January 2014 at a speed of 45 kilometers per second—faster than any object orbiting the Sun—before shattering in three explosions.

In 2022, Space Command reported in a letter to NASA that it was 99.999% certain the meteor was from beyond the Solar System. It also released details about the three explosions showing the meteor deeply penetrated the atmosphere before breaking up, which Loeb says indicates it was even tougher than the iron objects from the Solar System that routinely strike Earth. “It was an outlier in material strength,” Loeb says. Space Command located the meteor’s track to an 11-kilometer-wide square of ocean. With the help of a seismometer on Manus, which recorded the explosions, Loeb and Siraj narrowed down its location to a 1-kilometer-wide strip.

Loeb acquired $1.5 million from cryptocurrency entrepreneur Charles Hoskinson to go look for the debris. In June, the M/V Silver Star began to trawl the ocean bottom off Manus. Over 2 weeks, Loeb and colleagues dragged a sled covered with neodymium magnets along the sea floor, 2 kilometers down, hoping the magnets would pick up metallic meteor fragments. After each 8-hour run, the magnets were scraped and vacuumed clean. Loeb says they were lucky to have found anything. “There could have been so many failure points.”

On the ship and back at Harvard, the team picked over the debris with tweezers and found, amid volcanic ash, nearly 700 “spherules,” tiny metallic pellets 1 millimeter or less across. Droplets of once-molten material, spherules routinely form in explosions of ordinary meteors, and also in volcanic eruptions.

Loeb delivered spherules to several labs for compositional analysis, including one run by Harvard geochemist Stein Jacobsen. Jacobsen found that the ratios of iron isotopes in the spherules largely matched those of the Sun—a mark against an interstellar origin. However, five of the spherules were unusually enriched in beryllium and lanthanum, and, to a lesser degree, uranium. This “BeLaU” fingerprint hasn’t been seen in records of known meteoritic spherules, Jacobsen says, although it does resemble some lunar samples. “The fact is we found something unusual that has not been seen.”

Beryllium is rare in the universe, and most is produced when cosmic rays collide with large atoms, chipping off atomic fragments such as beryllium. On a long interstellar journey, an object would have more exposure to cosmic rays than any rock from the Solar System, and more time to produce beryllium, Loeb says. “Is beryllium a flag for interstellar travel?”

The three enriched elements also have an affinity for binding with iron. The authors suggest they may have crystallized from a magma ocean that covered the surface of a primitive celestial body with an iron core.

Jacobsen himself acknowledges the pattern does not on its own indicate that the spherules came from outside the Solar System. Planetary embryos with an iron core could have been numerous in the early days of the Solar System. Many known meteorites are thought to have such an origin, even if their detailed composition isn’t known. “Occam’s razor,” Schiller says. “There is no evidence it comes from outside the Solar System.”

Indeed, the team needs to do more to prove the spherules came from space and are not volcanic, Nittler says. So far, only one of the anomalous spherules has the isotopic pattern in its iron expected if it was heated by a fiery passage through the atmosphere. The paper also did not consider how volcanic eruptions can interact with other rocks to create strange combinations of elements, says Frédéric Moynier, a cosmochemist at the Paris Institute of Planetary Physics. Given the claims, he adds, “I don’t think it would pass any thorough review process.”

Finally, the central pillar of Loeb’s argument that the meteor was interstellar—its high speed—does not seem to be as certain as Space Command’s 99.999% estimate. A new study out this month in The Astrophysical Journal examined 17 known fireballs captured by both classified U.S. sensors and independent observations. The study showed that the government sensors often overestimated speed, with the errors getting worse the faster things got. “A third of the time, the numbers are just way off,” says Steve Desch, an ASU astrophysicist.

Desch says a meteor hitting the atmosphere at 45 kilometers per second would probably be completely vaporized, leaving barely any solid debris at all. He also says the link between the spherules and the 2014 fireball is tenuous. Even if the spherules hit the ocean where Loeb says, they would have likely drifted by tens of kilometers in ocean currents before settling on the seabed, Desch argues. Loeb’s team collected too few control samples from elsewhere on the sea floor to be sure their finds were unusual, Desch says. “They knew what they were looking for and that makes it prone to confirmation bias.”

Jacobsen says the spherules could yield more clues. He wants to look for other isotopic variations in trace elements such as neodymium, which could indicate formation around another star. But given the spherules’ small size—one weighed only 27 micrograms—teasing out that signal could be a challenge.

Jacobsen says he typically does much more work before submitting a paper. “It’s not my style—normally I work on something a bit longer.” But Loeb wanted to do something quickly, “so that’s what we did,” he says.

Quelle: AAAS