Source: Newsweek

Avi Loeb, who is Harvard’s Frank B. Baird Jr. professor of science, made global headlines in 2017 after announcing the discovery of ‘Oumuamua—the first interstellar object found in our solar system. Now, Loeb and his student Amir Siraj have found evidence of an interstellar meteor. Their findings, which have been submitted for publication in The Astrophysical Journal Letters, indicate there could be many other interstellar objects in our solar system, with some hitting Earth.

Loeb told Newsweek he had been preparing for an interview when he was asked about a meteor. This prompted him to start looking into literature on meteors, and he found a catalog spanning the past three decades. From this, he asked Siraj to look at the orbits of the fastest meteors. He measured three components of velocity, three components of position and the time of the impact.

“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?” Siraj said.

As a result, they discovered a meteor from 2014 that, because of its orbit and speed, appeared to originate from a region of space outside the solar system. It was traveling at about 37 miles per second and, after tracing the orbit, they suggest it came from the deep interior of a planetary system, or from a star in the disk of the Milky Way. 

“We found that one of these meteors…had to have been traveling extremely fast in order to hit the Earth at the direction and speed that it did,” Siraj said. “In fact, it was traveling so fast that it must have been unbound from the solar system—meaning that it, like ‘Oumuamua, originated from outside of the solar system.”

Such a high ejection speed could be achieved only if an object was coming from the innermost cores of planetary systems, Loeb explained. For sun-like stars, this would be an orbit within that of Mercury, while for dwarf stars it would be from within the habitable zone.

“This is a new way to learn about interstellar objects,” Loeb said. “Traditionally, scientists have used the sun as a ‘lamppost’ and then look for objects based on their reflected sunlight. This is how ‘Oumuamua was detected by Pan-STARRS, which is effective for objects larger than 100 meters in size.” 

The meteor, however, was much smaller—about a meter in diameter, with an approximate mass of 500 kilograms. Loeb said interstellar meteors probably hit Earth about once every 10 years, but this is a lower limit because the detection method loses efficiency at high speeds.

The discovery, Siraj noted, raises the possibility that there may be meteorites—meteors that survive passage through Earth’s atmosphere—that come from beyond our solar system. A meteorite like this, he said, would have isotope ratios (a chemical makeup) that would be different from ones coming from within the solar system.

“This paper is exciting because this meteor is the second object discovered that originated from outside of our solar system and the first such object discovered to have collided with the Earth,” Siraj continued. “Its detection challenges current theories of planetary system formation, and its unusual origin has untold implications for better understanding planetary systems outside of our own.”  

He added, “Perhaps most exciting, however, is the fact that this paper promotes a new way of searching for interstellar objects that could lead to important discoveries in the future.”

Loeb said the detection method could result in an alert system that could lead to observations of interstellar meteors. Because of the speeds these objects would be traveling, this could be fairly straightforward, he said.

Siraj said they now hope to continue investigating the population of interstellar objects “to better understand worlds different from our own.”

Not everyone is convinced, however. Alan Fitzsimmons of the Astrophysics Research Centre at Queen’s University Belfast in Northern Island, told Newsweek: “This would be a very interesting object if it can be confirmed that the resulting fireball was indeed interstellar. Unfortunately, the paper does not give any uncertainties on the speed and trajectory of the fireball.”

He continued: “There are many meteors seen with apparently interstellar trajectories. But pretty much all of them are consistent with belonging to our solar system, if you take into account the problem of measuring something for maybe a second and trying to track its trajectory back through the solar system. So my opinion is ‘not proven.'”