It is possible that the Earth has already been visited by an object external to our solar system, a meteor that burned in the atmosphere of the planet in 2014, say the astronomers. If confirmed, it would be the first known interstellar object to have entered the atmosphere.
The first known interstellar visitor to the Earth was the approximately 400 meter wide asteroid called 'Oumuamua. It sank some 24 million kilometers from the planet in October 2017 (SN: 11/25/17, p. 14). Their sharp angle approach to the solar system and their equally strange departure led astronomers to suggest that "Oumuamua could have been anything from a spongy skeleton of a comet to an alien spacecraft (SN Online: 02/27/19).
If there were an interstellar intruder, the astronomers reasoned, there would probably be more, including some who collided with Earth.
So the astronomer Avi Loeb and the university student Amir Siraj, both from Harvard University, looked in a NASA catalog of meteors that burned in the Earth's atmosphere to see if any had taken a trajectory similar to that of Oumuamua
The pair identified a 0.9-meter wide object that disintegrated in January 2014 in the sky over the South Pacific, off the northern coast of Papua New Guinea. The meteor had approached the sun at a speed of 60 kilometers per second, suggesting that it was not limited by the sun's gravity. The execution of that meteor's orbit over time shows that the object probably originated outside the solar system, possibly in the inner part of another planetary system in the thick disk of the Milky Way, astronomers report online April 15. in arXiv.org.
That origin could mean that the object came from the habitable zone of another star, the region around a star where temperatures are adequate for liquid water, and perhaps life. "If an interstellar object comes from another planetary system, it can bring life to the solar system from the outside," says Loeb.
This particular object was so small that it burned in the Earth's atmosphere, so it could not have sent microbes to the surface of the Earth, the team says. But as the duo found only one interstellar meteor in a database spanning decades, Loeb and Siraj estimate that Earth could be reached by one every 10 years. That would mean that some 450 million interstellar meteors could have hit the Earth throughout its history of approximately 4.5 billion years. "We do not need to bring life once a decade, we only need it once for a few billion years," says Loeb.
If scientists can identify one of these visitors before they enter the Earth's atmosphere, they might discover its composition by studying the light of the meteorite as it burns. "In hindsight, it's obvious that this should be a very good way to find an interstellar object and learn about its composition," says Loeb.
This is not the first time that astronomers have been searching for interstellar meteors, says astronomer Eric Mamajek, who is not convinced that the 2014 find is the real deal.
"The result is interesting, but it's based on measurements for a single event," says Mamajek, of NASA's Jet Propulsion Laboratory and Caltech in Pasadena, California. "Was the event a statistical coincidence or an actual interstellar meteorite?" The answer seems to be inaccessible government sensors or a fine spray of powdered dust that fell on the Pacific. "