“A11pl3Z”: Why this third interstellar object fascinates astronomers

An interstellar object was detected on the evening of Tuesday, July 1st, at the same distance from Earth as Jupiter (600 million kilometers!). The discovery was made in Hawaii by NASA astronomers belonging to the ATLAS asteroid monitoring project. Named A11pl3Z, it becomes the third object of this type observed by a human being.

Its speed, close to 60 kilometers per second, and its trajectory indicate that the object "is not in orbit around the Sun, but comes from interstellar space and will return there," according to the head of planetary defense at the European Space Agency (ESA), Richard Moissl.

It has been classified as a comet by the International Astronomical Union's Minor Planet Center. This is because it is not a dead body, as is the case with asteroids, but rather an object with an active nucleus, composed partly of ice. But in many ways, this comet differs from those we know, which notably cause meteor showers in summer, one of the most famous of which is Halley's Comet.

One of the major differences concerns its origin. The best-known comets come from the outer reaches of our solar system. Beyond the Kuiper Belt, where Pluto is located, they cluster in the Oort Cloud. This region is more than 5,000 astronomical units (AU) from Earth, or 5,000 times the distance between Earth and the Sun (about 150 million kilometers). A11pl3Z comes from even further away, beyond the Oort Cloud, more than 100,000 AU.

Nor is it an extrasolar comet, which orbits a star, but rather a body located between two systems. Thus plunged into darkness, the comet can only be visible when it enters our solar system. This explains why it is only the third object of this type to have been observed, after Oumuamua in 2017 and 2I/Borisov two years later.

The appearance of Oumuamua, which means "messenger" in Hawaiian, gave rise to several hypotheses about its origin. It was very long and flattened, about 150 meters long, 100 wide, and 20 deep, and did not contain luminous dust, like most comets. Above all, its speed was much higher, estimated at 87 kilometers per second, and its trajectory had changed as if to move away from the Sun.

Several scientists had seriously considered the possibility of an extraterrestrial vessel. Subsequent studies proved that it was indeed an interstellar comet. How then can we explain this astonishing behavior? The answer lies in the object's composition.

Like all comets, Oumuamua has a nucleus composed of ice. As the media outlet Futura-sciences explains, cosmic rays produced by the Sun convert the water molecules present in the ice into hydrogen. This gas remains trapped in the ice and then bursts forth as the comet approaches the Sun. As it breaks free, it produces a thrust that accelerates the comet's speed and thus modifies its orbit.

A11pl3Z is thought to be larger than Oumuamua, and likely the largest such object ever observed. The comet is thought to be between 10 and 20 km in diameter, but could be smaller if it is indeed composed of ice, which reflects more light and thus gives an impression of size. Jonathan McDowell, an astronomer at the Harvard-Smithsonian Center for Astrophysics, believes " it's mostly ice rather than rock."

This comet will be observable until next year. This is a boon for astronomers, who will therefore have more time to study it. Observations could make it possible to detect precursors of life, such as amino acids. According to Mark Norris, an astronomer at the University of Central Lancashire in the UK, this would give researchers "much more confidence that the conditions for the emergence of life exist in other star systems."

The scientist also assures that the new Vera C. Rubin Observatory, located in Chile, will soon be able to discover interstellar bodies every month. This will provide a way to learn more about these objects from elsewhere, which are still largely unknown.