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We May Have Had an Interstellar Visitor for Eons and Scientists Are Stumped

"I think it's going to be inconclusive, if not impossible to demonstrate conclusively," one physicist said of the possible interstellar comet.
We May Have Had an Interstellar Visitor for Eons and Scientists Are Stumped
Image: NASA/ STEREO
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A comet that may have come from interstellar space just made its latest dramatic swing around the Sun, an encounter that was imaged in mesmerizing detail by a space observatory. 

The origins of Comet 96P/Machholz (96P) have puzzled scientists for decades since its discovery. It is a four-mile-wide “sungrazer” object with a host of weird properties that suggest it may be an interloper from another star system. For instance, 96P’s composition is extremely unique and its orbit is highly tilted, causing it to pass closer to the Sun than almost any other comet. 

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These features, among others, suggest that 96P may have been rerouted into our solar system by a chance encounter with Jupiter after its voyage across interstellar space. In an ironic twist, however, the comet's interactions with Jupiter have obscured its backstory and raised the possibility that its oddities could simply point to an origin in an unusual part of our own solar system, or as part of a local celestial body that broke apart.  

“Because of its interactions with Jupiter, I think it's going to be inconclusive, if not impossible to demonstrate conclusively, that it came from another solar system because that dynamical history is lost,” said Matthew Knight, a professor in the physics department at United States Naval Academy who has published research about 96P, in a call with Motherboard.

For the time being, we’ll have to make do with the incredible images of the comet’s closest passes around the Sun, known as the “perihelions” of its five-year orbit. The comet completed its most recent perihelion on Tuesday, when it passed about three times closer to the Sun than Mercury. The approach was observed by the Solar and Heliospheric Observatory (SOHO), a joint space mission between NASA and the European Space Agency, which has watched 96P’s perihelions since it was first launched 1995.  

96P was first discovered by the amateur astronomer Donald Machholz, who spotted it with binoculars from California in 1986. More recent observations have revealed that the object is bizarrely depleted in common materials, like carbon and cyanogen, and does not appear to possess the dusty atmosphere, or “coma,” seen around other comets in the solar system.  

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These features hint that the comet might be a completely stripped down nucleus with a “primordial and possibly…interstellar origin,” according to a 2019 study in Astrophysical Journal, co-authored by Knight, that was based on observations of 96P’s 2017 perihelion.

Those observations also revealed that the comet’s colors appear similar to those of ‘Oumuamua, which was the first unambiguous interstellar object ever seen in the solar system. ‘Oumuamua hurtled through our system in late 2017, and also displayed weird features and motions that led to some speculation that it was a piece of alien technology, though most scientists think it had a natural origin.

The intriguing similarities “between the nucleus of 96P and 'Oumuamua might strengthen the argument for 96P having had an interstellar origin” meaning that its “surface properties were acquired during millions of years of cosmic-ray bombardment between systems, rather than having been due to evolution in our solar system,” according to the 2019 study. 

Ultimately, though, Knight said the mystery of 96P’s origin may never be solved, in part because it is not the easiest object to study. Its angled orbit never takes it particularly close to Earth, and its lack of activity makes it tricky to observe except when it's close to perihelion. Though space observatories such as SOHO can study it, many of the comet’s secrets, including its origin, are still locked away.

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Even if we were to launch an intercept mission to the comet, we might not be able to answer tantalizing questions about the comet’s past.

“I think [an intercept mission] can tell you that it's very strange,” Knight said of 96P. “I don't think it would conclusively tell you if it was interstellar, or not. But it would certainly be fascinating to go and see this object. I'd love to see it up close.”

Regardless of whether we learn more about the backstory of this elusive comet, scientists are likely to spot many more interstellar objects in the coming years, following the discovery of the first known interstellar comet, called 2I/Borisov, and the first known meteors that likely traveled from another star system before hitting Earth. Many more of these cosmic interlopers are expected to be spotted by next-generation telescopes, such as the Vera C. Rubin Observatory in Chile.

“Right now, we have such a small number of statistics that things are all over the map,” Knight said. “Borisov is kind of what we expected, but ‘Oumuamua is just so funky that it threw us off our game, so to speak.” 

“The work that came out of ‘Oumuamua in particular pegged the number that we might find in the Vera Rubin era at probably about one a year,” he concluded. “I think it'll be really fascinating to see—as interstellar comets number three, four, or five, and so on, come in—where they're falling in that spectrum between crazy and sort of ho-hum.”