Ever since astronomers first detected interstellar comet 3I/ATLAS in July, speculation about its true nature has run wild. Some experts speculate it isn’t a comet at all but rather an extraterrestrial spacecraft sent to sniff around our solar system.
New evidence has dumped cold water on this provocative hypothesis. MeerKAT, a radio telescope operated by the South African Radio Astronomy Observatory, recently detected a radio signal from 3I/ATLAS. Before you get excited, this isn’t a technological radio signal used for transmission. It’s a natural radio emission and some of the strongest evidence yet that 3I/ATLAS is a naturally occurring comet.
The astronomers who identified the signal posted a brief description of their findings on The Astronomer’s Telegram, a website where researchers announce new astronomical discoveries. They explain that MeerKAT detected lines of radio absorption by hydroxyl radicals (OH) at two different frequencies: 1,665 megahertz and 1,667 megahertz. This indicates that 3I/ATLAS was behaving like a normal comet as it zipped around the Sun last month.
D.J. Pisano, a researcher and professor of extragalactic multi-wavelength astronomy at the University of Cape Town, reported the findings alongside several collaborators. The findings have not yet been peer reviewed.
What MeerKAT’s discovery means
Between July and October, astronomers kept a close eye on 3I/ATLAS as it approached the Sun. MeerKAT observed the comet on October 24—just five days before 3I/ATLAS reached its closest point to our home star, also known as perihelion.
The closer a comet is to perihelion, the faster it sublimates due to increasing solar radiation. This is when ice on the comet’s surface rapidly transitions from a solid to a gas without entering an intermediate liquid state. It’s also what gives comets their signature comas and tails.
During sublimation, each frozen water molecule (H2O) on the comet’s surface splits into a hydroxyl radical (OH) and a hydrogen atom (H). Thus, hydroxyl radicals are a product—and therefore an indicator—of cometary sublimation.
If 3I/ATLAS were a metal spacecraft, telescopes wouldn’t detect these molecules. Previous failed attempts to spot them helped fuel speculation that this interstellar object could be technological—a hypothesis first proposed by Harvard astronomer Avi Loeb and colleagues. In a blog post about MeerKAT’s new findings, Loeb acknowledged the new findings and the apparent natural cometary phenomena at play but still did not explicitly rule out a technological explanation.
Not just a comet
Just because 3I/ATLAS is almost definitely a natural comet, that doesn’t mean it isn’t extraordinary. It’s only the third interstellar object ever discovered by astronomers, and its highly unusual characteristics offer a glimpse of the far-off solar system it hails from.
Astronomers have found evidence to suggest that 3I/ATLAS contains one of the highest carbon dioxide to water ratios ever seen in a comet and that it could be older than our solar system. Another preprint study shows that 3I/ATLAS exhibits “extreme negative polarization” that suggests it is a completely new type of comet—unlike any observed before.
3I/ATLAS is now departing our solar system, but astronomers—and several deep-space probes—will still have opportunities to study it before it disappears. The more information scientists gather about this interstellar visitor, the more it seems to surprise us.
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