Ask Onix
Mercury: The Solar System's baffling outlier
Mercury, the smallest and innermost planet in our Solar System, has long puzzled scientists with its unusual composition and orbit. A forthcoming space mission aims to shed light on how this enigmatic world came to exist.
A planet that defies expectations
At first glance, Mercury appears unremarkable-a barren, cratered sphere with no water, a thin atmosphere, and no signs of life. Yet its very existence challenges our understanding of planetary formation. Despite being 20 times less massive than Earth and barely larger than Australia, Mercury is the second densest planet in the Solar System, thanks to a massive metallic core that makes up 85% of its radius.
Its orbit, hugging the Sun at a distance where no planet should easily form, adds to the mystery. Planetary scientists have struggled to explain how Mercury ended up in its current state. "It's kind of embarrassing," admits Sean Raymond, a planetary formation expert at the University of Bordeaux. "There's some key subtlety that we're missing."
Theories behind Mercury's formation
Astronomers have proposed several hypotheses to explain Mercury's origins, none of which fully account for its peculiarities. One leading theory suggests that Mercury was once a much larger planet, possibly twice its current size, before a catastrophic collision with another Mars-sized body stripped away its outer layers, leaving only its dense core.
However, this scenario raises questions. Such a violent impact should have vaporized volatile elements like potassium and thorium, yet NASA's Messenger mission detected these compounds on Mercury's surface. "Something that close to the Sun shouldn't be rich in volatiles," notes David Rothery, a planetary geoscientist at The Open University.
Alternative theories propose that Mercury formed from iron-rich material near the Sun or that it migrated from a different region of the Solar System. Some researchers even speculate that Mercury might be the remnant core of a gas giant whose atmosphere was stripped away-though this idea is considered unlikely due to the immense gravity of such planets.
BepiColombo: A mission to unlock Mercury's secrets
A joint European-Japanese mission, BepiColombo, launched in 2018 and is set to arrive at Mercury in November 2026 after a thruster issue delayed its journey. The mission consists of two spacecraft that will separate upon arrival to study the planet's surface composition, gravity, and magnetic field.
"BepiColombo will perform additional measurements that can tell us about the origin of the planet," says Nicola Tosi, a planetary scientist at the German Aerospace Centre. Key objectives include analyzing Mercury's surface for signs of a past magma ocean-a potential remnant of a violent collision-and refining our understanding of its core structure.
Early images from the spacecraft's flyby have already revealed a surface marked by ancient lava flows and dramatic contraction as the planet cooled over billions of years. These observations could provide crucial clues about Mercury's formation history.
Broader implications for planetary science
Understanding Mercury's origins isn't just about solving a Solar System mystery-it could also inform the study of exoplanets. "Mercury is probably the closest planet that we have to an exoplanet," says Saverio Cambioni of MIT, given its unusual characteristics.
Observations of other star systems suggest that iron-rich "Super Mercuries" are relatively common, accounting for 10-20% of known exoplanets. Yet their formation remains just as puzzling. If Mercury's origins are better understood, it could help scientists interpret similar worlds beyond our Solar System.
Future exploration and unanswered questions
While BepiColombo promises to deliver new insights, some scientists dream of even more ambitious missions, such as landing on Mercury or returning samples to Earth. "What we really want is a sample of Mercury," says Rothery, though no such mission is currently planned.
In the meantime, researchers are investigating whether rare meteorites called aubrites might be fragments of a proto-Mercury. Camille Cartier, a petrologist at the University of Lorraine, is leading a study to determine if these meteorites share Mercury's chemical signatures. "We should have strong evidence in favor or not of this hypothesis," she says.
For now, Mercury remains an enigma-a tiny, dense world that shouldn't exist but does. Its secrets could redefine our understanding of how planets form, both in our Solar System and beyond.
