Mercury: The Planet That Shouldn't Exist

Far smaller and closer to the Sun than it should be, Mercury has long baffled astronomers because it defies much of what we know about planet formation. A new space mission arriving in 2026 might solve the mystery.

At a cursory glance, Mercury might well be the Solar System's dullest planet. Its barren surface has few notable features, there is no evidence of water in its past and the planet's wispy atmosphere is tenuous at best. The likelihood of life being found amidst its scotched craters is non-existent. Yet, look closer and Mercury is a fascinating, improbable world that is shrouded in mystery.

Planetary scientists remain flummoxed by the very existence of the closest planet to our Sun. This peculiar planet is tiny, 20 times less massive than Earth and barely wider than Australia. Yet Mercury is the second densest planet in our Solar System after Earth due to a large, metallic core that accounts for the majority of its mass.

Mercury's orbit – hugging close to our Sun – is also in a weird location that astronomers can't quite explain. All this ties together into one crucial point – we have no idea how Mercury formed. As far as we can tell, the planet simply shouldn't exist.

"It's kind of embarrassing," says Sean Raymond, an expert in planetary formation and dynamics at the University of Bordeaux in France. "There's some key subtlety that we're missing."

The mystery of where Mercury came from, how it formed and why it looks like it does today is one of the grandest mysteries in the Solar System.

Some answers, however, might be on the horizon. A joint European and Japanese mission called BepiColombo launched in 2018 and is currently on its way to Mercury. The probe will be our first visitor to the planet in more than a decade. When it enters orbit in November 2026, after a thruster problem delayed its journey, one of its key goals is to try and work exactly where Mercury came from.

Working out how Mercury formed is not just important for understanding more about the origins of our own Solar System, but also for studying planets around other stars – exoplanets – too.

"Mercury is probably the closest planet that we have to an exoplanet", due to its unusual formation, says Saverio Cambioni, a planetary scientist at the Massachusetts Institute of Technology in the US. "It's a fascinating world."

Astronomers first realised something was wrong with Mercury after Nasa's Mariner 10 spacecraft flew past the planet three times in 1974 and 1975, which were humanity's first visits to the Solar System's innermost world. Those flybys provided initial gravity measurements of the planet, providing a glimpse inside Mercury for the first time and revealing its bizarre innards.

Earth, Venus and Mars all have iron-rich cores that make up about half of their radius. On Earth, this is separated into a solid inner core and liquid outer core, which churns to produce our world's protective magnetic field. Above is the mantle and then the crust, where we live.

Mercury is completely different. Here the planet's core makes up about 85% of its radius, with only a thin rocky mantle and crust on top. This is what lies behind the planet's incredible density, but why its structure ended up like this isn't entirely clear. "The formation of Mercury is a major problem," says Nicola Tosi, a planetary scientist at the German Aerospace Centre in Berlin. "It's still unclear why Mercury looks like it does."

A later mission to Mercury, Nasa's Messenger mission that orbited the world between 2011 and 2015, only raised more questions. Orbiting just 36 million miles (60 million km) from the Sun, temperatures during the day on Mercury can reach highs of up to 430C (800F) while at night they can plunge as low as -180C (-290F).

Yet despite these extreme temperatures, Messenger found that Mercury had volatile elements such as potassium and radioactive thorium on its surface, which should have been boiled away by the Sun's radiation long ago. Complex molecules such as chlorine and even water ice trapped in the planet's shadowed polar craters, were also revealed to be hiding on the planet's surface.

- Author: Jonathan O'Callaghan, BBC

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