A black-and-white image of Mercury 2025

A black-and-white image of Mercury partially in shadow, captured by NASA’s Messenger spacecraft. (Credit: NASA)NASA

Much smaller and nearer to the Sun than expected, Mercury has long puzzled astronomers as it contradicts most of our understanding about how planets form. A new space mission set to arrive in 2026 may unravel this enigma.

At first glance, Mercury might seem like the Solar System’s most unremarkable planet. Its desolate surface has little of note, there is no sign of past water, and the planet’s thin atmosphere is barely there. The chance of finding life among its scorched craters is zero. However, upon closer inspection, Mercury is a captivating, improbable world wrapped in mystery.

Planetary scientists are still perplexed by the very existence of this closest planet to our Sun. This odd world is extremely small, with a mass 20 times less than Earth’s and barely wider than the continent of Australia. Yet, Mercury is the second densest planet in our Solar System after Earth, owing to a massive, metallic core that constitutes most of its mass.

Mercury’s orbit – clinging tightly to our Sun – is also in a strange location that astronomers cannot fully explain. All of this converges on one crucial point: we do not know how Mercury formed. As far as we can tell, this planet simply should not be.

“It’s somewhat embarrassing,” says Sean Raymond, an expert in planetary formation and dynamics at the University of Bordeaux in France. “There’s some key nuance that we are missing.”