Mercury in Retrogade? 2026 Space Mission Will Unveil Whether The Planet Should Exist

A European-Japanese space mission will reach Mercury in 2026, carrying instruments designed to answer a question that has quietly unsettled planetary science for decades: how does a planet like this exist at all?

Small, metal-rich, and orbiting perilously close to the Sun, Mercury defies the standard models that explain how rocky worlds form.

The mission, known as BepiColombo, will provide the most detailed look yet at the planet's interior, surface, and magnetic field, potentially forcing scientists to rethink how planets are built.

Mercury is a cold, desolate planet that appears to have little surface, no sign of water, and a thin atmosphere that can be observed. Its high temperatures, up to 430 °C during the day, and extremely low temperatures, down to -180 °C at night, make it appear hostile and desolate, with a surface covered with craters and ancient lava flows.

However, the depth beneath this simple surface is at odds with established planetary physics and raises doubts about whether it exists.

It's A Paradox, An Enigma Itself

Although it is the most minor adopted planet, with less than 20% of Earth's mass, and the closest to the Sun, Mercury is the second-densest planet mainly because it has a large, metallic core that accounts for nearly 85% of its radius.

The dominance of this core is also odd. The majority of rocky planets, such as Earth and Venus, are much more balanced internally, with a large rocky mantle. This unusual formation suggests an extraordinary history of formation, one that scientists are keen to know.

Theories of Mercury's origin struggle to explain the planet's current state within existing models of planetary formation.

According to conventional accretion theories, planets form in a protoplanetary disk of dust and gas, which gradually accumulates mass through collisions and mergers.

The proximity of Mercury to the Sun is a paradox because the planet's composition and size do not fit this paradigm, and given that the conditions in the inner Solar System could hardly have resulted in the formation of such a small, metal-rich planet so close to the Sun.

Some of the earliest information came from the NASA Mariner 10 spacecraft in the 1970s, which revealed a dense core of Mercury and a strange internal structure.

Subsequently, a more detailed consideration was made in the Messenger mission (2011-2015), which identified volatile elements (such as potassium and thorium) on the surface materials that at that early time should have been vaporised by the intense solar radiation, which casts doubt on the early history and surface composition of Mercury.

The presence of water ice in the shadowed polar craters also raised questions about existing theories, suggesting that Mercury might have had processes or origins capable of supporting the retention of surface-bound volatiles despite its proximity to the Sun.

Several Theories and Debates Emerge

Several theories have been developed to describe the oddities of Mercury. One of the most notable theories is that Mercury was originally a larger planet, perhaps comparable to Mars, that was once located farther from the protoplanetary disc, in a colder region.

What could have caused such an enormous loss of its surface early in its history? A massive collision might have removed its outer layers, leaving it to appear like a heavy-metal core as it does now.

This giant-impact scenario is consistent with the high concentrations of some elements, such as potassium and thorium, which are more typical of planets formed farther away.

This impact hypothesis, however, is not without problems. Such a collision would require a very high velocity, in the range of 100 km/s, which is not possible since the early Solar System objects are likely to have similar orbital directions.

The impact should also have eliminated volatile components. Still, Messenger was able to identify these on the surface of Mercury, indicating that they must have been formed differently or that the volatile retention would have to have been substantial following the impact.

According to other models, Mercury could have formed from material in an inner, swirling part of the protoplanetary disk, where solar outbursts and high temperatures favoured the accumulation of iron-rich, volatile-depleted material. Such a process could explain Mercury's large metallic core without the need for a catastrophic collision, but it raises the question of why Mercury did not keep growing or why it remained so small.

The other theories include Mercury as the core of a gas giant that lost its atmosphere to solar winds, but this is a speculative idea, since it is challenging to remove such large gaseous clouds.

Other scientists suggest that the present location of Mercury is a consequence of planetary migration- first as Mercury condensed nearer to the Sun in a region of dense and rich iron material, then flew away, possibly because of gravitational interactions with other planets.

The unusual properties of mercury render it a valuable tool in the study of exoplanets, notably those larger and heavier than Mercury, commonly known as Super Mercuries because of their dense, iron-rich compositions.

It has been observed that such planets may be widespread in our galaxy, but the mechanisms of their formation remain poorly understood.

2026 Space Mission To Unveil Mercury At Its Core

The answers to these questions will start appearing as the joint European-Japanese mission BepiColombo arrives in 2026. The mission consists of two spacecraft: the Mercury Planetary Orbiter, provided by ESA, and the Mercury Magnetospheric Orbiter, provided by JAXA, and will carry out detailed mapping of Mercury's surface composition, gravity field, and magnetic properties.

The study of Mercury's interior structure, especially its core composition and state, which preserves traces of its formation, is also one of the main tasks of BepiColombo.

The mission will be able to provide evidence of past magma-ocean or stripping events by analysing surface and subsurface material. Its gravity measurements will help define the distribution and density of the planet's materials, limiting models of its internal evolution.

The instruments on the spacecraft will also examine the volatile elements and mineralogy on the planet's surface to resolve the paradox of water and other water-loving elements surviving in such a hostile environment.

Already, the flybys have shown a surface full of scars and old lava flows, hinting at the past activity of the geology, but the ultimate revelations will come when the orbiting devices are put into use.

Though BepiColombo will be a significant step toward understanding Mercury, direct samples of Mercury are the ultimate aim for scientists seeking definite answers. A sample-return or lander mission has been suggested since time immemorial, but it is technically and economically difficult.

At the same time, meteorites called aubrites, which are thought to have formed in the crust of Mercury, can provide a possible, however indirect, insight into the composition of Mercury.

Mercury's existence has long challenged the idea that planets follow neat, predictable rules. When BepiColombo enters orbit in 2026, it will not determine whether Mercury 'should' exist, but it may finally explain how it does.