The mystery of a planet that shouldn’t exist. Scientists still don’t know how it was created

A planet full of puzzles

Planetary scientists are still puzzled by the very existence of the planet closest to our Sun. This unusual world is tiny, twenty times less massive than Earth and only slightly wider than Australia. Despiteč therefore, Mercury is the second densest planet in our solar system, right after Earth, thanks to the huge metallic core that makes up most of its mass. Its orbit, which hugs the Sun, is also in a strange location that astronomers cannot fully explain. It all boils down to one key fact – we have no idea how Mercury was formed. For all we know, that planet simply shouldn’t exist.

​- That’s a bit embarrassing. We are obviously missing some key subtlety – says Sean Raymond, an expert on planetary formation and dynamics from the University of Bordeaux in France.

The mystery of where Mercury came from, how it was formed and why it looks the way it does today is one of the biggest puzzles of the Sun’s system. Solving this problem is not only important for understanding the origin of our system, but and for the study of planets around other stars, so-called exoplanets.

​- Mercury, due to its unusual origin, is probably the closest we have to an exoplanet. It’s a fascinating world – says Saverio Cambioni, a planetary scientist from the Massachusetts Institute of Technology (MIT).

A mission that promises answers

Some answers may be on the horizon. A joint European-Japanese mission called BepiColombo, launched in 2018, is currently on its way to Mercury. The probe will be our first visitor to that planet in more than ten years.

When it enters orbit in November 2026, after a thruster problem delayed its journey, one of the key goals will be to try to figure out where exactly Mercury came from.

It first became clear to astronomers that something was wrong with Mercury after NASA’s Mariner 10 spacecraft flew past the planet three times in 1974 and 1975. These flybys provided the first measurements of gravity, offering the first insight into the planet’s interior and revealing its bizarre structure. Earth, Venus and Mars have iron-rich cores that are about half their radius. With Mercury, the situation is completely different. Its core is about 85% of the planet’s radius, with only a thin rocky mantle and crust on top. This explains the incredible density of the planet, but why its structure is like that remains unclear.

​- The formation of Mercury is a big problem. It is still unclear why Mercury looks the way it does – says Nicola Tosi, a planetary scientist from the German Aerospace Center in Berlin.

A later mission, NASA’s Messenger that orbited the planet between 2011 and 2015, only raised new questions. Although daytime temperatures on Mercury can reach 430°C, and at night can drop to -180°C, Messenger found volatile elements such as potassium and radioactive thorium on the surface, which should have evaporated long ago under the influence of the Sun’s radiation.

Giant collision theory

Discoveries like this reinforced the idea that Mercury doesn’t really belong in its current home. For years, astronomers have been trying to explain its location in a region where a planet like it could not easily form. No matter what parameters dynamicists change in their models, they simply cannot get Mercury as we see it today.

​- It’s a real pain. You always get zero Mercury – ž but Raymond.

One of the most considered scenarios is the idea that Mercury is someoneć was much larger, perhaps twice as massive and almost the size of Mars, and orbited further from the Sun. According to this theory, at some point in the first ten million years of its existence, this “proto-Mercury” was struck by a massive object. The collision stripped the planet of its outer layers – the crust and mantle – leaving only the dense, iron-rich core we see today.

​- The general interpretation is that Mercury suffered a giant impact that removed most of the mantle – says Alessandro Morbidelli, a planetary dynamicist from the Observatory of the Côte d’Azur in Nice.

However, this theory also has problems.

Such a collision would require an incredibly violent impact at speeds in excess of one hundred kilometers per second, an unlikely scenario. In addition, such an event should also remove all volatile elements, which makes Messenger’s revelations even more confusing.

Alternative hypotheses

There are other ideas. Maybe Mercury was not the target, but the very missile that crashed into another world, like Venus, before ending up in its current position.

Another possibility is that there was no giant impact at all, but that Mercury was formed from material closer to the Sun that was naturally richer in iron. There is also a theory of planetary migration, according to which all rocky planets formed much closer to the Sun, and then gradually moved away, leaving Mercury behind.

All this provides astronomers with many clues, but without consensus. BepiColombo may offer some answers. When the mission, which consists of two linked spacecraft from the European and Japanese Space Agency, enters orbit, the spacecraft will separate. He will use his instruments to map the composition of the surface, study gravity and the weak magnetic field.

If Mercury is someoneć was bigger so it was “peeled”, this should have created a temporarily melted mantle, an ocean of magma, the evidence of which we could still see today. BepiColombo will be looking for just such evidence.

In search of pieces of Mercury

To truly understand the origin of Mercury, scientists dream of landing on the planet and returning samples to Earth. Although such a mission is not planned, the best hope is to find a meteorite that hit Mercury.

There is a theory that a rare class of meteorites on Earth, called aubrites, are pieces of the supposed proto-Mercury. That idea remains “wild speculation,” but it’s tempting nonetheless. Camille Cartier, a petrologist from the University of Lorraine in France, is leading a study to investigate this possibility.

The question of the formation of Mercury is key to understanding the origin of planets in general. Was Mercury just a complete coincidence, the result of a lucky collision in our solar system, or something more ordinary?

For now, the mystery of its origin remains. On the surface, Mercury may seem like a gray and boring world, but deep down, this enigmatic planet could be one of the most fascinating places in the Solar System.

​- It is possible that Mercury is simply an incredible planet, one that in most timelines should not exist – but in ours it does – concludes Jennifer Scora, an expert on planet formation from the University of Toronto.