Experts say planet that ‘doesn’t fit’ could offer new insights into how solar system formed

A mission to Mercury, one of our solar system’s least studied planets, is about to embark on its seven-year journey.
Experts say BepiColombo could not only shed light on the mysteries of our neighbourhood’s smallest planet, but also offer new insights into how the solar system formed and even provide vital clues as to whether planets found orbiting other stars – so-called exoplanets – could be habitable.
“If we want to understand our Earth and how life can [begin] on Earth and maybe on other planets we have to understand our solar system,” said Joe Zender, deputy project scientist for BepiColombo. But while much progress has been made in the matter, Zender says there is a snag. “There is one problem really, which is Mercury – Mercury doesn’t fit.”
Among the puzzles is the surprisingly high density of Mercury and that it is thought to have a core that is at least partly molten.
And with some exoplanets orbiting very close to stars cooler than our own, finding out more about the first rock from our sun has become a pressing matter.
Due to launch from Kourou, French Guiana, late on Friday night local time, the BepiColombo mission is a €1.6bn (£1.4bn) joint venture between the European Space Agency (ESA) and the Japanese space agency, Jaxa.
While the mission has a hefty price tag, it could be argued it is something of a bargain, because BepiColombo doesn’t just involve one orbiter, but two. Once the spacecraft has been delivered into orbit around Mercury by the ESA-built Mercury Transfer Module, it will split to lose a protective sunshield and release the Mercury Magnetospheric Orbiter, built by the Japanese, and the Mercury Planetary Orbiter, built by the Europeans.
Only two previous missions have examined the planet’s properties – Nasa’s Mariner 10 probe, launched in 1973, and the more recent Messenger mission, which launched in 2004. The latest endeavour takes its named from the late Giuseppe “Bepi” Colombo, a professor at the University of Padua who was a key figure in devising Mariner 10’s Mercury fly-bys.
Both previous missions threw up intriguing results, including that Mercury has a magnetic field. This was a surprise given its leisurely pace of rotation – it takes 59 Earth days to spin on its axis – and the idea that because of its small size, the planet’s core would have cooled and become solid, precluding the existence of a magnetic field. What’s more, Mercury’s magnetic field is offset by 20% of the planet’s radius, meaning certain features at the south pole are different to those at the north.
Mercury was also found to have an exosphere – a very thin layer above the surface composed of atoms and molecules that have come from the crust and solar wind.
But before BepiColombo can probe such phenomena, it must negotiate two major hurdles.
“The heat is really severe, 450C on one side, but don’t forget the other side is -180C,” said Dr Suzanne Imber of the University of Leicester, who is a co-investigator on MIXS – one of the Mercury Planetary Orbiter’s 11 instruments – and also worked on Messenger. “[Our spacecraft] is going from one to the other over a few tens of minutes … our instruments have to operate around room temperature.”
The Japanese orbiter will spin 15 times a minute to avoid being toasted, like a kebab on a barbecue, while the European orbiter will be wrapped in a special multi-layer blanket and have a radiator for protection.
Getting to Mercury is also a head-scratcher.
“Mercury is a small body close to the sun, so you could fly straight towards Mercury and get there in a few months, but you can’t stop because the sun’s gravity sucks you in,” said Imber.
The answer is to arrive slowly via an elegant series of laps, with the spacecraft swinging by Earth, Venus and Mercury before entering an orbit around the closest planet to the sun in late 2025, then splitting and beginning operations in early 2026.
“We are passing once by the Earth, twice Venus and six times Mercury,” said Zender. “We are going so close to the individual planets that we can use their gravitational forces to change our direction for the next step.”

Artist’s impression of the BepiColombo spacecraft shortly after launch.
Artist’s impression of the BepiColombo spacecraft shortly after launch. Photograph: ESA/ATG medialab

Once in orbit around Mercury, the instruments will set to work. An x-ray telescope, MIXS will help to shed light on the makeup of the planet.
“We are going to know in really incredible detail what the surface of Mercury is made of,” said Imber, adding that the team can probe deeper down, too. “When you form an impact crater what you find is that layers of material from under the surface are lifted by the impact and land on the surface,” she said.
Prof Nicolas Thomas, co-project manager of the Bela instrument aboard the Mercury Planetary Orbiter, said he wants to investigate the planet’s curious surface features.
“A planet will shrink as it cools and Mercury has cooled a lot – we think the planet has cooled in such a way that its radius has been reduced by 8km over its history,” he said. “There are huge great cliffs that we suspect have been created by this process where this shrinking has gone on but some bits have shrunk and other bits haven’t shrunk quite so much.”
It is a phenomenon Bela will help probe.
“What we do is that we take a seriously large, very scary powerful laser and we fire it up to about 1,000km away from the planet and then we wait for 6-10 milliseconds and we look at the reflected pulse,” said Thomas. By looking at the time it has taken for the light to return, the team can calculate the contours of the surface below, essentially mapping Mercury.
While the BepiColombo mission is expected to last for two years once the orbiters are in position around Mercury, the team say it might continue for longer. Once over, the orbiters will be crashed into the planet’s surface.
For now, though, the team are focused on the launch. “You’ve put 15 years of your life into one of these experiments,” said Thomas. “And it’s sitting there on top of a controlled explosion. So yeah, that’s nerve-wracking.”