Observations of other stars suggest Super Mercuries might be quite common in our galaxy, says Cambioni, accounting for perhaps 10 to 20% of all planets out there. That’s a bit of a problem because, like Mercury, we don’t know how they form – they are too big to form via any collision scenario, for example. “They are uncomfortably common,” says Cambioni.
There is another theory for how Mercury came to be – one where the inner planets didn’t form where they are now, but instead moved around a bit. In one model of the Solar System, the inner planets of Mercury, Venus, Earth and Mars might have formed in two distinct rings of material around the Sun. Earth and Venus formed alongside Mercury in the inner ring, before they “migrated away and left Mercury behind”, says Raymond, because of its lower mass.
Modelling by Matt Clement, a planetary dynamicist at the University of Oxford in the UK, suggests the rocky planets could have all formed much closer to the Sun, within Mercury’s current orbit, before moving outwards. “Mercury gets kicked out of the action and runs out of material,” he says. The idea doesn’t quite solve why Mercury would have such a large core, unless it moved into a region of the Solar System that was richer in iron, but it does solve why the planet is the size it is and its distance from Venus. “I think you need migration,” says Clement.
There are some more unusual ideas too. What if Mercury isn’t a rocky planet per se, but the naked core of a gas giant planet like Jupiter that had its atmosphere ripped away? While such an idea has been touted, Cambioni thinks it is unlikely. “It is very hard to remove the atmosphere of a Jupiter-sized planet,” he says, due to their immense gravity.