The smallest planet in the solar system, according to new research, could have been formed by a collision with another planet billions of years ago.

A monstrous collision almost tangentially with a planet comparable in size to Earth destroyed the stony shell of proto-Mercury. This hypothesis explains well that such a small planet has a huge iron core.
Scientists estimate that the mass of the core of Mercury is about 60 percent of its mass, while its closest neighbors - Venus, Earth and Mars, this ratio is about 30%
When the Messenger interplanetary station orbited Mercury in 2011, many scientists assumed that a monstrous collision had once destroyed the planet's mantle. However, in this case, the crust of Mercury should have lost most of the light chemical elements.
When Messenger reached Mercury, we unexpectedly found large amounts of potassium and sulfur on the planet. This has left us with a puzzle: how a planet with such a large iron core can have large reserves of volatile chemical elements.
Patrick Peplowski, Johns Hopkins University
The answer most likely lies in the collision of proto-Mercury at the very beginning of its formation with a large celestial body about 4.5 billion years ago.
As a result of such a collision, not only Mercury could have formed, but also Mars, and some large asteroids such as Vesta and Psyche.
With the help of computer models, scientists calculated that Mercury should have collided with a planet whose mass was 0.85 Earth masses at an angle of 34 degrees.
What happened to the "killer planet" after hitting tiny Mercury and where it might be now, scientists hope to find out as a result of further research.