Mercury, the smallest planet and the closest to the sun, has an unexpectedly weak magnetic field, and European researchers have fingered the sun as the culprit.

Planetary magnetic fields are generated by flows in the hot, liquid iron cores of rocky planets. Based on its size and density, Mercury should field strengths similar to those on Earth -- yet the planet's field is 150 times weaker than Earth's.

Researchers in Germany say the sun's solar wind -- a constant stream of charged particles -- plays a large role in that.

Mercury, at an average distance from the sun of 36 million miles -- around one third of the distance from Earth to the sun -- is much more exposed to these particles, they said.

"We must keep in mind that Mercury strongly interacts with the surrounding solar wind," researcher Daniel Heyner, lead author of the study published in Science, said in a release from the Max Planck Institute for Solar System Research.

This interaction drives strong electrical currents in the magnetosphere of the planet, creating magnetic fields that counteract and cancel the internal dynamo effect of the flowing iron core, scientists said.

"The dynamo process in Mercury's interior is almost nipped in the bud by the interaction," researcher Karl-Heinz Glassmeier at the Technische Universitat Braunschweig said.