The planets then migrate inwards to the star where they eventually form a stable orbit.

The planet could have formed further from its current position, as a gas giant, and migrated inwards with the other gas giants.

It is believed that these planets formed out past the snow line and migrated in to their current position.

This planet, like b and c, has likely migrated inward.

As these planets migrated, resonances would be "swept" through the asteroid belt and Kuiper belt.

The resulting drag caused a transfer of angular momentum, and as a result the planets gradually migrated to new orbits.

There is an alternative reason why Jupiter-like planets around cooler stars may migrate in closer to the star than such planets around hotter stars.

All the planets from b to at least f must have migrated inward.

Alternatively the planet could have formed further from Gliese 876, as a gas giant, and migrated inwards with the other gas giants.

They theorize the planets formed farther from their star and then migrated inward, likely through interactions with the disk of material from which they originated.