01
Planetary systems form in disks around young stars, but planets do not necessarily remain where they first assembled.
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02
A planet exchanges angular momentum with surrounding gas and launches density waves that can move it inward or outward.
supported03
Hot Jupiters lie far inside likely giant-planet formation zones, resonant multiplanet systems preserve migration signatures, protoplanetary disks show rings and gaps, and population statistics disagree with models in which planets remain fixed at birth radii.