Young stars are surrounded by discs of gas and dust from which planets form. The gas orbits in a smooth, predictable pattern that radio interferometers can map through the Doppler shift of molecular lines.
A planet embedded in the disc disturbs that flow: it carves a gap, and near it the gas velocity shows a localised kink — a departure from the smooth rotation of a few hundred metres per second.
Finding the kink means finding a planet that is still being assembled, too faint and too enshrouded in dust to see by any other means.
One planet in the entire catalogue rests on this. It is the newest method here and the only one that watches planet formation as it happens.
The arithmetic
Unperturbed rotation
vKepler(r) = √(GM* / r)
The baseline. Everything in the disc should follow this; the detection is the residual after subtracting it.
Mass from the disturbance
δv / vK ∼ (Mp / M*)1/2
Approximate, and model-dependent — masses from this method carry large uncertainties.
HD 163296
Disc mapped in
CO emission with ALMA
velocity resolved to ~50 m/s
Kink found at
~260 AU from the star
far outside where other methods look
Inferred mass
~2 Jupiter masses
model-dependent
A planet still in the act of forming, detected purely from how it stirs the gas around it.
What it does well
Detects planets still forming, invisible to every other method
Works at very wide separations
Reveals how planets interact with the disc they came from
Where it cannot help
Requires ALMA-class interferometry
Masses are model-dependent and uncertain
Only applies to young stars with discs
A single detection so far
Landmark discovery
HD 163296 — a kink in the CO gas velocity betraying an unseen forming planet.
Instruments
ALMA
Archive name
Disk Kinematics — the value in NASA's discoverymethod field