ALMA captures spiral motion that gives rise to new planets for the first time ever

A peer-reviewed study shows spiral patterns in a young star’s disk actually moving over time. The array turned snapshots into a short view that reveals their winding motion. 

The star, called IM Lup, lies in the constellation Lupus. Using data gathered over seven years, astronomers tracked changes in its surrounding disk. Measurements from the European Space Agency’s (ESA) Gaia telescope show that the star is roughly 515 light-years, or 158 parsecs, from Earth.

Lead author Tomohiro C. Yoshida of The Graduate University for Advanced Studies SOKENDAI, led the analysis. The group compared images with consistent methods to track changes in the spiral ridges.

Why spirals matter

Spirals in protoplanetary disks are not just pretty. They trace how gas and dust move and pile up, which can speed up the growth of planets.

Several engines can draw spirals. An embedded planet can tug the disk, a passing star can stir it, or a tilted inner disk can cast shadows that look like swirls.

The team focused on motion, not just shape. They asked whether the pattern rotates like a solid bar or if it shifts at the same speed as the local orbit of the disk material.

What the stop motion shows

The animation links images taken years apart. The spiral crests drift to new angles each time, and the rate of that drift changes with radius.

The drift matched the orbital speed expected in the disk. That is Keplerian rotation, so the pattern is not a rigid wheel and it does not behave like a simple wake forced by a single planet.

Keplerian rotation and what it tells us

Keplerian rotation means the closer parts of a disk go faster and the farther parts go slower. That rule sits at the heart of orbital mechanics. If a spiral keeps pace with that local orbital speed, theory points to the disk’s own gravity as the driver. In other words, the pattern emerges when the disk is heavy enough for self-gravity to matter.

This state is called gravitational instability. This instability can raise spiral arms, stir turbulence, and sometimes fragment a disk. The IM Lup result lines up with those predictions. The match in speed is the key clue.

How ALMA made it possible

At the core of the effort is ALMA, a network of 66 antennas set on a dry plateau in northern Chile at about 16,400 feet (5,000 meters). The array works as one telescope, and is tuned to millimeter and submillimeter wavelengths that trace cold gas and dust. 

Those wavelengths carry the signatures of the building blocks of planets. Sharp, stable imaging lets scientists revisit the same disk years later without losing track of faint structures.

ALMA’s time coverage turns static pictures into a time series. That is how a set of images becomes a short, silent movie of a planet-forming environment.

Ruling out other causes

A planet can launch spirals, but those patterns usually spin with the planet like a lighthouse beam. That creates a nearly fixed pattern speed, not a local orbital match at every radius.

Flybys and shadows have different fingerprints too. They do not make the whole pattern keep pace with the changing orbital speed from inner to outer disk. IM Lup’s spirals did not act like any of those models. They followed the local speed, which is what the gravitational instability scenario predicts.

Why this matters for new planets

When a disk is near instability, small clumps and pressure bumps can grow. Those features can trap pebbles, and that can build the cores of planets faster than slow, gentle sticking would allow.

Wide orbits are especially tricky for core accretion. The local material is sparse and the clock runs fast before the gas fades. The new result provides data that support a more dynamic path. It shows a disk where gravity is already sculpting the structure at planet-forming scales.

Context from recent surveys

High-resolution ALMA surveys have mapped rings, gaps, and spirals across many disks, revealing how these patterns often trace areas of changing pressure, regions that trap dust, and places where hidden planets may be forming.

IM Lup has been a touchstone in that work. It is massive, extended, and chemically rich, so it serves as a clean case to test ideas about how structure links to planet formation.

What comes next

Time domain studies of disks are young. As more archival data and new cycles pile up, teams can tighten the rotation curves for spirals and compare them with models.

Longer baselines will also reveal any small departures from perfect Keplerian speed. That would help weigh the disk more precisely and test how close it sits to fragmentation.

The study is published in Nature Astronomy.

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