Astronomers use ALMA to map millimeter and submillimeter light from dusty disks and to analyze molecular lines from their gas. Continuum images show where emitting dust is concentrated; spectral lines reveal gas properties and, when measured at fine enough frequency intervals, motion within and around a disk. These observations expose structures and dynamics, but a ring or gap is evidence to interpret—not proof by itself that a planet caused it.
What ALMA observes
ALMA is an array of radio telescopes that observes at millimeter and submillimeter wavelengths. Its published receiver overview gives a system range of about 35–950 GHz, corresponding to wavelengths from 8.6 mm to 0.32 mm. The range describes the receiver system; it does not mean every band is available for every observing run. ALMA’s receiver overview is the reference for the technical range, while operational availability can change.
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At these wavelengths, astronomers can study emission from dust grains and from molecules in the gas. The observing setup—frequency, target, and desired spatial and spectral detail—is chosen to fit the scientific question. ALMA’s star and planet formation capability overview describes how these observations contribute to research on disks and their surroundings.
What continuum images and spectral lines reveal
Continuum traces dust emission
Continuum images combine emission across a range of nearby frequencies rather than isolating one molecular transition. In a planet-forming disk, the resulting brightness pattern can show where millimeter-emitting dust is concentrated and reveal rings, gaps, or asymmetries. Brightness is an observable, not a direct, assumption-free measurement of total dust mass: its interpretation depends on physical conditions and models.
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Spectral lines trace gas and motion
A spectral-line observation isolates emission at frequencies associated with particular molecules. Researchers use molecules and isotopologues such as CO, its isotopologues ¹²CO and ¹³CO, and CS to investigate gas and physical conditions. AGE-PRO also used N₂H⁺ alongside CO to improve gas-mass estimates. Different tracers sample different conditions and locations, so comparing them adds context that a continuum image alone cannot provide.
Frequency shifts and line profiles encode line-of-sight velocities. By measuring a line across the disk, astronomers can map rotation and other gas motions rather than seeing only a static brightness pattern. Line strength and derived gas mass, like continuum brightness and dust mass, require interpretation rather than a direct one-to-one conversion.
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Why spatial and spectral resolution matter
Angular resolution describes how finely an observation can distinguish features on the sky. Higher angular resolution can separate smaller structures, subject to source distance and the observation itself. Spectral resolution describes how finely the data separate frequencies; for a line, this affects the ability to distinguish small velocity differences.
These are independent choices: a study may need fine spatial detail, fine velocity detail, or a balance between them. The exoALMA survey reported 100 milliarcseconds (about 14 au at typical source distances) of angular resolution and 26 m/s of spectral resolution for its dataset. Those are study-specific values, not a guarantee for all ALMA observations. The exoALMA survey release describes the observations and results.
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How observations become a disk map
- Choose the question and target. A team decides whether it needs to map dust, trace particular gas, measure motion, or compare disks across a population.
- Select the observing setup. Researchers choose frequencies and suitable molecular lines, along with the angular and spectral resolution needed to answer the question. Not every project uses the same bands or tracers.
- Calibrate and image the data. The measured signals are processed into continuum images or spectral-line data cubes, which preserve information across both position and frequency. Methods vary by project; exoALMA reports developing alignment and cleaning methods to combine observations taken at different times.
- Compare tracers and interpret the patterns. Scientists examine dust and gas maps, line profiles, and velocity structure, then compare them with physical models. The resulting interpretation is constrained by what was observed and the assumptions used to connect emission with disk properties.
What ALMA observations have shown
HL Tau: resolving rings and gaps
ALMA’s landmark 2014 image of HL Tau used a 15 km baseline and achieved roughly 35 milliarcseconds of resolution, revealing bright concentric rings separated by gaps. The image demonstrated how fine angular resolution can expose disk substructure. The pattern alone does not establish that every gap was carved by a planet. ALMA’s HL Tau release presents the observation.
PDS 70: dust near a known planetary system
A Band 3 ALMA observation at a wavelength of 3 mm reported localized dust accumulation outside the orbits of PDS 70’s two known planets. It offers a useful example of studying remaining disk material in a system where planets are already known; the continuum structure is not, on its own, a new planet detection. The PDS 70 release describes the result.
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exoALMA: combining structure and kinematics
Alongside continuum, exoALMA observed ¹²CO, ¹³CO, and CS in a survey designed to map disk density, temperature, and velocity structure. Its dataset-specific resolution figures illustrate how an observing program can combine spatial and spectral detail to study more than dust morphology. The survey release reports its approach and findings.
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1Repair Windows errors before they cause bigger problems2Fix the driver behind crashes, sound loss and screen glitches3Clear out junk files and repair common Windows errorsAGE-PRO: comparing disks across populations
Not every ALMA project aims for a highly detailed image of one disk. AGE-PRO observed 30 protoplanetary disks around Sun-like stars in Ophiuchus, Lupus, and Upper Scorpius. The sample spans objects reported as younger than one million years to older than five million years, allowing a survey-style comparison of gas and dust evolution. ALMA’s 2025 release reports 12 papers in a special issue of The Astrophysical Journal. The AGE-PRO release gives details of the survey.
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L1527 IRS: watching a disk take shape
ALMA’s 2026 eDisk result for L1527 IRS resolved an approximately 16 au-wide transition zone between infalling envelope material and orderly disk rotation. This traces the formation of a disk around a young protostellar system; it is an earlier stage than the more evolved planet-forming disks discussed in the other examples. ALMA’s eDisk release describes the result.
Orion: a specialized view of ionized gas
ALMA has also detected H41α recombination-line emission at 3.1 mm from ionized shells around proplyds in Orion. This extends the picture beyond dust continuum and molecular gas, showing how ALMA can investigate disks exposed to nearby massive stars. It is a specialized case rather than the usual focus of planet-forming disk observations. The Orion release reports the detection.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What rings and gaps mean—and what they do not
Rings, gaps, and asymmetries are observed patterns. They can help researchers test ideas about how disks evolve and how planets may form, but morphology alone does not identify the mechanism that produced a feature. Astronomers weigh continuum and line data, account for the conditions each tracer probes, and compare the observations with physical models before drawing conclusions. A striking ALMA image is therefore a map of evidence, not automatically a picture of planets forming in real time.
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