ALMA tomography reveals hidden planets in protoplanetary disks

Star system: HD 135344 A | Distance from the Sun: 440 light years

The exoALMA Large Program has uncovered many detailed structures in the dust and gas of protoplanetary disks, where planets are forming around young stars. By using a technique called tomography—studying how molecular lines change at different locations and velocities—astronomers can now detect the presence of planets hidden within these disks.

The study shows that even with a few hours of ALMA telescope time, it is possible to spot the signatures of planets more massive than about one-thousandth of their host star’s mass. These signatures include not just changes in the speed of gas (deviations from Keplerian motion), but also local broadening of molecular lines, which helps pinpoint the planet’s position. The researchers also developed a new way to distinguish between features caused by planets and those caused by disk instabilities, using the shape of the observed spectral lines. Applying this method to real ALMA data, they found evidence for three massive planets in the disk of HD 135344B and complex spiral structures in MWC 758, likely caused by a companion object.

Key findings
- Tomographic analysis of molecular lines can reveal hidden planets in protoplanetary disks
- Planets more massive than 0.1% of the star’s mass can be detected with moderate ALMA observations
- Localized line broadening and velocity changes help locate embedded planets
- A new diagnostic using line skewness distinguishes between planetary and instability-driven features
- Evidence for three massive planets was found in HD 135344B, while MWC 758 shows spiral structures likely caused by a companion

Scientific study

exoALMA XX: Tomographic Detection of Embedded Planets in Protoplanetary Disks (external link)