Plasma Radio Emission Reveals the Growth of the Protoplanet PDS 70c
An international team of astronomers has published the most detailed study to date of radio emission from a forming planet. The research article, co-authored by dr. Ondřej Chrenko from the Astronomical Institute of Charles University, was selected as a highlight in the scientific journal Astronomy & Astrophysics.
While thousands of exoplanets have been discovered over recent decades, detections of protoplanets – planets still in the process of formation – remain so rare that they can be counted on one hand. These giant protoplanets (similar to Jupiter) are embedded in gas-dust disks from which they accrete material. That material forms a rotating circumplanetary disk1, which regulates their further growth and may also enable the formation of moons.
PDS 70, a young star 370 light-years away in the constellation Centaurus, is known for hosting two directly imaged protoplanets. Only the outer one, designated PDS 70c, shows radio emission that was thought to arise from the thermal radiation of dust grains accumulated in its circumplanetary disk. PDS 70c is thus the only known forming planet that allows direct study of the circumplanetary environment.
An international team, including dr. Ondřej Chrenko from the Astronomical Institute, has now published a detailed analysis of the radio emission from PDS 70c. Using interferometric observations with ALMA (Atacama Large Millimeter/submillimeter Array), the astronomers measured the planet’s brightness in four frequency bands (97.5, 145, 343.5 and 671 GHz). If the radio emission originated from dust, the flux should show a smooth trend across the bands. Surprisingly, however, the protoplanet was undetectable at the highest frequencies.
“Our observations suggest that the circumplanetary environment of PDS 70c is far more depleted in dust than expected,” says lead author Oriana Domínguez-Jamett, a doctoral student at the Universidad de Chile. “The data are best explained by radio emission from ionized gas, possibly produced by shocks as material falls onto the planet’s disk.”
“A thin plasma layer with a low ionization fraction, glowing like a veil on the surface of the circumplanetary disk, accounts for the fading signal at higher frequencies,” adds dr. Ondřej Chrenko. “These results advance our ability to study how giant planets grow, how much dust surrounds them, and how their moons may form.”
Radio emission from the PDS 70 system detected by ALMA in three frequency
bands. The circular inset shows the protoplanet PDS 70c surrounded by a circumplanetary disk and the drop in its signal in band B9. Credit: O.
Domínguez et al. – N. Lira – ALMA (ESO/NAOJ/NRAO)
The study was selected as one of the highlights of the latest issue of Astronomy & Astrophysics and was also featured in a press release by the ALMA observatory.
The research of dr. Ondřej Chrenko was supported by the Czech Science Foundation (grant 21–23067M) and the Charles University Research Centre program (No. UNCE/24/SCI/005).
Original
Study:
O. Domínguez-Jamett et al. Multi-frequency observations
of PDS 70c: Radio emission mechanisms in the circumplanetary environment,
Astronomy & Astrophysics, 2025, 702, A18. https://www.aanda.org/articles/aa/full_html/2025/10/aa54485-25/aa54485-25.html
1 A protoplanetary disk orbits a central star, whereas a circumplanetary disk orbits a protoplanet. It can be imagined as a “disk within a disk,” with a typical size related to the planet’s gravitational sphere of influence..
