ALMA pinpoints the radio signature of planetary growth

New multi-frequency observations reveal that the growing giant planet PDS 70c shines in radio waves not from dust, but from ionized gas in its environment.

Highlights

• Unprecedented multi-frequency radio observation of a forming planet: ALMA observed PDS 70c in Bands 3, 4, 7, and 9, revealing new details about its environment.
• Not dusty, but gaseous: The radio signal originates from ionized gas, not from the dusty disk astronomers anticipated.
• Likely origin in a circumplanetary disk: The emission most likely comes from the surface of a small disk surrounding the planet, where mass from the environment is deposited.
• Clues to planet and moon formation: These findings present the first radio spectral fingerprint of a circumplanetary environment, offering insights into how giant planets develop and how moons may form.

Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have obtained an unprecedented multi-frequency view of a forming planet in the nearby star system PDS 70. The new study, led by postgraduate student MsOriana Domínguez-Jamett (Universidad de Chile) and published in Astronomy & Astrophysics, shows that the planet PDS 70c emits radio signals produced by ionized gas rather than the dusty disk expected around such a young world.

PDS 70, a young star 370 light-years away in the constellation Centaurus, is famous for hosting two directly imaged protoplanets. Among them, PDS 70c is thought to be surrounded by a circumplanetary disk — a disk of gas and dust feeding the planet and possibly forming moons. Until now, the exact origin of its radio emission remained a mystery.

Using new ALMA observations in Bands 4 (145 GHz), 7 (343.5 GHz), and 9 (671 GHz), together with archival Band 3 data (97.5 GHz), the team detected a compact source at the position of PDS 70c in three of the bands. Intriguingly, they found no signal in the highest frequencies (Band 9). This “drop” in brightness challenges the idea that the emission originates solely from thermal dust. Instead, the results are best explained by partially optically thick free-free emission — radio light generated by the collisions of electrons and ions. In simple terms, the radio light from PDS 70c mainly comes from the surface of a small disk surrounding the planet. This gaseous disk shines because its surface is ionized by the impact of infalling material,  making it appear as a faint, glowing veil around the young planet.

“Our observations suggest that a standard dusty disk does not surround PDS 70c,” says lead author Oriana Domínguez-Jamett. “Instead, the signal points to ionized gas, possibly heated in shocks as material falls onto the planet’s disk. This means the planet is depleted of dust by at least a factor of a thousand compared to expectations.”

By comparing the spectrum with simple models, the researchers demonstrate that a very low ionization fraction can explain the observed turnover in emission. This marks the first time the radio emission mechanism in a circumplanetary environment has been identified.

“This is a breakthrough in our ability to study how gas giant planets grow and how their moons may form,” adds advisor Simon Casassus (Universidad de Chile). “ALMA can now not only detect circumplanetary disks but also determine what powers their emission.”

“These results highlight ALMA’s unique ability to probe the environment of forming planets,” says John Carpenter, ALMA Observatory Scientist. “By distinguishing between dust and gas emission, we gain a direct view of how young planets gather material and how future moon systems begin to form.”

The findings provide key new constraints on the density, temperature, and ionization state of the material around forming gas giants. They also highlight the unique potential of ALMA to explore the final stages of planet growth.

Additional Information

The results of this study appear in the Astronomy & Astrophysics as "Multi-frequency observations of PDS70c: Radio emission mechanisms in the circumplanetary environment" by O. Domínguez et al.

The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of the European Southern Observatory (ESO), the U.S. National Science Foundation (NSF), and the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Republic of Chile. ALMA is funded by ESO on behalf of its Member States, by NSF in cooperation with the National Research Council of Canada (NRC) and the National Science and Technology Council (NSTC) in Taiwan, and by NINS in cooperation with the Academia Sinica (AS) in Taiwan and the Korea Astronomy and Space Science Institute (KASI).

ALMA construction and operations are led by ESO on behalf of its Member States; by the National Radio Astronomy Observatory (NRAO), managed by Associated Universities, Inc. (AUI), on behalf of North America; and by the National Astronomical Observatory of Japan (NAOJ) on behalf of East Asia. The Joint ALMA Observatory (JAO) provides the unified leadership and management of ALMA's construction, commissioning, and operation.

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