Giant Planet PDS 70c Gaining Mass From Gas, Not Dust, Reveals New Radio Signals

A Planet’s Radio Chatter Reveals Ongoing Growth

Astronomers have detected a radio signal emanating from the young giant planet PDS 70c, but the signal’s characteristics are unexpected. Rather than the steady radio emissions expected from dust surrounding the planet, the signal originates from ionized gas, suggesting PDS 70c is actively gathering material and offering a glimpse into the final stages of planet formation. This discovery, made using the Atacama Large Millimeter/submillimeter Array (ALMA), challenges previous assumptions about the composition of disks around young planets and provides novel insights into how moons might form.

How PDS 70c’s Radio Signal Defies Expectations

PDS 70, located 370 light-years away in the constellation Centaurus, is a young T Tauri star known for hosting two directly imaged protoplanets, PDS 70b and PDS 70c. Scientists had anticipated that the radio emissions from PDS 70c would be produced by dust within its circumplanetary disk – a ring of gas and dust surrounding the planet. However, observations across four radio bands revealed a different story. The signal appeared in three bands but faded at the highest frequency, a pattern inconsistent with a dusty disk. A dusty disk would typically brighten steadily at higher radio frequencies.

The team, led by Oriana Domínguez-Jamett at the Universidad de Chile, determined that the missing high-frequency glow was best explained by gas heated and electrically charged as material fell onto the planet’s disk. This ionized gas emits radio waves through a process called free-free emission, where electrons pass close to ions. The findings are detailed in a study published in Astronomy & Astrophysics.

The Significance of Ionized Gas

The detection of ionized gas is significant since it indicates that PDS 70c is still actively accreting material. Material doesn’t fall directly onto the planet; instead, it slows down and spreads out in a small disk. When this material impacts the disk, it heats the surrounding gas and strips electrons from atoms, creating the ionized gas that emits the observed radio signal. This process suggests that the planet is still in the process of growing, and the circumplanetary disk is a dynamic environment where material is being deposited.

PDS 70c’s mass is estimated to be approximately 0.952+0.065−0.063 M☉, and it’s about 5.4 million years old. The star itself has a mass of 0.76 M☉ and is located 366.6 ± 0.8 light-years from Earth, according to data from the PDS 70 Wikipedia page. The discovery builds on earlier observations that identified a likely circumplanetary disk around PDS 70c, but the new radio data provides a more detailed picture of the disk’s composition and dynamics.

Implications for Moon Formation

The circumplanetary disk around PDS 70c isn’t just key for the planet’s growth; it also serves as the birthplace of moons. Previous estimates suggested that the dust around PDS 70c held only a small amount of mass, roughly equivalent to the mass of our Moon or slightly more. The new radio results reinforce this idea, indicating that moon building around young giants occurs in a confined and relatively short-lived environment. The fact that much of the observed glow comes from gas, rather than dust, further constrains the amount of material available for moon formation.

ALMA’s Role and the Importance of Multi-Frequency Observations

This discovery was made possible by the capabilities of ALMA, which allowed the team to observe PDS 70c across multiple radio bands within a short timeframe. This approach was crucial because PDS 70c has shown radio variability in the past, and using data from different dates could have led to misleading results. By comparing nearly simultaneous signals, the team was able to pinpoint the turn in the signal at higher frequencies, where dust should have remained brighter. ALMA’s ability to separate faint structures in cold cosmic material is proving invaluable for studying the final stages of giant planet growth.

Dust Depletion and What It Means

The team’s analysis suggests that PDS 70c is significantly depleted in dust – by a factor of at least 1,000 compared to expectations. This doesn’t mean the disk has vanished entirely, but it does mean that gas now dominates the radio signal. Domínguez-Jamett stated, “Our observations suggest that a standard dusty disk does not surround PDS 70c.” This finding challenges the conventional understanding of planet formation and suggests that the process may be more dynamic and gas-rich than previously thought.

Looking Ahead: Future Observations and Tests

Future observations will focus on determining whether the same gas glow appears around other young giants or if PDS 70c is an unusual case. Researchers also plan to monitor the signal over time to see if it changes as fresh material crashes into the disk. This would allow them to use radio light as a direct measure of planetary feeding, rather than relying on snapshots of leftover debris. Better measurements at the same frequencies will also help to refine the understanding of how much of the glow comes from the disk surface alone. The ability to track the last stages of giant planet growth, as demonstrated by ALMA, opens up new avenues for understanding the formation of planetary systems.