The Youngest Planetary Disks Ever Seen

How long does planet formation take? Maybe not as long as we thought, according to new research. Observations with the Atacama Large Millimetre/submillimetre Array (ALMA) show that planet formation around young stars may begin much earlier than scientists thought.

These new results were presented at the American Astronomical Society’s 243rd Meeting. Cheng-Han Hsieh, a Ph.D. candidate at Yale, presented the new observations. “ALMA’s early observations of young protoplanetary disks have revealed many beautiful rings and gaps, possible formation sites of planets,” he said. “I wondered when these rings and gaps started to appear in the disks.”

Hsieh is referring to the well-known ALMA images of protoplanetary disks that have been making news for a few years now. These images show the protoplanetary disks around young stars with gaps that scientists think are where planets are forming.

ALMA captured these high-resolution images of nearby protoplanetary disks in 2018 as part of its Disk Substructures at High Angular Resolution Project (DSHARP). The gaps indicate where planets are forming and ‘sweeping’ their lanes of material. Credit: ALMA (ESO/NAOJ/NRAO), S. Andrews et al.; NRAO/AUI/NSF, S. Dagnello

But this earlier image and ones like it are images of Class 2 disks. The new images show Class 0 and Class 1 disks, which are younger. The Classes refer to the age of the stars that host the disks. In fact, at these young ages, they’re not even called stars; they’re called young stellar objects (YSOs.)

A Class 2 YSO is a protostar with a visible photosphere. But the new images show YSOs and disks that are Class 0 and Class 1. At these young ages, the YSOs are still in the collapsing and formation stages.

A YSO is only a Class 0 object for about 10,000 years and a Class 1 object for a few hundred thousand years. So, finding rings and gaps in the disks around these extremely young stellar objects is a surprising development, to say the least. If planets are forming this early in a solar system’s life, it challenges our entire understanding of how planets form.

There are two theories for how planets form: core accretion and gravitational instability.

In core accretion, a rocky core forms from colliding planetesimals, and when it has sufficient mass, it attracts a gaseous envelope. Scientists think this is how large gas giants like Jupiter form.

In gravitational instability, a protoplanetary disk becomes massive enough that it’s unstable and gravitationally bound clumps will form. The clumps and fragments go on to form planets.

This simple illustration shows how the two planet-forming theories work. The core accretion model is considered a bottom-up process, and the disk instability model is considered a top-down model. Image Credit: NASA/ ESA/ A. Feild

“It is difficult to form giant planets within a million years from the core accretion model,” said Cheng-Han Hsieh.

At the AAS Press Conference, where he presented his work, Hsieh was asked if these images capture the first stages of planet formation. Is it possible that the process begins even earlier, and we just can’t see it?

“Our survey is limited by angular resolution,” Hsieh explained. “Our angular resolution is around 15 AU, so we can only detect substructures such as rings and gaps larger than 8 AU. So, to put it into perspective, the distance between the Sun and Saturn is 9 AU. So if there’s a gap or ring larger than the distance between the Sun and Saturn, then those substructures we can detect.”

“We don’t see any substructures in the earliest systems, and this might be because the substructures are smaller early on,” Hsieh said.

Ethan Siegel from “Starts With A Bang” asked Hsieh another important question. “It’s cool that you see structure appearing in these early disks,” said Siegel. “Is there any evidence either for or against that what we’re seeing is a planet-forming structure as opposed to a transient feature being carved in the disk that will be washed out during the evolution of this planetary system as some simulations indicate?”

“For simulations, if you have Earth-sized or Neptune-sized planets inside the protoplanetary disk, they will start to accrete gas from the surrounding gas, and then over time, they will carve out gaps and rings,” Hsieh said. “On the other hand, different instabilities like perturbations in density or temperature can also cause substructures. So, it is very difficult, from observations, to determine whether or not these substructures are definitely caused by a planet or they’re coming from instabilities.”

Hsieh also explained that it’s difficult to tell for certain and that it’s the subject of much research. He also explained how important it is to detect the gaps and rings, no matter if they’re planets or some type of instability.

This ALMA image shows the protoplanetary disc surrounding the young star HL Tauri. ALMA reveals some of the substructures in the disk, like gaps where planets may be forming. Only better observations can eventually tell us whether these gaps are planets, but even if they’re not, the presence of gaps indicates that the disk has calmed enough for planets to form. Image Credit: ESO/ALMA

“But it’s still very important to determine when these substructures form because even though we don’t know whether there’s a planet inside, it still gives us a time scale for planet formation,” Hsieh explained. That’s because neither planets nor any substructure can form until the disk settles down and turbulence subsides. So even if these aren’t actually planets, their presence indicates that the protoplanetary disk has calmed enough for structures and planets to start to form.

If they are planets, then the images show that they can begin to form within about 300,000 years into the life of the young stellar object that hosts the disk. Only future observations can tell us if these are actually planets.

A paper presenting these images and results is being prepared for publishing.