Seeing the Moment Planets Start to Form

Nature makes few duplicates, and planets are as distinct from one another as snowflakes are. But planets all start out in the same circumstances: the whirling disks of material surrounding young stars. ALMA’s made great progress imaging these disks and the telltale gaps excavated by young, still-forming planets.

But new images from ALMA (Atacama Large Millimeter/submillimeter Array) show a star and disk so young that there are no telltale gaps in the disk. Is this the moment that planets start to form?

Stars form first, and planets follow. Stars form in a cloud of interstellar gas called a giant molecular cloud. They start out as young protostars that begin to rotate, and pancake-like disks of gas and dust follow suit. These whirling protoplanetary disks contain the material from which planets form. Inside the disks, matter begins clumping together into protoplanets and planetesimals, excavating lanes in the disks. When a protoplanetary disk is at the correct viewing angle, some telescopes can spot these widening lanes, if not the planets themselves.

Seeing inside the disks is challenging. The immense amount of dust extinguishes most of the light. But ALMA, the ESO’s Atacama Large Millimeter/submillimeter Array, was built to sense what little light comes from these disks. As the telescope’s name makes clear, ALMA sees wavelengths of light around one millimetre, which is roughly between infrared light and radio waves.

This image shows four of the many protoplanetary disks that ALMA’s imaged over the years. Each gap in the disk is probably a planet forming. Credit: ALMA (ESO/NAOJ/NRAO) S. Andrews et al.; NRAO/AUI/NSF, S. Dagnello

The route to understanding planet formation goes right through all that dust. The physical properties of the dust grains play a big role in how planets form and what types form. The new study uses ALMA observations to characterize the disk around the young protostar DG Taurus (DG Tau.) It’s a class I-II protostar, only about one million years old. DG Taurus is “… one of the most promising targets for studying the early stages of the dust disk,” the study states.

The research is titled “Dust Enrichment and Grain Growth in a Smooth Disk around the DG Tau Protostar Revealed by ALMA Triple Bands Frequency Observations,” and it was published in The Astrophysical Journal. The lead author is Satoshi Ohashi from the National Astronomical Observatory of Japan and the RIKEN Cluster for Pioneering Research.

A critical part of understanding protoplanetary disks and the planets they give rise to is the size and distribution of the dust grains in the disk. The planet formation process can alter the distribution, so finding a smooth disk with no planets, like the one around DG Tau, is important in understanding how planets eventually form and how different types form in different locations.

“ALMA has so far succeeded in capturing a wide variety of disk structures and has revealed the existence of planets,” said lead author Ohashi. “On the other hand, to answer the question, ‘How does planet formation begin?’, it is important to observe a smooth disk with no signature of planet formation. We believe that this study is very important because it reveals the initial conditions for planet formation,” Professor Ohashi said.

“A uniqueness of the DG Tau disk is the smooth morphology of the disk structure in the early stage of the star formation,” the researchers explain. Recent ALMA images have shown us that protoplanetary disks contain a variety of structures like rings, spirals, and crescents. “In contrast, the DG Tau disk shows no significant substructures even though the disk mass, dust size, and turbulence strength are similar to others such as HL Tau.” HL Tau is widely known in astronomy for being one of the first protoplanetary disks to be imaged accurately, showing the gaps and rings that indicate planet formation.

This early ALMA image of HL Tauri shows the protoplanetary disk surrounding the young star. The observations reveal substructures within the disk that were never been seen before and even show the possible positions of planets forming in the dark patches within the system. DG Tau shows one of these features. Credit: ALMA (ESO/NAOJ/NRAO)

DG Tau has no rings or gaps in its disk, leading astronomers to conclude that no planets have formed yet. “This suggests that the DG Tau disk may still be in the early stages of planet formation,” the authors explain.

One of the ways the research team measured the dust around DG Tau is with polarity. Polarity is an important tool for astronomers. Dust grains aren’t spherical, so they tend to be aligned with interstellar magnetic fields. That alignment can polarize starlight that passes through the dust, and polarimetry can reveal some of the dust’s structure. Polarimetry allowed the researchers to measure dust surface density, temperature, and grain size.

This figure from the research illustrates some of the findings. Although the researchers found that DG Tau’s disk is a smooth structure without significant gaps or ring substructures, the intensity slope transitions at about r = 45 au. Their model implies that the dust size and surface density change at this position. Beyond this line, complex organic molecules may be found. Image Credit: Ohashi et al. 2023.

What did they find?

DG Tau’s disk is smooth and thin, but only to a point. At about 40 to 45 AU, the dust size and distribution changes. This could be because of the carbon monoxide snowline, and beyond that line, the dust size increases. The researchers think this is because frozen CO molecules are “stickier” than CO² molecules inside the CO frost line. So beyond the line, complex organic molecules (COMs) can form on the surface of the frozen CO² molecules.

COMs are normally defined as carbon-bearing molecules with six or more atoms. What role these COMs might play in the appearance of carbon-based life like Earth’s is hotly debated, but they definitely play a role in the complex chemistry needed for life. It’s too soon to conclude what finding them beyond a 40-45 AU line means in a very young disk, but it’s definitely interesting.

Scientists think that CO is necessary for planet formation, and finding it and understanding it in a young protoplanetary disk is important. Its importance only increases because of its tendency for potentially life-enabling COMs to form on its frozen surface. CO’s frost line, like water’s frost line and the frost lines of other compounds, will move as the DG Tau system evolves. Our own Solar System’s water frost line is at about 5 AU now, but was between Mars and Jupiter at about 2.7 AU when the Solar System formed.

DG Tau’s CO frost line, and the system’s other frost lines, will move over time, but how much and when are open questions. Their locations will influence the types of planets that form and can influence the presence of organic compounds. So, finding this system and its smooth disk gives scientists a clean starting point to work from. They can plot the growth of the protostar, the evolution of the disk, and the formation of planets. (If humanity endures long enough.)

Each planet that forms is distinct from others. And each one is a chance for life. Not only to appear and then languish in the mud and muck for a couple of billion years, but to eventually evolve complexity. That’s how we got here.

If we’re ever going to have a full understanding of how planets like Earth form and how life arises, then research like this will play a foundational role.

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