Astronomers See a Newly Forming Planetary Disk That’s Continuing to Feed On Material from its Nebula

Over the last few years, astronomers have observed distant solar systems in their early stages of growth. ALMA (Atacama Large Millimeter/submillimeter Array) has captured images of young stars and their disks of material. And in those disks, they’ve spotted the tell-tale gaps that signal the presence of growing young planets.

As they ramped up their efforts, astronomers were eventually able to spot the young planets themselves. All those observations helped confirm our understanding of how young solar systems form.

But more recent research adds another level of detail to the nebular hypothesis, which guides our understanding of solar system formation.

A new study presents evidence showing that the nebular hypothesis needs updating. The study is titled “A Case of Simultaneous Star and Planet Formation.” The lead author is Felipe Alves, an astronomer from the Center for Astrochemical Studies (CAS) at the Max Planck Institute for Extraterrestrial Physics (MPE). The study is published in The Astrophysical Journal Letters.

The nebular hypothesis was first developed by German philosopher Immanuel Kant, an important person during the Enlightenment. In his 1755 work “Universal Natural History and Theory of the Heavens,” he proposed that the planets formed from gas and dust orbiting the Sun. He was correct, of course. Over the years, the hypothesis has been refined and added to. The modern version may be more correctly called the solar nebular disk model (SNDM) or solar nebular model.

There are other proposed solar system formation theories, but the large majority of scientists work with the SNDM.

The SNDM says that stars form from giant molecular clouds, which are unstable. Instabilities cause matter to clump together, and over time the clumps join together and gather even more material. Eventually, enough gathers to trigger fusion, and a star comes into being.

The young star is surrounded by a rotating disk of gas and dust called a protoplanetary disk. Planets form out of that material, and as they form, they carve gaps in the disk. Astronomers have imaged many of these disks and their gaps and have even spotted individual planets. Those observations also show that the original mother cloud of gas that the star formed from has cleared out.

ALMA’s high-resolution images of nearby protoplanetary disks, which are results of the Disk Substructures at High Angular Resolution Project (DSHARP). The gaps are created by forming planets. Credit: ALMA (ESO/NAOJ/NRAO), S. Andrews et al.; NRAO/AUI/NSF, S. Dagnello

“We present a new case of star and planet formation happening in tandem.”

Paola Caselli, director at MPE and head of the CAS group.

The idea grew that there was a dividing line between a star gathering material from the cloud and planets’ formation. Planets form after the star has formed, according to those observations, and that became a part of the SNDM to many.

That’s where this study comes in.

In this study, astronomers spotted a system where the protoplanetary disk is in the planet-forming stage while the young star is still accreting material from the original cloud.

“We present a new case of star and planet formation happening in tandem,” states Paola Caselli, director at MPE and head of the CAS group. “Our observations strongly indicate that protoplanetary disks keep accreting material also after planet formation has started. This is important because the fresh material falling onto the disk will affect both the chemical composition of the future planetary system and the dynamical evolution of the whole disk.”

The team of astronomers used ALMA to probe the tip of the Pipe Molecular Cloud (Pipe Nebula) and examine the young stellar object [BHB2007] 1, also called simply BHB1. BHB1 is inside Barnard 59, a part of the vast dust cloud called the Pipe Nebula.

On the left is a picture of the Milky Way, with an orange clump of gas and stars in the center, the Pipe Molecular Cloud. On the right is a close up of part of that region, called Barnard 59. The young star in this study, BHB1, is inside Barnard 59. Image Credit: Left: ESO/VVV Team/A. Guzmán. Right: By European Southern Observatory – http://www.eso.org/public/images/eso1233a/; see also https://www.flickr.com/photos/esoastronomy/7787363490/, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=30171793

The ALMA observations showed the expected disk of material around the star, where planets are forming. But it also showed large filaments of gas still feeding into the disk and the star. The researchers identified these as accretion streamers, where material from the ambient cloud is still being fed into the protostar. In their paper, they write that “If these features are indeed filaments, they appear to be orbiting the system like large propellers or gas streamers accreting into the disk.” 

BHB1 is about one million years old, and at that stage of evolution, the disks are normally already formed and mature enough for planet formation. The astronomers were surprised to see these streamers.

“We were quite surprised to observe such prominent accretion filaments falling into the disk,” said Alves in a press release. “The accretion filament activity demonstrates that the disk is still growing while simultaneously nurturing the protostar.”

Artistic rendition of a protoplanet forming within the accretion disk of a protostar Credit: ESO/L. Calçada http://www.eso.org/public/images/eso1310a/

Though they interpret them as filaments feeding into the star and the disk, they also point out that there may be other interpretations. “Alternatively, we could be seeing a limb-brightened large-scale (>1000 au) outer disk or flattened envelope, where the central channels are too optically thin to observe in emission,” they write.

In any case, they confirm that these filaments of gas are not out-flowing. “What we can say with certainty is that we see large-scale emission associated with the disk, based on its velocities that is moving too slowly to be an outflow, and is oriented parallel to the disk plane rather than along a conventional outflow axis.”

This false-colour image shows the filaments of accretion around the protostar [BHB2007] 1. The large structures are inflows of molecular gas (CO) nurturing the disk surrounding the protostar. The inset shows the dust emission from the disk, which is seen edge-on. The “holes” in the dust map represent an enormous ringed cavity seen (sideways) in the disk structure. Image Credit: MPE

The ALMA observations uncovered something else. The team found an enormous cavity in the disk, 70 AU in diameter. The cavity encompasses a zone of hot molecular gas. Data from those observations, combined with supplementary data from the Very Large Array (VLA), suggest that a substellar object resides there. It could be either a brown dwarf or a young giant planet. According to the cavity size, the team says that the object within must be in the range of 4 to 70 Jupiter masses. That’s an admittedly wide range for a planet, but the team writes that “While the range is large, all masses point to a super-Jupiter sized companion…”

In their paper, the authors write: “As described in this Letter, the system presents a complex morphology, with a clean and wide gap in the dust millimetre continuum, surprising for such a young object. Within the gap, there appears to be gas, and some kind of localized warm emission, also seen with the Karl G. Jansky Very Large Array (VLA). Furthermore, this system does not appear to be “finished” with accretion from the molecular cloud environment, as we see large-scale, velocity-coherent filaments in the ALMA 12CO data.”

The localized warm emission they’re referring to is the suspected substellar object.

The ALMA 12CO data refers to carbon monoxide observations where the carbon is the carbon 12 isotope. 12CO is used to trace the disk’s molecular gas. Other CO isotopes are used, with each one having different properties and observations of each one revealing different gas velocities, rotations, and morphologies.

This figure from the study shows different observations of the disk and filament. a on the upper left shows the clear ring-shaped structure of the disk. b in the middle left shows 12CO ALMA observations of the disk. c shows 18CO ALMA observations of the disk. d on the right shows the large scale filaments feeding into the disk and the young star in 12CO ALMA observations. Image Credit: Alves et al, 2020.

In their conclusion, the team of researchers sum up their findings. “We report the discovery of a disk with a wide gap, even though the disk itself still appears to be fed by extended filaments detected in molecular gas. As a result, this system asks the question, can planets form before the disk itself is fully formed?”

This study says yes, but it’s not a definitive answer. It’s just the beginning of an answer. “Our data are well represented by a model of a protoplanetary disk carved by a giant planet or brown dwarf from which bright non-thermal emission is produced,” they write.

This isn’t the first time astronomers have found what appears to be a star and its planets growing simultaneously. Earlier this year, another team of researchers working with ALMA observed the young stellar object IRS 63. That team found that the young protostar is still growing while the protoplanets are still growing.

The young star IRS 63 has baby planets forming around it while the star itself is still forming. Image Credit: Segura-Cox et al, 2020.

These new results mean that our old understanding, which may still apply to some solar systems, is not universal. It also brings up some other possibilities.

These early-forming planets may eventually spiral into their still-forming stars and be destroyed. That hasn’t been seen yet, but who knows?

If that’s the case, then our own Solar System may have spawned more planets, now long gone. Our dear old Earth may have had siblings that we’ll never know about. Siblings that had the misfortune of being formed too soon and being swallowed up by our Sun.

We’ll never know.

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Evan Gough

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