In recent years, astronomers have observed distant solar systems in their early stages of growth. ALMA (Atacama Large Millimeter / Submillimeter Array) took pictures of young stars and their disks of material. And on these disks they discovered the tell-tale gaps that indicate the presence of growing young planets.
Eventually, as they stepped up their efforts, astronomers were able to spot the young planets for themselves. All of these observations have helped to confirm our understanding of the formation of young solar systems.
However, recent research adds another level of detail to the nebula hypothesis that guides our understanding of the formation of the solar system.
A new study shows that the fog hypothesis needs to be updated. The study is entitled "A Case of Simultaneous Star and Planet Formation". The main author is Felipe Alves, an astronomer from the Center for Astrochemical Studies (CAS) at the Max Planck Institute for Extraterrestrial Physics (MPE). The study was published in The Astrophysical Journal Letters.
The fog hypothesis was first developed by the German philosopher Immanuel Kant, an important figure during the Enlightenment. In his 1755 work, "Universal Natural History and Theory of Heaven," he suggested that the planets are formed from gas and dust that orbit the sun. He was right, of course. Over the years the hypothesis has been refined and supplemented. The modern version can more correctly be called the Solar Nebular Disk Model (SNDM) or the Solar Nebular Model.
There are other proposed theories about the formation of the solar system, but the vast majority of scientists work with the SNDM.
The SNDM says that stars form from huge molecular clouds that are unstable. Instabilities cause matter to clump together, and over time the clumps combine and collect even more material. Eventually enough to start the fusion, enough accumulates and a star is formed.
The young star is surrounded by a rotating disk of gas and dust called the protoplanetary disk. Planets are formed from this material, and as they form they carve gaps in the disk. Astronomers have mapped many of these disks and their gaps and even discovered individual planets. These observations also show that the original mother gas cloud from which the star formed has dissolved.
ALMA's high-resolution images of nearby protoplanetary plates, the results of the Disk Subructures at High Angular Resolution Project (DSHARP). The gaps are created by the formation of planets. Photo credit: ALMA (ESO / NAOJ / NRAO), S. Andrews et al .; NRAO / AUI / NSF, S. Dagnello
"We present a new case of star and planet formation in tandem."
Paola Caselli, director at MPE and head of the CAS group.
The idea arose that there was a dividing line between a star collecting material from the cloud and the planetary formation. According to these observations, planets form after the star is formed, and that became a part of the SNDM for many.
This is where this study comes in.
In this study, astronomers discovered a system in which the protoplanetary disk is in the planet formation stage while the young star is still accumulating material from the original cloud.
"We are presenting a new case of star and planet formation in tandem," explains Paola Caselli, director at MPE and head of the CAS group. “Our observations strongly suggest that protoplanetary disks continue to accumulate more and more material even after planets have started to form. This is important because the fresh material falling on the disk affects both the chemical composition of the future planetary system and the dynamic development of the entire disk. "
The team of astronomers examined the tip of the Pipe Molecular Cloud (Pipe Nebula) with ALMA and examined the young star object (BHB2007) 1, also simply called BHB1. BHB1 is located in Barnard 59, part of the giant cloud of dust called the Pipe Nebula.
On the left is an image of the Milky Way galaxy with an orange lump of gas and stars in the middle, 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 in Barnard 59. Photo credit: Left: ESO / VVV Team / A. Guzmán. Right: From the 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 form. But there were also large gas filaments that were still feeding into the disk and star. The researchers identified these as accretion streamers, in which material from the surrounding cloud is still fed into the protostar. In their work they write: "If these features are actually filaments, they seem to orbit the system like large propellers or gas streamers that collect in the disc."
BHB1 is about a million years old, and at this stage of evolution the disks are usually already formed and mature enough for planets to form. The astronomers were surprised to see these streamers.
"We were quite surprised to see such prominent accretion filaments fall into the disc," Alves said in a press release. "The accretion filament activity shows that the disc is still growing while nurturing the protostar."
Artistic rendering of a protoplanet forming inside the accretion disk of a protostar
Credit: ESO / L. Calçada
Although they interpret them as filaments feeding into the star and disk, they also indicate that there may be other interpretations. "Alternatively, we could see a large-area (> 1000 au) outer disk brightened by limbs or a flattened shell in which the central channels are too optically thin to be observed during emission," they write.
In any case, they confirm that these gas filaments do not flow away. “What we can say with certainty is that we are seeing a large scale emission associated with the disk based on its speeds moving too slow to be a drain and parallel to the disk plane, not along it is aligned with a conventional drainage axis. ”
This false color image shows the accretion filaments around the protostar (BHB2007) 1. The large structures are inflows of molecular gas (CO), which nourish the disc surrounding the protostar. The inset shows the dust emission from the pane, which can be seen at the edge. The “holes” in the dust map represent a huge annular cavity that can be seen (on the side) in the disc structure. Photo credit: MPE
The ALMA observations revealed otherwise. The team found an enormous cavity in the disk with a diameter of 70 AU. The cavity includes a zone of hot molecular gas. Data from these observations, combined with additional data from the Very Large Array (VLA), suggest that there is a substellar object there. It could either be a brown dwarf or a giant young planet. Based on the cavity size, the team says the object must be in the range of 4 to 70 Jupiter's masses. That is an admittedly long range for a planet, but the team writes: "While the range is great, all the masses point to a companion the size of Super Jupiter …"
In their paper, the authors write: “As described in this letter, the system has a complex morphology with a clean and wide gap in the dust millimeter continuum, which is surprising for such a young object. Inside the gap there appears to be gas and some sort of localized warm emission that can also be seen in the Karl G. Jansky Very Large Array (VLA). In addition, this system does not appear to be "done" with accretion from the molecular cloud environment, as we see large, velocity-coherent filaments in the ALMA 12CO data. "
The localized warm emission they refer to is the presumed substellar object.
The ALMA 12CO data refer to carbon monoxide observations where the carbon is the carbon 12 isotope. 12CO is used to track the disc's molecular gas. Different CO isotopes are used, each with different properties and observations, and showing different gas velocities, rotations and morphologies.
This figure from the study shows various observations of the disk and filament. one top left shows the clear ring-shaped structure of the disc. b in the middle left shows 12CO ALMA observations of the disc. c 18CO ALMA shows observations of the disc. d On the right-hand side you can see the large-area filaments that feed into the disk and the young star in 12CO ALMA observations. Photo credit: Alves et al., 2020.
Finally, the research team summarizes their results. “We report the discovery of a disk with a large void, even though the disk itself still appears to be fed by extended filaments that have been detected in molecular gas. As a result, this system raises the question of whether planets can form before the disc 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 is represented well by a model of a protoplanetary disk carved from a giant planet or brown dwarf that is producing a bright non-thermal emission," they write.
This is not the first time astronomers have found a star and its planets growing at the same time. 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.
Baby planets are forming around the young star IRS 63, while the star itself is still forming. Photo 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. There are some other possibilities as well.
These early-forming planets can eventually transform into their still-forming stars and be destroyed. That hasn't been seen yet, but who knows?
If so, our own solar system may have produced more planets that have long since disappeared. Our dear old earth may have had brothers and sisters we will never know about. Siblings who were unfortunate enough to be formed too early and swallowed up by our sun.
We will never know.