Planet formation remains one of the mysteries of astrophysics. Find out how a new study sheds light on the process and offers a new method for future research in the field.
One odd thing about growing up during the Space Race is that I can’t remember when I found out there were other planets. It feels as though I’ve always realized that Earth is just one planet out of many orbiting an unremarkable star of which there are countless others.
Everyone had stars and planets on their minds in the 60s and 70s. Yet, nobody seemed to be curious about where worlds like Earth come from.
Even today, planet formation is essentially a mystery. Scientists explain the formation of our solar system using a model called the nebular theory.
Model Called the Nebular Theory
The concept originated with Emanuel Swedenborg in 1734. Immanuel Kant (yes, that Kant) elaborated on the idea in 1755. They both believed that gas clouds rotate and flatten into disks to form the stars and planets in solar systems.
Pierre-Simon Laplace developed a similar theory in 1796. He pictured the sun’s atmosphere stretching all the way out to the edge of the solar system before contracting and cooling to form a disk.
The idea of planets forming out of what scientists call a protoplanetary disk fell in and out of favour. James Clerk Maxwell famously rejected the idea, believing that the material in such a spinning cloud couldn’t condense to cause planet formation.
Observing Disks Made of Gas and Dust Around Young Stars
Soviet astronomer Victor Safronov confirmed the nebular theory in 1969. Astronomers have confirmed Safronov’s model by observing disks made of gas and dusk around numerous young stars in our galaxy.
Now, researchers with the National Radio Astronomy Observatory have shed new light on the planet formation process. The Astrophysical Journal published their findings in two research papers.
The astronomers focused on a young star system called Elias 2-27. It’s about 400 light-years from Earth in the constellation Ophiuchus, the serpent bearer.
The team found that Elias 2-27 showed signs of gravitational instabilities. This happens when an infant star’s disk is carrying a large proportion of the mass of the star system.
Teresa Paneque-Carreno is a Ph.D. candidate at the University of Leiden and the European Southern Observatory. She led the research team, and she’s also the lead author of the Astrophysical Journal paper on planet formation.
“How exactly planets form is one of the main questions in our field,” Paneque-Carrero explained. However, there are some key mechanisms that we believe can accelerate the process of planet formation.” Gravitational instability is one of those mechanisms and was the focus of this study.
“One of the Main Questions in Our Field”
Paneque-Carreno outlined the results from the team’s observations of Elias 2-27 this way. “We found direct evidence for gravitational instabilities in Elias 2-27, which is very exciting because this is the first time that we can show kinematic and multi-wavelength proof of a system being gravitationally unstable. Elias 2-27 is the first system that checks all of the boxes.”
Elias 2-27 is an interesting specimen for another reason. Paneque-Carreno explains, “There may still be new material from the surrounding molecular cloud falling onto the disk, which makes everything more chaotic.” It also has extensive spiral arms, almost like a galaxy would have.
One reason planet formation is still poorly understood is that there’s been no way to directly measure the protoplanetary disk’s mass. However, the National Radio Observatory’s Atacama Large Millimeter/submillimeter Array (ALMA) in Chile has changed that.
Largest Radio Telescope Array in the World
Because ALMA is the most extensive radio telescope array globally, it delivers much greater sensitivity to researchers. ALMA’s added sensitivity enabled the team to observe the disk’s dynamic processes, density, and mass.
“This finding lays the foundation for the development of a method to measure disk mass that will allow us to break down one of the biggest and most pressing barriers in the field of planet formation,” explained Bernadette Veronesi, a postdoctoral researcher at Ecole normale superieure de Lyon. Veronesi is the lead author of the other Astrophysical Journal article, which is about the measurement method the team developed.
Veronesi went on to say, “Knowing the amount of mass present in planet-forming disks allows us to determine the amount of material available for the formation of planetary systems and to better understand the process by which they form.”
Better Understand the Planet Formation Process
Elias 2-27’s chaotic nature gives it another intriguing feature. Unlike most protoplanetary disks, it lacks vertical symmetry.
Panaeque-Carreno explains, “The Elias 2-27 star system is highly asymmetric in the gas structure. This was completely unexpected, and it is the first time we’ve observed such vertical asymmetry in a protoplanetary disk.”
There’s another new mystery arising from the study. “While gravitational instabilities can now be confirmed to explain the spiral structures in the dust continuum surrounding the star,” Paneque-Carreno added, “there is also an inner gap, or missing material in the disk, for which we do not have a clear explanation.”
Two Groundbreaking Discoveries
As a result of all this work, the team has made two groundbreaking discoveries. One is that Elias 2-27’s spiral arms seem to be due to gravitational instability. The other is that the disk mass measurement technique they’ve developed can be used to study other protoplanetary disks. That’s why the team published two separate papers.
These discoveries were no easy feat. “Studying how planets form is difficult because it takes millions of years to form planets,” Paneque-Carreno said. “This is a very short time-scale for stars, which live thousands of millions of years, but a very long process for us.”
For as long as humans have walked the Earth, they’ve wondered how our world began. We’ve told one another stories to make sense of the world and our place within it.
One Step Closer to Planet Formation Story
These discoveries take us one step closer to having a fact-based story about planet formation that we can all share globally. These are the stories we need to anchor us in troubling times.
Paneque-Carreno wrapped up by calling for further research, saying, “It’s like looking at a crime scene and trying to guess what happened. Our observational analysis paired with future in-depth analysis of Elias 2-27 will allow us to characterize exactly how gravitational instabilities act in planet-forming disks and gain more insight into how planets are formed.”
We always have more to learn if we dare to know.
Study of Young Chaotic Star System Reveals Planet Formation Secrets
Spiral Arms and a Massive Dust Disk with non-Keplerian Kinematics
A Dynamical Measurement of the Disk Mass in Elias 2-27
Astrobiology: Three Questions We Need to Answer
Planets Form from Stardust: Rapidly and Frequently
Exoplanet Water Common Yet Rare