Photo of earth and celestial bodies taken from space

Groundbreaking paper in the journal Nature by professor Ian Stephens illuminates formation of planets

January 10, 2024
By: Nancy Sheehan

Ian W. Stephens, assistant professor of physics and astronomy, is the lead author of a new paper in the scientific journal Nature that helps to explain the fascinating process of how dust grains evolve to form planets and other celestial bodies.

The paper, Aligned grains and scattered light found in gaps of planet-forming disk, presents data that, in its clarity and completeness, went far beyond what other scientists have done in the field. Stephens is the lead author and the paper is co-authored with an international team of scientists from universities in the United States, Argentina, Japan, China, Australia, Mexico, and France.

“I looked at the famous source HL Tau in polarization at an unprecedented resolution of 5 au [1 au is the distance from the earth to the sun]. This is by far the best map of polarization toward a circumstellar disk,” Stephens said, of the research for which he led an international team of scientists.  

Here’s an attempt at translation for the non-scientist: HL Tauri (abbreviated as HL Tau) is indeed famous in astronomy circles. It is a young star in the constellation Taurus, about 480 light-years from Earth. And by ‘young’ we mean no more than about a million years old. Despite its relative youth, the star is already surrounded by a well-developed circumstellar disc made of gas and dust that rotates around the star. HL Tau’s disc is in the process of making planets, asteroids, and comets. It is the brightest young circumstellar disc that can be seen from a telescope.

Planets and asteroids grow within the rings and gaps of these circumstellar discs. The dust grains in the disc coagulate and collide, gradually getting bigger and bigger. Eventually, the dust grains stick together, and the accumulation and growth of these dust grains forms larger objects like planets and asteroids and over millions of years. However, the exact mechanisms and details of how and when this growth occurs are still not fully understood.

To gain a clearer understanding of the process, Stephens captured images of HL Tau’s disc taken with the powerful ALMA telescope in the Atacama Desert of Chile. This high-altitude location sits above a large portion of the atmosphere, which can greatly reduce what can be seen by telescopes, and it offers an exceptionally clear view of the skies.

Stephens and his team were looking to understand the polarization of light emitted by the dust grains in the circumstellar disc’s rings and in the gaps between them. The polarization of the light can provide information about the shape and alignment of the dust grains. 

By studying the polarization of the grains, the researchers were able to gain insights into the shape, size, and alignment of these planetary building blocks. The study found that the dust grains in the disc are elongated and aligned in a certain direction, possibly due to their own aerodynamics. 

This new information is crucial for understanding the early stages of planet formation and provides valuable insights into the processes that shape our solar system and others. Previous observations did not distinguish between the polarization in the gaps and the rings due to the limited resolution. The study found that the polarization in the gaps was even stronger than in the rings, despite the gaps having less dust. This discovery challenges previous assumptions about the nature of the gaps in stellar discs.

ALMA, which stands for Atacama Large Millimeter/submillimeter Array, is the largest radio telescope in the world. Located in the Andes Mountains at about 17,000 feet elevation, the telescope serves researchers from Europe, North America, East Asia, and Chile, according to the ALMA observatory website. Telescope operators receive about 1,800 requests for observations from scientists every year. They accept only a few hundred of them, and Stephens’ proposal was among those deemed worthwhile.

Ian Stephens holding a globe.

Ian Stephens holding a globe

“I asked for a lot of time compared to a typical proposal,” he said. “To construct, ALMA had a $1.4 billion price tag on it. It’s an expensive telescope and continues to be expensive to operate. I basically asked for a little over a day’s worth of observations. I don’t even want to know how much that costs.” 

Before taking on Stephens’ proposal, ALMA had observed polarization of many circumstellar disks. However, the polarized signal emanating from the discs is very weak, so it was hard to do it at a resolution high enough to reveal the level of detail that Stephens and his team wanted. “We knew that the polarized signal should depend on the substructure, that is, rings and gaps in the disk, but observations of polarization at such high resolution is very difficult,” Stephens said. “But I found that it was possible to do this if I targeted the brightest proto-planetary disk possible and stared at it with ALMA for over a day.”

The data collected was from a large amount of dust, providing a significant amount of information. The study focused on a specific wavelength (submillimeter, 870 µm) that allowed for the detection of light emission from the dust grains. The study’s findings obviously are impactful for scientists, but Stephens said they also are important for the public for several reasons: 

  •   Understanding planet formation: The observations of the proto-planetary disc provide valuable insights into the early stages of planet formation. By studying the dust grains and their properties, scientists can learn more about how and when planets, including our own, may have formed. 
  •   Advancing astronomical knowledge: The high-resolution polarization observations conducted by ALMA are groundbreaking and push the boundaries of our understanding of disc constituents. These findings contribute to the overall body of astronomical knowledge and help scientists refine their models and theories. 
  •   Technological advancements: The ability to observe polarization at such high resolution and detail is a testament to the capabilities of the ALMA telescope and Stephens’ groundbreaking work will likely lead to a shift towards more aggressive polarization observations in the field. Researchers will recognize the importance of high-resolution observations and the need to gather detailed information about polarization in discs and gaps. 
  •   Inspiring curiosity and wonder: Discoveries like these can inspire awe and curiosity in the public. They highlight the beauty and complexity of the universe and encourage people to learn more about the wonders of space and the scientific advancements that enable us to explore it. Overall, these findings contribute to our understanding of the universe, inspire scientific curiosity, and showcase the capabilities of modern astronomical research.

That sense of curiosity and wonder is what keeps Stephens looking toward the heavens. “A lot of astronomy is about figuring out our origins, and this study tells you an awful lot about what happened here because Earth obviously went through a similar process,” he said. “Historically, we’ve always had huge curiosity about what’s out there so this is just a continuation of that curiosity with more detail to help us better understand where we came from and to someone like me, that’s fantastic.” 

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