What spiral 'arms' around a star tell us about planet formation

Elias 2-27, seen by researchers at the ALMA observatory in Chile, may hold clues to helping understand how planetary formation works.

Jeff Fitlow/Rice University
Elias 2-27, shown with spiral arms similar to many galactic structures.

Spirals can be found throughout nature, from the smallest sea shells to the largest galaxies. As stars from from clouds of gas, the gravitational pull of a star will collapse the gases into a disk around it, often forming spirals in a manner similar to the galactic-scale disk of our own Milky Way.

Scientists have observed these stellar spirals before. But the spiral around Elias 2-27 is a little different.

Elias 2-27 is a young star, at least a million years old and about half the size of our own sun. Yet the protoplanetary disk of gas spiraling around the star is unusually massive, and may hold clues about planetary formation, a subject that scientists still know little about.

"The standard theory of planet formation is called 'core accretion,' where a planet core grows out of smaller particles and planetesimals," study lead author Laura Pérez, an astronomer at the Max Planck Institute for Radio Astronomy in Bonn, Germany, told Space.com. "Once the core is large enough, it quickly accretes gas from the disk and forms a planet with an atmosphere – think of Jupiter, with its massive inner core and then its massive atmosphere. However, this standard picture fails at large distances from the star. There are not enough dust particles and gas for core accretion to be proceed."

Scientists have observed spirals around stars like this before, but never with a star as young as Elias 2-27. Previous observations showed hazy spirals, often after planets had already formed, leaving scientists guessing about the early stages of the process.

The data for the phenomenon was gathered by the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, and published in the journal Science. Among the data is the first ever high-resolution photo of the spiral disk around a star's mid-plane. 

"When the project started, we wanted to see how the dust grain in these disks evolve with time," study co-author Adrea Isella said in a statement. "Planets start to form from micro-sized particles that stick together, and Laura and I thought we might be able to measure the size of the dust particles based on their distance from the star. If you look at two or three different wavelengths of light from the disk, then you can measure grain sizes as a function of radius. But when we got the data, we got this beautiful spiral."

The spiral structure is likely caused by the clumping of materials in a phenomenon known as density waves. Faster-moving matter gets caught behind slower-moving manner, causing "arms" in the disc to separate from each other. As the disk turns around the star, these arms curve, forming a distinctive spiral. This areas of density in the arms may help provide material for planetary formation farther away from the star than earlier models would indicate. 

There's still a great deal of work to be done in order to understand what exactly is going on at Elias 2-27.

"Fortunately, the power of ALMA will be used in the future to answer this puzzle," Pérez told the National Radio Astronomy Observatory. "ALMA will further dissect this and other similar disks in an upcoming large program, helping astronomers understand the seemingly chaotic forces that eventually give rise to stable planetary systems like our own."

You've read  of  free articles. Subscribe to continue.

Dear Reader,

About a year ago, I happened upon this statement about the Monitor in the Harvard Business Review – under the charming heading of “do things that don’t interest you”:

“Many things that end up” being meaningful, writes social scientist Joseph Grenny, “have come from conference workshops, articles, or online videos that began as a chore and ended with an insight. My work in Kenya, for example, was heavily influenced by a Christian Science Monitor article I had forced myself to read 10 years earlier. Sometimes, we call things ‘boring’ simply because they lie outside the box we are currently in.”

If you were to come up with a punchline to a joke about the Monitor, that would probably be it. We’re seen as being global, fair, insightful, and perhaps a bit too earnest. We’re the bran muffin of journalism.

But you know what? We change lives. And I’m going to argue that we change lives precisely because we force open that too-small box that most human beings think they live in.

The Monitor is a peculiar little publication that’s hard for the world to figure out. We’re run by a church, but we’re not only for church members and we’re not about converting people. We’re known as being fair even as the world becomes as polarized as at any time since the newspaper’s founding in 1908.

We have a mission beyond circulation, we want to bridge divides. We’re about kicking down the door of thought everywhere and saying, “You are bigger and more capable than you realize. And we can prove it.”

If you’re looking for bran muffin journalism, you can subscribe to the Monitor for $15. You’ll get the Monitor Weekly magazine, the Monitor Daily email, and unlimited access to CSMonitor.com.