Spiral Arms Discovered in Planet-Forming Disk
According to a new paper co-authored by UA researchers, an ultra-rare spiral structure lives inside a planet-forming disk, some 400 light-years from here.

By Emily Litvack, University Relations - Communications
Oct. 9, 2015


Astronomers add one rare, majestic piece to the puzzle of figuring out how rare planets such as Earth are in the Milky Way.
Astronomers add one rare, majestic piece to the puzzle of figuring out how rare planets such as Earth are in the Milky Way.

Using a powerful, state-of-the-art new instrument, Kevin Wagner, a first-year graduate student in University of Arizona's Department of Astronomy, and his adviser Daniel Apai were hunting for exoplanets — planets that orbit stars outside of our own solar system.

But as Wagner and his colleagues pored over images of a young star some 400 light-years from Earth and twice the size of the sun, they noticed something curious.

It's not uncommon for young stars such as HD100453 to be encircled by planet-building disks. The disk that orbits this particular star is like most others, too: It's a gigantic, orbiting cosmic pancake of gas and dust. But inside it, they discovered, lives an odd, beautiful, symmetrical, two-armed spiral structure.

Up to now, after observing hundreds of young stars, astronomers discovered such spirals only in two other stars — and the one around HD100453 is by far the closest to Earth and most symmetrical in shape. Each of the two arms of the disk is about 3 billion miles, or about 40 times longer than the Earth-sun distance.

Compared to our own solar system, the gap in this disk ends at about the orbit of Uranus, and the spirals extend to about the orbit of Pluto, suggesting that HD100453 may resemble the young solar system and it may be where scientists should look for ongoing formation of giant planets.

Using the planet-finding SPHERE instrument on the European Southern Observatory's Very Large Telescope, the research team took the first high-resolution, high-contrast images of HD100453 and discovered the unique spiral protoplanetary disk.

"(We are) trying to figure out how planetary systems form, but we can't watch it happen because it takes tens of millions of years," Wagner said.

Instead, by studying multiple systems in different stages of evolution, researchers are able to make inferences on their general evolution from young disks of star- and planet-forming material to mature planetary systems.

This disk has not been imaged previously, and the spiral structure likely indicates interaction with unseen planets, according to Apai and other astronomers, although so far the observations have not been sensitive enough to detect them.

"Directly imaging planets around other stars is exciting, but very challenging — even giant planets are about a million times fainter than their host stars," Apai said. "The rare disk structures, such as Kevin's majestic two-armed spiral, are our best indicators of where the just-forming planets may hide."  

Finding out how many systems are like our own is an important part of answering the question of how rare planets such as Earth are in the Milky Way. Investigating how, when and where planets form in the disks around young stars will help pin down that number.  

The team's images also show a large gap in the disk, seen for the first time in this system and probably suggesting the presence of one or two massive, undetected planets, which could be driving the spiral arms and quickly clearing the disk of material, Wagner said.

He explained that while the possibilities are compelling, the only way to know for sure is to revisit the spiral by taking even more sensitive images to search for the planets themselves.

"We can't wait to see what our next, more detailed images will reveal," Apai said.

Apai, principal investigator and assistant professor of astronomy and planetary sciences, led the proposal and the observations, while Wagner performed the data reduction and analysis and is lead author on the paper. They are members of the major NASA-funded project Earths in Other Solar Systems team.

Apai, Wagner and astronomers Markus Kasper of the European Southern Observatory and Massimo Robberto of the Space Telescope Science Institute will have their results published as a letter in the Astrophysical Journal.

Ultimately, this kind of imaging and analysis is about answering big questions, Wagner said.

"Is our solar system rare or typical? Are there others like ours?"