Part of Planet Formation Possibly Seen in Real-time

Although it may take millions of year for swirling clusters of interstellar gas and dust to become a mature planet, scientists have discovered that rapid changes can be observed even within a fraction of that time span.?

Over the course of five months, the researchers observed that the infrared light from a disk of gas and dust around LRLL 31, a young star, tended to vary in unexpected ways. This suggests that another star ? or perhaps a planet ? is shoving the clump of planet-forming material around, which causes its thickness to vary as it spins around the star.

The observations were made using NASA's Spitzer Space Telescope. Before Spitzer was launched in 2003, only a few transitional disks with gaps or holes were known. With Spitzer's improved infrared vision, dozens have now been found. The space telescope caught the warm glow of the disks, which allowed researchers to map out their structures.

"We don't know if planets have formed, or will form, but we are gaining a better understanding of the properties and dynamics of the fine dust that could either become, or indirectly shape, a planet," said James Muzerolle of the Space Telescope Science Institute. "This is a unique, real-time glimpse into the lengthy process of building planets."

Some have theorized that as dusty grains swirling around a star in a disk, and begin to bulk up in size on their way to becoming a planet, they carve out gaps in the dust until a transitional disk takes shape with a large doughnut-like hole at its center. Eventually, this disk fades and a new type of disk emerges, made up of debris from collisions between planets and asteroids and comets. Over time, a more settled and mature solar system like our own forms.

Muzerolle and his team focused on a family of young stars in the IC 348 star-forming region of the constellation Perseus, many with known transitional disks. The stars are about two to three million years old and about 1,000 light-years away. Although a few of the stars showed surprising hints of variations, the astronomers decided to study LRLL 31 over a five month period.

Light from the inner region of the star's disk changes every few weeks -- sometimes in only one week. Both the intensity and the wavelength of infrared light varied over time. For instance, when the amount of light seen at shorter wavelengths went up, the brightness at longer wavelengths went down, and vice versa.

Muzerolle and his team think that a companion to the star circling in a gap within the system's disk could explain the findings.

"A companion in the gap of an almost edge-on disk would periodically change the height of the inner disk rim as it circles around the star: a higher rim would emit more light at shorter wavelengths because it is larger and hot, but at the same time, the high rim would shadow the cool material of the outer disk, causing a decrease in the longer-wavelength light. A low rim would do the opposite. This is exactly what we observe in our data," said Elise Furlan, a co-author from NASA's Jet Propulsion Laboratory.

Researchers plan to follow up with ground-based telescopes to see if a companion is tugging on the star hard enough to be perceived. The companion would have to be close in order to move the material around so fast -- about one-tenth the distance between Earth and the sun.

NASA?s Spitzer Space Telescope will again be used, this time to observe the system in its "warm" mission to see if the changes are periodic, as would be expected with an orbiting companion.

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