Planets Likely Cause Of Star’s Disk Gap
November 10, 2012

Multiple Planets Likely Caused Star’s Massive Protoplanetary Disk Gap

redOrbit Staff & Wire Reports — Your Universe Online

A large gap discovered in the protoplanetary disk of a Sun-like star may have resulted from the birth of multiple planets, an international team of researchers claim in a new study.

Jun Hashimoto of the National Astronomical Observatory of Japan (NAOJ), Ruobing Dong of Princeton University, and colleagues used state-of-the-art equipment to examine PDS 70 (an object discovered by the Pico dos Dias Observatory in Brazil in the 1990s).

By analyzing images of PDS 70, a young star that is approximately 10 million years old and has about the same mass as the Sun, they discovered it has the largest gap ever found in the protoplanetary disk of similar stars, the NAOJ said in a statement.

A protoplanetary disk is where planets form and the massive gap between its inner edge and the central part of the star could have been created by the gravitational force of those newborn worlds, they said. However, the size of the gap leads them to believe that no single planet, no matter how heavy it is, could have been solely responsible for its formation. The researchers surmise it may have been the result of multiple planets.

"The high contrast images from the observations allowed the researchers to study the details of the disk, which then enabled them to directly reveal the site of formation of one and possibly more planets," the NAOJ statement said. "The research team is now attempting to detect those planets."

"Protoplanetary disks occur around many Sun-like stars; they are composed of gas and dust that surround the stars and provide the materials out of which planets like the Earth form," it added. "Researchers conduct observations of protoplanetary disks to understand their evolution and the formation of planets within them. Disks around heavier stars tend to be more extended and brighter, hence easier to study in detail; those around less massive stars pose more of a challenge to research."

Studying the disks around less massive, Sun-like stars is the goal of the Strategic Exploration of Exoplanets and Discs with Subaru (SEEDS) project, which was launched in 2009. As part of that project, the researchers opted to choose PDS 70, a star with 90% of the Sun's mass that is located in the constellation Centaurus.

Previous research had suggested there might be a disk present on the star, but scientists had not previously been able to determine specifics about that structure. Now, however, Hashimoto, Dong, and colleagues have used the High Contrast Instrument for the Subaru Next Generation Adaptive Optics (HiCIAO) to locate a low-density space between PDS 70 and the inner edge of the surrounding disk, with a radius as large as 70 astronomical units (AU).

"The huge size of the gap in the disk around PDS 70 led the team to question how the gap formed. By studying the details of the spectral energy distribution (plotting the brightness of light vs. wavelength) of the star itself and the disk, they found another disk at a distance of only 1 AU," the NAOJ said. "The inner, much smaller disk is very close to the star, but the current observation does not clearly show that part of the disk because it is behind HiCIAO's mask that blocks the bright light from the central star."

"Gravitational forces from the planet(s) embedded in the disk might account for this kind of gap in the disk, because they could pull away the material from the disk, and the clearing of the material means less infrared radiation from that area," they added. "It would be very difficult for a single planet to create the giant gap in the disk around PDS 70. The research team thinks that more than one planet could be responsible for creating the gap."

Their research, which was partially funded by a Grant-in-Aid for Science Research in a Priority Area from the Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan; The Mitsubishi Foundation, Japan; and the National Science Foundation (NSF); has been published in The Astrophysical Journal Letters.