Direct Images of Disks Illuminate Planet Formation

Direct images of disks unravel mystery of planet formation

High-contrast near-infrared imaging of LkCa 15 disk. The bottom image shows Neptune’s orbit in the Solar System as a scale comparison. Credit: Subaru Telescope Facility, NAOJ

The fruits of the SEEDS (Strategic Explorations of Exoplanets and Disks with Subaru) project, led by Motohide Tamura of NAOJ (National Astronomical Observatory of Japan), are accumulating. Composed of over 100 scientists and 25 institutions, the international consortium of researchers supporting the project has announced another set of stunning findings obtained with the recently commissioned Subaru Telescope instrument HiCIAO (High Contrast Instrument for the Subaru Next Generation Adaptive Optics), an upgraded version of its predecessor CIAO (Coronagraphic Imager with Adaptive Optics).

Their initial announcement of a significant discovery came in December 2009: an exoplanet candidate around a Sun-like star. Now they announce another remarkable discovery: The telescope has directly and sharply imaged two young stars that reveal how planets may have formed from their disks.

No other telescopes, whether ground-based or in space, have ever penetrated so close to central stars, showing the details of their disks.

One of the images is of the disk of the very young star AB Aur in the constellation Auriga. The star is only about one million years old and is surrounded by its protoplanetary disk, a structure of gas and dust from which planets are likely to form.

The image displays high spatial resolution as well as high contrast within the disk near its central star; it reveals, for the first time, the fine structures of a disk orbiting closer to its star than Neptune’s distance from the Sun in our solar system. AB Aur’s disk has double rings that are tilted from the equatorial plane; a void of material between the rings; and a center that does not coincide with the position of the star. These irregularities suggest that at least one giant planet is affecting the features of the disk.

Near-infrared (1.6 micron) images of AB Aur. Credit: Subaru Telescope Facility, NAOJ

The other image from the project is that of another young star, LkCa 15, which is several million years old. Although previous works speculated that there was a gap in its disk, this is the first direct imaging of a central gap in the disk. The lack of material in the vicinity of the central star implies that a giant planet is sweeping up the disk’s leftover materials that the central star did not swallow.

Overall, these first two direct images of the detailed structure of the protoplanetary disks provide strong evidence for planet formation within a system on the scale of our solar system.

The influence of heavy planets is likely to have produced the irregular features of the disks. The SEEDS project will continue for five years, allowing astronomers to search the universe for exoplanets and their protoplanetary disks, in an ongoing quest to decipher how planetary systems form. Recent and future findings from the project may eventually contribute to our understanding of how Earth-like habitable zones develop.