Where's the planet that should be to blame for this star's carved-out disk?
How do planets form? This deceptively simple question has sparked a new field in astronomy over the last two decades. Astronomers have used telescopes all over the world (and in orbit!) to study all stages of planet formation, from disks of material around infant stars to the scattered debris and planetesimals in systems such as Fomalhaut’s. The goal of all these studies is the development of a coherent scenario that explains the behavior and evolution of planet-forming disks around young stars and links them to the exoplanets observed around older stars (such as those glimpsed by Kepler).
New results on a particularly exciting system, V1247 Orionis, may shed light on an important, seldom-observed stage of planet formation. Stefan Kraus (Harvard-Smithsonian Center for Astrophysics, University of Michigan, and University of Exeter, UK) and his colleagues observed in visible to far-infrared wavelengths (about 400 nm to 100 microns) the star V1247 Orionis, a young star hiding in Orion’s Belt that is part of the extensive star-forming association there. V1247 Ori has a mass about twice that of the Sun but is very young, with an age from 5 to 10 million years. The star was known to harbor a disk, but Kraus and his team used a slew of ground-based instruments in the Northern and Southern Hemispheres to better understand the disk’s properties.
Disks form around young stars with the material left over from star formation. The most widely accepted planet-formation scenario predicts that it takes a few million years to build planets from these disks, with dust grains gradually clumping together to form larger objects. Once these planetesimals gain enough mass, they begin to gravitationally interact and disrupt the disk they formed from, creating gaps in an otherwise smooth pancake.
In the last few years, observations of very young stars with disks have shown tantalizing evidence of just this scenario. Most notably, observations of the star LkCa15 revealed a planetary companion carving out a gap in the star's disk.
Kraus and his team deduced the presence of a gap spanning from about 0.2 to 46 astronomical units in the disk of V1247 Ori, slightly larger than the distance between the Sun and the Kuiper Belt’s inner edge and about the same size as the gap around LkCa 15. Yet when they used the Keck 10-meter telescope to look in the gap, they couldn’t find any protoplanets. Instead, they detected a complex, perhaps clumpy distribution of diffuse dust. Because the astronomers observed at a range of infrared wavelengths (the gap is most readily visible in the 1.5 to 13 micron range), they were able to probe a larger variety of dust temperatures than usual, which might explain the novel results.
V1247 Ori is the first star of the team’s observing campaign, and it’s possible that the star is experiencing a normal, early stage of disk clearing. But astronomers expect to find a planet if there’s a gap, and that gap "should" be clear. Perhaps the dusty haze is a short-lived transition period between gap and planetesimal formation.
An unexpected result from this analysis concerns the diffuse material’s composition. Previous observations of other holey disks have pointed to a silicate-based dust around these stars, like pulverized rock. However, in the case of V1247 Ori, the dust is probably carbon-based. Carbon is the building block of organic material, and is also thought to be prevalent in some exoplanets (such as 55 Cancri e). One other system, Beta Pictoris, shows signs of carbon-rich debris.
As astronomers gather more information on the planet-formation process, new questions and details continue to emerge. The type of investigation led by Kraus and his collaborators paves the way for studies with new telescopes, such as JWST and ALMA. These telescopes will be capable of directly observing the locations of planet formation, and the future looks bright for unraveling the mysteries of this process.
Reference: S. Kraus et al. "Resolving the gap and AU-scale asymmetries in the pre-transitional disk of V1247 Orionis." Accepted to the Astrophysical Journal, posted to arXiv.org 9 April 2013