Spirals, Gaps, and Cavities in Protoplanetary Disks: Are We Watching Planets Forming in Action?

Planets, like the ones in our own solar system, form in protoplanetary disks surrounding newly born stars. We are about to enter a golden age in the field of planet formation and protoplanetary disks, thanks to a fleet of new instruments with unprecedented resolving power that have just come online in the past few years. At near-infrared wavelengths, technological advances in adaptive optics and the use of differential imaging techniques have enabled us to achieve angular resolutions of 0.05 arcsec and inner working angles of 0.1 arcsec. At mm wavelengths, the Atacama Large Millimeter Array (ALMA) has brought us over one order of magnitude improvement over existing facilities in both resolution and sensitivity. Now for the first time in history, we are able to directly detect signals from planet formation activities on a solar system scale in nearby protoplanetary disks. Recent observations using the most powerful telescopes on Earth, such as Subaru, VLT, Gemini, and ALMA, have revealed exciting fine structures, such as spiral arms, gaps, and cavities, in many systems. These structures have been widely speculated to be related to ongoing planet formation activities, though the mechanisms are currently in debate. I will review the major discoveries in observations of disks made in the past few years, and discuss their possible origins; particularly, whether they are induced by embedded planets currently forming in these disks.