Protoplanetary disks surrounding the stars: The birth cradle of planets

The classical theory of planet formation can be traced back to the German philosopher Kant, who speculated that the Sun and the major planets were created in a cloud of faint gas and dust that gradually collapsed and flattened. In the theory, the planets are predicted to move about the Sun in low eccentric orbits and they are nearly coplanar. Nowadays, astronomers have developed this theory to further improve our understanding of planet formation process based on modern astronomical observations and high-speed computations. The widely accepted model of planet formation suggests that molecular clouds collapse to form the protostars in the central region, while the gas and dust remain in the protoplanetary disk surrounding young stars. In the subsequent process, the gas and dust in the protoplanetary disk can be further cooled down and gradually condensed into larger particles, then grow into planetesimals; the planetesimals can form planetary embryos through mutual gravitational interaction and frequent collisions, finally produce terrestrial planets and gas-giants. In this so-called core-accretion scenario, there exists a key issue to be first solved-the gas-giants such as Jupiter should be formed right before the protoplanetary disk entirely dissipates, because these gaseous planets, which initially start from a terrestrial core of a mass up to ten Earth masses, can only accumulate hydrogen and helium when the protoplanetary disk is still present. Based on observations of the protoplanetary disks and the age of young stars, the astronomers have shown that the lifetime of protoplanetary disks is roughly estimated to be 1-10 million years. This indicates that the formation process of gas-giants needs to be supremely fast. The entire formation scenario from the gas and dust in the molecular cloud to more than a dozen Earth mass planetary cores and then to the gas-giants should be completed in approximately 10 million years. In 2014, the ALMA (Atacama Large Millimeter/submillimeter Array) imaged the first protoplanetary disk around the HL Tauri system with high-resolution, which shows a surprising outcome of a series of concentric bright rings separated by gaps in this system. Such kind of structure was simply previously predicted by the theory or direct numerical simulations. According to the planet formation, when a planet grows up to a certain mass, it will open an annular gap in the protoplanetary disk around the star. Since the observed structure of the HL Tauri disk is probably related to a potential planetary system in the disk, this indicates that the planets scratch the disk. Such high-resolution images have brought a deep understanding of protoplanetary disk and planet formation. In the last few years, more and more gas and dust disks have been observed surrounding other young stars by ALMA and other ground-based projects. These observations do provide us the morphology of planet-forming regions, thereby showing a variety of patterns of spiral structure and ring structure, etc. The rich images of planet-forming regions can help us have a better understanding of the formation of our own solar system planets and other systems beyond us.