One of the most exciting recent developments in astronomy is the observations of protoplanetary disks in space, such as the disk that formed our solar system. The formation of disks in space has long fascinated astronomers (Section 4-2). Once astronomers settled on the protoplanetary disk hypothesis during World War II, they went to work on disk formation models. The Hubble Telescope confirmed the protoplanetary disk hypothesis with images of disks in the late 20th century. Since 2011, the ALMA array has collected many detailed images of protoplanetary disks.
One of the most amazing theoretical advances in astronomy was Immanuel Kant’s conception of the nebular hypothesis, which was that the solar system, galaxy, and the rings of Saturn formed in a disk around a central large mass.
The modern version of the nebular hypothesis is the protoplanetary disk model. Disks form in the cloud of gas, ice, and dust around young stars, and particles in the disk atmosphere fall to the disk midplane. Planetestimals and then planets form in the disk. Recent observations of disks around other stars, as well as the configuration of solar system planets, indicate that the circumsolar disk around our sun was unusual.
An atmosphere of gas surrounds the midplane dust layer in disks. Depending on time and intensity of radiation from a star, disk atmospheres slowly vanish.. Disks have three classifications. Protoplanetary disks have complete disk atmospheres, transition disks have partial disk atmospheres, and dusty disks have no disk atmospheres.
Observations of disks and exoplanets around other stars, as well as the configuration of solar system planets, indicate that the circumsolar disk around our sun was unusual.
The architecture of the inner solar system (arrangement of the four inner terrestrial planets) has puzzled astronomers for decades. Mars has only 10% of the mass of Earth, but protoplanetary disk models predict that Mars should be as massive as the Earth because Mars' position in the protoplanetary disk should have allowed it to grow as large as the Earth.
During the second day or age, Moses described the natural formation of an expanse. There are grammatical reasons to conclude that the expanse had the shape of a disk. Moses also described a miracle in which God separated the inner part of the disk from the outer part of the disk.