How do gaseous planets form?

The first planets outside our solar system were not discovered until the late 20th century, and they immediately challenged everything we thought we knew about the formation and evolution of planetary systems. As these discoveries accumulate, our understanding of how planets can form becomes clearer.

The main current theory on the formation of star systems is the nebular hypothesis. It was originally developed as early as the 18th century, when the German philosopher Immanuel Kant and the French astronomer and mathematician Pierre-Simon Laplace proposed it, independently of each other. Over the years it was abandoned and almost forgotten, until it resurfaced, in its updated version, in the late 1960s.

According to this theory, the interstellar medium contains huge clouds of gas, consisting mainly of molecular hydrogen and called “molecular clouds”. These clouds serve as sort of “birth centers” for stars, which form from cloud material, which also includes dust. The nebular hypothesis states that when a new star – i.e. a new sun – forms, not all of the material in the cloud is absorbed by the star. Some of the gas and dust that is not absorbed by the star accumulates around it in the form of a flat disc, called the “protoplanetary disc”. Planets will later form from this protoplanetary disk.

Stars form by a process of inward collapse following the disintegration of the cloud. When the mass of a molecular cloud is large enough, or following an external disturbance such as a collision with another cloud, it becomes gravitationally unstable and breaks up into many smaller fragments. Each of these cloud fragments collapses due to its own gravitational force. These, too, split into even smaller fragments which continue to crumble until a star forms from each one. The unaccreted remnants of the cloud form a rotating circumstellar disk around each star, so the protoplanetary disk is a byproduct of the star formation process.

According to the nebular hypothesis, planets form by a process opposite to that which formed their sun: they do not collapse inwards but rather grow from the inside out. It is a slow and continuous process in which the cosmic dust grains of the protoplanetary disk collide with each other and gradually consolidate into small pebbles.

These pebbles, in turn, continue to clump together to form large mile-sized chunks of rock called planetesimals, or in other words, ultimately small fractions of a planet. A planetesimal is large enough to pull solids and gases found in the protoplanetary disk by the force of its gravity, at a rapid rate, and grow into a protoplanet. The larger the protoplanet, the more effectively it cleans up disk matter around its orbit and even accretes smaller planets.

At relatively close distances from the star, the temperature of the disk is above the freezing temperature of volatile compounds such as water, methane and ammonia. Therefore, these volatile substances exist in a gaseous state and cannot condense on dust particles and pebbles and thus enlarge the core of the planet. This is also the explanation, according to this hypothesis, why the planets closest to the Sun in our solar system – Mercury, Venus, Earth and Mars – are rocky planets.

Since the temperature of the disc decreases as the distance from the star decreases, at distances greater than the “frost line” – the particular distance at which the temperature of the disc corresponds to the freezing temperature of these volatiles – there is much more solid matter that can participate in the planet formation process. Therefore, at these distances, much larger protoplanets, with a core composed of rock and ice, can form.

Giant protoplanets are able to accrete an incredible amount of gas from the disk and become gas giants, such as Jupiter and Saturn. At even greater distances, the density of disc material is lower. Therefore, smaller ice giants, such as Uranus and Neptune, will form there. This model, which explains the formation of gas giants via the accretion of large amounts of gas onto the initial core of the protoplanet, is called the core accretion model.

These dynamics are highly consistent with the structure of our solar system, leading to the conclusion that gas giants can only be found beyond the frost line. But surprisingly, the first planet discovered around a sun-like star, 51Pegasi b, is a gas giant that orbits very close to its sun, at a distance almost 19 times smaller than that separating Earth’s orbit from the Sun.

For this discovery, researchers Michel Mayor and Didier Queloz, received the 2019 Nobel Prize in Physics. Since then, many gas giants which have short orbital periods and are therefore close to their suns, have been discovered and named from informally “Hot Jupiters”, after the planet Jupiter. The main explanation is that these gas giants originally formed far from their star, beyond the frost line, and then migrated to a closer orbit.

A new discovery, published a few months ago in Nature Astronomy, challenges the basic accretion model as an accepted explanation for the formation of gaseous planets. It turns out that the nebular hypothesis could support an alternative explanation for the formation of the gas giants, known as the disc instability model. According to this model, gas giants can form directly from the collapse of disintegrated disk fragments, without the need to initially form a protoplanetary core, in the same way that planets form from fragments of molecular clouds.

In a study led by astronomer Thayne Currie, using the Subaru Telescope at the National Astronomical Observatory of Japan in Hawai’i, researchers discovered a gaseous protoplanet near the star AB Aurigae, found in the constellation Auriga. It is a very young star in astronomical terms – only 1 to 5 million years old, and it is indeed still surrounded by a protoplanetary disk.

In photos obtained from the Subaru Telescope in Hawaii and the Hubble Space Telescope, researchers identified the protoplanet as a bright orange speck on the disc, which appears to change position along its orbital path around the star in images taken in different years.

The researchers estimated that the mass of this protoplanet is nine times greater than that of Jupiter and that its distance from its star is 3.1 times greater than the distance of Neptune from the Sun. At such a distance from the star, the disk is not dense enough to form gas giants by coagulation, as suggested by the Core Accretion Model, supporting the idea that this giant planet formed because of the disc instability.

Since AB Aurigae’s planetary system is still very young, the possibility that the gas giant coagulated closer to the star and then migrated outward can be ruled out. Additionally, the researchers identified spiral arms connecting the protoplanet to regions near the disc. In their opinion, this result is also consistent with the predictions of the disk instability model.

This is the first direct evidence of the formation of gaseous planets from the collapse of unstable regions of the protoplanetary disk. It therefore appears that gas giants can form in different and diverse ways. The more planetary systems we discover, the deeper our understanding of how our solar system and other planetary systems formed.