The dust that made us: planets and stars

Protoplanetary disks are the dusty disks left behind after an interstellar gas cloud has become dense enough to collapse into a protostar core (pre becoming a fully formed star), and it is from the material in these disks that planets are made.

Low density gas in the pace between stars, known as the interstellar medium, is mainly made from hydrogen molecules, but can also include water or carbon monoxide. When large groups of molecules become attracted to each other by gravity, they can eventually form dense clumps and collapse under their own weight, becoming so hot that the protons fuse with each other and begin to burn, eventually forming a protostellar core. Gas continues to swirl around the young star, which feeds off this gas to grow in mass, and shoots jets of excess material from its north and south poles. Through increasing angular momentum, the dusty gas sphere enveloping the protostar core eventually flattens into a disk, in a similar way to how a twirling ballerina's skirt flattens when rotating quickly.

The diagram below shows the process of star/planet system formation.

Credit: Frontiers for Young Minds, authors/illustrators Majken Brahe, Ellegaard Christensen

This diagram shows the timeline of star/planet formation starting from the beginning as a collapsing molecular cloud, through the disk phase and eventually arriving at the stage of a host star orbited by planets after millions of years. The sketches are not to scale, but an approximation of size for each step is given, where 1AU is the distance from Earth to the Sun, or about 150 million km.

Stars evolve first from gas and dust in the interstellar medium, and a few million years later planets can form from the surrounding dust grains leftover. Our Solar System is one such example of this.

One potential way of spotting planets in these young stellar systems is from the rings they might trace out in the dusty gas as they orbit their young host star, accumulating material as they sweep across the dust filled disks. The most famous image of this process, taken by the ALMA Observatory, shows the protostar HL Tauri and its disk, which contains multiple ring-like gaps in the dust that astronomers have suggested may be caused by orbiting planets.

Credit: ALMA (ESO/NAOJ/NRAO). This is the sharpest image ever taken by ALMA . It shows the protoplanetary disk surrounding the young star HL Tauri. These new ALMA observations reveal substructures within the disc that have never been seen before and even show the possible positions of planets forming in the dark patches within the system.

Protostellar disks are at the intersection of star and planetary research, as they overlap in their formation history, where stars across the universe evolve into systems such as our Solar neighbourhood. Planets that orbit stars other than the our host star, the Sun, are called exoplanets, and several thousand exoplanets have been discovered to date. Some astronomers have dedicated their careers to searching for exoplanets which could be similar to Earth in other star-planet systems. Given that there are estimated to be billions of star/planet systems in our Milky Way galaxy alone, and estimates of billions to trillions of galaxies in the universe, it could just be that somewhere far far away, an Earth-like twin planet is orbiting its own version of what we call the Sun, having formed from a similar protoplanetary disk.