For the longest time, one of the biggest mysteries about our solar system was its formation. As we pointed our telescopes to other stars in the sky, we began discovering new star systems filled with exoplanets (1). Telescopes like the Kepler space telescope helped boost the number of exoplanet discoveries, but the question of star system formation still had no answer. As observation technology advanced, scientists were able to analyze different star systems in even greater detail. With some luck and patience, we were finally able to look at protoplanetary disks that were just beginning to form planets. These star systems helped explain how our solar system might have formed and why planets form.
Now, let’s start at the end of a supernova. The gases that were spread out from the death of a previous star form into a cloud, which is called a nebula. Higher density areas of gas can start to collapse onto itself due to gravity and begin to spin. A protostar emerges as more and more gas condenses at the center of the cloud. Due to the energy generated by the angular momentum and the density of the gases, the protostar experiences extreme internal pressure and heat, which kickstarts the process of nuclear fusion. Nuclear fusion allows the protostar to start converting hydrogen into helium to generate heat. At this point, we can start to call this collapsed nebular cloud a protoplanetary disk (2).
Protoplanetary disks have been tricky to observe in the past as the disk has to be oriented perpendicular to our viewing angle from telescopes. Since there are so many large telescopes searching the skies nowadays it has become increasingly easier to discover systems that are properly oriented for our observation. One such system was detected and observed by the ALMA telescope in Chile, which was pointed towards the HD 163296 system (once again what a creative name). Previous methods of planet detection used measurements of star brightness, and a dip in brightness would indicate a planet transiting its star. Instead, the ALMA telescope was able to directly image the protoplanetary disk, which clearly shows where the planets are located (see image below). As planets begin to form within a protoplanetary disk, they start to clear out their orbit of gas and rock. This leaves gaps in the disk, and each contains a newborn planet. In the case of HD 163296, there are three planets forming: the first planet is at 80 AU, the second is at 140 AU, and the third is at 260 AU, which is beyond the main part of the protoplanetary disk. These three planets all have a similar mass to Jupiter (3).
Another more recent observation of a protoplanetary disk happened at Europe’s Very Large Telescope. This new system is called PDS 70, and it has one lonely planet forming at around 20 AU away from its sun. Unlike the planets observed by ALMA, this planet is a much more massive gas giant that is several times larger than Jupiter. The picture below shows that this gas giant has cleared most of its orbit as the gap is extremely prevalent (4).
Likewise, we can apply this knowledge to our solar system. Around 4.5 billion years ago our sun was born from a nebula, which allowed the formation of the protoplanetary disk. Rocky planets condensed and formed within 4 AU of our sun and gas planets formed beyond the ice line (5), which is over 4 AU away from the sun. The planets cleared their orbits of particles and gases, which allowed each planet to gain more mass. At the end of the solar system formation, we were left with eight main planets and many more dwarf planets (2).
- Exoplanets: Planets that are found orbiting other stars.
- Ice Line: The boundary in a star system where gas giants condense beyond the line and rocky planets form before the line.