Planet Formation

Our own Solar System and its planets were formed about 4.6 billion years ago. While astronomers can study asteroids, the leftover of this planetary formation, impact craters, and other clues about the creation of the Solar System, it is impossible to truly turn back the clock. Studying a range of planets, including exoplanets, at different stages of their evolution is one way astronomer can determine how planets form.

The birth of a solar system

An artistic representation of a protoplanetary disk around a young star. (Credit: NASA)

One of the most fundamental questions a planetary scientist might ask is “how do planets form?”. This seemingly simple question has a very complicated answer. Since our Solar System is roughly 4.6 billion years old, it is challenging to find concrete evidence of how it formed. One thing we do know is that planets likely form in the dusty and gassy discs (called protoplanetary discs) that surround newborn stars.

There are two main theories about how planets form. The first potential process is called core accretion and states that planets form out of tiny particles. Small dust grains collide, stick together, and grow bigger and bigger until they become roughly a kilometer in size. At this point, we would call such a baby planet a planetesimal.

The second theory states that the protoplanetary disc is not smooth everywhere, and certain dense regions in the disc might collapse under their own gravity, forming a planetesimal directly in place. This theory is called the gravitational instability method of planet formation.

Both theories have their own challenges and advantages in explaining where planetesimals come from, but it is not totally clear which is the dominant formation method. Real planet formation may happen via one of these two processes, or a combination of both. In any case, once a population of planetesimals form, they can grow and collide and interact to create full-sized planets.

Modern computer simulations are able to model this planetesimal evolution. As they grow, the planetesimals build enough mass that nearby material gets pulled towards them by gravitational attraction. This may cause a phase of runaway growth that can create giant gas planets. Other planetesimals might struggle to reach this critical mass or reach it only after a nearby giant planet has already scooped up all the available gas. These ones may collide with one another and remain small rocky planets. In a relatively short period of time (a few million years), most of the protoplanetary disc material is either collected inside young planets and planetesimals or is ejected out of the system entirely.

How do we know?

How can we test these theories? For a long time, the only evidence available to us was in certain components of the Solar System that can keep a stable record for at least 5 billion years. Meteorites that fall to Earth’s surface may have been floating through space since they formed during the early stages of the Solar System. Their interiors tell us what kind of materials were available when the Solar System was created. Some of the comets we see entering our Solar System were initially kicked out when the planets were forming and are only just returning. The material in their glowing tails can provide a pristine snapshot of the outer Solar System’s chemical makeup.

We can also look at ancient craters to help establish a timeline of events. Craters on Earth are erased by erosion, life, and plate tectonics far too quickly to map out a 5 billion year history. However, places like the Moon or Mercury have none of these issues. The crater record reveals how common large impacts were in the past. It makes sense that in the very early phases of the Solar System’s formation, there were many impacts since there was still a lot of debris floating around. Interestingly, crater records show there was a second peak of impact rates much later called the “Late Heavy Bombardment”.

Any theory of how our Solar System formed needs to address all these pieces of evidence. Does it create asteroids and comets like we observe? Does it produce planets like the ones we have and in similar orbits? Can it explain the Late Heavy Bombardment?

Planetary baby pictures

A direct image of a planet forming around the star PDS 70 (whose light is blocked by a coronagraph) taken by the SPHERE instrument at the VLT telescope. (Credit: ESO/A. Müller et al.)

In recent years, telescopes have improved to the point that we can take detailed pictures of new stars and their protoplanetary discs by looking at sub-millimetre, millimetre, and radio emissions. We use a method called interferometry to take such high-resolution images. This technique uses many different telescopes that can be separated by several kilometers, all acting together like one giant telescope. These images map the warm dust particles in these discs that emit radiation at longer wavelengths.

The details we are discovering are striking and hint at how discs may develop and evolve in other systems. We can clearly see structures in the discs that may be causing, or are caused by, brand new planets! It is reasonable to assume that our own Solar System looked something like these when it was newly forming. Much of today’s planetary formation research focuses on refining our theoretical models so that they match the increasingly detailed observations.

Planet formation at iREx

Some of the research that is developing and refining planet formation and evolution models is being carried out by iREx members. In some cases, they are figuring out how the protoplanetary disc can form into rings and planets. They are also looking into how formation conditions affect the resulting planet and how it would appear in our telescopes. Some groups even model how planets might migrate around their systems as they evolve, and what that means for the whole system. To learn more, we invite you to read their profiles: