Planetary nurseries: the discs from which they grow

Planets form in discs of dust, gas and debris, known as protoplanetary discs. These are particularly fascinating to look at, although not typically seen in mainstream astrophysics pictures, which usually focus on galaxies or stars, or the planets within our own solar system, skipping out other planetary systems entirely. For this reason, today's blog post is dedicated to these faintly glowing discs of dust where, with little fuss or exploding, planets subtly accrete material from their surroundings from which to grow.

Debris discs are the remnants of planetary formation. Astronomers find these discs interesting because the study of these dusty, gaseous environments around stars can help us understand the birthplace of planetary systems, including our own Solar System. As mentioned in previous blog posts, stars form from clumps of collapsed molecular gas made up from molecules of the interstellar medium. These hot, swirling chaotic spheres of gas spin through angular momentum conservation, becoming increasinly flat spheres that resemble a pancake-like disc of dusty gas. It is from this messy and gritty environment that planets are born, so protoplanetary discs are the birthplaces of the planets we see today. As a planetary system gets older and settles down, the dust and debris begins to clear, potentially becoming swept up by growing planets, or gravitationally drawn in.

A complication of debris discs around stars, taken by the ALMA observatory. Image credit: ALMA(ESO/NAOJ/NRAO)/S. Marino et al.

The above image shows a compilation of 24 debris discs, the leftovers of planet formation around stars. In this image, the orange shows the distribution of dust within these discs, and the blue ones show the distribution of gas in 6 of these. Fun fact: the chap featured in the credits above was a researcher at both Cambridge and Exeter, where I met him for a brief chat about this very topic!

These discs glow as they are heated by the central star, which warms the surrounding gas and dust, hence emitting this radiation as light in the submillimetre range. For those unfamiliar, the submillimeter wavelength range sits between the infrared range and the radio range of the electromagnetic spectrum. As such, these are observed using telescopes such as ALMA, the Atacama Large Millimetre/submillimetre Array, a huge array of antennae scattered across northern Chile, which has been hugely influential in the study of planet formation and comets.

The Atacama Large Millimetre/submillimetre Array in Chile. Image credit: Y. Beletsky (ESO)

The origins of gas in debris discs remains uncertain. Potentially, it could be the leftover original (or primordial, for fancy astronomer speak) gas that was around the host star from the beginning, or maybe it is gas released later in the disc's life as dust grains collided with eachother and fragmented.  The two top right images show the debris disc around a star called HD 121617, where the orange is brighter on one side, shows a higher concentration of dust grains there. Potentially, if the gas here was of sufficient density, it could form a trap for accumulating dust particles, and this critical density corresponds to the density of gas from a primordial origin. Even our own Solar System has a debris disc, known as the Kuiper Belt, home to many asteroids and comets that orbit the Sun way beyond the furthest planet, Neptune. One theory suggests that the gravitational influence of large planets such as Neptune prevented dust and pebbles in the nearby region from clumping together to form larger bodies.

In the early stages of planet formation, planetary discs decrease in temperature with increasing distance away from the hot central star. As such, different molecules exist in different states on either side of their respective snowlines: the region where the temperature is such that a molecule changes in state from solid to liquid. (For a more detailed explanation of this with sketches, please find my long blog post on my favourite modules from my planetary science degree at Cambridge here). The snowlines for different molecules, such as CO2 or water, exist at slightly different radii from the central host star, since these molecules have different freezing/boiling temperatures. This is rather handy because if a planet is covered in liquid water, a potential theory for how this happened is that it may have originated further out in the disc, covered in ice, but as it migrated inwards and got warmer, the ice melted into a liquid state and formed oceans. Similarly, planets with a very watery atmosphere may be the result of further inward migration where oceans were boiled and any surface water was heated into a gas state. Studies of planetary discs therefore tell us about the habitats of planets where they are born, and the potential journeys made later in their lives, with any new molecules picked up along the way. An astronomical history of the physics and chemistry that has caused a planet to end up how it is today. :)