Why are there so many different kinds of planet?

A new study reveals fine-grain differences in the chemistry of protoplanetary compounds: the ingredients for building a planet.

Protoplanetary discs provide the raw materials that planets are assembled from. Within this swirling skirt of dust and gas surrounding a newly forming star, grains collide and coalesce together to create pebbles, which themselves amalgamate into large boulders, then planetesimals, and finally fully grown planets.

Advertisement

The exact composition of the local protoplanetary neighbourhood therefore determines the composition of each planet and is ultimately why, for example, Earth turned out so different from Mars or Neptune.

Illustration showing a star surrounded by a protoplanetary disk. Credit: NASA/JPL-Caltech
Illustration showing a star surrounded by a protoplanetary disk. Credit: NASA/JPL-Caltech

The problem for scientists who are trying to understand the factors affecting the final outcome of planetary systems is just how much variation there might be in the chemical composition – not only between different protoplanetary discs, but also within the same disc as you move outwards from the central star.

However, a huge jump has been made in how to understand these questions now a large partnership of astronomers has used the ALMA (the Atacama Large Millimeter/submillimeter Array) to scrutinise systems in the process of forming planets.

A dark storm spotted on Neptune by the Hubble Space Telescope. Credit: NASA, ESA, STScI, M.H. Wong (University of California, Berkeley) and L.A. Sromovsky and P.M. Fry (University of Wisconsin-Madison)
How did Neptune end up being so different from Earth?. Credit: NASA, ESA, STScI, M.H. Wong (University of California, Berkeley) and L.A. Sromovsky and P.M. Fry (University of Wisconsin-Madison)

ALMA offers high resolution radio observations of the distribution of molecules in protoplanetary discs.

The results of this ambitious programme are being published as a whole series of 20 separate papers in a special edition of The Astrophysical Journal.

One study was led by Charles Law, a PhD student at the Harvard and Smithsonian Center for Astrophysics. Law’s team created high-resolution maps of the distribution of 18 different compounds within the protoplanetary discs of five newly forming stars.

These compounds included carbon-rich compounds such as hydrocarbons like c-C3H2, oxygen-rich species like carbon monoxide CO, and molecules containing nitrogen, such as nitriles like HC3N – some of which are thought to be important in the origins of life.

The raw data, collected by ALMA from 2018 to 2019, required a 100 terrabyte hard drive to store it all.

The antennas of the Atacama Large Millimeter/submillimeter Array (ALMA) in the Chilean Andes. Credit: ESO/C. Malin
The antennas of the Atacama Large Millimeter/submillimeter Array (ALMA) in the Chilean Andes. Credit: ESO/C. Malin

Law then began the task of processing and analysing all this interferometer data to generate the separate maps of each different compound.

Law’s team discovered that the chemicals within the discs are not stirred uniformly throughout – there is a variation in the concentrations of different compounds.

His molecular maps showed a diversity of distribution patterns: some compounds were present throughout the disc except for a clear gap, while others existed in concentric rings around the star, and others still were present close to the star but dropped off further out.

Protoplanetary disk around young star HD 163296 ALMA, 15 SEPTEMBER 2021 Credit: ALMA (ESO/NAOJ/NRAO)/D. Berry (NRAO), K. Öberg et al (MAPS)
Protoplanetary disk around young star HD 163296, photographed by ALMA, 15 SEPTEMBER 2021. Credit: ALMA (ESO/NAOJ/NRAO)/D. Berry (NRAO), K. Öberg et al (MAPS)

Such structures within the disc could be due to differences in the dust particles within the disc, or compounds freezing onto the surface of dust grains, or being created or destroyed by ultraviolet radiation from the star.

What these maps make abundantly clear is that different planets form in molecular soups with varying ingredients, depending on their location within the disc.

This has huge implications for what sorts of planets will form at what distance from their star, and what their composition will be – all factors that will have a profound impact on our ideas of what a planetary system ‘should’ look like, and how habitable to life they might be.

Advertisement

Lewis Dartnell was reading Molecules with ALMA at Planet-forming Scales (MAPS) III: Characteristics of Radial Chemical Substructures by Charles J Law. Read it online at arxiv.org.