A new study by an international team (including IoA researchers) may have shed light on a long-standing mystery of planet formation: why are young planetary systems so rich in gas?

To understand why this is such an important question, let's step back a little. The formation of planets is one of the most important processes for astronomers to understand. This is because it touches on some of the most profound mysteries of all. One of humanities biggest questions is whether we are alone in the Universe. Astrophysicists have now discovered almost 4000 exoplanets located outside of our Solar System, and we are closer than ever to finding an answer to this mystery. The answer will probably not come from a direct detection of extraterrestrial life, but rather from a good understanding of nearby planetary systems. For example, we can now study exoplanets in enough detail that we can tell whether their surfaces are hospitable for life. We are also able to look for molecules that would be produced by living organisms, and to check that the planet is not being blasted by harmful radiation. We therefore live at a unique moment in history, where we have the ability to answer existential questions concerning the presence of life on other planets.

As planets are challenging to detect (they are far smaller and dimmer than stars, after all), astronomers often study planetary systems in formation by looking at the gas and dust mixed up with the young planets. Studying the gas content of more mature planetary systems has only become possible in the last few years, thanks to the latest generation of millimetre-wavelength observatories (such as ALMA, the Atacama Large Millimeter Array). Initially, astronomers were surprised to find gas in mature planetary systems. It was generally thought that a gas disc would only exist in the very earliest stages of a planetary system's life (the so-called 'protoplanetary disc' phase, which lasts just a handful of million years), and would be long gone in older systems. But the observations were clear: older planetary systems, aged between 10 Million and 100 Million years old, have gas discs too. This was not predicted by models of planet formation, and astronomers have been hard at work ever since trying to understand where it comes from.

There are two possible solutions to the problem. Either these gas discs are left-over remnants of the original protoplanetary disc, or they are created separately at a later date (maybe being formed from gas evaporating from the system's rocky bodies). This 'secondary origin' model agrees with some observations -- it matches the amount of gas we see, for example in the systems where low quantities of gas is detected -- but up to now there has been a big problem with the idea.

To understand the problem, we need to look at the chemical composition of the gas. The gas in young forming planetary systems is mostly hydrogen (H2), but also contains other molecules like carbon monoxide (CO). And the CO is critically important: H2 is all but invisible to astronomer's telescopes, so the carbon monoxide molecules are what astronomers are actually looking at when they measure gas discs. The issue is that CO is a very fragile molecule. It is easily broken up by high-energy ultraviolet (UV) photons from space. For CO to survive it must be protected, or 'shielded' (typically by a blanket of H2 molecules). It is thought that any primordial CO would exist alongside protective hydrogen envelope, allowing it to survive, while any CO that came later (secondary origin) would be unprotected and wouldn't last long.

So this is the problem. Astronomers observe mature planetary systems with massive gas discs (observed in CO). The amount of gas present and its distribution suggest that the discs aren't a left-over remnant from the system's earliest times, but were formed later. But any CO that arrives late should get destroyed by energetic UV photons from space.

This new study provides a novel solution which resolves the paradox. It turns out, Carbon monoxide can look after itself! CO, evaporating from volatile-rich planetesimals, initially gets destroyed by UV photons (as expected). But the fragments left over after a molecule is destroyed -- carbon and oxygen atoms -- build up and form their own protective shield. Once the initial CO has been destroyed, and the 'shield' has built up, CO discs are protected and free to grow.

The authors tested their new shielding model on the system around the star HD 131835. CO was already observed in this system. So to test the theory, they used ALMA to obtain new resolved observations of carbon atoms which should be protecting the CO. And the theory works! Firstly, the mass of carbon atoms in the system is indeed high enough to act as a 'shield' for CO. And secondly, the amount of CO is compatible with the 'secondary source' model for the gas disc. While this is the only massive system with observed carbon atoms, the authors also look at other massive gas discs and show that the observed CO masses are compatible with their shielding model. The paper also makes predictions of how much shielding carbon there should be in these systems, which can be tested at a later date.

Lead author Quentin Kral, astronomer at the Paris Observatory (previously at the Institute of Astronomy, Cambridge) said "This is an exciting discovery that resolves an important mystery in planet-formation theory. Thanks to our work, we can now understand the origin of these massive gas discs around mature planetary systems, which in turn gives access for the first time to the composition of the exoplanetesimals that release this gas, which we will soon compare to the composition of planetesimals in our own Solar System".

Read the paper here: https://arxiv.org/abs/1811.08439