Almost 4 000 exoplanets have now been discovered. To clarify the issues involved in the search for extra-terrestrial life in the Universe, a crucial preliminary step to do even before thinking about observing them directly, is to characterize the planetary systems in our Galaxy.
One way to to this is to study very young planetary systems in the process of formation (the protoplanetary phase), via an analysis of their gas and dust content, a medium in which orbit protoplanets which are being born.
With the new generatioin of observatories working in the millimeter domain (as for example the ALMA radiotelescope), the study of more mature planetary systems and their gaseous composition becomes possible.
Astronomers are focusing on this evolutionary step which follows the protoplanetary stage precisely because planets have been formed. Here, the observations are clear : these more evolved planetary systems, whose age lies between 10 and 100 million years, also have gas discs, even though planetary formation models predict the contrary.
Astronomers are intrigued by the presence of this gas. Two hypotheses have for a long time been advanced :
Either these gas discs are the remains of a young protoplanetary phase ;
or they have another origin : they could have been created later and the gas would have evaporated from the planetesimals, rocky bodies created by the coalescence of dust grains to make compact bodies which orbit in these systems somewhat like the asteroid or Kuiper belt in our system. The search for carbon monoxide (CO) is one way by which astronomers could determine the composition of planetesimals in evolved planetary systems.
Increasing numbers of massive CO discs are being discovered, and over ten are know so far. The quantity and distribution of the gas which is there suggests that these discs are not the young protoplanetary phase but are the product of a later degassing (secondary origin).
Nevertheless, since the CO model is fragile, even if it was produced relatively late, it should have been destroyed by the continuing UV radiation in space. How can one explain this paradox ?
A new model : the shield
The new work furnishes an explanation : the carbon monoxide can protect itself. The CO which evaporates from the planetesimals is initially destroyed by the UV photons (as expected). After the destruction of the CO molecules, the ambient fragments — carbon and oxygen atoms — create their own protective shield. Once a large mass of CO has been destroyed, the shield becomes sufficiently thick as to protect the CO ; and so the CO disc starts to grow and to spread.
The authors of this work have tested their model via observations with ALMA of the system surrounding the star HD 131835. The CO had already been observed in this system. Thus, to test their theory, they analyzed the carbon atoms supposed to constitute the CO shield.
Theory corroborated by observation
Observations have corroborated the theory. Firstly, the mass of carbon atoms in the system is effectively sufficiently large to constitute a shield for the CO. And finally, the quantity of CO observed is compatible with the hypothesis that the gas does have a ’secondary’ origin and that the observed gas has thus really been caused by the planetesimals of the system.
The paper also makes a prediction for the quantity of carbon in other systems where massive discs have been observed, a prediction which could well be tested in the near future.
"This very important discovery solves a deep mystery in the theory of planetary formation. Thanks to our work, we now understand the origin of these massive gas discs which are observed around mature planetary systems. This, for the first time, furnishes information about the composition of exoplanetesimals which relinquish gas in these systems, which one can very well compare with the composition of planetesimals in our solar system », notes Quentin Kral, astronomer at the Paris Observatory and first author of the paper.
Reference
This work is published in a paper entitled « Imaging [CI] around HD 131835 : reinterpreting young debris discs with protoplanetary disc levels of CO gas as shielded secondary discs » by Q. Kral, et.al. in the November 22nd 2018 issue of Monthly notices of the Royal Astronomical Society.
