Title: Water in the terrestrial planet-forming zone of the PDS 70 disk
Authors: G. Perotti, V. Christiaens, Th. Henning, B. Tabone, L. B. F. M. Waters, I. Kamp, G. Olofsson, S. L. Grant, D. Gasman, J. Bouwman, M. Samland, R. Franceschi, E. F. van Dishoeck, K. Schwarz, M. Güdel, P.-O. Lagage, T. P. Ray, B. Vandenbussche, A. Abergel, O. Absil, A. M. Arabhavi, I. Argyriou, D. Barrado, A. Boccaletti, A. Caratti o Garatti, V. Geers, A. M. Glauser, K. Justannont, F. Lahuis, M. Mueller, C. Nehmé, E. Pantin, S. Scheithauer, C. Waelkens, R. Guadarrama, H. Jang, J. Kanwar, M. Morales-Calderón, N. Pawellek, D. Rodgers-Lee, J. Schreiber, L. Colina, T. R. Greve, G. Östlin, and G. Wright
First Author’s Institution: Max Planck Institute for Astronomy, Königstuhl 17, D-69117 Heidelberg, Germany
Status: Published in Nature, Volume 620, p.516-520 (Open Access)
Everywhere there is life on Earth, there is water. It is a key ingredient in the complex chemistry involved in creating life, and is crucial to the creation of terrestrial planets like Earth (between Mercury and Earth-sized) and sub-Neptunes (between Earth and Neptune-sized). Understanding the origin of water is therefore important to solve. The authors of today’s paper may have found an answer to this very question – where does the water for Earth-like planets come from?
Using the James Webb Space Telescope, an international team of scientists have observed a protoplanetary disc – a huge circumstellar disc of dust, gas and debris left over from star formation – around the star PDS 70. This disc is of interest as it hosts two nascent giant planets, which could mimic the origins of the solar system. The team uncovered an exciting discovery in the place where we expect terrestrial planets to form: water! Previous Spitzer space telescope observations of PDS 70 had not detected water, pointing at an inner disc devoid of molecular gas. Now, JWST’s powerful Mid-InfraRed Instrument (MIRI) has elucidated the nature of water abundance around PDS 70; MIRI has previously been used to explore the chemistry of discs around M-Dwarf stars, covered here by Astrobites. But how does MIRI do this?
An observational technique called ‘spectroscopy’ is used: gas molecules in the disc absorb light from the star and re-emit it at certain wavelengths, which we understand thanks to our detailed quantum mechanical models and extensive lab experiments. This emission can come from electrons changing their energy levels within the molecules, as well as the vibrations of the molecule itself. Emissions appear as a spike at characteristic wavelengths in the MIRI data, allowing scientists to get a breakdown of the composition of the disc (Figure 1).
Armed with this knowledge, the scientists built a “slab model” to model the emission of light from certain molecules, and then compare it against the data from MIRI. These models can constrain the abundance of a species of interest (such as water) in the disc, as they rely on the number density of gas molecules, as well as their temperature. With this, the researchers performed a chi-squared fit to find the number density and temperature of water molecules that best matched the observed data (Figure 2).
The temperature constraints allowed the team to infer where the water is in the disc – and it turned out to be right where terrestrial planets are expected to form! What’s more is that the team also discovered Carbon Dioxide (CO2) emission, although carbon-based molecules are common in protoplanetary discs, so why is this interesting? Well, it reveals key information about what is happening to the water!
Normally, the sheer power of the star is enough to decompose CO2 into its constituent components through photodissociation from ultraviolet (UV) radiation. Water, however, is an excellent absorber of UV radiation and rescues CO2 from its untimely demise! Since only some of the water is lost to photodissociation, plenty still remains available for planet formation.
Where did this water come from, anyway? It may have been delivered on rocks as they are transported from the outer disc, but the huge planet-carved gap in PDS 70 suggests otherwise – we don’t see these rocks. The authors suggest that smaller, unobservable rocks could be responsible, or there might be local chemical reactions producing the water (Figure 3).
Understanding the origin of water in PDS 70 could reveal important insights into the formation of our own solar system, and where the water on Earth came from. With its UV-shielding effects, water could act as a vanguard for more complex molecules that are needed on Earth to create life millions of years later.
Astrobite edited by Neev Shah & Karthik Yadavalli.
Featured image credit: ALMA (ESO/NAOJ/NRAO)/Benisty et al.
Edit 6th March: Corrected that Spitzer had not previously detected any water around PDS 70.
