Terrestrial and sub-Neptune exoplanets are expected to form in the inner regions of protoplanetary disks around their parent stars. Water plays a key role in their formation, although it is yet unclear whether water molecules are formed in situ or transported from the outer disk. The planetary system PDS 70 is a benchmark target to search for water in a disk hosting an 8-billion-km planet-carved gap separating an inner and outer disk. Using the data from the Mid-InfraRed Instrument (MIRI) onboard the NASA/ESA/CSA James Webb Space Telescope, astronomers have now detected water vapor in the inner disk of PDS 70. This is the first detection of water in the terrestrial region of a disk already known to host two or more protoplanets.
This artist’s concept portrays PDS 70 and its inner protoplanetary disk. Image credit: NASA / ESA / CSA / J. Olmsted, STScI.
PDS 70 is a K7-type star located 370 light-years away from Earth in the constellation of Centaurus.
Also known as V* V1032 Cen and IRAS 14050-4109, this star is cooler than our Sun and is estimated to be 5.4 million years old.
PDS 70 hosts two protoplanets and a huge circumstellar disk of dust and gas in which a large region from 20 to 40 AU is cleared of dust.
The inner planet, PDS 70b, is located within the disk gap at a distance of about 22 AU from the star, similar to the orbit of Uranus in our Solar System.
The outer planet, PDS 70c, is located near the outer edge of the disk gap at 34 AU from the star, similar to Neptune’s distance from our Sun.
Astronomers haven’t yet detected any planets forming within the inner disk of PDS 70.
However, they do see the raw materials for building rocky worlds in the form of silicates.
“We find a relatively high amount of small dust grains,” said Radboud University astronomer Rens Waters, co-author of a study published in the journal Nature.
“Combined with our detection of water vapor, the inner disk is a very exciting place.”
The discovery raises the question of where the water came from.
Dr. Waters and colleagues considered two different scenarios to explain their finding.
One possibility is that water molecules are forming in place, where we detect them, as hydrogen and oxygen atoms combine.
A second possibility is that ice-coated dust particles are being transported from the cool outer disk to the hot inner disk, where the water ice sublimates and turns into vapor.
Such a transport system would be surprising, since the dust would have to cross the large gap carved out by the two giant planets, PDS 70b and PDS 70c.
Another question raised by the discovery is how water could survive so close to the star, when the star’s ultraviolet light should break apart any water molecules.
Most likely, surrounding material such as dust and other water molecules serves as a protective shield.
As a result, the water detected in the inner disk of PDS 70 could survive destruction.
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G. Perotti et al. Water in the terrestrial planet-forming zone of the PDS 70 disk. Nature, published online July 24, 2023; doi: 10.1038/s41586-023-06317-9
