“Wet” planetary nursery discovered

Artist’s rendering of the water-rich inner disc of PDS 70 and the two gas giant planets that have already formed. © MPIA

A birthplace of livable planets? Astronomers using the James Webb Space Telescope have discovered water in the protoplanetary disk of a distant star. The steam floats in an area in which typically Earth-like planets form. Thus, they could benefit directly from the considerable water reservoir. The discovery sheds light on the initial conditions that can lead to the formation of "blue" rocky planets with potentially habitable features, the researchers say.

On our planet, it has become the elixir of life: all terrestrial organisms depend on water. It has not yet been finally clarified why the earth has comparatively plentiful reserves of the versatile hydrogen-oxygen compound. Even their building blocks could have contained water, but many astronomers assume that the later delivery by asteroids was decisive. "We may now have found evidence that water could be one of the earliest components of rocky planets and is already present at their birth," says lead author Giulia Perotti from the Max Planck Institute for Astronomy (MPIA).

Signature of the Elixir of Life

The team made the discovery in the system of the relatively young star PDS 70, which is about 370 light-years away from us. It still has a protoplanetary disk of dust and gas. In the outer areas, however, two gas giant planets have already formed - PDS 70 b and c. They have accumulated dust and gas around their orbits as they have grown, creating a wide annular gap in the protoplanetary disk. Astronomers' attention turned to the remaining disk area in the central zone around the star. They targeted this region with the Mid-Infrared Instrument (MIRI) on board the James Webb Space Telescope (JWST). The recordings enabled spectral analyzes of the light from the disc, which can provide information about the components of the material.

As the team reports, the signature of significant amounts of water emerged. According to the data, the H₂O is in the form of vapor at a temperature of 330 degrees Celsius. What is special is that the "wet treasure" wafts around the star in an area that corresponds to where the rocky planets, including Earth, are located in our solar system. "This discovery is extremely exciting as it probes the region where Earth-like rocky planets typically form," says co-author Thomas Henning of MPIA. As the team points out, they have now, for the first time, detected water in a protoplanetary disk at a more advanced stage of development.

How could the water survive?

Because previous studies failed to detect water in the central regions of similarly evolved disks, astronomers suspected that the compound there could not withstand the harsh stellar radiation and decayed into hydrogen and oxygen. However, the current discovery shows that this does not always have to be the case. Unless it's an extreme exception, it could mean that many Earth-like planets forming in these zones are already being born with a key ingredient that enables life, the scientists say.

Because the water find was rather unexpected, the team of astronomers also explored scenarios to explain its origin. One possibility, then, is that the water is a remnant of an originally water-rich nebula that preceded the disk stage. It seems conceivable that surrounding dust and the water molecules themselves acted as a shield from the destructive power of stellar radiation. At least some of the water discovered near PDS 70 may have survived the destruction, the scientists say.

Another source could also be gas flowing from the outer edges of PDS 70's disk into the more central areas. Under certain circumstances, water vapor could form from oxygen and hydrogen gas. In addition, water-rich dust particles originating from the remaining outer dust ring could be entrained by the suction of the moving gas. Only when they get close to the star could their ice then evaporate.

"The truth probably lies in a combination of all of these possibilities," says Perotti. "Nevertheless, it is likely that one of these mechanisms plays a crucial role in refilling the water reservoir of the PDS 70 disk. In the future it will be a question of finding out which one it is," says the astronomer. The team is now hoping for further observation data with the James Webb Space Telescope, which should provide even more detailed images of the inner disk of PDS 70.

Source: Max Planck Institute for Astronomy, specialist article: Nature, doi: 10.1038/s41586-023-06317-9

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