The PDS 70 disk, with its artistic rendering depicted here, has yielded an interesting discovery through observations made by the James Webb Space Telescope (JWST) and the MPIA-led MINDS research collaboration. They have detected water in the inner region of the gas and dust disk surrounding the young star PDS 70. It’s significant because this area is where terrestrial planets typically form. The presence of two gas giant planets has created a wide gap in the disk during their growth. This marks the first time water has been detected in a disk hosting multiple planets. If rocky planets form in this water-rich inner disk, it would greatly enhance their potential habitability. This finding suggests that water could be supplied to potentially habitable planets during their formation and not solely through later impacts of water-bearing asteroids. The existence of water is crucial for life as we know it on Earth. Scientists have debated how water reached our planet and whether this process could apply to rocky exoplanets in other star systems. The prevailing idea has been that water is delivered by water-bearing asteroids colliding with young planets. However, this new discovery suggests that water could be one of the initial ingredients present in rocky planets from the start. Giulia Perotti, an astronomer at the Max Planck Institute for Astronomy (MPIA), states, “We now may have found evidence water could also serve as one of the initial ingredients of rocky planets and be available at birth.” The research article detailing this find will be published in the journal Nature. The video above illustrates the detection of water near the star PDS 70 using the MIRI instrument on board the JWST. The animation begins with a view of the starry sky, then zooms in on PDS 70, highlighting the positions of the two gas-rich planets. Finally, a section of the spectrum revealing water signatures obtained by MIRI is shown. The discovery of water in the inner region of the PDS 70 disk was made possible through observations made by the MIRI instrument on board the JWST. This region corresponds to where rocky planets orbit the star in our own solar system. The analysis indicates that the water is in the form of hot vapor with temperatures reaching around 330° Celsius (600° Kelvin). Thomas Henning, the Director of MPIA and co-author of the research article, describes this finding as extremely exciting since it provides insight into the formation of rocky planets similar to Earth. The MINDS program, a JWST guaranteed-time program involving research institutions from 11 European countries, aims to identify the characteristics of gas and dust disks around young stars, shedding light on the conditions that influence planetary composition. PDS 70 is an interesting object as it represents a relatively old disk, approximately 5.4 million years old, where water has been detected. Over time, the amount of gas and dust in planet-forming disks diminishes due to processes such as a central star’s radiation or wind, as well as the growth of dust particles into larger bodies that eventually form planets. Previous studies failed to detect water in the central regions of similarly evolved disks, leading to the assumption that these regions were dry and devoid of water. However, studying PDS 70 with the MIRI instrument on the JWST challenges this hypothesis, suggesting that the inner regions of evolved, dust-depleted disks may contain water after all. If this is the case, it raises the possibility that many terrestrial planets forming in such environments could begin their existence with a readily available water source, thus increasing the chances of habitability. While no planets have been discovered near the center of the PDS 70 disk thus far, two gas giant planets, PDS 70 b and c, have been detected farther out. These planets have accumulated the surrounding dust and gas as they orbited the star during their growth, resulting in a large gap in the disk. However, rocky planets forming closer to the star in a water-rich environment would benefit from having access to water early on in their life cycles. Hence, in addition to water transport through water-bearing asteroids, this discovery suggests the possibility of a sustainable mechanism for water provision to planets from birth itself. The MINDS program will continue to investigate whether water is common in the terrestrial planet-forming zones of evolved disks around young stars or if PDS 70 is an exception. The origin of the water detected near PDS 70 is still a subject of investigation. The MINDS team is considering several possibilities to explain this finding. One hypothesis involves water being retained from a previously water-rich nebula that existed before the disk stage. Water is commonly found in its frozen state, coating tiny dust particles. When exposed to the heat near a forming star, the water evaporates and blends with other gases. However, water molecules are vulnerable and can break down into smaller components, such as hydrogen and oxygen, upon exposure to the damaging UV radiation emitted by the nearby star. Nonetheless, the water molecules, along with surrounding dust particles, can act as a protective shield. As a result, it’s possible that some of the water detected near PDS 70 has managed to survive destruction. Another potential source is the influx of gas from the outer regions of the PDS 70 disk. Under specific conditions, oxygen and hydrogen gas can combine to form water vapor. Additionally, the movement of gas may transport water-rich dust particles from the prominent outer dust ring. The faintness of the central star prevents it from evaporating the water ice in that ring. Once the dust grains enter the inner disk closer to the star, the ice transitions into a gas. Perotti suggests that the truth likely lies in a combination of these scenarios and that further research is needed to determine the predominant mechanism responsible for maintaining the water reservoir of the PDS 70 disk. While JWST and MIRI have provided valuable insights, they offer only a partial view. Astronomers employ various types of observations across different wavelengths to gain a comprehensive understanding, much like using multiple colors to complete a painting. The MIRI instrument’s spectrograph allowed the team to analyze the infrared radiation emitted by PDS 70, separating it into smaller wavelength ranges to identify different water signatures. This enabled them to calculate temperatures and densities. The astronomers have supplemented their findings with additional observations from ground-based telescopes, and they eagerly await further JWST observations that could provide detailed images of the inner PDS 70 disk. These images may reveal clues about additional terrestrial planets or slightly larger sub-Neptunes forming within the water reservoir. The ongoing progress of the MINDS program will contribute to our understanding of whether water is prevalent in the terrestrial planet-forming zones of evolved disks around young stars or if PDS 70 represents an exceptional case.
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