Using the James Webb Space Telescope, the research collaboration led by MPIA at MINDS discovered water in the inner region of a disk of gas and dust around the young star PDS 70. Astronomers expect terrestrial planets to form in that zone. This is the first detection of such a disk with at least two planets. Any rocky planets formed in the inner disk would benefit from a large local reservoir of water, improving the chances of later habitability.
This finding offers evidence of a mechanism for supplying water to potentially habitable planets early in their formation, in addition to the late impacts of water-bearing asteroids.
Water is essential for life on Earth. However, scientists debate how it got to Earth and whether that process could also make rocky exoplanets around other stars habitable.
The preferred mechanism is a supply of water-bearing asteroids that bombard the surface of the young planet. “We may now see evidence that water can also be one of the initial ingredients of rocky planets and used at birth,” said Giulia Perotti, an astronomer at the Max Planck Institute for Astronomy (MPIA) in Heidelberg, Germany. He is the lead author of a research article that appears in the journal NATURE which reports on the detection of water in the planet-forming disk of the young star PDS 70, about 370 light-years away.
Water in the inner disk of the PDS 70
MIRI (Mid-InfraRed Instrument) observations aboard the James Webb Space Telescope (JWST) discovered water near the disk center, near the host star PDS 70. In the solar system, this is the region where rocky planets orbit the sun. According to the analysis, water is in the form of hot steam, burning at a temperature of about 330° Celsius (600° Kelvin).
“This discovery is very exciting, because it probes the region where rocky planets similar to Earth are usually formed,” pointed out MPIA Director Thomas Henning. He is a co-author of the underlying article, Co-PI (principal investigator) of MIRI, and the PI of the MINDS (MIRI Mid-Infrared Disk Survey) program. MINDS is a JWST guaranteed-time program involving research institutes from 11 European countries. This survey aims to identify the properties of disks made of gas and dust around young stars, which can teach us about the conditions that determine the composition of planets that may form there.
PDS 70 is the first relatively old disk—approximately 5.4 million years old—in which astronomers have found water. Over time, the gas and dust content of the planet-forming disks decreased. Either the radiation of the central star or the wind removes material such as dust and gas, or the dust grows into larger objects that eventually become planets. Since previous studies failed to identify water in the central regions of the same evolved disks, astronomers suspect that it could not survive the intense radiation of the star, leading to the dry rocky environments that formed the planet.
Observing PDS 70 with MIRI aboard JWST was key to challenging the hypothesis. As a result, the contents of the changed and exhausted disks may not be very dry. If so, many terrestrial planets formed in those zones could be born with essential ingredients to sustain life.
The water supply of the terrestrial planets—nature versus nurture
However, scientists have not found any planets near the PDS 70 disk center so far. Instead, astronomers detected two gas-giant planets ahead, PDS 70 b and c. They accumulate surrounding dust and gas while orbiting their host star during their growth, creating a vast annular gap that is almost devoid of any detectable material.
However, any rocky planets that formed in a water-rich environment close to the star would benefit from a supply of water at the start of their life cycles. Therefore, in addition to the water brought to the first dry rocky planets by a long process involving asteroids as a relatively random system of cosmic transport, this new result opens the door for a potential sustainable mechanism that provides water to the planets that have already been born.
It is not difficult to imagine that such a scenario would improve the chances of finding habitable rocky planets with abundant water that could support life. The progress of the MINDS program will eventually show whether water is common in terrestrial planet-forming zones in evolved disks around young stars or whether PDS 70 is just an exception.
What is the source of water?
Since the presence of water was somewhat unexpected, the MINDS team investigated several scenarios to explain their findings.
One possibility involves water being a remnant from an earlier water-rich nebula before the disk stage. Water is very common, especially in its frozen state, covering small dust particles. When heated near a forming star, water vaporizes and mixes with other gases. Unfortunately, water molecules are relatively fragile and break down into smaller components like hydrogen and oxygen when hit by harmful UV radiation from a nearby star. However, the surrounding material such as dust and the water molecules themselves act as a shield. As a result, at least some of the water found near PDS 70 may have survived the breach.
Another source could be gas coming in from the outer rims of the PDS 70 disk. Under certain conditions, oxygen and hydrogen gas can combine to form water vapor. In addition, the drag of moving gas may pull water-rich dust particles that migrate from the prominent outer dust ring. The central star is so faint that it cannot vaporize water ice at the distance of that ring. When the dust grains enter the inner disk near the star, the ice turns into gas.
“The truth is probably a combination of all the options,” Perotti said. “However, it is likely that one mechanism plays an important role in maintaining the water reservoir in the PDS 70 disk. The future task is to find out which one it is.”
To complete the picture
JWST and MIRI are powerful tools. However, they only provide some aspects of the whole picture. Like a painting that needs many different colors to convey its message, astronomers apply different types of observations and cover many wavelengths to gain information and complete their understanding.
In this case, the team used MIRI’s spectrograph to decompose the infrared radiation received from PDS 70 into signatures in smaller wavelength ranges—like distinguishing one color into many different shades. In this way, the team isolated several individual water signatures that they used to calculate temperatures and densities.
Astronomers are now taking additional observations using ground-based telescopes to complete the picture. Additionally, they eagerly await another set of JWST observations that will provide detailed images of the inner PDS 70 disk. And perhaps, its structure will reveal signs of additional terrestrial planets or the relatively larger sub-Neptunes formed within the water reservoir.
More information:
Giulia Perotti, Water in the terrestrial planet-forming zone of the PDS 70 disk, NATURE (2023). DOI: 10.1038/s41586-023-06317-9. www.nature.com/articles/s41586-023-06317-9
Provided by the Max Planck Society
Citation: JWST observations find water for the first time in the inner disk around a young star with giant planets (2023, July 24) retrieved July 24, 2023 from https://phys.org/news/2023-07-jwst-disk-young-star-giant.html
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