Have you ever wondered how planets like Earth get their water? How do they become habitable for life as we know it? These are some of the questions that astronomers have been trying to answer for decades, and they may have just found a clue in a distant star system.
In this article, we will explore the latest discovery of water in the inner region of a disk of gas and dust around a young star called PDS 70, which hosts at least two giant planets. We will also learn how this finding offers evidence of a mechanism to supply water to potentially habitable planets already during their formation, in addition to later impacts of water-bearing asteroids. Finally, we will discuss the implications of this discovery for our understanding of planet formation and the origin of life.
Section 1: The Young Star and Its Planets
PDS 70 is a very young star in the constellation Centaurus, located nearly 400 light-years away from Earth. It has a mass of about three-quarters of our sun and is approximately 5.4 million years old. That may sound old to us, but it’s actually very young for a star. For comparison, our sun is about 4.6 billion years old.
PDS 70 is not alone. It has a disk of gas and dust around it, which is the material from which planets are born. This disk has a radius of about 140 astronomical units (AU), which is about 140 times the distance between the Earth and the sun. The disk also has a large gap in it, which indicates that some of the material has been cleared out by the gravitational influence of planets.
Indeed, PDS 70 has at least two confirmed planets orbiting it, named PDS 70b and PDS 70c. They are both gas giants, similar to Jupiter in size and mass. They are also very young, estimated to be less than 10 million years old. They are still growing by accreting gas and dust from the disk around them.
PDS 70b and PDS 70c are among the first planets to be directly imaged by telescopes, meaning that we can see them as separate points of light from their star. This is very difficult to do because planets are much fainter and closer to their stars than stars are to each other. Most of the planets we know outside our solar system have been detected indirectly, by measuring how they affect their stars’ light or motion.
PDS 70b and PDS 70c were first detected in 2018 and 2019, respectively, by using a technique called coronagraphy, which blocks out the bright light of the star and reveals the fainter planets around it. They were observed by the Very Large Telescope (VLT) of the European Southern Observatory (ESO) in Chile, using an instrument called SPHERE (Spectro-Polarimetric High-contrast Exoplanet REsearch).
Section 2: The Discovery of Water
The discovery of water in the disk around PDS 70 was made by using another instrument on the VLT, called MIRI (Mid-Infrared Instrument). MIRI is part of the James Webb Space Telescope (JWST), which is a joint project between NASA, ESA (European Space Agency) and CSA (Canadian Space Agency). JWST is the successor of the Hubble Space Telescope, and it is designed to observe the universe in infrared light, which is invisible to our eyes but can reveal hidden features of celestial objects.
MIRI was installed on the VLT for testing purposes before being launched into space with JWST later this year. It was used to observe PDS 70 in February 2023, and it detected water vapour in the inner region of the disk, within about 10 AU from the star. This is the first detection of water vapour in a disk that has planets.
Water vapour is not uncommon in space. It can be found in interstellar clouds, comets, asteroids and even some exoplanets. However, finding water vapour in a planet-forming disk is very interesting, because it implies that water can be incorporated into planets during their formation.
Water is essential for life as we know it. It is also a key ingredient for making rocky planets like Earth. Water can act as a lubricant that helps dust grains stick together and form larger bodies called planetesimals. These planetesimals can then collide and merge to form larger bodies called protoplanets, which can eventually become full-fledged planets.
Water can also affect the temperature and chemistry of the disk, and influence the migration and evolution of planets. For example, water can freeze and form ice beyond a certain distance from the star, called the snow line. This can increase the amount of solid material available for planet formation, and lead to the formation of giant planets like Jupiter and Saturn.
Section 3: The Origin of Water on Planets
The discovery of water vapour in the disk around PDS 70 offers evidence of a mechanism to supply water to potentially habitable planets already during their formation, in addition to later impacts of water-bearing asteroids.
The origin of water on Earth and other rocky planets is still a matter of debate. One possibility is that water was present in the disk from which the planets formed, and was incorporated into them as they grew. Another possibility is that water was delivered to the planets later by comets and asteroids that collided with them.
Both scenarios may have played a role, but the relative contribution of each one is unclear. Some studies suggest that most of the water on Earth came from asteroids, while others suggest that most of it came from the disk. The answer may depend on the specific conditions and history of each planetary system.
The detection of water vapour in the disk around PDS 70 suggests that water can be present in the disk from an early stage of planet formation and that it can survive the high temperatures and radiation near the star. This means that water can be incorporated into planets as they form, especially in the inner region of the disk where terrestrial planets are expected to form.
However, this does not exclude the possibility that water can also be delivered to planets later by comets and asteroids. In fact, some studies suggest that comets and asteroids may have different isotopic compositions of water than the disk and that these differences can be used to trace the origin of water on planets. For example, some studies suggest that Earth’s water has a similar isotopic composition to that of carbonaceous chondrites, which are a type of asteroid rich in organic matter and water.
Therefore, by measuring the isotopic composition of water in different parts of the disk and in different types of comets and asteroids, we may be able to determine how much water came from each source, and how it was distributed among the planets.
Section 4: The Role of JWST
The detection of water vapour in the disk around PDS 70 was made possible by using JWST’s MIRI instrument on the VLT. MIRI is designed to observe infrared light with wavelengths between 5 and 28 micrometres. This range covers several important features of celestial objects, such as dust emission, molecular lines and ice bands.
MIRI has several advantages over other instruments for observing disks around young stars. First, it has a high sensitivity and resolution, which allows it to detect faint signals and resolve small details. Second, it has a coronagraphic mode, which enables it to block out the bright light of the star and reveal the fainter disk and planets around it. Third, it has a spectroscopic mode, which allows it to measure the spectrum of light coming from different parts of the disk and identify its chemical composition.
MIRI was able to detect water vapour in the disk around PDS 70 by using its spectroscopic mode. It measured the spectrum of light coming from different regions of the disk and compared it with models of how different molecules emit or absorb light at different wavelengths. By doing so, it identified a characteristic feature of water vapour at about 17 micrometres.
MIRI also detected other molecules in the disk around PDS 70, such as carbon monoxide (CO), carbon dioxide (CO2) and methane (CH4). These molecules can provide information about the temperature, density and chemistry of the disk, as well as its interaction with stellar radiation.
MIRI is one of four instruments on JWST, along with NIRCam (Near-Infrared Camera), NIRSpec (Near-Infrared Spectrograph) and NIRISS (Near-Infrared Imager and Slitless Spectrograph). Together, they cover a wide range of wavelengths from 0.6 to 28 micrometres and offer various modes for observing different types of celestial objects.
JWST is expected to launch in October 2023, after several delays due to technical issues and budget constraints. It will be placed in an orbit around the second Lagrange point (L2) of the Earth-sun system, which is about 1.5 million kilometres away from Earth. From there, it will have an unobstructed view of the sky and will be shielded from the heat and light of both Earth and the sun by a large sun shield.
Section 5: The Implications of the Discovery
The discovery of water vapour in the disk around PDS 70 has important implications for our understanding of planet formation and the origin of life. It shows that water can be present in the inner region of a disk that has planets and that it can be incorporated into them as they form. This means that water may be more abundant and widespread among planets than previously thought.
Water is not only essential for life as we know it but also for making planets habitable. Water can moderate the climate of a planet, by creating a greenhouse effect that keeps the surface warm, and by transporting heat from the equator to the poles. Water can also create a protective layer that shields the planet from harmful radiation and impacts. Water can also enable chemical reactions that produce organic molecules, which are the building blocks of life.
The discovery of water vapour in the disk around PDS 70 also opens up new possibilities for detecting and characterizing exoplanets. By using JWST and other infrared telescopes, we may be able to measure the water content and distribution in different disks and planets, and compare them with our own solar system. We may also be able to detect signs of water in the atmospheres of exoplanets, which could indicate their potential habitability.
The discovery of water vapour in the disk around PDS 70 is a remarkable achievement that demonstrates the power and potential of JWST. It is also a testament to the curiosity and creativity of astronomers who are constantly exploring the wonders of the universe. It is a reminder that we are not alone in this vast cosmos, but part of a cosmic story that began with stars and planets, and may end with life.
