- 3797 - WATER - found on Exoplanets, On Earth? Astronomers have found evidence that two exoplanets orbiting a red dwarf star are "water worlds," planets where water makes up a large fraction of the volume. These worlds, located in a planetary system 218 light-years away in the constellation Lyra, are unlike any planets found in our solar system.
--------------------- 3797 - WATER - found on Exoplanets, On Earth?
- Exoplanets Kepler-138c and Kepler-138d are about one and a half times the size of the Earth and could be composed largely of water. Water wasn't directly detected, but by comparing the sizes and masses of the planets to models, they conclude that a significant fraction of their volume, up to half of it, should be made of materials that are lighter than rock but heavier than hydrogen or helium, which constitute the bulk of gas giant planets like Jupiter. The most common of these candidate materials is water.
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- With volumes more than three times that of Earth and masses twice as big, planets Kepler-138c and Kepler-138d have much lower densities than Earth. This is surprising because most of the planets just slightly bigger than Earth that have been studied in detail so far all seemed to be rocky worlds like ours. The closest comparison to the two planets would be some of the icy moons in the outer solar system that are also largely composed of water surrounding a rocky core.
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- Researchers caution the planets may not have oceans like those on Earth directly at the planet's surface. The temperature in Kepler-138c's and Kepler-138d's atmospheres is likely above the boiling point of water, and we expect a thick, dense atmosphere made of steam on these planets. Only under that steam atmosphere there could potentially be liquid water at high pressure, or even water in another phase that occurs at high pressures, called a supercritical fluid.
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- Recently, another team at the University of Montreal found another planet, called TOI-1452 b, that could potentially be covered with a liquid-water.
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- In 2014, data from NASA's Kepler Space Telescope allowed astronomers to announce the detection of three planets orbiting Kepler-138, a red dwarf star in the constellation Lyra. This was based on a measurable dip in starlight as the planet momentarily passed in from of their star, a transit.
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- Re-observing the planetary system with the Hubble and Spitzer space telescopes between 2014 and 2016 caught more transits of Kepler-138d, the third planet in the system, in order to study its atmosphere.
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- Astronomers were surprised to find that the Hubble and Spitzer observations suggested the presence of a fourth planet in the system, Kepler-138e. This newly found planet is small and farther from its star than the three others, taking 38 days to complete an orbit.
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- The planet Kepler-138e is in the habitable zone of its star, a temperate region where a planet receives just the right amount of heat from its cool star to be neither too hot nor too cold to allow the presence of liquid water.
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- With Kepler-138e now in the picture, the masses of the previously known planets were measured again via the transit timing-variation method, which consists of tracking small variations in the precise moments of the planets' transits in front of their star caused by the gravitational pull of other nearby planets.
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- They had another surprise: they found that the two water worlds Kepler-138c and d are "twin" planets, with virtually the same size and mass, while they were previously thought to be drastically different. The closer-in planet, Kepler-138b, on the other hand, is confirmed to be a small Mars-mass planet, one of the smallest exoplanets known to date.
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- The origin of Earth’s water has been another enduring mystery. There are different hypotheses and theories explaining how the water got here, and lots of evidence supporting them. But water is ubiquitous in protoplanetary disks, and water’s origin may not be so mysterious after all.
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- Other young solar systems have abundant water. In solar systems like ours, water is along for the ride as the young star grows and planets form. The evidence is in Earth’s heavy water content, and it shows that our planet’s water is 4.5 billion years old.
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- The formation of a solar system starts with a giant molecular cloud. The cloud is mostly hydrogen, water’s main component. Next are helium, oxygen, and carbon, in order of abundance. The cloud also contains tiny grains of silicate dust and carbonaceous dust. The history of water in our Solar System, this is where it starts.
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- Stars form in molecular clouds, vast conglomerations of mostly hydrogen. Out here in the cold reaches of a molecular cloud, when oxygen encounters a dust grain, it freezes and adheres to the surface. But water isn’t water until hydrogen and oxygen combine, and the lighter hydrogen molecules in the cloud hop around on the frozen dust grains until they encounter oxygen. When that happens, they react and form water ice, two types of water: regular water and heavy water containing deuterium.
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- Deuterium is an isotope of hydrogen called heavy hydrogen. It has a proton and one neutron in its nucleus. That separates it from “regular” hydrogen, called protium. Protium has a proton but no neutron. Both these hydrogen isotopes are stable and persist to this day, and both can combine with oxygen to form water.
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- Gravity begins to exert itself in the cloud as matter clumps in the center. More mass falls into the center of the molecular cloud and starts forming a protostar. Some of the gravity is converted into heat, and within a few astronomical units (AU) of the cloud’s center, the gas and dust in the disk reach 100 Kelvin.
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- 100 K is bitterly cold in Earthly terms, only -173 degrees Celsius. But in chemical terms, it’s enough to trigger sublimation, and the ice changes phase into water vapour. The sublimation occurs in a hot corino region, a warm envelope surrounding the cloud’s center. Though they also contain complex organic molecules, water becomes the most abundant molecule in corinos.
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- Water is abundant at this point, though it’s all vapour. In step two, the protostar hasn't begun fusion yet. But it still generates enough heat to sublimate the water ice on dust grains into vapor. They call it the protostar phase.
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- Next, the star begins to rotate, and the surrounding gas and dust form a flattened, rotating disk called a protoplanetary disk. Everything that will eventually become the solar system’s planets and other features is inside that disk.
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- The young protostar is still gathering mass, and its life of fusion on the main sequence is still well in its future. The young star generates some heat from shocks on its surface, but not much. So the disk is cold, and the regions furthest away from the young protostar are the coldest.
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- The water ice that formed in step one is released into gas in step two but recondenses again in the coldest reaches of the protoplanetary disk. The same population of dust grains is again covered in an icy mantle. But now, the water molecules in that icy mantle contain the history of the water in the Solar System. \
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- As the protostar continues to gather mass, it begins to rotate. The gas and dust form a rotating disk centred on the star. The water vapour from step two recondenses, and the dust grains are again covered in icy mantles. But this time, the water ice retains a record of what it's been through.
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- In step four, the Solar System begins to take shape and resemble a more fully-formed system. All the things we’re accustomed to, like planets, asteroids, and comets, start forming and taking up their orbits. They originate from those tiny dust grains and their twice-frozen water molecules.
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- This is the situation we find ourselves in today. While astronomers can’t travel back in time, they’re getting better at observing other young solar systems and finding clues to the entire process. Earth’s water contains a critical hint, too: the ratio of heavy water to regular water.
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- Some detail is left out of the simple explanation given so far. When water ice forms in step one, the temperature is extremely low. That triggers an unusual phenomenon called super-deuteration. Super-deuteration introduces more deuterium into the water ice than at other temperatures.
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- Deuterium was only formed in the seconds following the Big Bang. Not much of it formed: only one deuterium for every 100,000 protium atoms. That means that if the deuterium was evenly mixed with the Solar System’s water, the abundance of heavy water would be expressed as 10^-5.
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- In a hot corino, the abundance changes. However, in hot corinos, the HDO/H2O ratio is only a bit less than 1/100. (HDO is water molecules containing two deuterium isotopes, and H2O is regular water containing two protium isotopes.)
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- The ratios contain such large abundances of deuterium because of super-deuteration. At the moment that ice forms on the surfaces of the dust grains, there’s an enhanced number of D atoms compared to H atoms landing on the grain surfaces. Abundant heavy water is a hallmark of water synthesis in the cold molecular cloud clump during the STEP 1 era.
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- The important thing so far is that there are two episodes of water synthesis. The first happens when the solar system hasn’t formed yet and is only a cold cloud. The second is when planets form. The two happen in different conditions, and those conditions leave their isotopic imprint on the water. Water from the first synthesis is 4.5 billion years old, and the question becomes, “How much of that ancient water reached Earth?”
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- In previous research, scientists compared those ratios with ratios in objects in our Solar System—comets, meteorites, and Saturn’s icy moon Enceladus. So they know that Earth’s heavy water abundance, the HDO/H2O ratio, is about ten times greater than in the Universe and at the beginning of the Solar System.
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- The results of all this work show that between 1% and 50% of Earth’s water came from the initial phase of the Solar System’s birth. The water in comets and asteroids (from which the vast majority of meteorites originate) was also inherited since the beginning in large quantities. Earth likely inherited its original water predominantly from planetesimals, which are supposed to be the precursors of the asteroids and planets that formed the Earth, rather than from the comets that rained on it.
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- Delivery by comets is another hypothesis for Earth’s water. In that hypothesis, frozen water from beyond the frost line reaches Earth when comets are disturbed and sent from the frozen Oort Cloud into the inner Solar System.
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- Earth’s water is 4.5 billion years olds. At least some of it is. Planetesimals probably delivered it to Earth, but exactly how that happens isn’t clear. There’s a lot more complexity that scientists need to sort through before they can figure that out.
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- These things are all wrapped up together in how life originated and how worlds formed. Water likely played a role in forming the planetesimals that delivered it to Earth. Water likely played a role in sequestering other chemicals, including the building blocks of life, onto rocky bodies that delivered them to Earth.
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- Water lies at the center of it all, and by showing that some of it dates back to the very beginnings of the Solar System, the authors have provided a starting point for figuring the rest of it out.
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- Here, we presented a simplified early history of the Earth’s water according to the most recent observations and theories,” they write. “A good fraction of terrestrial water likely formed at the very beginning of the Solar System’s birth when it was a cold cloud of gas and dust, frozen and conserved during the various steps that led to the formation of planets, asteroids, and comets and was eventually transmitted to the nascent Earth.”
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December 19, 2022 WATER - found on Exoplanets, On Earth? 3797
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