WATER IS ‘COMMON’ ON ALIEN WORLDS, SCIENTISTS SAY IN FINDING THAT COULD TRANSFORM SEARCH FOR EXTRATERRESTRIAL LIFE
Water is "common" on alien worlds, scientists have found in a study that could change our understanding of how planets form and where we might find alien life.
The discovery comes from the most extensive survey of the chemical compositions of planets ever conducted, and challenges our search for water in our own solar system and elsewhere.
Water is thought to be a key component of extraterrestrial life, and so finding it elsewhere in the universe is likely to be central to discovering whether aliens exist elsewhere in the universe.
The researchers used data from 19 exoplanets to get detailed measurements of the chemical and thermal properties of exoplanets. They looked at a wide variety of different worlds, from relatively small "mini-Neptunes" only 10 times bigger than our Earth to "super-Jupiters" that are as big as 600 of our own planet, and from places that are between 20C and 2000C.
They found that water was "common" across many of those exoplanets. But they also discovered that there was less of it on those planets than expected, and there was great variety between the different kinds of worlds.
"We are seeing the first signs of chemical patterns in extra-terrestrial worlds, and we're seeing just how diverse they can be in terms of their chemical compositions," said project leader Dr Nikku Madhusudhan from the Institute of Astronomy at Cambridge.
In our solar system, there is much more carbon relative to hydrogen in the atmospheres of the giant planets than there is in the Sun. That is thought to have come about at the formation of the planets, when large amounts of ice and other particles were pulled into the planet.
Researchers think that there will be a similar situation on other giant exoplanets. If that is true, there should also be large amounts of water.
Using data from a huge array of different telescopes, both in space and on the ground, the researchers found that water vapour was present in 14 of th 19 planets, and that there was also an abundance of sodium and potassium in six planets.
But they also found that there was less oxygen relative to other elements, and that they might have formed without gathering significant amounts of ice.
"It is incredible to see such low water abundances in the atmospheres of a broad range of planets orbiting a variety of stars," said Dr Madhusudhan.
The new data gives us a detailed understanding of exoplanets that we don't even have of our nearest neighbours, scientists said.
"Measuring the abundances of these chemicals in exoplanetary atmospheres is something extraordinary, considering that we have not been able to do the same for giant planets in our solar system yet, including Jupiter, our nearest gas giant neighbour," said Luis Welbanks, lead author of the study and PhD student at the Institute of Astronomy.
The discovery changes our understanding both of the prevalence of water on alien planets but also challenges our understanding of how those distant worlds might have formed.
"Given that water is a key ingredient to our notion of habitability on Earth, it is important to know how much water can be found in planetary systems beyond our own," said project leader Dr Madhusudhan.
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BREATHABLE ATMOSPHERES MAY BE MORE COMMON IN THE UNIVERSE THAN WE FIRST THOUGHT:
The existence of habitable alien worlds has been a mainstay of popular culture for more than a century. In the 19th century, astronomers believed that Martians might be using canal-based transport links to traverse the red planet. Now, despite living in an age when scientists can study planets light years from our own solar system, most new research continues to diminish the chances of finding other worlds on which humans could live. The biggest stumbling block may be oxygen—human settlers would need a high oxygen atmosphere in which to breathe.
So how were we so lucky to evolve on a planet with plenty of oxygen? The history of Earth's oceans and atmosphere suggests that the rise to present-day levels of O₂ was pretty difficult. The current consensus is that Earth underwent a three-step rise in atmospheric and oceanic oxygen levels, the first being called the "Great Oxidation Event" at around 2.4 billion years ago. After that came the "Neoproterozoic Oxygenation Event" around 800 million years ago, and then finally the "Paleozoic Oxygenation Event" about 400 million years ago, when oxygen levels on Earth reached their modern peak of 21%.
What happened during these three periods to increase oxygen levels is a matter for debate. One idea is that new organisms "bioengineered" the planet, restructuring the atmosphere and oceans through either their metabolisms or their lifestyles. For example, the rise of land plants roughly 400 million years ago could have increased oxygen in the atmosphere through land-based photosynthesis, taking over from photosynthetic bacteria in the ocean which have been the main oxygen producers for most of Earth's history. Alternatively, plate tectonic changes or gigantic volcanic eruptions have also been linked to the Earth's oxygenation events.
This event-based history of how oxygen came to be so plentiful on Earth implies that we're very fortunate to be living on a high-oxygen world. If one volcanic eruption hadn't happened, or a certain type of organism hadn't evolved, then oxygen might have stalled at low levels. But our latest research suggests that this isn't the case. We created a computer model of the Earth's carbon, oxygen and phosphorus cycles and found that the oxygen transitions can be explained by the inherent dynamics of our planet and likely didn't require any miraculous events.
One thing we think is missing from theories about Earth's oxygenation is phosphorus. This nutrient is very important for photosynthetic bacteria and algae in the ocean. How much marine phosphorous there is will ultimately control how much oxygen is produced on Earth. This is still true today—and has been so since the evolution of photosynthetic microbes some three billion years ago.
Photosynthesis in the ocean depends on phosphorus, but high phosphate levels also drive consumption of oxygen in the deep ocean through a process called eutrophication. When photosynthetic microbes die, they decompose, which consumes oxygen from the water. As oxygen levels fall, sediments tend to release even more phosphorus. This feedback loop rapidly removes oxygen. This meant that oxygen levels in the oceans were able to change rapidly, but they were buffered over long timescales by another process involving the Earth's mantle.
Throughout Earth's history, volcanic activity has released gases that react with and remove oxygen from the atmosphere. These gas fluxes have subsided over time due to Earth's mantle cooling, and our computer model suggests this slow reduction along with the initial evolution of photosynthetic life was all that was necessary to produce a series of step-change increases in oxygen levels.
These stepped increases bear a clear resemblance to the three-step rise in oxygen that has occurred throughout Earth's history. The model also supports our current understanding of ocean oxygenation, which appears to have involved numerous cycles of oxygenation and deoxygenation before the oceans became resiliently oxygenated as they are today.
What is really exciting about all of this is that the oxygenation pattern can be created without the need for difficult and complex evolutionary leaps forward, or circumstantial catastrophic volcanic or tectonic events. So it appears that Earth's oxygenation may have been inescapable once photosynthesis had evolved—and the chances of high oxygen worlds existing elsewhere could be much higher.
