You would have hundreds, or even 1,000 kilometres of high-pressure ice at the bottoms of oceans. Thats a dead planet. So what will it take to identify a live planet, one with an Earth-like mix of continents and seas, not too wet, not too dry? Given the range of possible worlds out there, there must be many like ours but how will we find them? The james Webb Space telescope (jwst) will dominate astronomy once it starts its five-to-10-year mission in 2020; it will be able to analyse the atmospheres of giant, neptune-size exoplanets, and might even spot some super Earths planets with masses two to 10 times that. But it will be too nearsighted to make out the atmospheres, let alone oceans, of such worlds. Its very difficult to look at something so small the size of Earth passing in front of its star and see that little sliver of atmosphere, says meinke, the jwst deputy project scientist. There are plans for future telescopes that should be able to do that, and I will make this prediction: I think well be able to do that within my professional career.
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And that, as mentioned above, would equal 400 Earth oceans, a life -crushing amount of water. So the only water worlds we can detect with existing technology would be unlikely to host any life. Thats state of the art right now, says Desch. We have the ability to look for water and we can see it when there is 10 per cent water, but that is way too much water. Seven such worlds orbit Trappist-1, a star 49 light-years away named by its Belgian discoverers for their favourite brand of beer. All of them are about Earths size, and three of them lie within their stars habitable zone, orbiting at distances that allow for the existence of liquid water. Theyre among the most tantalisingly earth-like worlds detected so far, but they might all be too wet or weighed down with ice for life. Gauging the compositions of remote planets from a few pixels-worth of light snagged by a telescope is an inexact science at best. Given those constraints, desch and his colleagues estimate that the outermost planets of Trappist-1 consist of 50 per cent ice; the innermost planets seem to be at least 10 per cent liquid water and ice. This is well beyond the extreme where you would cover continents, says Desch.
With current technology, astronomers would be unable to tell if an exoplanet such as Earth had any water at all. Astronomers use two basic techniques to determine the compositions of exoplanets. First, they estimate a planets size by observing how much light it blocks as it passes in front of its star. Then they measure the ever-so-slight wobble imparted to the star by the orbiting planet, which yields the planets mass. Dividing the planets mass by its volume gets the density, which in turn gives astronomers a rough idea of the percentages of gas, rocky matter and water in the planet. Think about how thin our ocean is, says Desch. It doesnt change earths resumes radius in any way. For now, he says, astronomers can tell if an exoplanet has oceans only if water accounts for about 10 per cent of its mass.
That would really screw things up because you would have no water -rock interactions, says Desch. As outlandish as those conditions seem, these worlds might be more common than rocky bodies such as Earth. Water and rock are probably equally abundant in planetary systems throughout the cosmos. In our own solar system, comets, some moons and the frozen denizens of the kuiper Belt out beyond Pluto are thought to consist of equal amounts of ice and rock. The outer planets are about 50 per cent ice, says Desch. Whats not normal is how dry earth. From our parochial perspective, earth seems to be the quintessential ocean planet the pale Blue dot dominated by its seas. But all those oceans amount to the thinnest of films on the planets surface. By mass, earth is only.025 per cent water.
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Phosphorus is super critical, says Tessa fisher, a microbial ecologist at Arizona State University who worked with Desch. In addition to rna and dna, it also makes up atp, which is the energy-carrying molecule for pretty much all biochemistry as we know. Terrestrial biochemistry as we understand it will not function in the absence of phosphorus. Desch and Fisher emphasise that their model doesnt rule out shrek the possibility of life on water worlds. The seas on such planets would probably contain some phosphorus, but not enough to support life on a scale that would leave a noticeable imprint on the atmosphere.
It would not have the 30 per cent oxygen atmosphere that you see on Earth, says Fisher. Its quite possible that a planet entirely covered by oceans could be inhabited. Its just that the life there would be so marginal its extremely unlikely we would be able to detect it from Earth with near-term technology. The most Earth-like worlds detected might all be too wet or weighed down with ice for life There probably are some worlds, though, with so much water that life would be impossible. Deschs team estimated that an Earth-size planet with 10 per cent of its mass in the form of water would be completely lifeless. Such a planet would have the equivalent of 400 of Earths oceans; the immense pressures at the bottom of its sea would create exotic dense forms of ice known as ice-six or ice-seven.
Desch and his colleagues have been running computer simulations of the exotic geophysical and atmospheric environments that might be found on alien worlds. Their goal is to provide a sort of planetary field-guide for future generations of exoplanet hunters desch calls it a periodic Table of Planets. It would rank the types of worlds most likely to contain evidence of life s byproducts in their atmospheres gases such as oxygen or methane, for example. Most importantly, those gases would have to be present in quantities large enough to be detectable by telescopes in the decades ahead. Says Desch: we need to prioritise observations for those planets where we might get the best indicators of life.
Water worlds, it turns out, might be one of the worst places to look. Deschs team created a computer model of a planet resembling Earth in nearly every respect: its size and its cosy not-too-cold, not-too-warm Goldilocks distance from a stable, sun-like star. Then they drowned that world with about five to seven times as much water as Earth has, enough to submerge all its continents. (If you put six oceans on Earth, that would cover everest, says Desch.) by drenching their virtual world, they eliminated a crucial life -sustaining process that we earthlings take for granted: the weathering of exposed rock. Without rain or running water to erode rock, the seas on the world created by deschs team contained very little phosphorus, an indispensable element for all life. Seawater itself is simply not acidic enough to dissolve phosphorus as efficiently as freshwater can.
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What makes water so essential? The chemical reactions that fuel the engines of life require a liquid to dissolve and transport molecules throughout a cell. Water is one of the best solvents known; it remains liquid over a wider range of temperatures than almost any other substance. Its possible that other liquids might fulfil water s role in some truly alien biochemistries lakes of liquid methane, for example, have been found on Titan. But so far there are no known exceptions to the life -needs- water rule. So wouldnt planets completely covered in that essential substance make ideal havens for life? Some recent research throws a giant wet blanket on such expectations: many worlds might actually have too much water for life to arise or to thrive mother if it did get started. More is not necessarily better, says Steven Desch, an astrophysicist at Arizona State University.
Hydrogen makes up 74 per cent of the ordinary matter in the Universe; oxygen is the third most-common element in existence. Combine the two, and you get. Astronomers have observed traces of water ice in craters on the moon and even on Mercury the planet closest to the sun. Its common in interstellar clouds and in the dusty disks of nascent planetary systems; it has already been detected in the atmospheres of some giant exoplanets. The study of exoplanets has been so explosive, says Bonnie meinke, deputy project scientist for nasas James Webb Space telescope, an exoplanet-spotting instrument scheduled for launch next year. In the last 20 years, weve gone from knowing a handful of exoplanets to knowing thousands. And we now know that, for every star you see in the night sky, theres probably a planet up there with that letter star. I think that we can expect that most of those little planets around those stars have some kind of water on them. And where theres water there might be life follow the water has long been an axiom among astrobiologists.
of them are moons! With so much water in our own neck of the cosmos, its almost certain that countless planets around other stars must have oceans too, not to mention retinues of wet moons. Astronomers have already tentatively identified a few water worlds beyond our solar system planets with no dry land at all. Its mind-boggling, says Christopher Glein,. Cassini mission scientist at the southwest Research Institute in San Antonio, texas. Its like inventing a new field of oceanography. In hindsight, maybe the presence of extraterrestrial oceans shouldnt have been such a surprise.
Astonishing and irrefutable evidence for Enceladuss briny deep came in 2005 when the. Cassini space probe captured brief images of geysers spouting ice and water vapour hundreds of miles into space. Cassini even flew right through the geysers in October 2015, skimming within 30 miles of the moons surface to sample their contents. To say that the abundance and ubiquity of liquid water in the outer solar system completely upended scientists expectations doesnt do justice to the discoveries. Before the revelations provided. Cassini, galileo and other probes, the consensus was stark: the moons around Jupiter and Saturn would look much like our own or those of Mars rocky, crater-pocked wastelands utterly hostile to life. Nobody expected that there were subsurface oceans, says Seth Shostak, an astronomer with the seti institute in mountain view, california. It extends our concept of habitability and where you might find life to worlds that we hadnt considered before.
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The largest and deepest body of water resumes known to us has never been sailed upon; it has no islands or shores, no wind-churned waves, no sunlight-silvered surface. This dark ocean cant be found on any map of Earth its more than 300 million miles away, on Europa, one of at least 69 moons that orbit Jupiter. Galileo spacecraft, which flew past Europa 11 times between 19, revealed that an immense salty sea lies beneath this moons smooth icy surface. Estimated to be 60 miles deep about eight times the maximum depth of the pacific it has two to three times as much water as all of Earths oceans combined. And Europa isnt some singularly soggy outlier. At least two additional jovian moons ganymede and Callisto have subsurface oceans. Titan and Mimas, which orbit Saturn, probably do, too. And theres no doubt that another Saturnian moon, Enceladus, harbours water beneath its frozen crust, probably a volume comparable to the Great lakes.