@非人間的すぎる

3 months ago

More questions than answers
In the thousands of years humanity has been contemplating the cosmos, we are the first people to know one thing for sure: The stars beyond our Sun are teeming with planets. They come in many varieties, and a good chunk of them are around the size of Earth. Like most scientific questions, though, getting an answer to this one just breeds more questions: Which, if any, of these exoplanets harbors some form of life? How quickly does life get its start? And how long does it last?

Where is everybody?
The universe's eerie silence has its own name – the "Fermi paradox." Physicist Enrico Fermi famously posed the question: "Where is everybody?" Even at slow travel speeds, the universe's billions of years of existence allow plenty of time for intelligent, technological lifeforms to traverse the galaxy. Why, then, is the cosmos so quiet?
Meanwhile, exoplanet discoveries over the past two decades have filled in a few of the terms in the much-debated Drake Equation – a chain of numbers that might one day tell us how many intelligent civilizations we can expect to find. Most of its terms remain blank – the fraction of planets with life, with intelligent life, with detectable technology – but the equation itself suggests we might one day arrive at an answer. It feels at least a little more hopeful than Fermi's silence.
We stand at a crossroads in the search for life. We've found thousands of planets in our Milky Way galaxy, a large fraction of them in Earth's size range and orbiting in their stars' "habitable zones" – the distance from the star at which liquid water could exist on the surface. We know the galaxy likely holds trillions of planets. Our telescopes in space and on the ground, and our remote-sensing technology, grow ever more powerful. Yet so far, the only life we know of is right here at home. For the moment, we're staring into the void, hoping someone is staring back.

Our notion of the range of environments where life could exist has also expanded thanks to the discovery on Earth of extremophile organisms that can thrive in places far hotter, saltier, acidic, and more radioactive than previously thought possible, including creatures living around undersea hydrothermal vents.

We’re now getting closer than ever before to learning how common living worlds like ours actually are. New tools, including machine learning and artificial intelligence, could help scientists look past their preconceived notions of what constitutes life. Future instruments will sniff the atmospheres of distant planets and scan samples from our local solar system to see if they contain telltale chemicals in the right proportions for organisms to prosper.

“I think within our lifetime we will be able to do it,” says Ravi Kopparapu, a planetary scientist at NASA’s Goddard Space Flight Center in Maryland. “We will be able to know if there is life on other planets.”

While humans have a long history of speculating about distant worlds, for much of that time actual evidence was in short supply. The first planets around other stars—known as exoplanets—were discovered in the early 1990s, but it took until the launch of NASA’s Kepler space telescope in 2009 for astronomers to understand how common they were. Kepler carefully monitored hundreds of thousands of stars, looking for tiny dips in their brightness that could indicate planets passing in front of them. The mission helped the number of known exoplanets rise from a mere handful to over

Kepler was built to help determine the prevalence of Earth-like planets orbiting sun-like stars at the right distance to have liquid water on their surface (a region often nicknamed the Goldilocks zone). While not a single extraterrestrial world has been a perfect twin of our own so far, researchers can use the sheer quantity of discoveries to make educated guesses as to how many might be out there. The current best estimates suggest that anywhere between 10% and 50% of sun-like stars have planets like ours, leading to numbers that make astronomers’ heads swim.

“If it’s 50%, that’s bonkers, right?” says Jessie Christiansen, an astrophysicist at Caltech in Pasadena, California. “There are billions of sun-like stars in the galaxy, and if half of them have Earth-like planets, there could be billions of habitable rocky planets.”

Is there anybody home?
Determining whether these planets actually contain organisms is no easy task. Researchers must capture the faint light from an exoplanet and spread it into its constituent wavelengths, scanning for signatures that indicate the presence and amount of different types of chemicals. While astronomers would like to focus on sun-like stars, doing so is technically challenging. NASA’s mighty new James Webb Space Telescope (JWST) is currently training its 6.5-meter mirror and unparalleled infrared instruments on worlds around stars smaller, cooler, and redder than our sun, known as M dwarfs. Such places might be habitable, but at the moment, nobody is really sure.

For liquid water to be present on their surfaces, planets around M dwarfs would need to orbit close to their stars—which tend to be more active than the sun, sending out violent flares that could strip away atmospheric gases and likely leave the ground a dry husk. JWST has been investigating Trappist-1, an M dwarf 40 light-years away with seven small rocky worlds, four of which are at the right distance to potentially have liquid water. The two closest exoplanets have already been shown to be devoid of atmospheres, but scientists are eagerly awaiting the results of JWST observations from the next three. They want to know if even those outside the habitable zone can have atmospheres.

There’s special interest in looking for other planets around M dwarf stars, because they are far more prevalent than sun-size stars. “If they find them to hold atmospheres, that increases the habitable real estate of the galaxy a hundredfold,” says Christiansen.

Once we’ve found a planet that looks a lot like Earth, then we’ll want to start hunting for chemical clues of life on its surface. JWST isn’t sensitive enough to do that, but future ground-based instruments like the Extremely Large Telescope, Giant Magellan Telescope, and Thirty Meter Telescope—which are expected to begin taking data in the 2030s—could tease out the chemical components of nearby Earth-like worlds. Information from more distant targets will have to wait for NASA’s next planned flagship mission, the space-based Habitable Worlds Observatory, expected to launch sometime in the late 2030s or early 2040s. The telescope will use either an external star shade or an instrument called a coronagraph to block the glaring light of a star and home in on dimmer planetary light and its potential molecular fingerprints.

Which chemicals in particular astronomers should be looking for remains a matter of debate. Ideally, they want to find what are known as biosignatures—molecules like water, methane, and carbon dioxide present in amounts similar to what we find on Earth. What that means in practice isn’t always clear, since our planet has gone through many periods when it contained life yet the quantities of different chemicals varied wildly.

“Do you want it to detect an Archaean Earth, like 2 or 3 billion years ago?” asks Kopparapu. “Or from the Neoproterozoic, where there was a snowball Earth? Or do you want to detect the current Earth, where there is a lot of free oxygen, ozone, water, and CO2?”

Can u pin me now?😊
Edit:this is only a bit of what I know lo

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