news

한어Русский языкEnglishFrançaisIndonesianSanskrit日本語DeutschPortuguêsΕλληνικάespañolItalianoSuomalainenLatina

Recently, CNN reported that the latest discovery shows that the thick acidic clouds of Venus contain "phosphine, a gas that means life on Earth", and another special gas component, ammonia, has also been found in the atmosphere of Venus. Both phosphine and ammonia are considered to be markers of life on exoplanets, and this discovery has triggered speculation about whether there is life on Venus.

So, are we alone in the universe?

The following content is excerpted from "The Ultimate Question of Astronomy: Who Are We, Where Do We Come From, and Where Will We Go", which has been edited and modified from the original text. The illustrations used in the text are all from the book. The publication has been authorized by the publisher.

"The Ultimate Question of Astronomy: Who are we, where do we come from, and where are we going", written by Neil deGrasse Tyson and James Treifel, translated by Fu Lei, Hu Fanghao and Wang Kechao, published by Jiangsu Phoenix Science and Technology Press in July 2023.

Anyone trying to answer questions like “What is life” and “Are we alone?” is inevitably limited by our own knowledge: the only major type of life we ​​know of or have studied so far exists only on Earth. But life on exoplanets could look and function unlike anything ever observed, and to continue searching for life there, we need to acknowledge our own myopic tendencies.

Long ago, before the advent of DNA sequencing and other biotechnologies, we used to divide life into two categories: plants and animals. But we have since learned that there is an amazing diversity of single-celled and multicellular organisms on this planet. Despite this, all known life forms on Earth, including animals, plants, protists, fungi, archaea, and bacteria, share a common basic chemical structure: they are all built on a backbone of carbon atoms. Therefore, it is understandable that people believed that all organisms must be built in this way - all life is carbon-based, just like the life forms in our world.

In Hollywood sci-fi movies, aliens usually appear in human-like shapes, which shows a tendency of self-preference. Why do aliens have to have teeth, shoulders and fingers like humans? Going further, why do aliens have to look like plants or animals on Earth? If aliens in the universe are even more different from us than we are from E. coli, then what will extraterrestrial life look like?

Are we alone? It is human nature to look up and think.

Two different life development paths from ours

Let us explore two different life development paths that are different from ours.

Life could be based on atoms other than carbon. One example popular with science fiction writers is silicon-based life.

Silicon is an attractive alternative to carbon because its electronic structure is similar to that of carbon.

It sits just below carbon on the periodic table, so it can also chemically bond to four different atoms, a useful property for building complex molecules like DNA. But silicon bonds tend to be stronger than carbon bonds, which makes it unlikely that silicon could form complex molecules, and therefore complex life.

The second way life could develop, as we know it, is that it could have arisen in liquid environments that were not made of water. We know of at least one place where lakes that were not made of water exist: Saturn's largest moon, Titan, the only known planet in the solar system with liquid flowing on its surface. As mentioned earlier, at -180 degrees Celsius on Titan's surface, lakes of liquid methane and ethane extend to its poles. By comparison, the coldest temperature ever recorded on Earth (measured in Antarctica) is -89 degrees Celsius.

In stark contrast to Titan's extremely cold environment, we can also imagine an exoplanet with a molten surface, where life thrives in a hot furnace. We just don't know what complex chemical reactions will occur under such extreme temperatures, and there may be completely unexpected things waiting for us to discover.

An artist's impression of the exoplanet 55 Cancri e. It orbits very close to its parent star and is tidally locked, so its entire star-facing surface is likely covered in boiling magma.

So far, we have only considered life based on chemical reactions, which we call chemical preference. However, imaginative scientists have speculated on the complex structures of completely different forms of life, such as the interaction between electric and magnetic fields, or the electrostatic forces between dust particles in interstellar clouds. What would such forms of life be like? If we could even perceive them with our dull human senses, they would probably be incomprehensible to all but the most open-minded thinkers.

The astonishing range of possible modes of life on countless exoplanets in the Universe provides us with a compelling case that life, intelligent or not, is not unique to Earth and could not have arisen elsewhere, even though life forms on Earth arose from a series of unlikely and rare events.

It goes without saying that we humans don't like to think of ourselves as alone. Long ago, we arranged beings in the sky - gods, demons, aliens... Our imagination is limitless.

The Sun's luster as a potential home for life is fading

It wasn’t until the 20th century that we had the technology to test our ideas about other life with science. In the 1700s, some astronomers thought the sun might harbor carbon-based life. Of course, this life didn’t live on the sun’s blazing surface, but in its solid interior, which they believed must exist. Some even imagined that if you pointed your telescope in the right direction, you could look through sunspots and see inhabited villages below. After all, we hadn’t mastered or understood the branch of physics called thermodynamics, which tells us that the heat from a boiling exterior would vaporize any village inside.

As time went on, the sun’s luster as a potential home for life faded, but other strange ideas popped up. In 1837, for example, Englishman Thomas Dick published a book with the grandiose title “The Landscape of the Celestial World, or The Wonders of the Planetary System, Explaining the Perfections of the Deities and the Diversity of the World.” In it, he claimed that we could find humans living in the rings of Saturn.

In Herbert George Wells’ 1901 novel “The First Men in the Moon” and the 1964 film of the same name, humans encounter insect-like moon people beneath the lunar surface.

By the early 20th century, many people still believed that there was life on the Moon, Mars, and Venus. In 1901, for example, Wells, a writer best known for his early work, War of the Worlds, told the story of an English gentleman who traveled to the Moon in search of a breathable atmosphere and encountered a race he called the Selenites. This belief gained a kind of authority when the famous American astronomer Percival Lowell began publishing books about his observations of Mars. Lowell imagined the Red Planet as home to a dying civilization, with a network of canals carrying water from the poles to the equator—another lost idea about Martian life.

Today we know that life (most likely microbial) is most likely to be found in the subsurface oceans of moons like Europa, and there is also a faint hope in aquifers beneath the surface of Mars.

Scientists who study life must work under obstacles unique to the field. In public, we celebrate the biodiversity of Earth, but in private, we lament that it can all be traced back to a single origin, a single example of life.

Earth is just one of more than 100 spherical bodies in the solar system that can be compared and contrasted. This, by the way, is why geology departments have become so rare in our universities—they have morphed into planetary science departments.

Biologists, however, do not have such a luxury. Every living thing on Earth operates in the same chemical way, controlled by its DNA molecules, making it clear that we all evolved from a single primitive ancestor cell that emerged in Earth's oceans billions of years ago.

Why is this important? Imagine if the only aquatic life you had ever seen was a goldfish. You would then take for granted that all aquatic life was orange vertebrates that loved fresh water and ate plants and insects. Imagine going to the beach for the first time one day and seeing a great white shark, then a jellyfish, then a crab. Everything you knew about aquatic life would need to be re-evaluated, and marine and freshwater biology would emerge.

The Allen Telescope Array (ATA), operated by the Search for Extraterrestrial Intelligence (SETI) Institute in California, continues to survey the sky for signs of intelligent life beyond our solar system.

How will our view of life change?

How would our view of life change if we discovered other life forms?

First, all life on Earth involves the chemistry of carbon atoms bonding together in a liquid water environment. As we will see in the rest of this chapter, almost all thinking about extraterrestrial life assumes that all life we ​​find elsewhere has this feature. This is the view from the goldfish mentioned earlier.

For someone who has never seen another aquatic creature, imagining extraterrestrial life is based on a goldfish. They may be able to imagine how life can survive in water, but imagining and searching for a shrimp, a coral, or a 50-ton whale requires more information, time, and especially imagination. Humans are prone to biases or favoritism in the absence of information. Discovering life elsewhere may (or may not) force us to abandon the source of these biases.

Carbon preference: Does life have to rely on carbon atoms? Both science fiction writers and serious scientists have thought about life based on other atoms such as silicon.

Water preference: Is water the only fluid that could support life? Ammonia and liquid methane are other possibilities, and chemists have added hydrogen sulfide, the gas responsible for the rotten egg smell we sometimes smell around hot pools, to the list.

Surface preference: Can life evolve only on the surface of a planet? In many places in the solar system, such as the moons of Jupiter and Saturn, most liquid water is not on the surface but in underground oceans. Also, could life evolve and thrive entirely in the atmosphere of a gas giant planet?

Stellar preference: Can life develop only on planets orbiting stars? After all, calculations show that there are probably more so-called rogue planets wandering in and beyond the Milky Way than there are planets orbiting stars. Can life develop without relying on stars as a source of energy? Can radioactive heat from the interior of a planet replace sunlight?

Chemical preference: We have to ask whether life must be based on chemistry. If life requires energy flow, some theoretical calculations suggest that the interaction of electric and magnetic fields could develop the level of complexity typically found in living systems.

It stands to reason that questioning each preference opens up new and increasingly surprising patterns of life. Where would you like to start?

The early Earth's surface was volcanic and lifeless, frequently bombarded by comets and meteors that brought the basic ingredients of life from elsewhere in the solar system.

If you’re planning on implementing a large search project, it helps to know exactly what you’re looking for.

People often confuse the search for extraterrestrial life with the search for extraterrestrial civilizations, so let's start with a thought experiment: How would alien visitors view our planet at different times in Earth's history?

For its first 500 million years, Earth was a hot, airless ball floating in space, devoid of life, let alone intelligent life.

For the next 3 billion-plus years, Earth will be a world of floating green goo. Floating relatively simple microbes harvesting energy from sunlight. There will be life on this world, but it will clearly not yet be what we would call intelligent.

At some point in the past few hundred million years, alien visitors would have discovered more complex life forms. When they crossed the threshold to intelligent life depends on what you consider intelligent life: worms, fish, dinosaurs, primates, or cats?

Extraterrestrial sociology requires comprehensive consideration of intelligent life forms from across the galaxy, and the Cosmic Bar in Star Wars is a perfect example of extraterrestrial sociology.

Rather than getting lost in a murky debate about the definition of intelligence, it’s better to look at how we’ve searched for life on exoplanets so far and compare that to how we’d search for intelligent life.

We basically usespectrumThe only way we can detect intelligent life in the universe is to look for electromagnetic signals emitted intentionally or accidentally from exoplanets, but that means we define intelligent life as having the ability to build a radio telescope. This also means that the long period of human history from the time of Homo habilis 2 million years ago to the 19th century would not be visible to alien observers if we use our own definition of intelligent life. Multicellular, or complex, life arose about a billion years ago, so intelligent life, defined as having the ability to emit radio signals, accounts for only a tiny fraction of the history of complex life on Earth—about 0.00001 percent—of the time.

Stromatolites are biogenic sedimentary structures built by primitive microorganisms, such as this one in Australia. Although they are rare today, the microbes that built them were once the dominant form of life on Earth 3.5 billion years ago.

Is it fair to assume, based on the limited data we have, that there is no intelligent life beyond Earth?

Even though we've sent a bunch of probes to Mars and have them roaming the surface and collecting data, scientists are still debating whether there are microbes on Mars. In other words, based on what we know now, our own galaxy may contain lots of planets floating with green goo, maybe some with dinosaurs, but none of them are sending us radio signals, or at least any that we can detect.

Original author/US] Neil deGrasse Tyson [US] James Treifel

Excerpt/He Ye

Editor/Zhang Jin

Introduction proofreading/Yang Li