Extraterrestrial life facts for kids
Extraterrestrial life means life that is not from Earth. It is also called alien life. Scientists think it's possible that planets similar to ours exist, and that life could have evolved there too. So far, no alien life has been found. However, it's possible that life once existed on Mars.
People have searched for signals from alien life forms. But no signals have been received yet.
The idea of life beyond Earth is not new. Many thinkers have wondered if other planets like ours could have life. We now know that many other star systems have exoplanets, which are planets outside our own solar system.
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What Life Needs to Start
Life on Earth needs water. Water acts like a solvent, meaning it dissolves things so that chemical reactions can happen. If other planets have enough carbon, other elements, and water, living things might form there. This would most likely happen on terrestrial planets, which are rocky planets similar to Earth in their chemical makeup and temperature.
Some scientists have wondered if life could use other liquids instead of water, like ammonia or hydrocarbons (such as methane). But water seems to be the best liquid for life.
About 29 different chemical elements are important for life on Earth. Most living things (about 95%) are made of just six elements: carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur. These six elements are the basic building blocks for almost all life on Earth.
Carbon is very special. It can form four strong chemical bonds with other atoms, including other carbon atoms. This allows carbon atoms to create complex 3D structures, like the ones found in nucleic acids (like DNA) and proteins. Carbon forms more compounds than all other elements combined! Because carbon is so versatile, it's probably the most important element for life, even on other planets.
Where Life Might Exist in Our Solar System
Some places in our Solar System could have environments where alien life might exist. This is especially true for places with possible subsurface oceans, which are oceans hidden under layers of ice or rock. If life is found elsewhere in our Solar System, scientists think it will likely be tiny extremophile microorganisms. Extremophiles are microbes that can live in very harsh conditions.
Life on other worlds would probably start with microbes. Any complex life would likely have grown from these tiny living things. Studying microbes on Earth helps us understand what kind of life might exist elsewhere.
Mars might have small underground areas where microbial life could live. Jupiter's moon Europa is thought to be the most likely place in the Solar System, besides Earth, to find extremophile microorganisms. This is because it might have a subsurface ocean.
The idea of panspermia suggests that life found elsewhere in the Solar System might have a common origin. This means life could have spread from one planet to another, like Earth seeding Mars, or Mars seeding Earth.
Venus
In the early 1900s, people often thought Venus was similar to Earth and could have life. But space missions have shown that Venus's surface is too hot and harsh for Earth-like life. However, high up in Venus's atmosphere (between 50 and 65 kilometers), the pressure and temperature are more like Earth's. Some scientists think that tiny, acid-loving extremophile microorganisms might exist in the acidic upper layers of Venus's atmosphere. Also, Venus probably had liquid water on its surface for millions of years after it formed.
Mars
People have wondered about life on Mars for a long time. Scientists believe liquid water existed on Mars in the past. Now, small amounts of liquid brine (salty water) can sometimes be found in shallow Martian soil. Scientists have also found methane in Mars's atmosphere. The source of this methane is a mystery, and some wonder if it comes from living things, though other explanations are possible.
There is evidence that Mars was once warmer and wetter. We've found dried-up riverbeds, polar ice caps, volcanoes, and minerals that form in water. Even today, conditions underground on Mars might support life. In 2013, the Curiosity rover found strong evidence of an ancient freshwater lake in Gale Crater. This lake could have been a good place for microbial life.
The Curiosity and Opportunity rovers are still searching Mars for signs of ancient life. They look for evidence of a biosphere (a place where life exists) based on different types of microorganisms. They also search for signs of ancient water, like plains related to old rivers or lakes that might have been habitable. Finding evidence of habitability, fossils, and organic carbon on Mars is a main goal for NASA.
Ceres
Ceres is a dwarf planet in the asteroid belt. It has a thin atmosphere of water vapor. Scientists may have also found frost on its surface, seen as bright spots. Because water is present on Ceres, some people think life might be possible there.
Jupiter System
Jupiter
In the 1960s and 1970s, Carl Sagan and others imagined tiny organisms living in Jupiter's atmosphere. However, Jupiter's strong radiation and other conditions probably don't allow for life as we know it. But some of Jupiter's moons might have places that can support life. Scientists think that warm, liquid water oceans might exist deep under the icy surfaces of three of Jupiter's largest moons: Europa, Ganymede, and Callisto. Future missions are planned to study if these places could support life.
Europa
Jupiter's moon Europa is a strong candidate for life because it likely has a liquid water ocean beneath its ice. If Hydrothermal vents exist at the bottom of this ocean, they could warm the water and support tiny microorganisms. It's also possible that Europa could support larger animals that use oxygen created by cosmic rays hitting its surface ice.
The chance of life on Europa grew in 2011 when scientists found huge lakes within Europa's thick ice shell. They saw that ice shelves around these lakes seemed to be collapsing into them. This could be a way for chemicals needed for life, created by sunlight on Europa's surface, to reach the ocean below.
In 2013, NASA reported finding "clay-like minerals" on Europa's icy surface. These minerals are often linked to organic materials. Scientists think these minerals might have come from an asteroid or comet collision. The Europa Clipper mission, planned for 2025, will study Europa's habitability. Europa's subsurface ocean is considered the best place to look for life.
Saturn System
Saturn's moons, Titan and Enceladus, are thought to have possible habitats for life.
Enceladus
Enceladus, a moon of Saturn, has some conditions needed for life. These include geothermal activity (heat from inside the moon) and water vapor. It may also have oceans under its ice, warmed by the pull of Saturn's gravity. In 2005, the Cassini–Huygens probe flew through one of Enceladus's geysers, which spews ice and gas. It found carbon, hydrogen, nitrogen, and oxygen – all key elements for life. The temperature and density of these plumes suggest a warm, watery source beneath the surface.
Titan
Titan, Saturn's largest moon, is the only moon in our Solar System with a thick atmosphere. Data from the Cassini–Huygens mission showed that Titan does not have a global ocean of hydrocarbons. However, it does have liquid hydrocarbon lakes in its polar regions. These are the first stable bodies of surface liquid found outside Earth. Studies of the mission's data show that the chemistry in Titan's atmosphere near the surface is consistent with the idea that organisms there, if they exist, could be using hydrogen, acetylene, and ethane, and producing methane.
Small Solar System Bodies
Small Solar System bodies, like comets and asteroids, have also been thought to host extremophiles. Some scientists have suggested that microbial life might exist on them.
Other Possible Water Worlds
Models of how heat is kept and produced inside smaller icy bodies in the Solar System suggest that several moons and dwarf planets might have oceans under their icy crusts. These include Rhea, Titania, Oberon, Triton, Pluto, Eris, Sedna, and Orcus. These hidden oceans could be about 100 kilometers thick. What's interesting is that these models suggest the liquid layers are directly touching the rocky core. This allows minerals and salts to mix efficiently into the water. This is different from larger icy moons like Ganymede or Titan, where layers of high-pressure ice might be between the liquid water and the rocky core.
Hydrogen sulfide has also been suggested as a possible liquid for life. It is very common on Jupiter's moon Io and might be in liquid form just below its surface.
Searching for Alien Life
Scientists are searching for extraterrestrial life in two ways: directly and indirectly. As of 2017, 3,667 exoplanets have been found in 2,747 systems. Also, other planets and moons in our own solar system might host simple life, like microorganisms.
Direct Search
Scientists look for biosignatures (signs of life) within our Solar System. They study planetary surfaces and examine meteorites. Some researchers claim to have found evidence that microbial life once existed on Mars. For example, an experiment on the two Viking Mars landers reported gases coming from heated Martian soil samples. Some scientists thought this was a sign of living microorganisms. However, other experiments on the same samples did not support this, suggesting a non-biological reaction was more likely. In 1996, a debated report claimed that structures looking like tiny bacteria were found in a meteorite called ALH84001, which came from Mars.
In 2005, two NASA scientists, Carol Stoker and Larry Lemke, suggested they might have found evidence of current life on Mars. They based their claim on methane found in Mars's atmosphere, which was similar to methane produced by some simple life forms on Earth. They also studied primitive life near the Rio Tinto river in Spain. NASA officials quickly distanced themselves from these claims, and Stoker later changed her initial statements. While the methane findings are still discussed, some scientists still believe life might exist on Mars.
In 2011, NASA launched the Mars Science Laboratory. This mission landed the Curiosity rover on Mars in 2012. The rover is designed to figure out if Mars could have supported life in the past or present, using many scientific tools.
The Gaia hypothesis suggests that any planet with a lot of life will have an atmosphere that is chemically out of balance. This imbalance can be detected from far away using spectroscopy (studying light). However, we need much better telescopes to find and study light from smaller rocky planets around other stars before we can use this method. The Carl Sagan Institute, founded in 2014, focuses on studying the atmospheres of exoplanets in circumstellar habitable zones (areas where life could exist). Future telescopes like WFIRST and ELT will help gather this data.
In 2011, NASA findings from meteorites found on Earth suggested that parts of DNA and RNA (like adenine and guanine), which are building blocks for life, might form naturally in outer space. In 2011, scientists also reported that cosmic dust contains complex organic matter. One scientist suggested that these compounds might have helped life start on Earth, saying that "life on Earth may have had an easier time getting started as these organics can serve as basic ingredients for life."
In 2012, astronomers at Copenhagen University made a world-first discovery. They found a specific sugar molecule called glycolaldehyde in a distant star system. This molecule was found around a young star system called IRAS 16293-2422, which is 400 light-years from Earth. Glycolaldehyde is needed to form ribonucleic acid, or RNA, which is similar to DNA. This discovery suggests that complex organic molecules can form in star systems before planets even exist. These molecules could then arrive on young planets early in their formation.
Images for kids
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Astronomers have discovered stars in the Milky Way galaxy that are almost 13.6 billion years old.
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A series of artist's ideas about how much water Mars had in the past.
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Living things produce many different signs that telescopes might be able to detect.
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The Green Bank Telescope is one of the radio telescopes used by the Breakthrough Listen project to search for alien communications.
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Artist's idea of Gliese 581 c, the first rocky exoplanet found in its star's habitable zone.
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Artist's idea of the Kepler telescope.
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The statue of Simandhara, an enlightened man in Jain mythology who is believed to live on another planet.
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Artificial Martian channels, drawn by Percival Lowell.
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The Wow! signal shown as "6EQUJ5". The original paper with Ehman's handwritten exclamation is kept by the Ohio History Connection. It was pointed towards the Proxima Centauri system. This signal was used to support the search for extraterrestrial intelligence.
See also
In Spanish: Vida extraterrestre para niños
