Five Scientific Reasons Extraterrestrial Life on Exoplanets and Mars Could Be Real

In 1977, researchers diving near the Galapagos Islands found something that shouldn't exist: entire ecosystems clustered around hydrothermal vents on the ocean floor, kilometres below where sunlight reaches. The organisms weren't surviving despite the conditions, the boiling sulphuric water, the crushing pressure, the total absence of photosynthesis. They were thriving because of them.

This discovery launched the field now called astrobiology, and it forced a complete revision of the checklist scientists use to define a habitable environment. Before 1977, the checklist was short: liquid water, moderate temperature, sunlight. After, it became almost uncomfortably open-ended.

Tardigrades, microscopic animals about half a millimetre long, can survive the vacuum of space, radiation doses that would kill a human in minutes, and temperatures from minus 272 degrees Celsius to 150 degrees above zero. They have been found in Antarctic ice, deep ocean sediment, and the upper atmosphere. Deinococcus radiodurans, a bacterium, can reassemble its own shattered DNA after absorbing radiation 3,000 times the lethal dose for humans. It has been found living inside nuclear reactors.

The biological lesson from extremophiles is not that life is tough in a general sense. It is that life, once it starts, finds ways to persist in conditions we previously assumed were sterile. That changes the probability calculation for every moon and planet scientists look at.

Jupiter's moon Europa is roughly the size of Earth's Moon. Its surface is a cracked sheet of ice, and for decades that surface was the whole story. Then the Galileo spacecraft, which orbited Jupiter from 1995 to 2003, measured Europa's magnetic field and found something unexpected: the field was being disrupted in a pattern consistent with a saltwater ocean beneath the ice, conducting electricity.

NASA's subsequent analysis confirmed it. Europa holds an estimated 3 times more liquid water than all of Earth's oceans, kept liquid not by solar heat but by tidal flexing, Jupiter's enormous gravity squeezes the moon's interior as it orbits, generating enough friction to maintain liquid water kilometres below the ice shell.

The ocean floor of Europa almost certainly has hydrothermal vents. The same chemistry that sustains life around Earth's deep-sea vents, mineral-rich water, chemical gradients, heat, is likely present there. NASA's Europa Clipper mission, launched in October 2024, is designed to fly past the moon 49 times and analyse its ice plumes, magnetic field, and surface chemistry in detail. The results will take years to process. The question it is asking is not whether conditions are theoretically possible. It is whether they are actively present.

In 2018, NASA's Curiosity rover drilled into 3.5-billion-year-old mudstone in Gale Crater on Mars and extracted something that had been sitting there since before complex life existed on Earth: organic molecules. The findings were published in Science by Jennifer Eigenbrode and colleagues at NASA's Goddard Space Flight Center.

Organic molecules are carbon-based compounds, the building blocks of life as we know it, though their presence does not confirm life. They can form through non-biological chemistry. What made the Gale Crater discovery significant was their preservation. Mars today is cold, dry, and bombarded by radiation that destroys organic material at the surface. The fact that these molecules survived for billions of years, sealed in rock, told scientists that Mars once had conditions capable of preserving biological signatures, and possibly creating them.

ISRO's Mangalyaan mission, which reached Martian orbit in 2014 and made India only the fourth space agency to reach Mars, carried a methane sensor. Methane on Mars has been detected intermittently by Curiosity and by the European Space Agency's Mars Express orbiter. On Earth, most atmospheric methane is biological in origin. On Mars, the source remains unconfirmed, geological processes can also produce it, but the intermittent spikes, first reported in a 2019 paper in Nature Geoscience by Christopher Webster's team at JPL, have not been fully explained by any non-biological model either.

NASA's Kepler space telescope, which operated from 2009 to 2018, changed the scale of the conversation entirely. Before Kepler, scientists knew exoplanets existed but had confirmed only a few hundred. Kepler watched 150,000 stars simultaneously, looking for the tiny dip in brightness that occurs when a planet crosses in front of its star. Over nine years it confirmed more than 2,600 exoplanets and identified thousands more candidates.

The number that matters most: Kepler data suggests that roughly 20 to 50 percent of Sun-like stars in the Milky Way have at least one Earth-sized planet in the habitable zone, the orbital range where liquid water could exist on a rocky surface. The Milky Way contains an estimated 200 billion stars. Even at the conservative end of that estimate, the number of potentially habitable planets in our galaxy alone runs into the billions.

Kepler's successor, the TESS satellite launched in 2018, has continued the census. The James Webb Space Telescope, operational since 2022, can now analyse the atmospheres of some of these planets directly, looking for biosignatures, chemical combinations like oxygen and methane that would be unstable unless something biological was continuously producing them. The first atmospheric readings of rocky exoplanets in habitable zones are already being processed.

In September 2020, a team led by Jane Greaves at Cardiff University announced they had detected phosphine in the atmosphere of Venus, at concentrations of about 20 parts per billion. The announcement, published in Nature Astronomy, immediately became one of the most contested findings in recent planetary science.

Phosphine is a molecule, one phosphorus atom bonded to three hydrogen atoms. On Earth, it is produced in two ways: industrial synthesis, and anaerobic biological processes. Certain microbes in oxygen-free environments produce it as a metabolic byproduct. The Venusian atmosphere, particularly a cloud layer between 48 and 60 kilometres altitude, has temperatures and pressures similar to Earth's lower atmosphere. Some researchers have proposed that microbial life could survive in those clouds, never touching the planet's infernal surface.

The controversy matters as much as the detection. Several subsequent analyses disputed the concentration estimate, and some argued the signal could be explained by sulphur dioxide or instrumental artefacts. Greaves' team revised their figures downward but maintained the detection was real. The scientific argument is ongoing. What it demonstrated is that Venus, long written off as a dead world, is back in the astrobiology conversation, and that the tools scientists now have are sensitive enough to detect trace biosignatures from Earth orbit.

The five lines of evidence don't converge on a single planet or a single mechanism. Extremophiles come from Earth's own geology. Europa's ocean sits under ice in the outer solar system. Mars holds ancient chemistry in its rocks. Kepler's numbers come from statistical astronomy across the galaxy. Venus offers an atmospheric chemistry puzzle that remains open. What they share is that each one arrived from a direction nobody predicted, which is, historically, exactly how biology tends to announce itself.