Introduction
In Five Cents What We Know About Life Beyond Earth, we’re separating places that might support life from evidence that life actually exists. We’ll explore ancient Mars, the oceans beneath icy moons, and planets around distant stars. Then we’ll look at why an intriguing chemical signal is not a discovery of organisms.
The key points
The best starting point is the distinction that runs through all of them: habitable does not mean inhabited. No life beyond Earth has been confirmed. What scientists have found are environments and chemical ingredients worth investigating. For life as we know it, the basic requirements include liquid water, usable energy, and suitable chemistry. Temperature, pressure, and radiation also matter. These requirements help researchers choose where to look and what instruments to send.
But even an environment with all the right ingredients might never have produced life. We still do not know how readily life begins, because Earth remains our only confirmed example. That also explains a common misunderstanding about the habitable zone around a star. It describes a range of orbits where liquid water could exist on a planet’s surface under suitable conditions. It does not tell us that water is actually there, or that the atmosphere is suitable.
And it is not the only place to search. A moon far from the Sun can retain an ocean beneath its ice, with internal heating helping keep the water liquid. So the search is broader than finding another planet that looks like Earth. Mars offers a chance to investigate a potentially habitable past. Its rocks preserve evidence of ancient rivers and lakes.
In Jezero Crater, Perseverance is exploring what was once a lake and delta. Sediments there could have preserved traces of ancient chemistry, and possibly biology. Features in a rock called Cheyava Falls have been described as potential biosignatures. That means they deserve investigation, not that microbes have been discovered. Biological and non-biological explanations still need to be tested.
Elsewhere on Mars, Curiosity has detected organic molecules. Here, organic means carbon-containing. It does not mean produced by an organism. Such compounds can form through chemistry without life. The important question is not simply whether a molecule exists, but how it formed, what surrounds it, and whether several observations support the same explanation.
The icy moons offer a different possibility
The icy moons offer a different possibility: environments that may be habitable today. Europa, orbiting Jupiter, has strong evidence for a salty ocean beneath its frozen surface. Europa Clipper will investigate whether conditions there could support life. It is not a mission designed to detect organisms.
Enceladus, orbiting Saturn, provides a particularly useful opportunity because it ejects material from its subsurface ocean into space. A spacecraft can sample those plumes without drilling through the ice. Cassini measurements revealed organic compounds and phosphorus salts in the expelled material. Phosphorus is important to life on Earth, so finding it helps establish that relevant ingredients are available.
But an ingredient is not an organism. These moons make compelling targets because they allow researchers to investigate water, chemistry, and possible energy sources together, rather than judging habitability from surface appearance alone. Beyond our Solar System, thousands of planets have been confirmed, but their distance changes how we investigate them.
For some, researchers study starlight passing through an atmosphere as the planet crosses in front of its star. Different molecules absorb different wavelengths, allowing scientists to infer what gases may be present. Those signals can be faint. Both the star’s behaviour and the models used to interpret the data introduce uncertainty.
K2-18 b illustrates the challenge. It has been proposed as a world with a hydrogen-rich atmosphere and an ocean, but that ocean is not directly observed. Claims of possible atmospheric molecules associated with life have also been disputed by independent analyses. The responsible conclusion is a contested signal, not confirmed life. Remote observations can identify promising targets, but establishing biology requires much more than one interpretation of a spectrum.
When you encounter a claim about alien life
When you encounter a claim about alien life, ask what was actually measured. Was it an environment, an ingredient, or a possible biosignature? A biosignature is a feature that could indicate past or present life, but context matters. Even oxygen, strongly associated with life on Earth, can arise through non-biological processes elsewhere.
A stronger case would combine independent measurements, environmental context, and careful testing of alternative explanations. Repeated observations help establish whether a signal is real. They do not automatically establish that its cause is biological. The evidence points to promising habitats and clues, not confirmed extraterrestrial life. Water and useful chemistry guide the search; a biological explanation needs multiple lines of support.
To explore this further, you can generate two new Five Cents podcasts: How Ocean Moons Stay Warm and How Telescopes Read Alien Atmospheres. And with that, you're up to speed in a few minutes.