In Greek mythology, Prometheus suffered terrible torment for stealing fire from the gods. It is therefore slightly ironic that these gods, or at least the planets identified with their Roman counterparts, in fact have no fire. This also applies to Apollo. While the Sun is often referred to as fiery, the fusion process that provides its heat is something quite different. It’s just billions of years of life on Earth that made fire possible, and if any of the planets we find around other stars are supporting fire, that would be a really big clue that they have too of life.
Fire generally requires the presence of oxygen. Although oxygen is the third most abundant element in the universe, it bonds so easily with others that it is rarely found in the molecular form that makes fire possible. A few other gases have been found that can burn particular materials, but these are generally rare and considerably less likely than oxygen to be found outside of a chemistry lab.
Neither oxygen nor these alternatives are present elsewhere in the solar system at the concentrations necessary for combustion. Indeed, there is no evidence of fire in the fossil record before the Middle Ordovician period. Although there had been oxygen in the atmosphere since the great oxidation event nearly two billion years ago, the concentration was too low to sustain the fire.
If all life on Earth suddenly disappeared, the oxygen in Earth’s atmosphere would gradually react with the rocks until it disappeared completely. As things stand, limited fuels are available to people living at high altitudes, as many do not burn as oxygen becomes scarce. Climb high enough and nothing burns at all.
Although the sustained presence of oxygen is considered a likely sign of life, it is not definitive proof on its own, which is why astronomers look for combinations of gases as biosignatures. With so much oxygen bound to the planets’ surfaces, it’s possible that some chemical process could cause its release, but it’s still unlikely to provide the abundance needed to sustain the fire.
In addition, fire also needs fuel (and heat). There is a pattern in what we burn: wood from trees, coal from ancient forests, and oil that was once just organisms. Most substances capable of undergoing the rapid oxidation process that produces flames are also products of life.
Methane and hydrogen both burn and can exist without life – they make up most of Jupiter’s and Titan’s lakes respectively. However, the combination of either with oxygen would be considered a much better indication of life forms that modify the atmosphere than oxygen alone.
Some phenomena can look like fire enough to fool our eyes, even beyond merging into stars. Erupting volcanoes can produce “fire fountains,” which Astronomy.com says likely occur on Jupiter’s moon Io. Tidal forces applied by Jupiter and the outer moons have melted the interior of Io, making eruptions nearly constant. However, fire fountains are not actual fires, but rather magma sprayed through a vent.
There will probably always be easier ways to test for the presence of life on new worlds, but fire might work in a pinch – as long as you can tell it apart from things that look alike.