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Distant moons, asteroids, and mysterious objects drifting beyond Pluto have yet to be explored.
There's no telling what we may yet discover — but life beyond Earth is a distinct possibility.
During a recent live TED Talk in New York, NASA planetary scientist Jim Green described the four most likely places that we could find life in the solar system.
Keep scrolling to find out what they are and why they're the most exciting spots to search for alien life in the solar system.
1. Mars appears pretty barren now, but there's evidence that it used be a watery world not unlike Earth.
Some scientists think the red planet may have been capable of supporting life: It once had oceans that covered 19% of its surface.
This artist’s impression shows how Mars may have looked about four billion years ago. The young planet Mars would have had enough water to cover its entire surface in a liquid layer about 140 metres deep, but it is more likely that the liquid would have pooled to form an ocean occupying almost half of Mars’s northern hemisphere, and in some regions reaching depths greater than 1.6 kilometres.
ESO/M. Kornmesser/N. Risinger (skysurvey.org)
But it turns out that Mars still has liquid water near its surface that occasionally bubbles up through the crust. Everywhere we find water on Earth, we find life.
2. Enceladus, which orbits Saturn, is another intriguing world to planetary scientists.
NASA/JPL/Space Science Institute
Below its icy crust is a giant subsurface ocean.
An artist's rendition of Enceladus' global liquid water ocean between its core and crust.
NASA/JPL-Caltech
There's evidence that these oceans have hydrothermal vents. That's huge because the hydrothermal ocean vents on Earth support a whole ecosystem of life.
Sea life swims near the hydrothermal vents of the East Diamante submarine volcano off the coast of the Marianas Islands.
usfwspacific/Flickr
It's already found the presence of sodium and organic molecules. Data from the latest flyby may help scientists figure out how active the ocean's hydrothermal vents are.
Image of Saturn's moon Enceladus, showing the "tiger stripes," long fractures from which the water vapor jets are emitted.
NASA/JPL/Space Science Institute
It's the only place in the solar system besides Earth that has liquid on its surface. Scientists think some form of life could have adapted to thrive in a methane world.
NASA
3. Europa harbors about twice as much water as there is on Earth.
Scientists think the cracks form when plumes of warm ocean water rise to the surface, where they create lakes of liquid water within Europa's icy crust. Eventually the ice above the lakes cracks and sinks and creates these chaos regions.
Right now NASA is planning a mission called the Europa Multi-Flyby Mission to land the first-ever probe on Europa.
This artist's rendering shows a concept for a future NASA mission to Europa in which a spacecraft would make multiple close flybys of the icy Jovian moon, thought to contain a global subsurface ocean.
NASA/JPL-Caltech
Such a spacecraft could help us learn the composition of Europa's icy crust — and maybe see if anything might swim below the surface
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