Where could alien life exist in our solar system?

Who hasn't looked into the sky and wondered if there's life out there, somewhere, looking back at us? Is it possible there's alien life closer to home — within the bounds of our solar system?

The short answer is yes, there are several places researchers are looking for signs of life within our solar system, and there are some hints that life might be hiding in unexpected places.

Is there life on Mars?

While there don't seem to be any "little green men" inhabiting Mars, it's possible there was microbial life in the past. Mars is currently a frigid desert, but thanks to Mars rovers that have studied the planet's rocks, we know that a long time ago, it had liquid water — an important condition for life.

"The Curiosity rover was built to search for habitable environments, the kind that could have supported microbial life on Mars in the past, if life ever arose on the Red Planet," said Amy Williams, a geology professor at the University of Florida and a member of both NASA's Curiosity and Perseverance science teams. "Curiosity's payload was selected to accomplish this task by looking for the conditions that we know life on Earth requires — liquid water, a source of carbon, and chemical energy to power a metabolism," she told Live Science.

In addition to studying the Martian environment, the Perseverance rover has been collecting promising rock samples to be returned to Earth for advanced analysis. The Mars Sample Return mission is currently under development by NASA and ESA to travel to Mars, collect the sample cores, and bring them back to Earth. (The Donald Trump administration has proposed cutting NASA funding to pay for this mission, however.)

"At least one of the cores collected has qualities that fit the definition of a possible indicator of ancient microbial life," Williams said of a particularly interesting rock sample. "Returning samples to Earth with the Mars Sample Return architecture would enable us to address some of humanity's most profound questions, including whether we are alone in the universe."

Related: How would we give aliens directions to Earth?

Could there be life on Venus?

Even though it's commonly called Earth's "twin," Venus is probably not the first place anyone might look for life in the solar system. Its surface temperature is hot enough to melt lead, and the average surface pressure is more than 90 times that of Earth. It's wrapped in thick, dense clouds that are mostly made from sulfuric acid, a highly corrosive substance that mixes with water to form acid rain on Earth, damaging flora, fauna and soil.

Despite those hostile conditions, extremophile microbial life might be able to survive high in Venus' atmosphere, where the temperatures and pressures aren't so extreme; perhaps life could find ways to protect itself from sulfuric acid's destructive nature. To find out, MIT is putting together the Morning Star Missions, which will travel to Venus' atmosphere with the ultimate goal of collecting a sample of the planet's clouds and returning it to Earth for better analysis.

"Microbial life might exist in Venus's clouds, despite their composition of concentrated sulfuric acid," Sara Seager, a professor at MIT and the director of the Morning Star Missions, told Live Science. "If we find compelling signs of life on Venus, it would upend our understanding of habitability and prove that life can exist in non-water solvents, opening up the range of worlds where life is possible."

When looking for life, scientists often focus on the search for liquid water because it's a requirement for life on Earth. Finding life on Venus would shatter that limitation — meaning alien life could be a lot stranger than we've thought and there could be a lot more places to look for it.

In 2020, a possible detection of phosphine gas in Venus' clouds became the center of a controversy about signs of life on Venus. Phosphine gas was an unexpected find because most known chemical processes that produce it come from living organisms or from extreme pressures on gas giants. However, the signal was small and critics said it might have been noise in the data. The Morning Star Missions to Venus will give us much more information about the planet's chemistry and possibly confirm if there's phosphine floating in the clouds.

Is there life on the moons of Saturn and Jupiter?

Further out in the solar system, there are several moons orbiting Saturn and Jupiter that could potentially host life, particularly Enceladus and Europa.

Saturn's moon Enceladus has a global subsurface ocean hiding underneath a thick icy shell. The moon constantly spits giant plumes of salty water into space that give scientists a rare opportunity to directly sample the interior of a celestial body other than Earth. The Cassini mission performed several flybys of Enceladus from 2005 through 2017, collecting images and even direct samples from the plumes that are still being analyzed. A 2023 paper in Nature announced that scientists had detected phosphates on Enceladus — meaning scientists have now detected all the basic chemical ingredients for life in the moon's ocean.

Several missions to study astrobiology on Enceladus, with the goal of collecting more samples from the plumes, have been in the works by teams at ESA and NASA, although they're still a long way from getting to the launch pad.

Like Enceladus, Jupiter's moon Europa is an icy world with a global subsurface ocean and water plumes erupting from its surface. Jupiter's massive gravitational pull on the moon provides energy and motion that fuels active geologic activity, and is thought to be the main reason the ocean is warm enough to be liquid. Interactions between the salty ocean and the rocky interior could possibly host life similar to the life found around hydrothermal vents on Earth.

ESA's Jupiter Icy Moons Explorer (Juice) launched on April 14, 2023 with an expected arrival at Jupiter in July 2031, where it will study Jupiter and three of its moons: Europa, Callisto and Ganymede. NASA's Europa Clipper mission launched on Oct. 14, 2024 to reach Jupiter in 2030 and conduct flybys of Europa to study if conditions for life are present.

Titan, another of Saturn's moons, is an extremely unique place in the solar system. It has a methane cycle like Earth's water cycle and a thick nitrogen and methane atmosphere. The Cassini mission performed over 50 flybys of Titan from 2004 through 2017 and launched the Huygens probe that landed on the moon's surface in 2005. These two missions collected data that indicated Titan's atmosphere could probably create complex molecules that are building blocks for life. NASA is putting together a car-sized octocopter to fly on Titan and collect samples at different landing sites.

Where else could life be in the solar system?

There are quite a few other places we might find life in the solar system, including dwarf planets like Ceres. Ceres is thought to have a lot of liquid water under an icy and rocky crust, thanks to images taken by NASA's Dawn mission which visited it in 2015.

The more we explore the solar system, the more places we find that have conditions that could support life. If there is life out there, it's likely not advanced enough to reach out to us, so we'll have to be the first to say hello.

Extraterrestrials quiz: Are you an alien expert, or has your brain been abducted?