The deeper we look into space, the more places we come across that maybe, just maybe, could host life. From our neighboring planets to distant galaxies sending out weird signals, the list of spots in space worth checking out just continues to grow.
The closest worlds we should check for signs of life are those next door that we've already been to, or at least our robots have.
Originally published April 19, 2017.
Update, Sept. 16, 2020: Adds new slides.
Yes, our nearest planetary neighbor is supposed to be a horrible, hot and toxic hellscape, but that's just below the clouds. Higher in the atmosphere it could be quite nice. In September 2020, scientists announced the landmark discovery of phosphine, a prime sign of potential microbial life, in this temperate zone. The rush is now on to get a closer look at what's going on. The planet wasn't always so inhospitable, so perhaps something managed to adapt? Scary to imagine what might have managed that, but you know you want to see it.
There's increasing evidence that Mars was once a lot more like Earth, with oceans on its surface. Today it's more harsh, but it's not out of the question that we could find some sort of microbes in Martian soil.
Dwarf planet Ceres in the asteroid belt is full of surprises. It started with those big bright spots that turned out to be salt deposits, and there's also a huge, strange pyramid-shaped mountain, plenty of water beneath the surface and even the building blocks of life. Some people already believe this huge rock is actually an alien ship. The evidence isn't there to support that theory, but the place does seem worth a closer look.
In 2019, a noted engineer suggested a "recently discovered group of nearby co-orbital objects is an attractive location for extraterrestrial intelligence (ETI) to locate a probe to observe Earth while not being easily seen." That's right. Some of those space rocks may actually be ancient spies keeping an eye on us for centuries, or longer.
We don't think of the largest gas giant planet around as a place to look for life, but science fiction author Ben Bova has other ideas.
"It's got all the ingredients, enough room and lots of energy," he said in 2016.
Bova briefly explained his notion of life-forms that might be able to live in the air or in water underneath Jupiter's dense deck of clouds. He referred me to a few of the novels from his "Grand Tour" series, including "Jupiter" and "Leviathans of Jupiter."
The storyline of the novels revolves around the existence of massive, city-size life-forms called Leviathans living in gigantic oceans that have condensed beneath the clouds of Jupiter.
Um, sure. Why not?
Saturn's satellite Titan is the rare moon in our solar system with an atmosphere, weather, seas and rivers. It sure looks like home, except it's freezing and the lakes are flammable. Whatever life could survive there would be awfully weird, but scientists would still love to send a submarine to see for themselves.
Like Europa, Saturnian moon Enceladus has an icy shell with plumes shooting into space. In 2015, the Cassini spacecraft actually flew through one of the plumes and found large amounts of hydrogen present in its hidden ocean. This suggests the watery world has just about all the ingredients required to support life.
Jupiter moon Europa not only hides a subsurface ocean beneath its icy shell, but geysers have also been spotted there, hinting that some sort of hydrothermal activity might be able to support marine life.
Jupiter's moon Callisto is another world that harbors an unseen ocean. Checking it for microbes or any other exotic life forms might be tough, though, because it would require drilling through its huge, rocky exterior.
Ganymede, Jupiter's largest moon, has long been suspected of harboring a subsurface ocean. In 2015, scientists said they could confirm a salty ocean beneath its frozen crust. It also has a thin oxygen atmosphere, adding to its intrigue.
This former planet is very cold, but it's also more interesting than we used to think, with hints of active geology, lots of ice and perhaps some hidden oceans of its own. Definitely worth adding to the life-prospecting itinerary.
In 2017, a bizarre, cigar-shaped object tumbled into our solar system, flew past the sun and then made its way back out to deep space, inexplicably speeding up as it left us. It may have just been a weird type of dead or dying interstellar comet, or it may have been a probe sent by a distant alien civilization as one Harvard astronomer famously suggested. We may never know, but its origins are worth continuing to ponder.
The Trappist-1 system is just 40 light-years away and hosts up to seven Earth-sized planets, all very close to each other and perfect for the space-faring civilization of our sci-fi dreams.
Wolf 1061 c is a "super-Earth" just 14 light-years away, making it one of the top five closest potentially habitable planets orbiting another star. We've known about it for a few years, and scientists have already started checking it for alien transmissions.
TOI 700-d is the first Earth-sized, habitable-zone planet found by the Transiting Exoplanet Survey Satellite (TESS), NASA's latest planet hunter. It's located 100 light-years away around a calm M dwarf star. While it's likely to be very different than Earth (it probably doesn't rotate, for starters), astronomers are eager to get a closer look at it with next generation telescopes like NASA's upcoming James Webb Space Telescope to see what's happening there (or not).
Mysterious signals known as "fast radio bursts" have baffled astronomers for over a decade. As the study of these enigmatic emissions evolved, a handful have been found to repeat, and a few of those have been traced back to distant sources. So far, the closest is a spiral galaxy similar to our own Milky Way.
Something weird is going on around the distant star KIC 8462852, also known as Boyajian's Star. After a few years of research, no one knows for sure what's happening, but one explanation that's yet to be completely ruled out is the far-out notion that a highly advanced society is building insanely huge megastructures in space that obstruct the star. Gulp.
There might not be aliens around Boyajian's Star, but an intriguing notion put forth by a group of Swedish researchers is to check stars that have inexplicably disappeared from the night sky over the decades. Either they died without anyone noticing, or perhaps they've been blocked from view by some crazy alien megastructure.
Kepler 186f was one of the first confirmed Earth-like, potentially habitable exoplanets. But at 500 light-years away, it no longer receives quite as much attention as targets closer to home.
Kepler-283c was discovered as part of a huge data dump from Kepler that included over 700 newly confirmed exoplanets. It is about twice the size of Earth and orbits much closer to its home star, which is 1,743 light-years from Earth.
Some of the strangest exoplanets can be found in the habitable zone of Gliese 667C , which is one of three suns in the triple-star Gliese 667 system. This probably makes for some interesting skyscapes from planets Gliese-667Ce and Gliese-667Cf, the two most likely planets in the system to harbor water (a third planet nearby is also in the habitable zone, but with slightly less favorable conditions for life.)
HD 40307g is another nearby super-Earth only 42 light years away that will be a top target for the next generation of telescopes.
Kepler 62f was one of the first exoplanets found by the space telescope beginning to approach the size of Earth. Years later, it's still among the best candidates that could be a sister planet to our own.
Gliese 581d is a potentially habitable planet just 20 light-years away. In 20 years we could probably know if it delivers on that potential.
The exoplanet LHS 1140b, which orbits a red dwarf star 40 light-years from Earth, may be the new holder of the title "best place to look for signs of life beyond the solar system." This world is a little larger and much more massive than Earth and has likely retained most of its atmosphere.
In 2019 K2-18b, some 110 light-years away, became the first super-Earth exoplanet known to host both water and temperatures that could support life.
A very strange signal reportedly came from the direction of the star HD 164595 in the constellation Hercules, which has at least one confirmed planet, a Neptune-size world in close orbit that would seem unlikely to support life as we know it. Then again, there could be other planets there we haven't seen yet...
There's a number of other potentially habitable exoplanets out there that kind of blend together, most of them discovered by the Kepler Space Telescope and a growing crop observed by TESS. To keep from getting monotonous, we're not listing them all, but the Planetary Habitability Lab does here.
When some of us think of life on distant exoplanets, it's hard not to picture dusk on the home world of one young Luke Skywalker. Recently scientists have considered how real planets in binary star systems, just like Tatooine of Star Wars fame, might be able to support life.
Their results were promising, partially validating a primary reason that at least one space nut hopes we continue looking everywhere for life: the hope that Yoda really is still out there somewhere.