Are we alone in the universe? Maybe. Maybe not.

Often while looking to the night sky smeared with stars, each unfathomably far away, both options seem as equally likely. Yet less than 30 years ago, astronomers still fiercely debated whether those same stars even housed planetary systems like our own, let alone life.

Then, in 1995, the debate ended. Astronomers found the first planet beyond our solar system, and the potential for life in a galaxy far, far away seemed just a bit more likely.

In this month's Century of Science theme, Other Worlds, Science News covers the massive expansion of our universe from the astronomer's perspective. In less than one hundred years, astronomers went from arguing whether anything existed beyond our galaxy, to cataloging a suite of <link explore whats-here exhibits henry-crown-space-center>new galaxies, solar systems and planets.

The most enticing of the new worlds are those with the potential to support life as we know it, so-called Earth-like planets. These rocks often set up in the "Golidlocks zone": the distance from their star that's not too cold, not too hot, but just right for liquid water to exist. Of the more than 5,000 known exoplanets, only 55 meet Earth-like conditions. Three of these are within the TRAPPIST-1 system, nearly 40 light-years away from OG Earth.

Astronomers discovered the TRAPPIST-1 system in 2016 using a duo of telescopes designed to pick up the teeny-tiny changes in a star's brightness when an orbiting planet crosses in front of it. This method, called "transiting," was the one used to confirm the existence of exoplanets, first picking up a Jupiter-like planet orbiting a star in the Pegasus constellation.

In the TRAPPIST-1 system, seven Earth-like planets orbit an ultra-cool red dwarf star that is less than 10% the mass of our own sun. Of those seven, three nestle within the Goldilocks zone. Add in some tidbits gleaned from our telescopes, and the possibility for liquid water on one or all three is more than a little alluring. Scientists have already pointed the James Webb Space Telescope at the TRAPPIST-1 system, hoping to glimpse liquid water on these rocky worlds.

But liquid water isn't necessarily needed for all life. All water really does is provide a medium and a promiscuous electrical charge to kickstart the biochemical reactions that keep Earth-like life humming along at a steady clip. In theory, any kind of medium that provides a dance floor on which chemicals can boogie would do, but that life might not look like life as we know it.

Given that, life - or something like it - <link explore whats-here giant-dome-theater worlds-beyond-earth>might exist in our very own solar backyard. Enter Titan.

Titan is the largest of Saturn's 83 moons, coming in at just a bit under half the size of Earth. At more than 880 million miles from the sun, Titan's atmosphere sits at a balmy -290° F. At this deep-freeze, water only exists as vapor in the highest reaches of Titan's atmosphere. Yet, lakes pockmark Titan's surface. These lakes are made not of liquid water, but of liquid methane.

After a tip from the Hubble Space Telescope suggested that Titan had methane lakes, the Cassini-Huygens mission spotted and confirmed them. The presence of methane lakes makes Titan the only other known place in our solar system to have stable bodies of liquid sloshing around on its surface.

No life on Earth (that we know of!) exists by using methane as a medium, but it's theoretically possible. This hypothetical lifeform would breathe H₂ instead of O₂, metabolize it to acetylene rather than glucose and exhale methane instead of carbon dioxide. While scientists haven't yet spotted such a lifeform yet, that hasn't stopped them from taking a peek at Titan's surface.

On January 14, 2005, the Cassini-Huygens mission provided scientists with their first glimpse of Titan's surface ... and it looked a lot like Earth when life first started popping up here: rocky with signs of erosion from rushing rivers. Throw in Titan's nitrogen-rich atmosphere and a jolt of UV radiation from the sun and life could find a way on the frigid moon.

Are we alone in the universe? Maybe the better question is, for how much longer will we be alone in the universe? Seems that might only be a matter of time.