When you hear astronomers speak of habitable<\/em> worlds \u2013 or worlds in the habitable zone<\/em> of their stars \u2013 do you think of alien civilizations? Or do you think of worlds where humans<\/em> might someday live? What do astronomers mean by habitable<\/em>? Perhaps not what you think.<\/p>\n To an astronomer, the word habitable<\/em> simply means a planet whose physical conditions might allow life \u2013 any form of life, perhaps microbial life \u2013 to exist there. Here\u2019s what NASA says about the word habitable<\/em>:<\/p>\n The standard definition for a habitable planet is one that can sustain life for a significant period. Based on our solar system, life requires liquid water, energy and nutrients. A \u2018habitable zone\u2019 is the region around a star where planets can receive the perfect amount of heat to maintain liquid water on their surfaces. <\/p>\n<\/blockquote>\n So our Earth, for example, is habitable<\/em>. It\u2019s the only planet in the habitable zone of our star, that is, the only one of our sun\u2019s planets where liquid water can exist on the surface. So Earth has the right conditions for life: liquid water, energy, nutrients. Other worlds orbiting distant stars also lie in the habitable zones of their stars. But do they have liquid water \u2026 enough energy reaching the surface for living things to grow, thrive and produce nutrients? We don\u2019t know for sure yet of any<\/em> worlds like that, beyond Earth.<\/p>\n EarthSky lunar calendars are back in stock! And we\u2019re guaranteed to sell out, so get one while you can. Your support means the world to us and allows us to keep going. Purchase here..<\/p>\n So it goes without saying that \u2013 so far \u2013 Earth is the only planet<\/em> we know that\u2019s inhabited<\/em>. And it\u2019s also the only planet<\/em> we know, so far, where human life can<\/em> walk around on the surface without spacesuits, breathing the air, drinking the water, eating the plants.<\/p>\n Then again, exoplanets \u2013 or planets orbiting distant sun \u2013 are not easy to find. As of late January 2024, we know only 5,572 confirmed exoplanets in 4,145 planetary systems, with 942 systems having more than one planet. Meanwhile, there are an estimated 100 to 400 billion<\/em> stars in our home galaxy, the Milky Way.<\/p>\n So when we in the science media mention habitable<\/em> exoplanets, please don\u2019t think it means we\u2019re suggesting that world might be an alternate Earth, or a world we might someday colonize<\/em>. Far from it! Likewise, habitable<\/em> doesn\u2019t mean that astronomers believe alien life exists on this or that distant planet. <\/p>\n So why discuss habitability<\/em> at all? The word has academic interest, certainly; professional astronomers are curious. And many in the public are curious also! We at EarthSky are curious. Aren\u2019t you? Is Earth alone in the Milky Way galaxy, or are other worlds capable of supporting life as we know it? Those are big and weighty questions, and it\u2019s exciting that astronomers are ferreting out the clues that might let us begin to answer them.<\/p>\n And sure. Maybe someday humans will<\/em> venture to a habitable exoplanet. And maybe someday we will<\/em> find life on a world orbiting the habitable zone of its star. But will this happen in our lifetimes?<\/em> Will it ever<\/em> happen?<\/p>\n Let\u2019s look at our own solar system. We know Earth is habitable. But what about the other planets and moons? Some scientists think Venus may have been habitable in the past, but climate change turned it into the scorching furnace we know today. It also appears that Mars may have been more habitable in the past, when an ocean covered much of its surface. Even Mercury may have habitable spots, such as in its salt glaciers.<\/p>\n Farther out in the solar system, Saturn\u2019s moon Enceladus is an exciting prospect for habitability. It\u2019s covered in a global ocean hiding beneath an icy crust. Jupiter\u2019s moon Europa may also have an ocean that could be habitable.<\/p>\n Many scientists are researching just what elements, abundances and combinations we should be looking for to find habitable exoplanets.<\/p>\n Some of the other key things scientists look for are terrestrial planets with the potential for water on their surfaces. Some of the candidate exoplanets for habitability include K2-18 b, which may have a deep hydrogen atmosphere and global water ocean. Another is Wolf 1069 b, an Earth-size and Earth-mass exoplanet orbiting in the habitable zone of its star just 31 light-years away.<\/p>\n A proposed mission, called the Habitable Worlds Observatory, would help astronomers study exoplanets in habitable zones. In August 2023, scientists and engineers gathered at Caltech to discuss the future mission, which would launch in the late 2030s or early 2040s. The National Academy of Sciences\u2019 Decadal Survey on Astronomy and Astrophysics \u2013 a roadmap of upcoming astronomy goals \u2013 picked the Habitable Worlds Observatory as their top priority in 2020. The observatory would be second in power only to the James Webb Space Telescope.<\/p>\n Bottom line: When astronomers say a distant world is \u2018habitable,\u2019 what do they mean? Do they mean humans could live there? Do they mean we\u2019ve discovered alien life? No to both. They simply mean that \u2013 on that distant world \u2013 the conditions<\/em> are right for life, possibly just microbial life, to exist. <\/p>\n <\/div>\n\n
Habitable doesn\u2019t mean inhabited<\/h3>\n
![\"Three](\"https:\/\/earthsky.org\/upl\/2019\/10\/habitable-zone-stars-distances.jpg\")
Habitability in our solar system<\/h3>\n
Habitable exoplanets<\/h3>\n
Plans for a dedicated observatory<\/h3>\n