The habitable zone is the region around a star where an orbiting planet could host liquid water and, therefore, possibly support life.
The habitable zone is also known as the "Goldilocks zone" because planets orbiting at that "just right" distance from a star are not too hot or too cold to host liquid water. If planets are closer to their star, the water turns to steam; if they're farther, it freezes.
Some researchers think the potential for liquid water has too simplified a picture of what it takes to support life. Venus, for example, is technically in the sun's habitable zone; its orbit keeps the planet within the area where liquid water could possibly exist. But in reality, the planet's carbon dioxide-rich atmosphere gives it the hottest surface temperatures of any planet in the solar system, and it's unlikely that life exists on its scorching surface or in its skies.
Related: The 10 most Earth-like exoplanets
What is the Goldilocks zone?
The Goldilocks zone is different around each star. Bigger, hotter stars like the sun, a G-type star, have a wider habitable zone, while smaller red dwarfs confine habitable planets to a narrower range, according to NASA (opens in new tab). But G-type stars are shorter-lived (on a galaxy timescale, that is) than some other types of stars. One abundant kind of star, K-type stars, can burn for tens of billions of years and, because of their stability, might have the most promising habitable zones.
But stars are individuals, too; for example, some red dwarfs are unpredictable, sending sterilizing flares toward their planets, while others are steady and reliably calm. So while two stars may have similar habitable zones and similarly sized planets, one star system might contain a much more promising candidate for life.
One example is the exoplanet TOI 700 d, discovered in 2020 by NASA's Transiting Exoplanet Survey Satellite (TESS). The star that TOI 700 d orbits could have been considered an unlikely home for life-bearing planets based on its type and its habitable zone alone. However, it’s a particularly “quiet star,” according to its discoverers, and has been stable long enough that on one of its planets, life could indeed have taken hold.
A star's habitable zone does not depend on the types of planets that are actually in the region. Gas giants are less likely to harbor life, according to NASA. So, in addition to searching for planets in stars' habitable zones, researchers look for worlds that resemble Earth in size, atmosphere or chemical composition.
Related: Superhabitable planets: Alien worlds that may be more habitable than Earth
Which solar system planets are in the habitable zone?
Venus, Earth and Mars all orbit within the sun's habitable zone.
Venus is just far enough away from the sun for the planet to have the possibility of hosting liquid water, according to NASA (opens in new tab). However, Venus' runaway greenhouse atmosphere makes the planet even hotter than Mercury, which is much closer to the sun. That atmosphere eliminates the possibility of life (as we would recognize it) on the planet's surface.
Earth is squarely in the middle of the sun's habitable zone. Some researchers say that isn't a coincidence; after all, we created the idea of "habitable zones" to look for planets like ours. To be fair, we have only one frame of reference. But as a result, we may be overlooking even better conditions for life than those on Earth because our view is too narrow.
Mars is at the far, chilly edge of the sun's habitable zone. But there is evidence that an ocean may have once flowed on Mars, and NASA's Perseverance roveris still busily sampling for microbial life that could be hidden below the planet's surface.
Why is the habitable zone important for the search for life?
Researchers use the idea of a habitable zone to narrow down which exoplanets might be good candidates in the search for extraterrestrial life.
As of early 2023, NASA's Kepler Space Telescope and TESS missions have detected and cataloged more than 5,000 exoplanets, according to the NASA Exoplanet Archive (opens in new tab), and those represent just a tiny fraction of the planets we're likely to discover as we keep looking. Kepler showed that, on average, every star has at least one planet.
Additional resources
If you're excited about the possibility of civilizations in space, check out what's new at the SETI Institute (opens in new tab), an organization dedicated to the search for extraterrestrial intelligence. Try your hand at a little exoplanet exploration of your own with NASA’s citizen science project (opens in new tab). And if you want to dive deeper into the search for habitable worlds, read "The Smallest Lights in the Universe (opens in new tab)" (Crown, 2020), an astrophysicist's memoir about searching the universe while appreciating life on Earth.
Bibliography
De La Torre, L. B. (2022, December 15). What is the habitable zone? Exoplanet Exploration: Planets Beyond Our Solar System. NASA/JPL. https://exoplanets.nasa.gov/resources/2255/what-is-the-habitable-zone (opens in new tab)
NASA Exoplanet Science Institute. Exoplanet and candidate statistics. (n.d.). NASA Exoplanet Archive. Retrieved January 27, 2023, from https://exoplanetarchive.ipac.caltech.edu/docs/counts_detail.html (opens in new tab)
NASA. (2021, April 2). The search for life. Exoplanet Exploration: Planets Beyond Our Solar System. https://exoplanets.nasa.gov/search-for-life/habitable-zone (opens in new tab)
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