What is the Habitable Zone?
We’re on the hunt for worlds that can sustain life! But how can we identify them from trillions of miles away?
We start the search within the habitable, or “Goldilocks,” zone around other stars – the not too hot, not too cold region where liquid surface water could exist on orbiting planets.
This artist’s impression shows a star with several planets within its habitable zone.
It sounds simple enough, but it’s surprisingly complicated. For one thing, the size and location of a habitable zone varies from star to star, mainly depending on how big and hot it is. It’s like standing near a bonfire; the smaller the flames, the closer you have to be to feel the heat.
This visualization of our solar system shows the rough location of the Sun’s habitable zone. Earth is nestled comfortably in the middle, and Venus and Mars fall within it as well. But since other factors also influence habitability, Mars and Venus are significantly less habitable than Earth.
Our Sun is a pretty average star with a mid-size habitable zone that extends from around Venus’s distance from the Sun to a bit past Mars.
Larger stars (which are more scarce) have wider habitable zones located farther from the star. But the stellar heavyweights burn through their nuclear fuel very quickly, so they die much younger –– maybe too soon for advanced life to have a chance to form on orbiting planets, even if they’re in the habitable zone.
This infographic compares the characteristics of three classes of stars in our galaxy: Sunlike stars are classified as G-type stars; stars less massive and cooler than our Sun are K dwarfs; and the very faintest and coolest stars are the reddish M dwarfs. The habitable zones, potentially capable of hosting life-bearing planets, are wider for hotter stars, which have shorter longevity. Planets in the habitable zones of the smallest stars are subject to the most radiation, which could make it difficult or impossible for life to thrive.
Smaller stars are far more common, with the littlest three types (shown above) adding up to around 90% of all the stars in our galaxy! They have habitable zones that are not only tiny but also very close in.
That’s a problem because the smallest stars often have the biggest tempers, lashing out with X-ray and ultraviolet radiation that could sterilize nearby worlds. And those planets tend to be tidally locked, like the Moon is to Earth, which means the same side always faces the star. Can you imagine living on a planet with eternal daylight on one side and an endless night on the other? That would make for some extremely weird weather!
This illustration shows one way that planet TOI-715 b –– a tidally locked super-Earth in the habitable zone around its star –– might appear to a nearby observer.
Out of more than 6,000 exoplanets discovered so far, around 150 are known to orbit within the habitable zone. Is Earth 2.0 among them?
This collage visualizes some of the more than 6,000 exoplanets confirmed to date.
Maybe! But it takes several more ingredients to truly make a world habitable for life as we know it. (Life as we don’t know it may exist, but we hunt for the familiar because we know it worked at least once –– on Earth –– and it gives us a solid starting point for our search).
Based on what we’ve observed in our own solar system, large, gaseous worlds like Jupiter seem far less likely to offer habitable conditions. Smaller rocky planets seem to offer gentler environments that would be more conducive to life.
This artist’s impression visualizes a variety of terrestrial planets.
And even for Earth-like worlds in the habitable zone of an average star, life would be tough without an atmosphere. An atmosphere acts as a security blanket that provides breathable air, regulates temperature, shields against harmful solar radiation, and helps any surface water remain stable for long periods of time. That’s one reason that Mars, with its barely there atmosphere, is much less friendly to life even though it’s in the Sun’s habitable zone.
This artist’s concept shows what the surface of a planet called TRAPPIST-1f may look like. It’s one of seven Earth-size planets in the system.
NASA missions kicked off the hunt for habitable worlds and will soon take them to the next level. Our now-retired Spitzer Space Telescope helped confirm the first known star system with seven Earth-sized exoplanets, Trappist-1, and studied rocky worlds. Then our Kepler space telescope, which is also now retired, showed that planets are even more common than stars in our galaxy, and proved that Earth-size worlds are abundant among them.
The Hubble Space Telescope kicked off the direct study of exoplanet atmospheres, which our James Webb Space Telescope now does in even more detail. And our TESS (Transiting Exoplanet Survey Satellite) is busily scouring the skies for nearby Earth-size planets, identifying the best targets for follow-up atmospheric studies.
In the coming decades, two more major missions will enter the scene: NASA’s Nancy Grace Roman Space Telescope and Habitable Worlds Observatory. Roman will survey the stars to find planets like those in our solar system, which other telescopes typically struggle to find, and showcase technology to directly photograph Jupiter-like exoplanets. That will provide a crucial stepping stone for the Habitable Worlds Observatory to photograph Earth-like planets for the first time ever. It will also look for signs of life called biosignatures by measuring atmospheric gases like oxygen and ozone that could signal the presence of living things.
By narrowing the search for habitable worlds, we keep moving ever closer to finding Earth 2.0! Follow along with NASA’s exoplanet discoveries here: https://science.nasa.gov/exoplanets/.
Make sure to follow us on Tumblr for your regular dose of space!
