Research, Feb. 2026
A catalog of rocky exoplanets in the habitable zone
Which planets are the best targets to search for life?
Gillis Lowry with A. Bohl, L. Lawrence, and L. Kaltenegger
We've found over 6000 planets that lie beyond our solar system, and most of them are deadly.
These planets orbit all sorts of stars, from small, slow-burning red stars to giant, fast-living blue-whites to middle-of-the-road, prime-of-their-life yellows like our sun. There are planets that see two suns in their skies, even three suns; there are planets that take one million years and planets that take two hours to circle around their stars; there are red-hot planets where gems may fall from clouds of metal and deep blue dots where glass may rain sideways.
Less extreme planets have a chance at hosting life, if all goes right. They’ll need to be not too far, not too close, but just the right distance from their host star: in the “Goldilocks” or “habitable” zone where liquid water could exist on a planet's surface without freezing or steaming away.
The boundaries of the habitable zone are different for every star. It's not just the amount of light the planet receives, but the color of that light; redder wavelengths tend to be more easily absorbed by a planet's atmosphere, heating it more easily. Some stars can also flare up, emitting dangerous amounts of light that could strip away the planet's atmosphere. Not all "host" stars are kind to their planets.
Since Earth is the only world we know for sure has life, it's hard to predict if these very different planets around very different stars are habitable. Until we can take closer looks and confirm which planets have atmospheres and liquid water, our models of the habitable zone are just educated guesses.
Unfortunately, we don't have unlimited telescope time to check every single planet, and "Earth-like" planets are often too tiny and dim for our current technology. If we want to know where to look—both to refine our estimates of the habitable zone and to hopefully find life itself—then what's a poor astronomer to do?
Make a catalog, of course!
My research with three colleagues checks all 6000+ planets in the NASA Exoplanet Archive to see which ones are in the habitable zone. We then list different types of habitable zone planets that astronomers should prioritize for future observations. This includes planets near the edges of the habitable zone (to help us refine our models), planets with very eccentric rather than circular orbits (to test whether planets that spend some time outside the habitable zone could still support life), planets at approximately the same position in their habitable zones as Earth is in the Solar System, and planets that are easiest to observe due to being far enough from their star, bright enough compared to their star, and somewhat nearby our Solar System.
Which rocky planets might be habitable?
The plot below shows all the planets we found to be "rocky" (below two times the radius or five times the mass of Earth) and within the habitable zone for their star.
A diagram depicting habitable zone boundaries across star type with data from this research (Bohl et al. (2026)). Earth is plotted alongside 45 exoplanets with radii less than 2 times or masses less than 5 times that of Earth, making them potentially rocky worlds. The "optimistic" habitable zone uses the examples of Venus and Mars to set limits on habitability—marking points at which a runaway greenhouse effect would trap too much heat (Venus) or a planet would be too cold to retain a thick atmosphere (Mars). The "conservative" habitable zone uses global cloud models to estimate the inner edge of the habitable zone, placing it much farther from Venus and closer to modern Earth. The boundaries of the habitable zone shift based on host star color, since different wavelengths of light will heat a planet's atmosphere differently.
Single-Celled or Intelligent Life?
Within our lists of planets to prioritize, we wanted to make sure that life would have had enough time to evolve. Using Earth's example, planets would need to be at least a billion years old (for the first single-celled life). We "intelligent" humans evolved after 4.5 billion years of Earth's history, so an older planet might have more advanced life than on Earth.
For our research, I searched through databases of astronomy papers to find estimates of host star ages, since planets tend to form at roughly the same time as their stars. It's very hard to estimate the ages of stars, however, so this part is mostly a proof-of-concept. Once astronomers have devised better age estimates for stars, we may be able to narrow down the thousands of discovered exoplanets first into the habitable zone planets, then just the habitable zone planets small enough to have a rocky surface, then just the rocky, potentially habitable planets that are old enough to support advanced life.
Mouse around on the plot below to see the age estimates for our rocky habitable zone planets. Look at those huge error bars!
Ages of rocky habitable zone planets
A diagram depicting ages of rocky habitable zone planets. Venus, Earth, and Mars are shown for reference.
Even if we don't precisely know most of these planets' ages, we can feel confident that none of these are too young to have life. We now have a list of planets that might be small enough, might be the right temperature, and might be old enough to at least support single-celled life.
That means it's time to figure out which planets we could actually take a closer look at. Detecting an atmosphere around such tiny planets is much harder than finding the planet itself, and detecting specific signs of life, like high amounts of oxygen, ozone, and methane, is much harder still.
Which planets are easiest to observe for signs of life?
We're still decades away from developing the technology to take photographs of tiny planets, so we have to use a more indirect way to look at their atmosphere: letting the host star light up the planet. This only works if the star and its planets align: if, from our point of view, the planet's orbit has no tilt at all, and we can see the planet pass behind and in front of its host star as it goes around.
When the planet passes in front (called "transiting"), some starlight passes through its atmosphere. This leaves an imprint on the light—similarly to how only certain colors can pass through a stained glass window, the light we end up collecting in our telescopes has certain colors missing. Each type of molecule has its own "fingerprint," because it always absorbs the same colors of light, no matter whether we see the molecule on Earth or another planet. We're able to see these fingerprints for gas giant planets, but for small, Earth-like worlds, our current telescopes are only powerful enough to see the closest ones.
Mouse around on the next plot to see our chances of observing these planets. "TSM" stands for "Transmission Spectroscopy Metric"—a calculated number based on how big the planet is, how hot the planet is, and how big, bright, and far away the star is. The Transmission Spectroscopy Metric tells us how easily we could detect the "fingerprints" I described above. The green dashed line represents the "minimum" for our current technology, defined by the creators of the TSM. Only the TRAPPIST-1 system of planets are above this line, and all the planets below it are likely too difficult to observe.
TSM and separation between planet and star in our sky
(transiting planets only)
A diagram of habitable zone planets that pass in front of their star ("transit"). Planets in the top right of the graph are the most observable (farther from their star and higher "Transmission Spectroscopy Metric"—based on how big and hot the planet is, and how big, bright, and far away the star is). Large, faint circles are gas planets; triangles are rocky planets whose error bars overlap the habitable zone; small circles are all other rocky planets. Earth is plotted as multiple "exoplanets" (circles with cross inside) to compare how easily we could observe a planet just like Earth from different distances. This is difficult, mainly because Sun-like stars with a habitable zone as distant as ours are also too big and bright for the planet to be seen, and planets orbit for a full year before we see them transit again.
The final graph below considers how far the planet is from its star and how bright the planet is compared to its (much, much brighter) star. Higher separation and higher planet brightness is important for "direct imaging"—taking a photograph of the planet. We're still many years from taking photos of tiny planets, but mission concepts such as Habitable Worlds Observatory are making this their primary science goal. It's important to start the conversation early so we can create—and continue to revise—our target lists for future missions.
Contrast and separation between planet and star
A diagram of all habitable zone planets. Planets in the top right of the graph are the most observable (farther from their star and less dim compared to their star). Large, faint circles are gas planets; triangles are rocky planets whose error bars overlap the habitable zone; small circles are all other rocky planets. Earth is plotted as multiple "exoplanets" (circles with cross inside) to compare how easily we could observe a planet just like Earth from different distances. Similarly to the previous graph, it is difficult to observe exoplanets like Earth because of how bright Sun-like stars are.
Even the "best" rocky planet targets in the graph above are 2 x 10⁷ times dimmer than their star—or, to flip it around, the star is 20 million times brighter! Even if the difference in brightness wasn't so extreme, we're viewing the planets from extremely far away, so we can barely make out any distance between the planet and its star (their angular separation, so small it must be measured in "milli-arcseconds"). A common analogy is that rocky planets are like fireflies next to a lighthouse; in this case, trying to observe our "best" candidates is like trying to see a firefly that's orbiting one inch from its lighthouse... while we're 56 miles away.
Judging by the two graphs above, the TRAPPIST-1 planets are pretty much our only targets for telescopes like the James Webb Space Telescope, at least among rocky habitable zone worlds.
Searching for life in the future
Our current telescopes are great, but they can only do so much. How can we observe more rocky planets with lower values on the TSM and contrast graphs? The best way is to build bigger telescopes in space and on the ground so we can collect more light from these planets. Once we've built better telescopes for direct imaging, we'll also be able to "zoom in" and actually see these tiny planets as separate dots from their star, rather than just blocky, noisy pixels.
In the meantime, I'm working on publishing my research that studies the ten most "Earth-like" planets from our rocky habitable zone planet list, including planet "e" from the TRAPPIST-1 system. I was interested in learning how much time it would take to detect atmospheric signs of life on these planets, using current capabilities of the James Webb Space Telescope. The answer, unsurprisingly, is "too much" time for any planet other than TRAPPIST-1 e, but estimating the actual numbers for these planets may be helpful in considering future telescope designs.
If you're interested in this work, you can follow me at the links on my homepage, or follow my fellow astronomers at the Carl Sagan Institute in their quest to find other pale blue dots. The search has barely even begun—in the next few decades, we may be able to say that out of hundreds of habitable zone planets, out of thousands of exoplanets found, out of hundreds of billions that surely exist in the Milky Way… there is one more corner of the universe that knows itself.
Notes
This research paper used the NASA Exoplanet Archive for planet characteristics, but also included stellar values from the Gaia spacecraft's Data Release 3 (DR3). We updated host star radius and temperature (in cases described in the methodology of our paper), and for many systems, this also caused an update in planet radius, since planet radii are often derived from the size of their host star. Some planets shift in and out of the habitable zone due to new Gaia DR3 temperatures, and some planets lose or gain the status of "rocky" based on new estimates of their radii. For this reason, some planets in our catalog are different from the lists of other habitable zone papers.
Download our final catalogs as CSV files here: https://zenodo.org/records/18134528
Acknowledgements
This research has made use of the NASA Exoplanet Archive, which is operated by the California Institute of Technology, under contract with the National Aeronautics and Space Administration under the Exoplanet Exploration Program.
This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the
institutions participating in the Gaia Multilateral Agreement.
Recent Venus and Early Mars boundaries from the habitable zone come from Kopparapu et al. (2013), while the more conservative inner limit comes from Ramirez & Kaltenegger (2018).
Header image by NASA / Caltech. Artist's impressions for exoplanets in final image from Pablo Carlos Budassi (Celestialobjects). All graphs were created by Gillis Lowry in Matplotlib, Photoshop, and Plotly.
Feel free to contact me at glowry@sfsu.edu if you have any questions!