Debates about extraterrestrial life never really go away, but some voices carry more authority than others when the subject turns to where life beyond Earth might actually exist.
One of those voices is Lisa Kaltenegger, an astronomer and director of the Carl Sagan Institute at Cornell. She literally wrote a book on the topic.
Kaltenegger is the author of Alien Earths, and she also recently served as senior author on a paper examining some of the most promising places to search for life. In discussing the strongest candidates, she pointed to three planetary systems and seven individual worlds that stand out as especially intriguing. Mars, however, does not make the list.
One system high on that list is TRAPPIST-1, an ultra-cool red dwarf star orbited by seven planets that are all roughly similar in size to Earth. NASA has said each of the seven could potentially host surface water, making them compelling targets for future study.
Even so, Kaltenegger narrows the field. In her view, only four of those seven planets look especially promising, because the innermost worlds are likely too hot to support life as we know it.
“Planet TRAPPIST-1 b (circles its star in 1.5 Earth-days): way too hot; TRAPPIST-1 c (2.4 days): too hot; TRAPPIST-1 d (4 days): quite hot; TRAPPIST-1 e (6.1 days): just right; TRAPPIST-1 f (9.2 days): just right; TRAPPIST-1 g (12.3 days): almost just right but a bit cold; TRAPPIST-1 h (18.8 days): probably too cold.
Another fascinating possibility is Proxima Centauri b, which Kaltenegger has described as ‘the planet next door’. It is close enough to have inspired plenty of science fiction, including comparisons to worlds seen in Avatar.
Proxima Centauri b was detected because it ‘tugs’ on Proxima Centauri, our nearest stellar neighbor, causing the star to wobble in a way telescopes on Earth can measure.
“About four light-years away from us in the constellation Centaurus, Proxima Centauri is the easiest destination for us to reach once we invent ships that can travel those vast distances. A red star about the same age as our Sun, it is part of a triple- star system, consisting of two yellow suns, Alpha Centauri A and B, and a red sun, Alpha Centauri C (Proxima Centauri).
“Proxima Centauri wobbles just the right amount. Its planet takes only eleven days to complete its path around its red, active sun, which bombards it with flares of intense radiation. The short time it takes to circle its star means that the planet is likely tidally locked, captured in synchronous rotation. That means that only one side of the planet ever sees the sun; you would have to walk away from the sunlit parts of the planet to experience dawn or dusk and then trek even farther to reach the part of the planet that is shrouded in perpetual darkness.”
That possibility of a world with one permanent day side and one permanent night side is one reason the planet continues to capture attention.
Kaltenegger also discusses the wider Alpha Centauri system in her book. Although neither Alpha Centauri A nor Alpha Centauri B has any confirmed planets, she notes that both stars ‘have inspired the imagination of sci-fi writers for decades’.
“If one of the two stars had a planet, that planet would see two suns in its sky (and a third dim red one very far away).
“In the 2009 3D-movie Avatar, a film written and directed by the science enthusiast James Cameron, the lush, inhabited fictional moon Pandora has an atmosphere poisonous to humans. It is a little smaller than Earth and circles a fictional gas giant, Polyphemus, around Alpha Centauri A.
“The idea of habitable moons as abodes for life is based on our hope of finding life on some of the moons in our own solar system. And if a habitable moon were massive enough—like the fictional Pandora— it should be able to provide environments similar to Earth’s if it gets comparable amounts of starlight.”
The third system she highlights is Kepler-62, located around 1,200 light-years from Earth and first identified in 2013.
Today, Kaltenegger says there are ‘most likely billions of rocky planets circling their stars at just the right distance for life, not too hot and not too cold’.
But Kepler-62 marked an important shift. Before it was discovered, astronomers had found planets in so-called Goldilocks zones mainly through the ‘wobble technique’. That method could reveal a planet’s mass, but it could not clearly separate rocky Earth-like worlds from smaller gas planets that would be inhospitable to life.
“Scientists believed that warm, rocky planets like Earth existed, but it was by no means a certainty,” she said.
“Two rocky, temperate worlds circling another star, news that everyone hoping to find life on other worlds had been sitting on the edges of their chairs waiting to hear. And suddenly, my research to find life in the cosmos went from visionary to practical, from far-fetched to applied, from future- oriented to needed- right- now.”
Since this conversation was first published, the search has become even more focused. In March 2026, Cornell researchers led by Kaltenegger released a new catalogue that identified 45 rocky worlds in the habitable zone, plus another 24 in a more conservative zone, out of more than 6,000 known exoplanets. The team said the list is meant to help astronomers decide where to spend precious observing time with telescopes such as James Webb and the next generation of space and ground-based observatories.
That broader target list does not replace the seven-world shortlist Kaltenegger discussed here; instead, it shows how quickly the field is moving from broad speculation toward a prioritized observing plan. Among the most practical nearby targets, TRAPPIST-1 remains especially important, and NASA said by late 2025 that Webb observations had already been reported for four of its seven planets, including several in the habitable zone. Proxima Centauri b also remains one of the nearest and most closely watched candidate worlds, while Kepler-62e and Kepler-62f continue to stand out as the system’s two temperate planets.