A habitable world next door?

Telescopes are made to see. It's even in the name. Telescope is a Renaissance neologism that was concocted from the Greek tele, far, and scopos, watcher, for one of Galileo's inventions.^[1] So it's frustrating that exoplanet astronomy has historically involved very little actual seeing. Sight requires contrast, and most exoplanets are far too dim compared to their stars for our telescopes to resolve. Of the 5600 or so exoplanets we know about, less than 100 — all gas giants — have been imaged directly. The rest we know only by the flickering light of their host stars; when a planet transits, or passes between the Earth and a distant star, the star appears to dim.

The astronomer of romantic imagination climbs the observatory steps at midnight to peer through his mechanical far-watcher and take in the night sky. The astronomer of modern reality — if she's interested in exoplanets — waits for a space telescope to beam down data that she can turn into graphs.

That's one reason it's so exciting that astronomers now think they've taken a picture of an exoplanet orbiting the closest Sun-like star to Earth, Alpha Centauri A. The results were announced on August 5 in a pair of preprints on the arXiv server, which were published by the Astrophysical Journal Letters on August 11.

The planet still needs to be confirmed; for now, it is still just an excitingly planet-like smudge of light. But if it really is there, it orbits in the habitable zone of its star, meaning that water could be liquid on its surface — if it had a surface. The planet is a gas giant, so not exactly a welcoming environment for life as we know it. But if it's anything like the gas giants we know from our solar system, it could host pretty big moons. An entire system of habitable little worlds could be spinning around right next door, within reach of ambitious but technically feasible interstellar mission concepts like Breakthrough Starshot.

Behold, perhaps the most exciting smudge you'll ever see:

How exciting is this news? Exciting, but don't get your hopes up that we'll be popping the alien champagne anytime soon. There's still a lot we don't know about this world, tentatively dubbed Alpha Centauri Ab or Rigil Kentaurus b — including whether it is really there at all.

Here's what you need to know about Alpha Centauri Ab, the (maybe) planet next door. 

How to take a picture of an exoplanet

Taking pictures of exoplanets is hard.

Exoplanets are very far away and they're very dim compared to stars. A typical Earth-like planet could be tens of millions to tens of billions of times dimmer than its sun. So, just as actual stars are visually washed away in the blazing light tsunami that is daytime sunshine on Earth, exoplanets usually get lost in the light of their host stars. This is why most exoplanets are discovered and studied using the transit method, which is technically easier. It's basically interstellar eclipse-gazing, and entails waiting for a planet to transit, or pass between the Earth and its star. Scientists measure how much starlight the transiting planet blocks to infer its properties.

Sometimes, though, direct imaging is possible. The conditions have to be just right. It's easier to image brighter planets, and bright means big, hot, or preferably both. Because hot planets are easier to see, you might think it'd be easiest to snap pictures of planets broiling in very tight orbits. But if a planet clings too close to its star, its weak glow is completely lost in the star's glare. The balance between brightness and distance works out to favor young (and therefore hot) gas giant planets in distant orbits — ideally around a nearby star. It also helps if the planet orbits face-on, meaning the planet's orbit traces out the ring of a bullseye rather than the thin edge of a dinner plate from our perspective.

Even in ideal conditions, stars would still obscure their exoplanets if astronomers didn't use opaque shields called coronagraphs to block out their light. JWST's coronagraph can block out about 99% percent of starlight. That's impressive, but remember that 1% of ten billion is still 100 million; even using coronagraphs, planets are still hard to make out. Coronagraphs can also block out planets with tight orbits.

The first planets discovered using direct imaging — a quartet of gas giants orbiting star HR 8799 — were spotted in images taken over 7 years by the Keck telescope in 2008. Since then, scientists have found about 50 more. All of them are gas giants at least twice the mass of Jupiter.^[2]

How scientists found Alpha Centauri Ab

The story of Alpha Centauri Ab begins one year ago. In August 2024 scientists spotted a smudge of light in images taken by the Very Large Telescope. It looked like it could be a planet. But the smudge was only observed once. The data wasn't enough to rule out other possibilities like dust or instrumental artifacts (which the authors pointed out clearly in the paper abstract; a habit I wish more exoplanet astronomers would cultivate).

Part of the problem is the Alpha Centauri system itself. While it's the closest star system to Earth, and close is good, it also poses a special challenge for exoplanet astronomers: it is not one star, but three. The naked-eye star we named "Alpha Centauri" is actually two stars, Alpha Centauri A and Alpha Centauri B. These two have a third and more distant companion, little red Proxima Centauri, which is too dim for the eye to pick out.

Alpha Centauri A and B orbit each other quite closely — their distance to each other is between 11 and 36 times the distance from Earth to the Sun. That'd place them roughly between the orbits of Saturn and Pluto relative to each other. Just imagine: instead of a little icy blob ringing in its Solar System, Alpha Centauri Ab would have a second sun.

As fun as that second sun might be for sci fi authors (the ternary Alpha Centauri system is the core premise of Liu Cixin's amazing Three Body Problem), it's kind of annoying for planet hunters. In direct imaging, more light means more trouble. It's hard enough to pick out a candle flame an inch away from one floodlight, let alone two. To look for planets orbiting Alpha Centauri A, astronomers had to combine coronagraph images masking its light with other images masking its companion that were taken in close succession.

The team hoped to confirm their planet using JWST. Typically, it's pretty hard to image habitable zone planets directly, even if they're big; a glance at the catalogue suggested that all confirmed, directly-imaged planets are frosty worlds with distant orbits beyond the habitable zone (please correct me if I missed one). But Alpha Centauri A is close enough to Earth that planets in its habitable zone are, theoretically, within visible reach of JWST's MIRI instrument.

Now, the JWST data is in. Astronomers report three sets of follow-up observations with JWST taken in August 2024, February 2025, and April 2025. The data still aren't conclusive, but the team seems to think they've found a planet given the text of their preprints and their excited comments to the press:

Once the planet had been uncloaked, “I was in complete awe”, says Sanghi. “I had to take a moment of silence to appreciate what I was seeing.” To make sure the faint planet wasn’t just distorted starlight, the team spent about a year reanalysing the data. Again and again, the blotch of light remained. “We couldn’t kill it,” Sanghi adds.

Alien planet glimpsed in star's 'habitable zone' by Jenna Ahart in Nature, August 7

Is Alpha Centauri Ab real?

If Alpha Centauri Ab is confirmed in follow-up observations, it'll be a big deal. Not only would it add a planet to our nearest star system, but it'd be the first time we'd directly imaged any planet in the habitable zone of another star.

It's hard to overstate the significance of such a discovery. Exoplanet astronomy is about understanding the universe, and having a gas giant to observe that's so close would be great for scientists interested in understanding how planets form in multiple star systems. But let's be honest: exoplanet astronomy is, to a large extent, an alien hunt. A gas giant with a potential gaggle of habitable moons orbiting the Sun-like star next door is the kind of landmark observation that ends up growing into its own subfield and motivating its own missions.

Unfortunately, we don't yet know if this planet is real. As the authors have been careful to emphasize in their papers, follow-up observations will be needed to confirm their light smudge as a planet. That's because, while JWST spotted the candidate planet in August 2024, the smudge had disappeared in its follow-up observations in February and April 2025. The research team says that this kind of disappearing act is pretty normal planet behavior; basically, it just moved out of view. It's easier to directly image planets with very distant, long-period orbits for this reason. They don't move out of the way very fast.

This stuff is just hard, and scientists need time — both human and telescope — to get it right. It's honestly remarkable that exoplanet astronomers can pull anything at all out of the data JWST is beaming back. I mean, look at this:

You start with a starburst, reduce it to a smear, and end up with a smudge that might just change how we think about our place in the universe. Incredible.

Is Alpha Centauri Ab habitable?

So, might there be anyone home? Could Alpha Centauri Ab be habitable?

Planetary habitability is complicated enough to occupy an entire subfield of astrobiology. But in practice, given how little we know about exoplanets, we reduce it to one factor: a planet's average distance from its host star. Alpha Centauri Ab falls within its star's conventional "habitable zone," meaning that liquid water would be stable on its surface, assuming a sufficiently thick atmosphere.

The problem is that Alpha Centauri Ab — if it exists — is all atmosphere and no surface. It's a gas giant with more heft than our solar system's heavyweight, Jupiter. Scientists have proposed some fun ideas for how aliens could survive on gas giants, including Carl Sagan's famous "floaters and sinkers" that he imagined drifting in Jupiter's cloud deck. And there's a vocal minority of astrobiologists who seem pretty convinced that there are microbes floating in the atmosphere of Venus. However, even if life could survive on Alpha Centauri Ab, it's hard to imagine how that life would get started in the first place. Habitability, the potential to support life that's already alive, and genesity,^[3] the potential to get life going and keep it going, are not the same thing. We don't know how life began on Earth, but none of our best ideas would work on a gas giant.

That's bad news for life on Alpha Centauri Ab itself. But I'd argue that finding a gas giant in the habitable zone next door is even more exciting than finding a rocky planet — because the gas giants in our solar system are swarming with moons.^[E1]

Jupiter, Saturn, Uranus, and Neptune all host large, rocky moons that might have what it takes to support life. The biggest one, Jupiter's moon Ganymede, is more massive than Pluto. Titan, Saturn's largest moon, has a thick atmosphere rich in organic molecules. It's like a weird, frosty funhouse mirror reflection of Earth with rain and rivers of liquid methane and ethane and rock made of ice. Europa and Enceladus, moons of Jupiter and Saturn, host vast, ice-covered oceans of liquid water. It's not a stretch to imagine that Alpha Centauri Ab could host several moons that could have atmospheres and oceans.

It's particularly exciting that Alpha Centauri A is a Sun-like, G-type star. The only potentially habitable rocky exoplanets we can study closely with JWST all orbit red dwarf stars. These puny stars are terribly misbehaved — I think of them as the yappy lap dogs of astronomy. Red dwarfs spew atmosphere-shredding radiation that could strip away the air of any planets close enough to be habitable. And because these stars are so dim, being in the habitable zone means being very close — close enough to become tidally locked, with a blazing "day side" that always faces the sun and a "night side" left to freeze in eternal night. Some scientists consider M-dwarf systems basically uninhabitable. But we know that G-type stars^[4] work for life.

Binary systems aren't the friendliest neighborhoods for planets to grow up in, but models suggest that planets — and presumably their moons — huddled within about 3 astronomical units^[5] of Alpha Centauri A can orbit in relative peace for at least a billion years or so. If Earth is anything to go by, that's plenty of time for life to emerge, evolve and maybe even spread. Plenty of studies have demonstrated that it's feasible for microbial life to hitch a ride on impact debris and sail to other worlds.

Perhaps Alpha Centauri Ab is not just one planet, but a whole system of little worlds, each its own experiment in building a biosphere.

Taking nearby aliens seriously

In a coincidence that's really just too good to be true, the habitable moon Pandora in James Cameron's Avatar orbits a fictional gas giant around Alpha Centauri A. The gas giant is called Polyphemus, for the cyclops in the epic Odyssey^[6] — I'd say that'd be a fun nickname for Alpha Centauri Ab if Polyphemus weren't known for trying to eat humans. If we are allowed to transcend the stubbornly impersonal exoplanet naming conventions, which I'd argue is worth considering for the world next door, maybe we should consider something less gory.

Polyphemus and Pandora are science fiction, of course. And anything that sounds too much like science fiction is easy to dismiss as silly and unscientific. However, if Alpha Centauri Ab is really confirmed, I think you'd just be wrong not to take the possibility that the Alpha Centauri system hosts alien life seriously. And I do mean seriously enough to design missions tailor-made for this system, including the ludicrous-sounding idea of eventually trying to send a probe there. It'd be a system with not one but two chances for life. The red dwarf Proxima Centauri also hosts a potentially habitable world, an Earth-sized rocky planet in the star's — admittedly alien and maybe quite hostile — habitable zone.

But let's not get ahead of ourselves. Step one is confirming that Alpha Centauri Ab is, indeed, a planet and not a spurious smudge. Assuming it is, the next obvious question to ask is whether it has any moons. Simple as it sounds, I anticipate this project will occupy exoplanet astronomy for decades.

If spotting an exoplanet is like seeing a candle flame next to a spotlight, spotting an exomoon is like spotting a firefly next to a candle next to a spotlight. It's hard. We haven't confirmed a single exomoon yet. But exomoon research seems to have really taken off in the last few years. Several exomoon candidates have already been identified. The first one discovered was Kepler-1625b I — a moon that'd be the size of Neptune orbiting a planet much larger than Jupiter. The group that spotted it, the Cool Worlds lab at Columbia led by David Kipping, is searching for exomoons around the Jupiter-like exoplanet Kepler-167e.^[7] It's one of several exomoon projects that have recently been awarded precious JWST time.

So it looks like astronomers are hopeful that JWST will be able to spot exomoons around certain planets. But "certain planets" might not include Alpha Centauri Ab. The methods being developed for JWST are mostly for moons around transiting planets, not for planets spotted via direct imaging. Rocky worlds are, with few and unproven exceptions, entirely beyond JWST's direct imaging capabilities. We'd have a much better shot of seeing moons around Alpha Centauri Ab with the Habitable Worlds Observatory, a bespoke direct imaging exoplanet mission that should be able to see exomoons around giant planets. But that mission is just a proposal right now, and if it flies it won't launch until the 2040s.

Still, I assume we will eventually be able to confirm exomoons around Alpha Centauri Ab — every gas planet in our solar system has moons, so "yes moons" seems like the most appropriate null hypothesis to me. The technical barrier should be surmountable with observatories that are already in the works. It will just take a while. My guess is that the PhD students starting exomoon projects today will be in middle age when it finally happens.

In the meantime, I really do think it'd be a good idea to get serious about planning an interstellar mission to Alpha Centauri.

Since the 70s at least, scientists have, from time to time, proposed technical concepts for missions to nearby stars. These concepts are ambitious and would require technological advances and truly obscene amounts of money, but they're not science fiction fantasies; they're based on technologies we already have. Project Daedalus, pitched in the 70s, is one example — though with a price tag that sci fi bloggers seem to agree would be in the 100s of trillions, it'd cost an order of magnitude more than the GDP of the United States.

A more realistic, more recent proposal is Breakthrough Starshot, a mission concept that proposes using laser-propelled light sails to launch a swarm of cheap, small probes to Alpha Centauri. It's a private venture endeavor that involves a few people who astronomers tend not to like that much including Avi Loeb, a Harvard astrophysicist with a habit of sparking media firestorms around aliens, and tech CEOs like Mark Zuckerberg. But we shouldn't throw interstellar missions out with the distasteful personality bathwater. The world of publicly-funded science would be remiss to treat interstellar missions like fancy toys for billionaires. Developing the technology needed for Starshot — solar sails, hyper-miniaturized instruments, ultra-sensitive radio telescopes, and huge arrays of lasers that can zap things in space — would be useful here on Earth and for solar system exploration. Harry Atwater and his colleagues at Caltech have been working on light sails with Starshot funding and it looks like they're making progress.

I'm skeptical Starshot as currently imagined will ever happen, not least because building a massive array of super-lasers capable of blasting spacecraft out of the sky would be, shall we say, geopolitically complicated. But getting the first probe to another system is the kind of civilizational triumph that one can imagine the whole world getting behind. I personally believe it'd be more worthwhile than establishing a Mars colony — and potentially much cheaper. Breakthrough Starshot is supposed to cost $10 billion. NASA estimates that even just the first, there-and-back mission to Mars would cost $500 billion. Elon Musk, who very much believes we can and should build a city on Mars, tweeted that such a project would cost hundreds of trillions of dollars. Would you rather have miserable Mars colonists or interstellar spaceflight?

There's another ambitious concept that wouldn't even require us to leave our own solar system. A Hubble-like telescope flown out to near the edge of the solar system could use the Sun as a gravitational lens to take pictures of exoplanets that would actually look like planets, not just smudges of light. The catch is that such a telescope could only look at one planet. It'd be a huge investment in getting a close look at just one world. But if there's a potentially habitable system next door, there'd be an obvious target.

Space science, many decades removed from its maverick youth, has become conservative. It sticks with what works, which is why we have so many 6-wheeled rovers on Mars. That's understandable given the price tags on big missions. But if the chance to snap pictures of a whole system of living moons around the nearest Sun-like star isn't enticing enough to risk failure for, I don't know what is.

If we want a Star Trek future, we need a Star Trek attitude. Kirk put it best in "Return to Tomorrow, an episode that aired before Neil Armstrong set foot on the Moon: risk is our business.

Footnotes

[1]: Lest you think this was a frivolous Greek borrowing, legend holds that telescope was actually coined by a Greek mathematician, Giovanni Demisiani. There's all sort of conflicting information online, so consume this trivia with caution.

[2]: You can find a table of all exoplanets confirmed by direct imagining on the NASA Exoplanet Archive.

[3]: Genesity is not a widely used term, yet. It was coined by Michael Wong, Stuart Bartlett, Sihe Chen, and Louisa Tierney in their 2022 paper in Life: "Searching for, Mindful of Lyfe's possibilities".

[4]: The G in G type stands for golden retriever, because they're the goodest stars.

[5]: An astronomical unit is a common unit of distance in planetary science. 1 AU is just the average distance from the Sun to the Earth — a handy measuring stick for planetary orbits.

[6]: For my fellow conlang nerds, the English name refers to the Na'vi name Naranawm, which means "great eye." Not exactly a cyclops, but close enough.

[7]: The grad student leading this study, Ben Cassese, was in my a capella group at Caltech. Small world.

[E1] Edit: Previously, this read "big, rocky moons" — thank you to Ray Pierrehumbert (@climatebook.bsky.social‪‬) for pointing out that the Solar System gas giants don't host any moons that planetary scientists would consider big and rocky. The heftiest moons are water worlds that would mostly evaporate if you brought them into the habitable zone. That makes it even more important to identify and closely scrutinize transiting gas giants in the habitable zones of other stars; we need to figure out if big moons can survive inside of the snow line

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