Featured Essays astronomy Bad News From Alpha Centauri A… There’s a planet in the habitable zone… but not an Earthlike planet. By James Davis Nicoll | Published on September 19, 2025 Credit: NASA/Caltech/IPAC Comment 0 Share New Share Credit: NASA/Caltech/IPAC Astronomers report that there very well might be a planet orbiting in or just outside Alpha Centauri A’s habitable zone. Alas, ordering the stokers to start shovelling seetee into the photon-drive may be premature. Alpha Centauri Ab is an interesting world but it isn’t Earth 2. But first, a refresher. At 4.3 light years1, Alpha Centauri is currently the closest star system to our own2. Alpha Centauri is a triplet star system, whose most distant component, Alpha Centauri C, is small, dim, distant from its two companions3, and irrelevant to this essay. Alpha Centauri A (whose proposed world is the subject of this essay) is slightly more massive than the Sun and half again as bright. B is a little less massive than the Sun and half as bright. A and B have an eccentric orbit around each other4, but not so closely as to preclude planets in either star’s habitable zone5. At least not directly. Because Alpha Centauri is nearby, because two of its components are Sun-like, and because SF authors have heard of the star system, SF authors have populated A and B with many habitable planets, some of which I’ve mentioned in an earlier essay6. Alas, nothing known about the exoplanet Alpha Centauri Ab (A is the star, b is the planet) suggests that it’s habitable. To begin with the minor issues, Ab is actually a bit outside A’s habitable zone, thus its estimated temperature of 225 K or about -50o C. Not necessarily a deal-killer. The estimate excludes, as far as I can tell, the impact of any greenhouse gases. As you know (Bob) without its atmosphere, Earth would be about 255 K or about -25o C7. The next issue is that Ab’s orbit is far more eccentric than that of Earth; it’s comparable to Pluto’s. This means the amount of light Ab receives from A varies considerably over the course of a single orbit. Ab spends a lot more time traversing the outer part of its orbit than it does the inner8. So, longer, colder winters than Earth and because the issue is distance and not axial tilt, all of Ab goes into the deep freeze at the same time. No migrating to the summertime hemisphere. This paper observes that “The S1+C1 candidate is in a highly inclined (≈50∘ or ≈130∘ with respect to the α Cen AB binary orbital plane) and eccentric (∼0.4) orbit, not unlike other S-type planets in close binary systems (e.g., HD 196885 Ab and γ Cep Ab), and is expected to undergo large amplitude von Zeipel-Kozai-Lidov (vZKL) oscillations.” This isn’t great, because it means that the eccentricity isn’t just extreme, but evolving. All of which is minor compared to the final detail: Ab is somewhat more massive than Saturn, which means it’s probably composed of hydrogen and helium. Maybe it’s an extremely large ice giant (ice giants are worlds like Neptune or Uranus, which have a significant amount of water, in states of matter unlike any with which we’re familiar) but probably not. In any case, Ab would be as uninhabitable as our Solar System’s gas or ice giants. Ab’s mass has another annoying consequence, which is that it’s clearing out adjacent orbits that might otherwise (assuming a multitude of counterfactuals) be taken by an Earth-massed planet. Perhaps Ab has Mars-plus massed moons and maybe one or more of them is a potential abode for life? The first thing that comes to mind is that no gas or ice giant in our system has moons quite that large in comparison to the primary. Still, if there’s one thing exoplanets have taught us, it is that our solar system is not the default model. Yet… even if such an Earth-like moon existed it would be subject to the very un-Earthlike conditions mentioned above. Ah, well. There’s always Alpha Centauri B… except that if B is large enough for to induce von Zeipel-Kozai-Lidov oscillations in Ab, then surely the more massive A will do the same for any hypothetical Bb? There are a lot more ways for a world to be uninhabitable than habitable, so it’s not that surprising9 that Alpha Centauri Ab seems to be a dud from that angle. It’s still an interesting system from an orbital dynamics perspective, but for habitable worlds, we will have to look elsewhere.[end-mark] That is about the height of 10^16 giraffes, stacked one on top of each other, and ignoring that such a mass of giraffes would collapse into an extremely (if only very briefly) distressed sphere. Yes, obviously the giraffes are in space suits. I’m not a monster. ︎Stars move. ︎C is almost 9000 times as far from AB as the Earth is from the Sun. I too am boggled that the star has not been stripped away from AB over the five billion plus years the system has been around. By the way, case matters. If I say A or B, I mean the stars but a lower-case b is an exoplanet. AB would be both Alpha Centauri A and B, but Ab would be the proposed planet orbiting A and Bb an exoplanet orbiting B. ︎The distance between A and B varies from about the distance between the Sun and Neptune and the Sun and Jupiter. ︎Looking at you, Procyon. ︎Or sometimes, in the case of authors who knew the name but nothing else about Alpha Centauri, the planet orbiting a singleton star called Alpha Centauri. ︎Back when the Sun was young, it was much dimmer than it is now. A back of the envelope calculation says the Earth, other factors aside, would have had about the same temperature as Alpha Centauri Ab. Why everything wasn’t frozen solid is a bit of a mystery. Alpha Centauri A being more massive than the Sun, its luminosity would have evolved even more and faster than the Sun’s. Ab might have been even farther from the habitable zone than it is today… except we know the orbit would have been very different billions of years ago. ︎Because A and B’s orbit around each other is eccentric, climate on Ab would be further complicated by B’s small but not negligible input as it varies over an eighty-year cycle. ︎What is astonishing, at least to me, is the number of red dwarfs, such as Alpha Centauri C, that have exoplanets in their Goldilocks zone. Red dwarfs are very dim and their potentially habitable zones are tiny. Is there something that favours planetary formation in or near that region of stellar systems? ︎The post Bad News From Alpha Centauri A… appeared first on Reactor.