Centauri Dreams

Early next week I’ll be discussing the winning entry in Project Hyperion’s design contest to build a generation ship. But I want to sneak in the just announced planet candidate at Alpha Centauri A today, a good fit with the Hyperion work given that the winning entry at Hyperion is designed around a crewed expedition to nearby Proxima Centauri. Any news we get about this triple star system rises immediately to the top, given that it’s almost certainly going to be the first destination to which we dispatch instrumented unmanned probes.

And one day, perhaps, manned ships, if designs like Hyperion’s ‘Chrysalis’ come to fruition. More on that soon, but for today, be aware that the James Webb Space Telescope is now giving us evidence for a gas giant orbiting Centauri A, the G-class star intriguingly similar to the Sun, which is part of the close binary that includes Centauri B, both orbited by the far more distant Proxima.

Image: This artist’s concept shows what the gas giant orbiting Alpha Centauri A could look like. Observations of the triple star system Alpha Centauri using the NASA/ESA/CSA James Webb Space Telescope indicate the potential gas giant, about the mass of Saturn, orbiting the star by about two times the distance between the Sun and Earth. In this concept, Alpha Centauri A is depicted at the upper left of the planet, while the other Sun-like star in the system, Alpha Centauri B, is at the upper right. Our Sun is shown as a small dot of light between those two stars. Credit: NASA, ESA, CSA, STScI, R. Hurt (Caltech/IPAC).

JWST’s Mid-Infrared Instrument (MIRI) once again proves its worth, as revealed in two papers in process at The Astrophysical Journal Letters. If this can be confirmed as a planet, its orbit appears to be eccentric (e ≈ 0.4) and significantly inclined with respect to the orbital plane of Centauri A and B. But we have a lot of work ahead to turn this candidate, considered ‘robust’ by the team working on it, into a solid detection.

The proximity of the central binary stars at Alpha Centauri makes this kind of work extremely difficult, one reason why a system so close to our own is only gradually revealing its secrets. Bear in mind that MIRI was able to subtract the light from both stars to reveal an object 10,00 times fainter than Centauri A. The Webb instrument took observations beginning in August of 2024 that posed a subsequent problem, for two additional observation periods in the spring of this year failed to find the object. Interestingly, computer simulations have clarified what may have happened, according to PhD student Aniket Sanghi (Caltech), co-first author of one of the two papers describing this work:

“We are faced with the case of a disappearing planet! To investigate this mystery, we used computer models to simulate millions of potential orbits, incorporating the knowledge gained when we saw the planet, as well as when we did not,.. We found that in half of the possible orbits simulated, the planet moved too close to the star and wouldn’t have been visible to Webb in both February and April 2025.”

Image: This 3-panel image captures the NASA/ESA/CSA James Webb Space Telescope’s observational search for a planet around the nearest Sun-like star, Alpha Centauri A. The initial image shows the bright glare of Alpha Centauri A and Alpha Centauri B, then the middle panel shows the system with a coronagraphic mask placed over Alpha Centauri A to block its bright glare. However, the way the light bends around the edges of the coronagraph creates ripples of light in the surrounding space. The telescope’s optics (its mirrors and support structures) cause some light to interfere with itself, producing circular and spoke-like patterns. These complex light patterns, along with light from the nearby Alpha Centauri B, make it incredibly difficult to spot faint planets. In the panel at the right, astronomers have subtracted the known patterns (using reference images and algorithms) to clean up the image and reveal faint sources like the candidate planet. Credit: NASA, ESA, CSA, STScI, DSS, A. Sanghi (Caltech), C. Beichman (JPL), D. Mawet (Caltech), J. DePasquale (STScI).

The combination of observations and orbital simulations indicates that a gas giant of about Saturn mass moving in an elliptical orbit within Centauri A’s habitable zone remains a viable option. Also fed into the mix were the parameters of a 2019 observation of Centauri A and B from the European Southern Observatory’s Very Large Telescope. It is clear that the point source referred to as S1 is not a background object like a galaxy or a foreground asteroid moving between JWST and the star. Its orbital parameters would make it quite interesting given the tight separation between Centauri A and B.

The second of the two papers clarifies the significance of such a find and the need to confirm it. The temperature calculated below is based on the photometry and orbital properties of the candidate object, with 200–350 K originally expected for a planet heated by Centauri A at 1.3 AU:

A confirmation of the S1 candidate as a gas giant planet orbiting our closest solar-type star,α Cen A, would present an exciting new opportunity for exoplanet research. Such an object would be the nearest (1.33 pc), coldest (∼225 K), oldest (∼5 Gyr), shortest period (∼2–3 years), and lowest mass (≲ 200 M⊕) planet imaged in orbit around a solar-type star, to date. Its extremely cold temperature would make it more analogous to our own gas giant planets and an important target for atmospheric characterization studies. Its very existence would challenge our understanding of the formation and subsequent dynamical evolution of planets in complex hierarchical systems. Future observations will confirm or reject its existence and then refine its mass and orbital properties, while multi-filter photometric and, eventually, spectroscopic observations will probe its physical nature.

The papers are Beichman et al., “Worlds Next Door: A Candidate Giant Planet Imaged in the Habitable Zone of α Cen A. I. Observations, Orbital and Physical Properties, and Exozodi Upper Limits,” accepted at Astrophysical Journal Letters (preprint); and Sanghi, et al., “Worlds Next Door: A Candidate Giant Planet Imaged in the Habitable Zone of α Cen A. II. Binary Star Modeling, Planet and Exozodi Search, and Sensitivity Analysis,” accepted at ApJL (preprint). The paper on the 2019 observation is Wagner at al., “Imaging low-mass planets within the habitable zone of α Centauri,” Nature Communications 10 February 2021 (full text).

Science fiction has always provoked interesting research. After all, many of the scientists I’ve spoken with over the years have been science fiction readers, some of whom trace their career choices to specific novels (Poul Anderson’s Tau Zero is frequently mentioned, but so is Frank Herbert’s Dune, and there are many others). This makes sense because there is a natural tension in exoplanet studies growing out of the fact that in most cases, we can’t even see our targets. Instead, we detect them through non-visual methods. True, we can analyze planetary atmospheres for some gas giant planets, but we’re only beginning to drill down to the kind of biosignature searches that may eventually flag the presence of life.

But fiction can paint a planet’s physics and visually explore its surface, modeling worlds in vast variety and sometimes spurring directions of thought that would otherwise remain unexplored. Consider Hal Clement, whose forays into planet-building included the remarkable Mesklin, a fast-rotating oblate world with an 18-minute day and surface gravity varying from 700 g at the poles to an almost bearable 3 g at the equator. Mission of Gravity, published as a serial in Astounding Science Fiction in 1953, involves an indigenous race’s interactions with a human crew at the equator. The encounter dazzled readers and led some into astrophysics.

These are unconventional aliens, and were particularly so in 1953, when communications between humans and tiny, flattened insect-like creatures seemed more at home in works of fantasy than what would become known as ‘hard science fiction’ (i.e., SF with a scrupulous reliance on proven physics). Clement’s novel was well received and spurred correspondence between the author and Robert Forward, who carried on the idea of extreme habitats in his novel Dragon’s Egg (1980). Both continued to ponder life in utterly extreme environments.

Gary Westfahl, the author of numerous titles of science fiction criticism including Hugo Gernsback and the Century of Science Fiction (McFarland, 2007) has dissected the hard science fiction genre in an essay in Science Fiction Studies. Westfahl makes the case that Mission of Gravity was “the first SF novel built on actual observational data involving another possible solar system.”

When I first read that, my thought was that it referred to Peter van de Kamp’s studies of Barnard’s Star at Swarthmore College’s Sproul Observatory in the 1930s and later. The detection of planets there proved erroneous, but so did a ‘detection’ at 61 Cygni. Clement seems to have used that supposed exoplanet as he modeled his world Mesklin. He wrote about his process in Astounding‘s issue of June, 1953 in which Mission of Gravity continued to be serialized.

I checked my collection of old magazines to find that issue, where he describes exactly how he built his planet. The details are fascinating, and available in some editions of Mission of Gravity. He’s not totally convinced that the 61 Cygni find is actually a planet — the object could not be seen, and the ‘detection’ was based on astrometry using photographs of this binary system. The paper, by Kaj Aage Strand, was painstaking, although the supposed planet turned out to be a chimera. Clement is not sure, but he accepts it as a planet for the purposes of the story: He writes:

If we assume the thing to be a planet, we find that a disk of the same reflecting power as Jupiter and three times his diameter would have an apparent magnitude of twenty-five or twenty-six in 61 C’s location; there would be no point looking for it with present equipment. It seems, then that there is no way to be sure whether it is a star or a planet, and I can call it whichever I like without too much fear of losing points in the game.

Image: Reproduction of diagrams by Hal Clement, originally published in his article “Whirligig World”, Astounding Science Fiction, June 1953. Top: Diagram of the cross-sectional shape of Mesklin, with approximate values for the effective surface gravity at various latitudes (in multiples of Earth gravity). The dashed lines are polar circles. The shaded circle in the middle represents the size of Earth on the same scale. Bottom: Diagram of Mesklin’s orbit, with approximate isotherms and times of crossing them. Credit: Wikimedia Commons.

These days we have to say that the first novel built on observational data of other stellar systems would have to be limited to a time after 1992, which is when Aleksander Wolszczan and Dale Frail found planets around the neutron star PSR B1257+12. Readers are welcome to name the novel (I don’t know the answer). This was, after all, the first time planets beyond our Sun were detected and confirmed, even if it would be another three years before we found 51 Pegasi b, the first planet around a main sequence star.

Robert Forward’s Dragon’s Egg takes astrobiology into even more extreme territory. He had been talking to Frank Drake, the first practitioner of SETI, who in 1973 was already thinking about life in highly unusual places, including settings on a neutron star. Let’s pause with Drake for a moment, because this is an interesting period in the history of science fictional ideas. Drake is quoted in Astronomy Magazine for December of 1973 as saying that life might well evolve in such a place.

In the exterior layers of these objects, we don’t have atoms…, but we do have atomic nuclei. And we have more varieties of atomic nuclei in a neutron star than we have varieties of atoms on our Earth. And from what we know of nuclear physics, those nuclei might combine together to form enormous supernuclei, or macronuclei, analogous to the large molecules which make up Earth life. And so as far as we know, it is possibly feasible to reproduce exactly the evolution which occurred on Earth but substituting for atoms and molecules, nuclei and macronuclei. So indeed there could be creatures on neutron stars that are made of nuclei. The temperatures are just right to make the required nuclear reactions go.

The combination of Clement’s planet Mesklin and Drake’s musings on neutron star life propelled Forward to re-examine the whole question and further refine Drake’s ideas. In Dragon’s Egg, the surface gravity on the neutron star is 67 billion times that of Earth. The local species is called the cheela, who are creatures the size of sesame seeds. The novel follows the development of their civilization from its earliest technologies to actual communications with a human-manned spacecraft in orbit around the star. For as the humans come to realize, the cheela experience the life and death of numerous generations in the span of mere hours.

So we see civilizational change in minutes. Forward had help with the structure of the novel from science fiction writer and editor Lester Del Ray, then working at Ballantine. He would eventually refer to the book as something of a textbook on neutron star physics “disguised as a novel.” None of that takes away from the sheer readability of this encounter with a species that within days achieves physics breakthroughs beyond those of the humans that are observing them. As with 1984’s Rocheworld, Forward’s prose is a bit clunky but his science is tight and his plot gripping.

Image: An combined image from multiple instruments showing a neutron star in the Small Magellanic Cloud. The reddish background image comes from the NASA/ESA Hubble Space Telescope and reveals the wisps of gas forming the supernova remnant 1E 0102.2-7219 in green. The red ring with a dark centre is from the MUSE instrument on ESO’s Very Large Telescope and the blue and purple images are from the NASA Chandra X-Ray Observatory. The blue spot at the centre of the red ring is an isolated neutron star with a weak magnetic field, the first identified outside the Milky Way. Credit: ESO/NASA, ESA and the Hubble Heritage Team (STScI/AURA)/F. Vogt et al. Acknowledgments: Mahdi Zamani.

We could go on with life in extreme environments as envisioned by science fiction (and I might mention Stephen Baxter’s Raft (Gollancz, 2018), where a rip in spacetime takes a human crew into a universe where the force of gravity is one billion times stronger than ours). Other readers will have their own favorites. I notice that some exoplanet and SETI researchers are following the lead of these novelists and taking a hard look at places we would consider hostile to any forms of life. As witness a recent paper from Brian Lacki and Stephen DiKerby on SETI at high energy levels.

And why not? We’re learning to think outside our usual preconceptions when it comes to habitability, and if we take seriously the idea of Kardashev Type II or III civilizations, we might well look for places where vast power resides in small spaces. Clément Vidal continues to make this point. Here the reference is his essential The Beginning and the End: The Meaning of Life in a Cosmological Perspective, Springer 2014. This is a key text for anyone serious about Dysonian SETI.

Can we learn to be as imaginative as some of the great science fiction authors? I think the wild variety of exoplanets thus far discovered demands that response from anyone pondering what might exist on everything from gas giant moons to desert worlds just barely touching the habitable zone. Keith Cooper gets into these questions in his fine Amazing Worlds of Science Fiction and Science Fact (Reaktion Books, 2025), where the link between the literature of the fantastic and cutting edge astrophysics is explicitly studied. I’ll be reviewing this one soon in these pages.

As to Lacki and DiKerby, they’re interested in exploring parts of the SETI landscape that have seen little attention. While our thinking about astrobiology naturally flows out of life as we already know it (and thus on Earth), what about those off-the-wall places where humans would instantly perish if they were so unwise as to get too near? Is a neutron star a SETI target? The accretion disk of a black hole? A binary X-ray pulsar?

We can posit strange lifeforms like those of Clement and Forward, but we can also add that places of high energy could be exploited by advanced civilizations that developed on far different worlds, cultures that are mining these high energy sources to drive civilizational projects whose intent may remain unfathomable. So without any knowledge of whether exotic life can be possible in, say, stellar plasma or on a neutron star’s surface, we might consider just what technosignatures would be possible if we found a culture at work in the places where the most extreme energies are available.

Lacki (University of Oxford) is part of the Breakthrough Listen team, while DiKerby is an astrophysicist at Michigan State University. I want to go through their paper next time as they push SETI concepts to the limit and ask what the result would look like.

The paper on high energy SETI is Lacki & DiKerby, “Possibilities for SETI at High Energy,” a white paper for NASA DARES (NASA Decadal Astrobiology Research and Exploration Strategy). Available here.