https://www.centauri-dreams.org/2025/04/08/on-sun-like-stars/ Centauri Dreams * Home * About * Administrative * Booklist * Contact Select Page [ ] On 'Sun-like' Stars by Paul Gilster | Apr 8, 2025 | Exoplanetary Science | 5 comments The thought of a planet orbiting a Sun-like star began to obsess me as a boy, when I realized how different all the planets in our Solar System were from each other. Clearly there were no civilizations on any planet but our own, at least around the Sun. But if Alpha Centauri had planets, then maybe one of them was more or less where Earth was in relation to its star. Meaning a benign climate, liquid water, and who knew, a flourishing culture of intelligent beings. So ran my thinking as a teenager, but then other questions began to arise. Was Alpha Centauri Sun-like? Therein hangs a tale. As I began to read astronomy texts and realized how complicated the system was, the picture changed. Two stars and perhaps three, depending on how you viewed Proxima, were on offer here. 'Sun-like' seemed to imply a single star with stable orbits around it, but surely two stars as close as Centauri A and B would disrupt any worlds trying to form there. Later we would learn that stable orbits are indeed possible around both of these stars, and in the habitable zone, too (I still get emails from people saying no such orbits are possible, but they're wrong). So just how broad is the term 'Sun-like'? One textbook told me that Centauri A was a Sun-like star, while Centauri B was sometimes described that way, and sometimes not. I thought the confusion would dissipate once I knew enough about astronomical terms, but if you look at scientific papers today, you'll see that the term is still flexible. The problem is that by the time we move from a paper into the popular press, the term Sun-like begins to lose definition. [eso2011a] Image:A direct image of a planetary system around a young K-class star, located about 300 light-years away and known as TYC 8998-760-1. The news release describes this as a Sun-like star, raising questions as to how we define the term. Image credit: ESO. G-class stars are the category into which the Sun fits, with its lifetime on the order of 10 billion years, and if we expand its parameters a bit, we can take in stars around the Sun's mass - perhaps 0.8 to 1.1 solar masses - and in the same temperature range - 5300 to 6000 K. Strictly speaking, then, Centauri B, which is a K-class star, doesn't fit the definition of Sun-like, although K stars seem to be good options for habitable worlds. They're less massive (0.45 to 0.8 solar masses) and cooler (3900 to 5300 K), and they're more orange than yellow, and they're longer lived than the Sun, a propitious thought in terms of astrobiology. So we don't want to be too doctrinaire in discussing what kind of star a habitable planet might orbit, but we do need to mind our definitions, because I see the term 'Sun-like' in so many different contexts. When I see a statement like "one Earth-class planet around every Sun-like star," I have to ask whether we're talking about G-class stars or something else. Because some scientists expand 'Sun-like' to include not only G- and K- but F-class stars as well. Why do this? Such stars are, like the Sun, long-lived. F stars are hotter and more massive than the Sun, but like G- and K-class stars, they're stable. Some studies, then, consider 'Sun-like' to mean all FGK-type stars. Some examples. 'COROT finds exoplanet orbiting Sun-like star' is the title of a news release that describes a star a bit more massive than the Sun. So the comparison is to G-class stars. 'Astronomy researchers discover new planet around a 'Sun-like' star' describes a planet around an F-class star, so we are in the FGK realm. 'First Ever Image of a Multi-Planet System around a Sun-like Star Captured by ESO Telescope' describes a planet around TYC 8998-760-1, a K-class dwarf. So there's method here, but it's not always clarified as information moves from the academy (and the observatory) into the media. Confounding the picture still further are those papers that use 'Sun-like' to mean all stars on the Main Sequence. This takes in the entire range of stars OBAFGKM. This is a rare usage, but there is a certain logic here as well. If you're looking for habitable planets, it's clear that stars in the most stable phase of their lives are the ones to examine, burning hydrogen to produce energy. No brown dwarfs here, but the category does take in M-class stars, and the jury is out on whether such stars can support life. And they take in the huge majority of stars in the galaxy. So if we're talking about hydrogen burning, the Main Sequence offers up everything from hot blue stars all the way down to cool red dwarfs. End the hydrogen burning and a different phase of stellar evolution begins, producing for example the kind of white dwarf that the Sun will one day become. A paper's context usually makes it perfectly clear which of the three takes on 'Sun-like' it is using, but the need to clarify the term in news releases, particularly when dealing with a wide range through F-, G- and K-class stars is evident. All of this matters to the popular perception of what exoplanet researchers do because it wildly affects the numbers. G-class stars are thought to comprise about 7 percent of the stars in the galaxy, while K-class stars take in about 12 percent, and M-class dwarfs as high as 80 percent of the stellar population. Saying there is an Earth-class planet around every Sun-like star thus could mean 'around 7 percent of the stars in the Milky Way.' Or it could mean 'around 22 percent of the stars in the Milky Way, if we mean FGK host stars. If we included red dwarf stars, it could mean 'around about 95 percent of the stars in the galaxy,' excluding evolved, non-Main Sequence objects like white dwarfs, neutron stars and red giants. Everything depends upon how the terms are defined. I keep getting emails about this. My colleague Andrew Le Page is a stickler for terminology in the same way I am, with his most trenchant comments being reserved for too facile use of the term 'habitable.' So we've got to be careful in this burgeoning field. Exoplanet researchers are aware of the need to establish the meaning of 'Sun-like' carefully. The fact that the public's interest in exoplanets is growing means, however, that in public utterances like news releases, scientists need to clarify what they're talking about. It's the same thing that makes the term 'Earth-like' so ambiguous. A planet as massive as the Earth? A planet that is rocky as opposed to gaseous? A planet in the habitable zone of its star? Is a planet on a wildly elliptical orbit crossing in and out of the habitable zone of an F-class host Earth-like? Let's watch our terms so we don't confuse everybody who is not in the business of studying exoplanets full-time. The interface between professional journals and public venues like websites and newspapers is important because it can have effects on funding, which in today's climate is a highly charged issue. A confused public is less likely to support studies in areas it does not understand. [tzf_img_post] 5 Comments 1. Henry Cordova on April 8, 2025 at 14:40 Some excellent observations. This is how I choose to think about it. All we know about the habitability of planets is derived from only one example, our own sun/earth system. We don't even know if our own planet and sun combination is in the middle range of "habitability". We may be the perfect "Goldilocks" example, or we may be at one of the extreme ends of some range of possibilities defined by stellar evolution and planetary system formation. If we ever get to explore a large sample of worlds we may find many like earth where life never arose, or we may find life common and flourishing in environments we would now consider hostile. We may even find both! We simply don't know. In order to speculate profitably on these possibilities we should first consider just what is the ideal for life, rather than the range of conditions that might favor it. We can say that there are certain general conditions that must be common to any world where life evolved, and then ponder just how far from some ideal life is still possible. Presumably, as we get further and further from this ideal, the probability of finding life, and its level of complexity, decreases. Again, our reasoning must begin from our own meager experience, one world, our own. First, we need an old star, fossil evidence suggests our own earth may have been inhabited by primitive microbes soon after it formed about 5 billion years ago, and we also know higher forms of life (multicellular) seem to have appeared about half a billion years ago. We also know that evolution is a slow process, so we are looking for old stars-about several billion years old. All else being equal, the more complex life manifestations (like intelligent species) would require older stars. There is no shortage of much older stars in our galaxy, but we are often reminded that even though these stars and their accompanying planets also have problems that make things problematic for their alleged inhabitants. We all know about tidal locking, atmosphere-destroying flares and other problems associated with red dwarfs. Massive stars tend to be younger, and more unstable, it is likely that their life cycles are on a more accelerated schedule than those of living organisms. They also evolve quickly, perhaps faster than their inhabitants. But massive stars are also brighter, so we see an exaggerated number of them because we can detect them at longer distances. Their time on the main sequence is less, less time for life to arise and flourish. But it is not just the stellar properties which constrain life, it is also the TYPE of life they shelter. We are familiar with a carbon/water form of organism that has been very successful here on earth, but there may be other forms of life out there. We've all heard of Silicon biology, ammonia breathers, sulfur eaters, hydrocarbon habitats and all sorts of other exotics that might push the environments where they are possible out to extremes. Perhaps they also evolve faster! But we really don't know. We have no evidence these exotics exist, and even if they are common in the universe, we just can't tell. We can't even tell if they are possible. We simply don't know anything about them. I suggest that speculating about them is pointless. I may be old-fashioned, but I'll stick to the original assumptions that we have been handed down by traditional SETI: carbon/water critters on old (>10^9 yr) F, G and K stars. I can't rule out a larger arena, and I certainly hope it exists, but anything else is purely speculative. The carbon/water assumption is expandable (there may be many alternative evolutionary paths for these life forms, but again, we simply don't know. A carbon and water form of life that has evolved alternatives to amino acids, DNA, RNA or any of the familiar metabolic and enzymatic pathways our molecular biologists have discovered is certainly possible, but it may not be very likely. Again, we simply don't know. We don't even have any way of knowing. There is another consideration. The universe is a violent, unstable place. Perhaps life has arisen on many worlds only to be sterilized quickly by a nearby supernova, asteroid strike or some other violent cosmic catastrophe. How common these events are and how deadly they may have been for life in the Galaxy is something we don't understand well, but this is an area where astronomical research is rapidly progressing, and we can hope for more constraints to our speculation. Maybe earth and sun just got lucky. It may very well be that life easily arises and evolves if the conditions are right, for long enough, but these benign conditions are extremely rare. So rare that perhaps we are truly alone. Again, we just don't know. I have been speaking at length here (as have we all) with very little in the way of quantitative detail or knowledge, basing most of my speculations on purely qualitative ideas. And I also recognize how my own ideas have changed over time (without the aid of any new data, I might add) from highly optimistic to grimly pessimistic. Its sad. I hope I'm wrong. I want the universe to be teeming with life and consciousness. But I'm afraid (again, without any real evidence) that its not. Like the song said; "I'm losing my religion". Reply 2. Michael Fidler on April 8, 2025 at 16:02 The FGK stars are the ones that would have planets that are compatible with our life style. That is what the general public would understand and support. The big draw is finding planets that we can colonize easily, these should be very common and have wide public support ... Reply 3. Alex Tolley on April 8, 2025 at 16:06 The popular conception of "Earth-class" and "Earth-like" is an inhabited world with continents, oceans, and a biodiverse biosphere with terrestrial-similar flora and fauna. In SciFi, is Arrakis (Dune) Earthlike? It was once before it became a planet-wide desert. But is Arrakis Earth-like by the time of the first novels, and does it return to that state by Leto's reign as a "god-emperor"? Is Arrakis more Mars-like - dry, but with a breathable atmosphere (despite the apparent lack of photosynthesizers)? "Habitable" strictly means that the surface is warm enough to maintain liquid water on the [rocky] surface. But if that requires a dense CO2 atmosphere, could it be inhabited with complex, aerobic life? Let us not forget that Earth has experienced what appears to be almost complete glaciation in the past. Would we consider that state, like Star Wars' Hoth, as Earth-like? In the Star Wars universe, Hoth is treated as if it were like the Arctic/Antarctic - habitable and inhabited like those regions, even if the rest of the needed biosphere was missing. Would a casual visitor on a planet resembling Earth in the Archaean, with only invisible prokaryote microbes, land surfaces devoid of any plants or animals, with an unbreathable atmosphere, consider such a world Earth-like, even with a G-class star in the sky, warming our visitor's face? We see the sun as "yellowish", and that is how children paint it, even though we know that it is white, just as the sun's temperature LEDs appears to our eyes. Our older tungsten bulbs are far warmer in color, and are the temperature of M-dwarfs, which are often depicted as red, like our sun in a perfect sunset. An O-class star would probably be very carefully avoided looking at. If a quiescent M dwarf had a very Earth-like planet orbiting it, it would probably seem to be rather like our sun towards dawn or sunset, although the flora might appear to be rather different in color. Squint, and such a world might feel very Earth-like despite its star not being "sun-like". Lastly, we are still biased in what Asimov called "planetary chauvinism" when Gerry O'Neill suggested we should populate space in giant habitats. Yes, those planets are needed to evolve life of some kind. But human eyes may only see those stars illuminating, either directly or indirectly, the interiors of such habitats. O-class stars would have their light filtered of converted to our solar spectrum, whilst M-class stars would have their light collected and emitted by interior lights mimicking the solar spectrum. The light from these stars would look very sun-like to the habitat occupants and very different from being viewed directly (if that was even possible or safe). Reply 4. Geoffrey Hillend on April 8, 2025 at 17:03 I agree that we should be precise on what we consider to be called Earth like stars which have to be G class stars. This is important because the larger gravity of G class stars and whether or not a planet might be tidally locked and have no rotation, etc. There is more material in the larger system and more Iron in G class stars than in smaller stars. This might effect the abundance of iron in exoplanets round F and M stars so these exoplanets have less iron. Don't assume that because there can be stable orbits for planets around Alpha Centauri A and B, that there must be exoplanets there. According to Open AI Chat GPT, that is not enough evidence. There is no five sigma evidence that there are planets around A and B Centauri. Google AI. According to Open AI Chat GPT, Rudolf Kippenhahn idea that stars at a certain distance apart like Centauri A and B can't form planets because the two stars grab all the angular momentum so there is no gas in the center. Modern computer simulations show this type of system can possible form planets, but the probability is much less than when the stars are further apart or close together. Consequently, it is harder for two accretions disks to form in such systems and it is also a three body problem and more disturbance and tidal forces can form in such a system. Quote Chat GPT: The stars are far enough apart that they each can have their own circumstellar disk, but the gravitational influence from the companion tends to truncate the disks, often reducing the material available for planet formation. This disruption can affect how dust grains clump and whether rocky planets (or gas giants) can ever reach full formation"So in these Sun-to-Saturn or larger separations, planet formation is possible, but more fragile and sensitive to conditions like eccentricity and disk mass." Ibid. "Intermediate binaries (like Alpha Centauri at 23 AU): Tricky. Planet formation is possible, but disks are disrupted, and planet-building is much harder. Ibid. It's not the worst case though: "Modern simulations recognize both regimes. The worst case for planet formation is actually intermediate binaries (a few AU separation), where the stars are too far to support a stable circumbinary disk, but too close to allow large circumstellar disks." ibid. Reply 5. Mike Serfas on April 8, 2025 at 19:52 In a sense, the most Sun-like of all the stars we know is class F. Namely, HD 162826, the best known sister of the Sun, is luminosity class F8 V. Emerging from the same stellar nursery as the Sun, though now 110 light-years away, that star has a spectrum that (naturally) seems encouraging for planet formation, though none have been observed that I know of. There is a speculation that the seeding of Earthly life could have been shared with the system. There are other stellar siblings that have been investigated; it would be great to see an update. Out there somewhere is believed to be one single twin of the Sun, which as the closest star to early Earth is perhaps a leading prospect for Earthly life abroad. Reply Submit a Comment Cancel reply Your email address will not be published. Required fields are marked * [ ] [ ] [ ] [ ] [ ] [ ] [ ] Comment * [ ] Name * [ ] Email * [ ] Website [ ] [ ] Save my name, email, and website in this browser for the next time I comment. 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