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<\/p>\nOn \u2018Sun-like\u2019 Stars
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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.<\/p>\n
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. \u2018Sun-like\u2019 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\u2019re wrong).<\/p>\n
So just how broad is the term \u2018Sun-like\u2019? 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\u2019ll 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.<\/p>\n
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Image:<\/strong>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.<\/p>\n 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\u2019s mass \u2013 perhaps 0.8 to 1.1 solar masses \u2013 and in the same temperature range \u2013 5300 to 6000 K. Strictly speaking, then, Centauri B, which is a K-class star, doesn\u2019t fit the definition of Sun-like, although K stars seem to be good options for habitable worlds. They\u2019re less massive (0.45 to 0.8 solar masses) and cooler (3900 to 5300 K), and they\u2019re more orange than yellow, and they\u2019re longer lived than the Sun, a propitious thought in terms of astrobiology.<\/p>\n So we don\u2019t 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 \u2018Sun-like\u2019 in so many different contexts. When I see a statement like \u201cone Earth-class planet around every Sun-like star,\u201d I have to ask whether we\u2019re talking about G-class stars or something else. Because some scientists expand \u2018Sun-like\u2019 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\u2019re stable. Some studies, then, consider \u2018Sun-like\u2019 to mean all FGK-type stars. <\/p>\n Some examples. \u2018COROT finds exoplanet orbiting Sun-like star\u2019 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. \u2018Astronomy researchers discover new planet around a \u2018Sun-like\u2019 star\u2019 describes a planet around an F-class star, so we are in the FGK realm. \u2018First Ever Image of a Multi-Planet System around a Sun-like Star Captured by ESO Telescope\u2019 describes a planet around TYC 8998-760-1, a K-class dwarf.<\/p>\n So there\u2019s method here, but it\u2019s 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 \u2018Sun-like\u2019 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\u2019re looking for habitable planets, it\u2019s 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.<\/p>\n So if we\u2019re 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\u2019s context usually makes it perfectly clear which of the three takes on \u2018Sun-like\u2019 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. <\/p>\n 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 \u2018around 7 percent of the stars in the Milky Way.\u2019 Or it could mean \u2018around 22 percent of the stars in the Milky Way, if we mean FGK host stars.<\/p>\n If we included red dwarf stars, it could mean \u2018around about 95 percent of the stars in the galaxy,\u2019 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<\/a> 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 \u2018habitable.\u2019 <\/p>\n So we\u2019ve got to be careful in this burgeoning field. Exoplanet researchers are aware of the need to establish the meaning of \u2018Sun-like\u2019 carefully. The fact that the public\u2019s interest in exoplanets is growing means, however, that in public utterances like news releases, scientists need to clarify what they\u2019re talking about. It\u2019s the same thing that makes the term \u2018Earth-like\u2019 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? <\/p>\n Let\u2019s watch our terms so we don\u2019t 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\u2019s climate is a highly charged issue. A confused public is less likely to support studies in areas it does not understand. <\/p>\n![\"\"](\"https:\/\/mycbh.com\/Games\/spaceperium\/wp-content\/uploads\/sites\/2\/2025\/04\/tzf_img_post-1.jpg\")
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