Some years back I read a science fiction story in which the planet where the action took place orbited an F-class star. That was sufficiently odd to get my attention, and I began to pay attention to these stars, which represent on the order of 3 percent of all stars in the galaxy. Stars like our G-class Sun weigh in at about 7 percent, while the vast majority of stars are M-dwarfs, still our best chances for life detection because of the advantages they offer to our observing technologies, including deep transits and lower stellar brightness for direct imaging purposes.
F-stars are intriguing despite the fact that they tend to be somewhat larger than the Sun (up to 1.4 times its mass) and also hotter (temperatures in the range of 6200-7200 K). Back in 2014, I looked at the work of Manfred Cuntz (University of Texas at Arlington), who had performed a study examining radiation levels in these stars and the damage that DNA would experience with an F-star in the sky at various stages of stellar evolution. We’re dealing here with a shorter life expectancy than the Sun, usually reckoned in the range of 2-8 billion years on the main sequence depending on mass.
We’re also dealing with a larger habitable zone, a width 1.5 to 4 times greater than in the case of the Sun, again depending on the mass of the star and the climate models used to calculate the HZ. So there are advantages, for in the 2014 work, Cuntz and team found that the outer regions of the HZ experience tolerable levels of UV radiation. Now Cuntz has pushed the F-star work forward with a new paper, working with lead author Shaan Patel, a UTA grad student, and colleague Nevin Weinberg. The new work embarks on a statistical analysis of planet-hosting F-class stars drawn from data in the NASA Exoplanet Archive, which is a resource I don’t link to often enough. Says Cuntz:
“F-type stars are usually considered the high-luminosity end of stars with a serious prospect for allowing an environment for planets favorable for life. However, those stars are often ignored by the scientific community. Although F-type stars have a shorter lifetime than our Sun, they have a wider HZ. In short, F-type stars are not hopeless in the context of astrobiology.”
Image: The habitable zone as visualized around different types of star. Credit: NASA.
206 planetary systems emerge from the investigation, of which 18 offer a planet in the liquid water habitable zone for at least part of its orbit. The authors break these worlds down into categories based on the amount of time each spends in the HZ. It’s worth noting that all the currently known planets in the habitable zone of F stars are Jupiter-class worlds, so what we are thinking about here in terms of astrobiology is habitable moons, about which interesting new work continues to emerge. I also assume we’ll be finding terrestrial-class worlds around these stars with deeper investigation.
The exo-Jupiter 38 Virginis (HD 111998) is noteworthy for spending the entirety of its orbit in the habitable zone, which most of these worlds do not. Now things get intriguing. There are reasons for including planets whose orbital eccentricity allows only partial passage through the HZ, drawing on previous research (citation below) on atmospheric conditions for Earth-class planets in extremely elliptical orbits. That 2002 study found that despite large variations in surface temperature, long-term climate depended on the average stellar flux over the entire orbit, meaning that planets not in but near the HZ may still be potentially habitable, at least for extremophiles.
And we can possibly extend our definition of habitable zone. From the paper:
As part of our study, we also consider cushions for both HZ limits. This approach is informed by previous studies given by Abe et al. (2011) and Wordsworth et al. (2013). The former work deals with climate simulations for “land planets” (i.e., desert worlds with limited surface water), which based on those models have a significantly extended inner HZ limit than planets with abundant surface water (akin to Earth). Moreover, Wordsworth et al. (2013) continued to explore the outer limit of HZs by considering the impact of CO2, including CO2 clouds. They found that in their models the outer HZ is notably extended, commensurate to the Martian orbit in the solar system.
Image: This is Figure 10 from the paper. Caption: Depiction of all 18 systems that spend at least part of their time within their respective HZs. Empty markers in panel (c) represent actual planetary mass values as opposed to minimum mass values, which are represented by filled in markers. Credit: Patel et al.
Consider that the lowest-mass planet currently in a habitable zone in all these systems has an estimated mass 143 times Earth and you’ll agree with the need to probe further into potentially habitable exomoons, about which we know next to nothing. Overall, with projects like the Habitable Worlds Observatory on the table, we should consider F-class stars as targets for deeper study. As lead author Patel says, “In future studies, our work may serve to investigate the existence of Earth-mass planets and also habitable exomoons hosted by exo-Jupiters in F-type systems.”
The paper is Patel et al., “Statistics and Habitability of F-type Star–Planet Systems,” The Astrophysical Journal Supplement Series Vol. 274, No. 1 (12 September 2024), 20 (full text). The paper on habitability in eccentric orbits is Williams & Pollard, “Earth-like worlds on eccentric orbits: excursions beyond the habitable zone,” International Journal of Astrobiology Vol. 1, Issue 1 (January, 2002), 61-68 (abstract).
As much as I like Star Wars, the idea of habitable exomoons would be rare, the result of orbital capture with forming insitu probably being impossible.
The problem with F stars is a planet moves too fast out of the habitable zone before intelligent life could evolve, but that does not rule out planets with life.
Something I’ve kept meaning to find out about moons of giant planets: does the nearby presence of a formidable gravity well mean these moons are less likely than rocky inner planets to be impacted by asteroids? If that’s how worlds get water delivered to them, are moons therefore disfavoured for life getting started because their host planet will gobble up the impactors?
The gravity well around F type stars is less for the mass as the luminosity is non linear to mass, ie. Double the mass of the star compared to ours and the luminosity is around 25 time higher. As for been more habitatable the solar wind for these f type stars is a lower amount which would reduce atmospheric striping of earth like worlds.
I actually meant the gravity well of the giant planet that these moons would be orbiting. Have the Jovian moons, for example, been blessed with as much water delivery from asteroid impacts as the earth was? Or does an asteroid sucked in to the Jovian system tend to hit Jupiter itself?
It appears that we have here (essentially) a database of discovered exoplanets around local F stars. And quite often when these collections are gathered the statistical methods make it difficult to determine much about them. For example, looking at the catalog of exoplanets surrounding F stars, one wonders how they were detected. F stars are significantly more luminous than they are massive with respect to the sun. Consequently, and F2V is about 1000 K hotter at its surface than the sun, about a 1.5 x as massive and about 5 times as luminous ( 5 Lsun). If we consider the Earth’s offset from our sun as an indicator, and with the local black body temperature of a solar source expanded to 1 AU, that would be a temperature close to 400 K . Earth temperature is a bit above water freezing for a number of thermodynamic reasons, if we take Earth’s 400 K offset as an initial reference point, that would be about 2.25 AUs out from an F star. And the Earth with a G star got to its equilibrium ( albedo, reflectance, greenhouse, chemistry, etc. after about 4 billion years). With an F star, all things the same, maybe that is about time to leave?
Well, then we have substantially more UV to deal with in the black body flux. Atmospheric chemistry and days at the beach are not going to be the same.
Nor is the length of the year. Period is proportional to radius to 3/2 over square root of mass. I get the year as 2.75 Earth years long. Crossing my fingers, of course. But the point is, if we are looking for HZ planets, I don’t think it that easy to detect transits about F stars. Four data points takes 11 years. Likely, the low hanging fruit are the jovian mass planets that cause some radial velocity curves – and at the short edge of the putative HZ.
On the other hand, since the HZ would be physically wider, a lot of planets could be packed within its bounds in comparison to our G star case. But if that is the case, then there could be a lot of junk floating around for a long time wrt the lifetime of an F star. And if orbits are as eccentric as the paper explored, who knows what might happen?
Going back to the transit proposition and habitable moons, the Earth-Moon case is probably about as observable as a binary system gets – at least so far. With a mass ratio of about 81 to 1, and separation of about 240,000 statute miles ( in this case convenient) the Earth is bout 3000 miles removed from its barycenter – and about 4800 km offset,,, The Earth moves around the sun at about 30 km/sec – so a Transit Time Start Variation could be as much as +/- 160 seconds. The moon would provide a small extinction before or after the Earth depending on their separation angles. It might reach the threshold of detection by itself, but still it would take years…. Longer with an F star at the same HZ distance. From what we have observed so far, there are exoplanets more massive than Jupiter, but if their mass ratios are like Jupiter’s to the Galilean moons, barycenter offsets are going to be small.
We need a break.
The observations show that most Jupiter-class planets are inside the HZ, no doubt due to orbital periods being long for the planet’s validation.
From the data of other, lower mass stars, do we have some estimate of the distribution of these gas giant planets across their systems?
Do we have any idea of the impact these planets have on rocky worlds that could exist in the system’s HZ? [ Do close orbiting gas giants preclude rocky planets in the HZ, or not?]
Is there any relationship between stellar mass and the mass of the largest planet? The data for F-class stars is in the Appendix, but I don’t want to spend time extracting the data, plotting it, and then comparing this across other star classes, if the answer is already known.
A.T.,
Your concerns are similar to mine. But I suspect that F star statistics will be similar to Ks and Gs in many ways. And in fact, starting with the earliest radial velocity detections in the mid to late 90s, those smaller, dimmer stars first revealed hot jupiters and then a number of terrestrial planets – inside and outside the orbits of the most prominent signals.
51 Pegasi and 55 Cancri would be illustrative examples.
55 Cancri Ab had a 14.5 day period, nominal eccentricity of 0.014. Resolved now to about 0.83 jovian mass. The star itself is bound with 55 Cancri B, about 1000 AUs away.
Subsequent to 55 Cancri Ab, the other exoplanets were detected as transits.
https://en.wikipedia.org/wiki/55_Cancri
Planet orbital periods in days:
e 0.73
b 14.6
c 44.4
f 259.9
d 5574
All the planets are rather large. f is about 40 some Earth masses.
But the good news is that a hot Jupiter did not clean out the habitable zone – or the unhabitable zones for that matter.
However, checking back on 51 Pegasi, another early exoplanet discovery with a hot Jupiter, the tally of exoplanets appears not to have gone past “b”.
In addition to these two systems, there were two other early systems that
revealed rather large exoplanets with doppler methods. 47 Ursae Majoris and 70 Virginis with one or two large planets at intermediate distance. In these particular cases there has not been a growth in planet numbers since.
Consequently, since F stars are less numerous than less massive stars, and terrestrial exoplanets do less to perturb their primaries ( doppler) or might not have as observable lines, and their surface extinction is less prominent and less frequent in transit – it might take a while to detect them.
The image charts a,c, and d show no correlations. c shows that jupiter+ mass planets are in the systems’ HZs. If we put Jupiter in our HZ, wouldn’t that destabilize the rocky worlds in our HZ? So while a moon might be habitable, there may not be any terrestrial-type worlds in the HZs.
As for the possibility of a habitable exomoon, we only speculate that Europa may be inhabited below a thick icy crust that protects it from Jupiter’s radiation fields. My question is for any Jupiter+ mass, how far out would a moon need to be to escape any equivalent radiation from the planet?
Perhaps we need to look for F-type stars where there is no Jupiter+ planet in the HZ to hold out hope for a terrestrial world in the HZ?
Hi Paul
Yes its a very interesting read.
I think you have the mass range about right too.
Cheers Edwin
The other problem with habitable moons is the ones we have in our solar system are tidally locked with their primary, which means their day/night cycle will be quite long compared to Earth. This means really high temps during the day and really cold temps at night. I don’t think this makes for good habitability.
All of our moons are between our moon and Mars in size. Does this suggest an upper limit in size for such moons? Or could they be Earth-sized? We don’t know as they are really hard to detect.
While our airless Moon with a 28-day day has extremes of temperatures, how would that differ with an atmosphere to distribute heat? Do we have models of heat and temperature distribution for planets with longer days? After all, Earth had a much shorter day 4 bn years ago (~ 10 hours). Has the day length 2.4x longer made the climate inhospitable? No. The temperatures are dependent on proximity to an ocean or sea, currents, clouds, winds, and vegetation. How much temperature extremes can be mitigated by such factors for a much longer day?
It would appear that a binary system like the Earth (81 to 1 on mass) is exceptional. The Outer Planets ranging between Neptune and Jupiter have moons as large as ours – or larger – with significantly more volatile material on their surfaces ( even atmospheres), but their mass ratios to their primaries are substantially lower.
Synchronous rotation with the satellite orbital period… With the Galilean satellites starting with Io, the revolutions or days vs. Earth’s are 1.77, 3.55, 7.15 and 16.87. Then for some of that the satellites are eclipsed by the planet in a way that our moon seldom is. If the planet is more massive than Jupiter and nearer to the Earth’s HZ, then one can imagine similar systems with shorter periods.
But in the case of Jupiter the magnetosphere is the big problem. The imminent Europa mission has highlighted how intense the magnetosphere flux is on the surface. Radiation hardness of the Clipper’s circuit boards is under review and it might be a show stopper for launch next month. To look at it in another way, If astronauts were living in a quonset hut on the surface they would not last long.
Still, with a hundred or more gas giants with satellites, maybe there might be one with a moon that has an atmosphere, a moderate mean temperature and
a relatively benign magnetosphere surrounding it.
A further reflection on modeling for stellar black body variations.
To first order, one can calibrate the HZ with the overall effective temperatures as described above. This would be for temperature contours of various levels such as 400 K, 350 K etc. But it might be worth considering an equivalence of UV at some bandwidth for a blackbody. For example, the original surface temperature of a star, the hotter it gets, the more UV is generated. Or to put it another way, an object 1 AU from where its flux is diffused to 400 K effective, there will be less UV in that flux than in the F star where a planet is backed off to the same radiative temperature. The Black body temperature Wien relation is an inverse of temperature and wavelength, but the distributive curve of radiation has another behavior – though formulaic. The question would be, what would be a good UV band to fix if the Earth’s flux is the standard? Then it might be possible to calculate equivalent stand off radii for various stellar luminosities and spectra. The likely result is that the effective temperature would vary from the 400 K nominal. Published HZs usually are not very clear on this matter since their presentations are more illustrative than analytical.
But since we have had no real field tests of habitability, both means of modeling tell us something about exoplanets in the meantime.
This is true. But the temperature swings would still be dramatic. Day night temperature swings on Earth can be 20-30 degrees. This swing would be much more with a 100 or 200 hour day.
A.L.,
Your argument is valid too. And I suspect that in the majority of cases the moons to be encountered among exoplanets will be affected by such observed circumstances. But there might be some that are not. And the fundamental reason would be “thermal inertia”.
To illustrate, were Titan one of the Galilean moons, I think its adjusted equilibrium temperature would not fluctuate as you suggest. But having it magically appear among these other icy moons would neglect elements of the jovian system’s own geological history. The Galilean moons might have passed through geological stages such as I am suggesting already.
Thermal inertia would be a lag in a planetary atmosphere ( and surface) in responding to an instantaneous change in heat flux – such as eclipse by the primary or planet such as Jupiter or Saturn.
Where I encountered the idea for the first time was in old softcover textbook titled, “Atmospheres” by Richard Goody and James C. G. Walker, About 150 pages and written in 1972, it was actually rather readable – even though it did not benefit from the wealth of potential illustrative examples we have now in exoplanets.
Thermal inertia, should it be significant in an exoplanet or exomoon’s atmosphere, would provide an exponential decay rate ( e.g., time to decrease absolute temperature of atmosphere, ocean or combination by 63 percent).
Even with the Galilean moons and Titan, there would be differences in overall estimates. Titan is the only example with an atmosphere of consequence and it is already at very low temperature. But since it is deep and dense, its response to occultation by Saturn would be sluggish in these terms. The surface or ocean could figure into this too. And then, of course, were Venus
shut off from the sun, it would take a very long time to cool down too.
So I hope Goody and Walker’s book and methods will come in handy in the near future should we spot some exomoons of significant size in the HZs of other stars.
AL, even with an atmosphere after a few days it would below zero IMO, atmospheres like ours doesn’t hold heat well at all even with winds moving the heat around. However water would help a lot to distribute heat but even then it’s going to be cold quite quickly.