Centauri Dreams

What catches your eye in this description of an exoplanetary system? Start with a ‘hot Jupiter,’ with a radius 0.87 times that of our Jupiter and an orbit of 7.1 days. This is WASP-132b, confirmed in 2016, and first discovered through the labors of the Wide-Angle Search for Planets program. Subsequent confirmation came through the CORALIE spectrograph installed on the Euler telescope at the European Southern Observatory’s La Silla site. This world orbits a K-class star 403 light years out in Lupus.

The CORALIE measurements gave hints of another giant planet in a long period orbit. The system came still further into focus in 2021, when observations from TESS (Transiting Exoplanet Survey Satellite) showed a transiting super-Earth with a diameter of 1.8 Earth radii in a tight orbit of 1.01 days. The mass of the planet, as measured by the HARPS spectrograph at La Silla, is six times that of Earth. So we have both a hot-Jupiter and a super-Earth hugging the star, along with an outer gas giant.

Image: The WASP-132 system was known to harbour WASP-132b, here in the foreground, a hot Jupiter planet orbiting around a K-type star in 7.1 days. New data confirms the system has more planets, including an inner super-Earth, here seen transiting in front of the orange host-star. Visible as a pale blue dot near the top right corner is also the giant planet WASP-132d discovered in the outskirts of the system. © Thibaut Roger – Université de Genève.

While the European Space Agency’s Gaia satellite continues to take astrometrical data on WASP-132, follow-up work has shown the super-Earth to have a density similar to Earth’s and a composition of metals and silicates fairly similar to our planet (remember, we have both radius and mass measurements to work with because of the multiple datasets from different detection methods). Meanwhile, the problem here should be apparent.

Ravit Helled (University of Zurich) and a co-author of the study offers this:

‘‘The combination of a Hot Jupiter, an inner Super-Earth and an outer giant planet in the same system provides important constraints on theories of planet formation and in particular their migration processes. WASP-132 demonstrates the diversity and complexity of multi-planetary systems, underlining the need for very long-term, high-precision observations.’’

All true, of course, but it doesn’t get across how unusual this finding is. For hot Jupiters as thus far observed have been relatively isolated from planets further out in their systems. That makes sense because the model for their formation involves migration, with the giant worlds forming far enough out from the star to feed off plentiful gas and dust in the protoplanetary disk, and then moving inward as the system takes form. Woe to inner planets, whose fate might include ejection from the system entirely.

WASP-132 shouldn’t have the system architecture it does given this theory of migration, meaning we have to re-examine the nature of migration, or ponder ways to achieve a planet of this size in a tight stellar orbit that leave migration behind altogether. The hot Jupiter here leads François Bouchy (University of Geneva), a co-author of the study, to say this:

“The WASP-132 system is a remarkable laboratory for studying the formation and evolution of multi-planetary systems. The discovery of a Hot Jupiter alongside an inner Super-Earth and a distant giant planet calls into question our understanding of the formation and evolution of these systems.”

To my knowledge, WASP-132 is the second example of a planetary system in this configuration. WASP-47 takes precedence in terms of discovery, having been first analyzed by the WASP team in 2012 (discovery of the hot Jupiter) and then expanded through work with K2 data in 2015. WASP-47, a G-class star in Aquarius some 880 light years away, hosts a super-Earth inside the hot Jupiter’s orbit, a hot Neptune outside its orbit, and an outer gas giant (‘warm Saturn’) within the habitable zone. The discovery paper of the smaller worlds at WASP-47 is worth quoting:

The continued existence of the companions in this system indicates that HEM [high eccentricity migration] ] cannot serve as the sole formation mechanism for hot Jupiters. HEM would likely have disrupted the orbits of the smaller planets. It is quite possible that there is more than one potential formation mechanism for hot Jupiters. Additionally, recent observations have identified an additional Jupiter-mass planet in a 571-day orbit (called WASP-47c; Neveu-VanMalle et al. 2015) in this system, making this the first hot Jupiter with both close-in companions and an external perturber. Future dynamical work will place limits on the architecture of this system.

The paper is Thibaut et al., ”Discovery of a cold giant planet and mass measurement of a hot super-Earth in the multi-planetary system WASP-132′,” Astronomy and Astrophysics Vol. 693 (15 January 2025), A144 (full text). On WASP-132, see Becker at al., “WASP-47: A Hot Jupiter System With Two Additional Planets Discovered by K2,” Astrophysical Journal Letters Vol. 812, No. 2 (12 October 2015), L18 (full text).

Last Friday’s post on K-dwarfs as home to what researchers have taken to calling ‘superhabitable’ worlds has caught the eye of Dave Moore, a long-time Centauri Dreams correspondent and author. Readers will recall his deep dives into habitability concepts in such essays as The “Habitability” of Worlds and Super Earths/Hycean Worlds, not to mention his work on SETI (see If Loud Aliens Explain Human Earliness, Quiet Aliens Are Also Rare). Dave sent this in as a comment but I asked him to post it at the top because it so directly addresses the topic of habitability prospects around K-dwarfs, based on a quick survey of known planetary systems. It’s a back of the envelope overview, but one that implies habitable planets around stars like these may be more difficult to find than we think.

by Dave Moore

To see whether K dwarfs made a good target for habitable planets, I decided to look into the prevalence and type of planets around K dwarfs and got carried away looking at the specs for 500 systems of dwarfs between 0.6 mass of the sun and 0.88.

Some points:

i) This was a quick and dirty survey.

ii) Our sampling of planets is horribly skewed towards the massive and close, but that being said, we can tell if certain types of planets are not in a system. For instance Jupiter and Neptune sized planets at approximately 1 au show up, so if a system doesn’t show them after a thorough examination, it won’t have them.

iii) I had trouble finding a planet list that was configurable to my needs. I finally settled on the Exoplanets Data Explorer configured in reverse order of stellar mass. This list is not as comprehensive as the Exosolar Planetary Encyclopedia.

iv) I concocted a rough table of the inner and outer HZ for the various classes of K dwarfs. Their HZs vary considerably. A K8 star’s HZ is between 0.26 au and 0.38 au while a K0’s HZ is between 0.72 au and 1.04 au. This means that you can have two planets orbiting at the same distance around a star and one I will classify as outside the HZ and the other inside the HZ.

v) Planets below 9 Earth mass I classified as Super-Earth/Sub-Neptune. Planets between 9 Earth masses and 30 are classified as Neptunes. Planets over that size are classified as Jupiters.

Image: An array of planets that could support life are shown in this artist’s impression. How many such worlds orbit K-dwarf stars, and are any of them likely to be ‘superhabitable’? Credit: NASA, ESA and G. Bacon (STScI).

What did I find:

By far the most common type are hot Super-Earths/Sub-Neptunes (SE/SNs). These are planets between 3 EM (Earth mass) and 6 EM. It is amazing the consistency of size these planets have. They are mostly in close (sub 10 day) orbits. There also appears to be a subtype of sub 2EM planets in very tight orbits (some quoted in hours) and given some of these were in multi-planet systems of SE/SNs, I would say these were SE/SNs, which have been evaporated down to their cores.

I also found 7 in the HZ and 2 outside the HZ.

I found 52 hot Jupiters and what I classified as 43 elliptical orbit Jupiters. These were Jupiter-sized planets in elliptical orbits under 3 au.

There were also 10 Jupiter classification planets in circular orbits under 3 au. and 3 outside that limit in what could be thought of as a rough analog of our system.

There were also 46 hot Neptunes and 14 in circular orbits further out, only one outside the habitable zone.

Trends:

At the lower mass end of the scale, K dwarf systems start off looking very much like M dwarfs except that everything, even those in multi-planet systems, is inside the habitable zone.

As you work your way up the mass scale, there is a slight increase in the average mass of the SE/SNs with 7-8 EM planets becoming more prevalent. More and more Jupiters appear, and Neptune-sized planets appear and become much more frequent. Also, you get the occasional monster system of tightly packed Jupiters and Neptunes like 55 Cancri.

An interesting development begins at about the mid mass range. You start getting SE/SNs in nice circular longer period orbits but still inside the HZ (28 in 20-100d orbits.)

Conclusions:

If we look at the TRAPPIST-1 system around an M-dwarf, its high percentage of volatiles (20% water/ice) implies that there is a lot of migration in from the outer system. If a planet has migrated in from outside the snow line, then there’s a good chance that even if it’s in the habitable zone, it will be a deep ocean planet.

Signs of migration are not hard to find. Turning back to the K-dwarfs, if we look at the Jupiters, only three show signs of little migration (analogs of our system). Ten migrated in smoothly but sit at a distance likely to have disrupted a habitable planet. Forty-three are in elliptical orbits, which are considered signs of planet-planet scattering.

Hot Jupiters can be accounted for by either extreme scattering or migration. As to inward migration, Martin Fogg did a series of papers showing that as Jupiter mass planets march inwards they scatter protoplanets, but these can reform behind the giant, and so Earth-like planets may occur outside of the hot Jupiter.

Neptunes in longer period circular orbits and the longer period SE/SNs all point to migration. These last groups are intriguing as they point to a stable system with the possibility of smaller planets further out. I would include the 7 planets in the habitable zone in this group. But if these planets all migrated inwards they may well be ocean planets.

K dwarfs have an interesting variety of systems, so they’d be useful to study, but I don’t see them as the major source of Earth analogs—at least not until we learn more.

We think of Earth as our standard for habitability, and thus the goal of finding an ‘Earth 2.0’ is to identify living worlds like ours orbiting similar Sun-like stars. But maybe Earth isn’t the best standard. Are there ways planets can be more habitable than our own, and if so where would we find them? That’s the tantalizing question posed in a paper by Iva Vilović (Technische Universität Berlin), René Heller (Max-Planck-Institut für Sonnensystemforschung) and colleagues in Germany and India. Heller has previously worked this issue in a significant paper with John Armstrong (citation below); see as well The Best of All Possible Worlds, which ran here in 2020.

The term for the kind of world we are looking for is ‘superhabitable,’ and the aim of this study is to extend the discussion of K-class stars as hosts by modeling the atmospheres we may find on planets there. While much attention has focused on M-class red dwarfs, the high degree of flare activity coupled with long pre-main sequence lifetimes makes K-class stars the more attractive choice, although less susceptible to near-term evaluation, as the paper shows in its sections on observability. It’s intriguing, for example, to realize that K-class stars are expected to live significantly longer than the Sun, as much as 100 billion years, and because they are cooler and less luminous than G-class stars, their habitable zone planets produce more frequent transits.

Image: This infographic compares the characteristics of three classes of stars in our galaxy: Sunlike stars are classified as G-stars; stars less massive and cooler than our Sun are K-dwarfs; and even fainter and cooler stars are the reddish M-dwarfs. The graphic compares the stars in terms of several important variables. The habitable zones, potentially capable of hosting life-bearing planets, are wider for hotter stars. The longevity for red dwarf M-stars can exceed 100 billion years. K-dwarf ages can range from 50 to 100 billion years. And, our Sun only lasts for 10 billion years. The relative amount of harmful radiation (to life as we know it) that stars emit can be 80 to 500 times more intense for M-dwarfs relative to our Sun, but only 5 to 25 times more intense for the orange K-dwarfs. Red dwarfs make up the bulk of the Milky Way’s population, about 73%. Sunlike stars are merely 8% of the population, and K-dwarfs are at 13%. When these four variables are balanced, the most suitable stars for potentially hosting advanced life forms are K-dwarfs. Image credit: NASA, ESA, and Z. Levy (STScI).

Let’s dig into this a little further. The contrast in brightness between star and planet is enhanced around K-dwarfs, and spectroscopic studies are aided by lower levels of stellar activity, which also enhances the habitability of planets. While an M-dwarf may be in a pre-Main Sequence phase for up to a billion years, K stars take about a tenth of this. They emit lower levels of X-rays than G-type stars and are also more abundant, making up about 13 percent of the galactic population as opposed to 8% for G-stars. With luminosity as low as one-tenth of a star like the Sun, they offer better conditions for direct imaging and their planets are far enough from the host to avoid tidal lock.

So we have an interesting area for investigation, as earlier studies have shown that photosynthesis works well under simulated K-dwarf radiation conditions. The authors go so far as to call these ‘Goldilocks stars’ for life-bearing planets, and there are about 1,000 such stars within 100 light years of the Sun, Thus modeling superhabitable atmospheres to support future observations stands as a valuable contribution.

The authors model these atmospheres by drawing on Earth’s own history as well as astrophysical parameters, finding that a superhabitable planet would be somewhat more massive than Earth so as to retain a thicker atmosphere to support a more extensive biosphere. Plate tectonics and a strong magnetic field are assumed, as are elevated oxygen levels that would “enable more extensive metabolic networks and support larger organisms.” Surface temperatures are some 5 degrees C warmer than present day Earth and increased atmospheric humidity supports the ecosystem.

The paper continues:

In terms of the atmospheric composition, key organisms and biological sources affecting Earth’s biosphere and their atmospheric signatures are considered. A superhabitable atmosphere would have increased levels of methane (CH4) and nitrous oxide (N2O) due to heightened production by methanogenic microbes, as well as denitrifying bacteria and fungi, respectively (Averill and Tiedje 1982, Wen et al. 2017). Furthermore, it would have decreased levels of molecular hydrogen (H2) due to higher enzyme consumption (Lane et al. 2010, Greening and Boyd 2020). Lastly…molecular oxygen (O2) levels could increase from present-day 21% by volume on Earth to 25% to reflect a thriving photosynthetic biosphere (Schirrmeister et al. 2015).

Given these factors, the authors deploy simulations using three different modeling tools (Atmos, POSEIDON and PandExo, the latter two to examine observability of transiting planets). Using Atmos, they simulate three pairs of superhabitable planets in differing locations in K-dwarf habitable zones, varying stellar radii and masses and star age. They focused on organisms and biological sources that had influenced Earth’s biosphere, including O2, H2, CH4, N2O and CO2 at a variety of surface temperatures.

The results offer what the authors consider the first simulated data on superhabitable atmospheres and assessments of the observability of such life. What stands out here is the optimum positioning of a superhabitable world around its star. Note this:

We find that planets positioned at the midpoint between the inner edge and center of the habitable zone, where they receive 80% of Earth’s solar flux, are more conducive to life. This contrasts with previous suggestions that planets at the center of the habitable zone—where our study shows they receive about 60% of Earth’s solar flux—are the most favorable for life (Heller and Armstrong 2014). Planets at the midpoint between the center and the inner edge need less CO2 for temperate climates and are more observable due to their warmer atmospheric temperatures and larger atmospheric scale heights. We conclude that a superhabitable planet orbiting a 4300K star with 80% of the solar flux offers the best balance of observability and habitability.

Image: An artist’s concept of a planet orbiting in the habitable zone of a K-type star. Image credit: NASA Ames/JPL-Caltech/Tim Pyle.

Observability presents a major challenge. Using the James Webb Space Telescope, a biosignature detection at 30 parsecs requires 150 transits (43 years of observation time) as compared to 1700 transaits (1699 years) for an Earth-like planet around a G-class star. That would be a mark in favor of K-stars but it also underlines the fact that studies of that length are impractical even with the anticipated Habitable Worlds Observatory. The JWST is working wonders, but clearly we are talking about next-generation telescopes – or the generation after that – when it comes to biosignature detection on potential superhabitable planets.

So what we have is encouraging in terms of the chances for life around K-class stars but a clear notice that observing the biosignatures of these planets is going to be a much harder task than doing the same for nearby M-class dwarfs, where extremely close habitable zones also give us a much larger number of transits over time.

The paper is Vilović et al., “Superhabitable Planets Around Mid-Type K Dwarf Stars Enhance Simulated JWST Observability and Surface Habitability,” accepted at Astronomical Notes and now available as a preprint. The earlier Heller and Armstrong paper is “Superhabitable Worlds,” Astrobiology Vol. 14, No. 1 (2014). Abstract. Another key text is Schulze-Makuch, Heller & Guinan, “In Search for a Planet Better than Earth: Top Contenders for a Superhabitable World,” Astrobiology 18 September 2020 (full text), which looks at candidates. Cuntz & Guinan, “About Exobiology: The Case for Dwarf K Stars,” Astrophysical Journal Vol. 827, No. 1 10 August 2016 (full text) should also be in your quiver.