Europa’s Patchy Plate Tectonics

I keep an eye on recent findings about Europa because fine-tuning procedures for the science that missions like Europa Clipper and JUICE (Jupiter Icy Moons Explorer) will perform at the Jovian moon is an ongoing process that doesn’t stop at launch. The more we learn now – the more anomalies we uncover or processes we begin to glimpse – the better able we’ll be to adjust spacecraft observing strategies to go after the answers to these phenomena. A new study teaches us a bit more about Europa’s plate tectonics, the only solid evidence of tectonics we know of other than Earth’s. And it will take new high-resolution imagery to confirm the theories put forth within it.

Appearing in the Journal of Geophysical Research: Planets, the paper looks at the processes that evidently govern the evolution of the fractured Europan surface the Galileo mission revealed to us back in the 1990s. What’s intriguing here is the identification of Europan tectonic plates in the context of deep time. If a tectonic plate shows a broad continuity, it is bounded by surface discontinuities. The authors show that a sequence can be established based on how discontinuities cut through surface features, and we can begin to make statements about how the surface changes.

Image: A complex pattern of ridges and bands named Arachne Linea is seen in this false-color image of Europa’s surface taken by the Galileo spacecraft on 26 September 1998. New research shows that this landscape was formed by the jostling of nearby tectonic plates. Credit: NASA/JPL-Caltech/SETI Institute.

The work, led by Geoffrey Collins (Wheaton College, MA), looks at three areas on Europa – spread out in terms of latitude so as to include the high northern and southern latitudes as well as the equatorial areas – to reconstruct what they would have looked like before surface plates moved. It’s fascinating to contrast the findings with what we see on Earth, because on Europa tectonic movement is scattered, with some areas showing no plates at all. That reveals a regional process in which plate travel distance can be less than 100 kilometers. Plate tectonics, in other words, occurs only in limited areas on Europa, covers only a small degree of motion across the surface, and only appears to operate intermittently. It does not appear to be happening now, at least in the areas the paper surveys.

I come back to Europa Clipper, the mission that should benefit from this analysis, for although we’ve studied plate tectonics on Europa before, the work has been hampered by lack of imaging data at high enough resolution to attain a widespread view. The authors argue that the improved imaging from missions like Clipper and JUICE will detect many more areas with plate-like motions to add to their three examples. This should highlight the fact that while Earth’s tectonic plates form a global system, Europa’s are sharply confined in a process that is likely local or regional.

Note the stop-and-start aspect of plate movement, as discussed in the paper (italics mine):

In all of the study areas, young ridges and ridge complexes overprint the plate boundaries. The young ridges do not accommodate offsets like those seen in the plate boundaries. Thus, whatever process was driving the plate motions came to an end, and is not actively driving plate motions today in any of the areas studied. The relationship between the Castalia Macula study area and the older Belus area to its north… demonstrates that the plate tectonic-like behavior on Europa did not occur all at the same time. Combined with the previous conclusion, we develop the view that plate tectonic-like behavior on Europa occurs in regional patches and turns on and off at different times in different places.

Image: This is Figure 19 from the paper, illustrating tectonic change over time. Caption: An area exhibiting plate-like motions north of Belus Linea is outlined by a red dashed line. The purple lines in the south are plate boundaries CM2 and CM4 from the Castalia Macula reconstruction… The blue line shows a ridge that is crosscut by CM2 and CM4, and extends all the way south to be crosscut by Acacallis Linea (off the southern edge of this figure). The Blue Ridge crosscuts the green ridge, which crosscuts all of the candidate plate boundaries in the area north of Belus. This shows that all of the plate-like activity in the area north of Belus is older than the activity in the Castalia Macula study area. Orthographic projection centered at 15°N, 135°E. Credit: Collins et al.

The study of these three areas of Europa implies that something stops plate motion from persisting and spreading, possibly because of the nature of the surface material or else the mechanism that forces plate motions in the first place. Here’s an interesting point that has further ramifications for future observations by our spacecraft: Is convergence – where surface material is lost and pre-existing terrain must be reconstructed – always apparent from the limited data we have available? We’ll need plenty of high resolution imagery from future spacecraft to make the call on that.

There is much for Europa Clipper to examine in terms of how tectonics functions here, not the least of which is the question of what drives the plate motions thus far observed on Europa. If plate tectonics is indeed as sharply limited as this study implies, just what is it that forces a plate movement and then apparently shuts it down?

And in terms of that ever-fascinating ocean below the ice, is surface material drawn in large quantities into the ice shell, and are there times when the lower crust or ocean is exposed while the plate motion occurs? Reconstructing plate motions is an open investigation with serious implications for habitability. Europa Clipper and the JUICE mission should give us data that sharply refine our modeling of surface and ocean.

The paper on Europa’s tectonics is Collins et al, “Episodic Plate Tectonics on Europa: Evidence for Widespread Patches of Mobile?Lid Behavior in the Antijovian Hemisphere,” Journal of Geophysical Research: Planets Vol. 127, Issue 11 (06 November 2022). Full text.


WASP-39b: JWST and Exoplanet Atmospheres

Although I often see the exoplanet WASP-39b referred to as a ‘hot Saturn,’ and sometimes a ‘hot Jupiter,’ the terms don’t really compute. This is a world closer to Saturn than Jupiter in mass, but with a radius somewhat larger than that of Jupiter. Hugging its G-class primary in a seven million kilometer orbit, it completes a circuit every four days. The system is about 700 light years from us in Virgo, and to my mind WASP-39b is a salutary reminder that we can carry analogies to the Solar System only so far.

Because we have nothing in our system that remotely compares to WASP-39b. Let’s celebrate the fact that in this exoplanet we have the opportunity to study a different kind of planet, and remind ourselves of how many worlds we’re finding that are not represented by our own familiar categories. I imagine one day we’ll have more descriptive names for what we now call, by analogy, ‘super-Earths’ and ‘sub-Neptunes’ as well.

I’ve seen WASP-39b referred to in the literature as a ‘highly inflated’ planet, which is an apt description. This is a transiting world, one that has been the subject of numerous studies that have produced insights into giant planet atmospheres under extreme conditions through the spectroscopic study of light from the star as it filters through the planet’s gaseous envelope. Even before the James Webb Space Telescope became available, we had learned a lot about its composition.

Now JWST data take our understanding to an entirely new level. Both the Hubble and Spitzer space observatories have gone to work on WASP-39b in recent years, finding interesting ingredients in an atmosphere that reaches 900 ?. What we have from JWST, as revealed in a set of five just released scientific papers, is a wide range of atoms and molecules that provides signs of active chemistry and even cloud activity. JWST’s NIRSpec (Near-Infrared Spectrograph), NIRCam (Near Infrared Camera ) and NIRISS (Near Infrared Imager and Slitless Spectrograph) all produced data in this work.

Image: A series of light curves from Webb’s Near-Infrared Spectrograph (NIRSpec) shows the change in brightness of three different wavelengths (colors) of light from the WASP-39 star system over time as the planet transited the star July 10, 2022. Credit: Illustration: NASA, ESA, CSA, and L. Hustak (STScI); Science: The JWST Transiting Exoplanet Community Early Release Science Team.

An international team is behind the set of new papers, with hundreds of scientists analyzing the data. Although carbon dioxide had been found in pre-JWST observations, it appears in the new data at higher resolution, even as sodium, potassium and water vapor detections confirmed earlier findings from other instruments. Methane and hydrogen sulfide do not appear. We also have the first detection in an exoplanet atmosphere of sulfur dioxide, produced from chemical reactions to the star’s light.

Thus we learn of a role for photochemistry in the atmosphere of a star-hugging gas giant. Natalie Batalha is an astronomer at UC-Santa Cruz who was deeply involved in the new WASP-39B observational effort:

“We observed the exoplanet with multiple instruments that, together, provide a broad swath of the infrared spectrum and a panoply of chemical fingerprints inaccessible until JWST. Data like these are a game changer.”

The sulfur dioxide detection is worth pausing on, as it may offer insights into planet formation. The passage below is from Shang-Min Tsai et al., one of the five papers now available in preprint form:

The accessibility of sulphur species in exoplanet atmospheres through the aid of photochemistry allows for a new window into planet formation processes, whereas in the Solar System gas giants, the temperature is sufficiently low that sulphur is condensed out as either H2S clouds or together with NH3 as ammonium hydrosulphide (NH4SH) clouds making it more difficult to observe. Sulphur has been detected in protoplanetary discs where it may be primarily in refractory form. As such, sulphur may not undergo the level of processing inherent in the evolution of more volatile species, making it a preferred reference element when tracing the formation history of solar system objects through analysis of elemental ratios. Such efforts for warm giant exoplanets are now a possibility thanks to the observability of photochemically produced SO2. The improved constraints on bulk planetary metallicity provided by the observable SO2 feature further provides information on planet formation histories such as the accretion of solid material.

Image: New observations of WASP-39b with the JWST have provided a clearer picture of the exoplanet, showing the presence of sodium, potassium, water, carbon dioxide, carbon monoxide and sulfur dioxide in the planet’s atmosphere. This artist’s illustration also displays newly detected patches of clouds scattered across the planet. Credit: Melissa Weiss/Center for Astrophysics | Harvard & Smithsonian.

All this is excellent news as we ponder the implications for JWST’s capabilities in relation to small, terrestrial worlds. The work is part of a program called Director’s Discretionary-Early Release Science (DD-ERS), which is designed to help the scientific community quickly get up to speed with the capabilities of the new instrument. Judging from these results, we can expect a string of new insights into nearby exoplanetary systems. Just what will JWST uncover, for example, in the TRAPPIST-1 system?

The two papers among the five that I’ve had the chance to go through are Tsai et al., “Direct Evidence of Photochemistry in an Exoplanet Atmosphere” (preprint) and Alderson et al., ”Early Release Science of the Exoplanet WASP-39b with JWST NIRSpec G395H” (preprint). The other three papers are: Feinstein et al., “Early Release Science of the exoplanet WASP-39b with JWST NIRISS (preprint); Ahrer et al., “Early Release Science of the exoplanet WASP-39b with JWST NIRCam (preprint); and Rustamkulov et al., “Early Release Science of the exoplanet WASP-39b with JWST NIRSpec PRISM” (abstract).


Interstellar Probe: Prospects for ESA Technologies

The Interstellar Probe concept being developed at Johns Hopkins Applied Physics Laboratory is not alone in the panoply of interstellar studies. We’ve examined the JHU/APL effort in a series of articles, the most recent being NASA Interstellar Probe: Overview and Prospects. But we should keep in mind that a number of white papers have been submitted to the European Space Agency in response to the effort known as Cosmic Vision and Voyage 2050. One of these, called STELLA, has been put forward to highlight a potential European contribution to the NASA probe beyond the heliosphere.

Image: A broad theme of overlapping waves of discovery informs ESA’s Cosmic Vision and Voyage 2050 report, here symbolized by icy moons of a gas giant, an temperate exoplanet and the interstellar medium itself, with all it can teach us about galactic evolution. Among the projects discussed in the report is NASA’s Interstellar Probe concept. Credit: ESA.

Remember that Interstellar Probe (which needs a catchier name) focuses on reaching the interstellar medium beyond the heliosphere and studying the interactions there between the ‘bubble’ that surrounds the Solar System and interstellar space beyond. The core concept is to launch a probe explicitly designed (in ways that the two Voyagers currently out there most certainly were not) to study this region. The goal will be to travel faster than the Voyagers with a complex science payload, reaching and returning data from as far away as 1000 AU in a working lifetime of 50 years.

But note that ‘as far away as 1000 AU’ and realize that it’s a highly optimistic stretch goal. A recent paper, McNutt et al., examined in the Centauri Dreams post linked above, explains the target by saying “To travel as far and as fast as possible with available technology…” and thus to reach the interstellar medium as fast as possible and travel as far into it as possible with scientific data return lasting 50 years. From another paper, Brandt et al. (citation below) comes this set of requirements:

  • The study shall consider technology that could be ready for launch on 1 January 2030.
  • The design life of the mission shall be no less than 50 years.
  • The spacecraft shall be able to operate and communicate at 1000 AU.
  • The spacecraft power shall be no less than 300 W at end of nominal mission.

This would be humanity’s first mission dedicated to reaching beyond the Solar System in its fundamental design, and it draws attention across the space community. How space agencies work together could form a major study in itself. For today, I’ll just mention a few bullet points: ESA’s Faint Object Camera (FOC) was aboard Hubble at launch, and the agency built the solar panels needed to power up the instrument. The recent successes of the James Webb Space Telescope remind us that it launched with NIRSpec, the Near-InfraRed Spectrograph, and the Mid-InfraRed Instrument (MIRI), both contributed by ESA. And let’s not forget that JWST wouldn’t be up there without the latest version of the superb Ariane 5 launcher, Ariane 5 ECA. Nor should we neglect the cooperative arrangements in terms of management and technical implementation that have long kept the NASA connection with ESA on a productive track.

Image: This is Figure 1 from Brandt et al., a paper cited below out of JHU/APL that describes the Interstellar Probe mission from within. Caption: Fig. 1. Interstellar Probe on a fast trajectory to the Very Local Interstellar Medium would represent a snapshot to understand the current state of our habitable astrosphere in the VLISM, to ultimately be able to understand where our home came from and where it is going.

So it’s no surprise that a mission like Interstellar Probe would draw interest. Earlier ESA studies on a heliopause probe go back to 2007, and the study overview of that one can be found here. Outside potential NASA/ESA cooperation, I should also note that China is likewise studying a probe, intrigued by the prospect of reaching 100 AU by the 100th anniversary of the current government in 2049. So the idea of dedicated missions outside the Solar System is gaining serious traction.

But back to the Cosmic Vision and Voyage 2050 report, from which I extract this:

The great challenge for a mission to the interstellar medium is the requirement to reach 200 AU as fast as possible and ideally within 25-30 years. The necessary power source for this challenging mission requires ESA to cooperate with other agencies. An Interstellar Probe concept is under preparation to be proposed to the next US Solar and Space Physics Decadal Survey for consideration. If this concept is selected, a contribution from ESA bringing the European expertise in both remote and in situ observation is of significance for the international space plasma community, as exemplified by the successful joint ESA-NASA missions in solar and heliospheric physics: SOHO, Ulysses and Solar Orbiter.

I’m looking at the latest European white paper on the matter, whose title points to what could happen assuming the JHU interstellar probe concept is selected in the coming Heliophysics Decadal Survey (as we know, this is a big assumption, but we’ll see). The paper, “STELLA—Potential European contributions to a NASA-led interstellar probe,” appeared recently in Frontiers of Astronomy and Space Science (citation below), highlighting possible European contributions to the JHU/APL Interstellar Probe mission, and offering a quick overview of its technology, payload and objectives.

As mentioned, the only missions to have probed this region from within are the Voyagers, although the boundary has also been probed remotely in energetic neutral atoms by the Interstellar Boundary Explorer (IBEX) as well as the Cassini mission to Saturn. We’d like to go beyond the heliosphere with a dedicated mission not just because it’s a step toward much longer-range missions but also because the heliosphere itself is a matter of considerable controversy. Exactly what is its shape, and how does that shape vary with time? Sometimes it seems that our growing catalog of data has only served to raise more questions, as is often the case when pushing into territories previously unexplored. The white paper puts it this way:

The many and diverse in situ and remote-sensing observations obtained to date clearly emphasize the need for a new generation of more comprehensive measurements that are required to understand the global nature of our Sun’s interaction with the local galactic environment. Science requirements informed by the now available observations drive the measurement requirements of an ISP’s in situ and remote-sensing capabilities that would allow [us] to answer the open questions…

We need, in other words, to penetrate and move beyond the heliosphere to look back at it, producing the overview needed to study these interactions properly. But let’s pause on that term ‘interstellar probe.’ Exactly how do we characterize space beyond the heliosphere? Both our Voyager probes are now considered to be in interstellar space, but we should consider the more precise term Very Local Interstellar Medium (VLISM), and realize that where the Voyagers are is not truly interstellar, but a region highly influenced by the Sun and the heliosphere. The authors are clear that even VLISM doesn’t apply here, for to reach what they call the ‘pristine VLISM’ demands capabilities beyond even the interstellar probe concept being considered at JHU.

Jargon is tricky in any discipline, but in this case it helps to remember that we move outward in successive waves that are defined by our technological capabilities. If we can get to several hundred AU, we are still in a zone roiled by solar activity, but far enough out to draw meaningful conclusions about the heliosphere’s relationship to the solar wind and the effects of its termination out on the edge. In these terms, we should probably consider JHU/APL’s Interstellar Probe as a mission toward the true VLISM. Will it still be returning data when it gets there? A good question.

IP is also a mission with interesting science to perform along the way. A spacecraft on such a trajectory has the potential for flybys of outer system objects like dwarf planets (about 130 are known) and the myriad KBOs that populate the Kuiper Belt. Dust observations at increasing distances would help to define the circumsolar dust disk on which the evolution of the Solar System has depended, and relate this to what we see around other stars. We’ll also study extragalactic background light that should provide information about how stars and galaxies have evolved since the Big Bang.

Image: A visualization of Interstellar Probe leaving the Solar System. Credit: European Geosciences Union, Munich.

The white paper offers the range of outstanding science questions that come into play, so I’ll send you to it for more but ultimately to the latest two analytical descriptions out of JHU/APL, which are listed in the citations below. To develop instruments to meet these science goals would involve study by a NASA/ESA science definition team, and of course depends on whether the Interstellar Probe concept makes it through the Decadal selection. It’s interesting to see, though, that among the possible contributions this white paper suggests from ESA is one involving a core communications capability:

One of the key European industrial and programmatic contributions proposed in the STELLA proposal to ESA is an upgrade of the European deep space communication facility that would allow the precise range and range-rate measurements of the probe to address STELLA science goal Q5 [see below] but would also provide additional downlink of ISP data and thus increase the ISP science return. The facility would be a critical augmentation of the European Deep Space Antennas (DSA) not only for ISP but also for other planned missions, e.g., to the icy giants.

Q5, as referenced above, refers to testing General Relativity at various spatial scales all the way up to 350 AU, and the authors note that less than a decade after launch, such a probe would need a receiving station with the equivalent of 4 35-meter dishes, an architecture that would be developed during the early phases of the mission. On the spacecraft itself, the authors see the potential for providing the high gain antenna and communications infrastructure in a fully redundant X-band system that represents mature technology today. I’m interested to see that they eschew optical strategies, saying these would “pose too stringent pointing requirements on the spacecraft.”

STELLA makes the case for Europe:

The architecture of the array should be studied during an early phase of the mission (0/A). European industries are among the world leaders in the field. mtex antenna technology. (Germany) is the sole prime to develop a production-ready design and produce a prototype 18-m antenna for the US National Research Observatory (NRAO) Very Large Array (ngVLA) facility. Thales/Alenia (France/Italy), Schwartz Hautmont (Spain) are heavily involved in the development of the new 35-m DSA antenna.

As the intent of the authors is to suggest possible European vectors for collaboration in Interstellar Probe, their review of key technology drivers is broad rather than deep; they’re gauging the likelihood of meshing areas where ESA’s expertise can complement the NASA concept, some of them needing serious development from both sides of the Atlantic. Propulsion via chemical methods could work for IP, for example, given the options of using heavy lift vehicles like NASA SLS and the possibility, down the road, of a SpaceX Starship or BlueOrigin vehicle to complement the launch catalog. The availability of such craft coupled with a passive gravity assist at Jupiter points to a doubling of Voyager’s escape velocity, reaching 7.2 AU per year. (roughly 34 kilometers per second).

As to power, NASA is enroute to bringing the necessary nuclear package online via the Next-Generation Radioisotope Thermoelectric Generator (NextGen RTG) under development at NASA Glenn. But improvements in communications at this range represent one area where European involvement could play a role, as does reliability of the sort that can ensure a viable mission lasting half a century or more. Thus:

Development and implementation of qualification procedures for missions with nominal lifetimes of 50 years and beyond. This would provide the community with knowledge of designing long-lived space equipment and be helpful for other programs such as Artemis.

This area strikes me as promising. We’ve already seen how spacecraft never designed for missions of such duration have managed to go beyond the heliosphere (the Voyagers), and developing the hardware with sufficient reliability seems well within our capabilities. Other areas ripe for further development are pointing accuracy and deep space communication architectures, thus the paper’s emphasis on ESA’s role in refining the use of integrated deep space transponders for Interstellar Probe.

Whether the JHU/APL Interstellar Probe design wins approval or not, the fact that we are considering these issues points to the tenacious vitality of space programs looking toward expansion into the outer Solar System and beyond, a heartening thought as we ponder successors to the Voyagers and New Horizons. The ice giants and the VLISM region will truly begin to reveal their secrets when missions like these fly. And how much more so if, along the way, a propulsion technology emerges that reduces travel times to years instead of decades? Are beamed sails the best bet for this, or something else?

The paper is Wimmer-Schweingruber et al., “STELLA—Potential European contributions to a NASA-led interstellar probe,” a whitepaper that was submitted to NASA’s 2023/2024 decadal survey based on a proposal submitted to the European Space Agency (ESA) in response to its 2021 call for medium-class mission proposals. Frontiers in Astronomy and Space Sciences, 17 November 2022 (full text).

For detailed information about Interstellar Probe, see McNutt et al., “Interstellar probe – Destination: Universe!” Acta Astronautica Vol. 196 (July 2022), 13-28 (full text) as well as Brandt et al., “Interstellar Probe: Humanity’s exploration of the Galaxy Begins,” Acta Astronautica Volume 199 (October 2022), pages 364-373 (full text).


KOBE: The Hunt for Habitable Zone K-dwarf Planets

From the standpoint of producing interesting life, K-dwarf stars look intriguing. Our G-class Sun is warm and cozy, but its lifetime is only about 10 billion years, while K-dwarfs (we can also call them orange dwarfs) can last up to 45 billion years. That’s plenty of time for evolution to work its magic, and while G-stars make up only about 6 or 7 percent of the stars in the galaxy, K-dwarfs account for three times that amount. We have about a thousand K-dwarfs within 100 light years of the Solar System.

When Edward Guinan (Villanova University) and colleague Scott Engle studied K-dwarfs in a project called “GoldiloKs,” they measured the age, rotation rate, and X-ray and far-ultraviolet radiation in a sampling of mostly cool G and K stars (see Orange Dwarfs: ‘Goldilocks’ Stars for Life?). Their work took in a number of K-stars hosting planets, including the intriguing Kepler-442, which has a rocky planet in the habitable zone. Kepler-442b is where we’d like it to be in terms of potential habitability, but it’s twice as massive as Earth, and it also raises the question of why it seems rare.

In other words, why do we have so few habitable zone planet detections around K-dwarfs? A new paper from astronomers with the European Southern Observatory points out that we have only a small number of such worlds at present. Have a look at the table below to see what I mean. Here we’re looking at known planets, which means either confirmed or validated, that are found within the ‘optimistic’ habitable zone of K-dwarfs with an effective temperature between 3800 K and 4600 K. ESO’s Jorge Lillo-Box and his fellow researchers go so far as to declare this lack of habitable zone worlds ‘the K-dwarf habitable zone desert’ in the paper, which has just appeared in Astronomy & Astrophysics (citation below).

Image: This is Table 1 from the paper, showing confirmed or validated K-dwarf planets in the habitable zone known as of May 2022.

A few points about all this. First of all, note the distinction above between ‘confirmed’ and ‘validated’ planets, two terms that are all too often conflated but mean different things. With the original Kepler mission (before the K2 extended mission), a planet candidate was a transit signal that passed various false-positive tests. A ‘validated’ planet is one that has been studied in follow-up observations and determined quantitatively to be more likely an exoplanet than a false positive. Thus ‘validated’ means a planet about which confidence of its existence is higher than a simple ‘candidate.’ Confirmation, as for example through radial velocity study of the transiting planet’s mass, is the final step in turning a validated planet into a confirmed one.

So what’s happening in the K-dwarf desert, or more to the point, why is the desert there in the first place? Expectations are high, for example, that a K-dwarf like Centauri B might host a planet in the habitable zone, such orbits being allowed even in this close binary system. The authors point out that the reason may simply be lack of observation. There is understandable emphasis on G-class stars because they are like the Sun, while M-dwarfs are highly studied because planets there are more readily detectable. What is needed is a dedicated program of K-dwarf observations.

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 15 to 45 billion years. And, our Sun only lasts for 10 billion years. Red dwarfs make up the bulk of the Milky Way’s population, about 73%. Sunlike stars are merely 6% 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, sometimes called orange dwarf stars. Credit: NASA, ESA, and Z. Levy (STScI).

Enter KOBE, standing for K-dwarfs Orbited By habitable Exoplanets. This is a survey, introduced by Lillo-Box and team, that monitors the radial velocity of 50 pre-selected K-dwarfs using the CARMENES spectrographs mounted on the 3.5m telescope at the Calar Alto Observatory in southern Spain. Given the capabilities of the instruments and planet occurrence expectations, the team believes it will find 1.68 ± 0.25 planets per star, with about half of these likely to be planets in the habitable zone.

The choice of K-dwarfs is interesting in various ways. I’ve focused on this class of star recently because while G-class stars like the Sun offer obvious analogies to our own Solar System, their size puts habitable zone orbits far enough from the star that a radial velocity campaign to detect them takes years. Bear in mind as well that, as I learned from this paper, G-class stars produce a lot more stellar noise than K-dwarfs, making detections more problematic, even with instruments like ESPRESSO.

With M-dwarfs as well, we run into intrinsic problems. They tend to be active stars, so that working at the levels needed to detect habitable zone planets likewise means extracting data from the noise (not to mention the effect of flares on potential habitability!) We’ll continue to put a lot of emphasis on M-dwarfs because with habitable zones closer to their smaller stars, any planets there are quite detectable, but given the advantages of K-dwarf detection, an effort like KOBE is well justified.

The authors make the case this way:

K-dwarfs have their HZ located at longer periods (typically 50-200 days), where planets can have their rotation and orbital periods decoupled, thus allowing the planet to have day-night cycles. Stellar activity and magnetic flaring is dramatically diminished for stars earlier than M3 and specially in the late K-type domain… Consequently, habitability is not threatened by these effects as much as it is in the HZ planets around M dwarfs. Besides, unlike in M-dwarfs, we can derive, in a standard way, precise and reliable stellar parameters, as well as chemical abundances that are relevant to a proper characterization of the planets and the star-planet connection…

Indeed, K-dwarfs show UV and X-ray radiation levels 5 to 50 times smaller when they are young than early M-dwarfs, an interesting point re habitability prospects. And as the authors note, this type of star offers ‘the best trade-off’ to detect biosignature molecules through direct imaging using next-generation space observatories.

So I’m all for KOBE and similar efforts that may arise to populate our catalog of habitable planets around this interesting kind of star. Because it focuses on the detection of new worlds in the habitable zone, KOBE rules out stars that have already had exoplanets discovered around them or are highly monitored by other surveys. The effort runs through 2024, and if things go according to expectation, we should wind up with about 25 new planets in the so-called K-dwarf habitable zone desert.

Image: The 3.5m telescope at Calar Alto Observatory under the Milky Way. Credit: CAHA.

It’s always useful to delve into anomalies that seem to be the result of observational biases in getting a more accurate picture of the systems in the stellar neighborhood. And while it’s a southern sky object and thus out of range of KOBE’s efforts at Calar Alto, I still have high hopes for Centauri B, the closest K-dwarf to Earth…

The paper is Lillo-Box et al., “The KOBE experiment: K-dwarfs Orbited By habitable Exoplanets Project goals, target selection, and stellar characterization,” Astronomy & Astrophysics Vol. 667 (15 November 2022) A102. Full text.


Simultaneous Growth of Planet & Star?

I’m interested in a new paper on planet formation, not only for its conclusions but its methodology. What Amy Bonsor (University of Cambridge) and colleagues are drawing from their data is how quickly planets can form. We’ve looked numerous times in these pages at core accretion models that explain the emergence of rocky worlds and gravitational instability models that may offer a way of producing a gas giant. But how long after the formation of the circumstellar disk do these classes of planets actually appear?

A planet like the Earth poses fewer challenges than a Jupiter or Saturn. Small particles run into each other within the gas and dust disk surrounding the young star, assembling planets and other debris through a process of clumping that eventually forms planetesimals that themselves interact and collide. Thus core accretion: The planet ‘grows’ in ways that are readily modeled and can be observed in disks around other stars.

But the gas giants still pose problems. Core accretion would suggest the growth of a solid core that gradually draws in mass until a dense atmosphere enshrouds it. But the core accretion process, according to the latest models, takes long enough that by the time it has finished, the disk is depleted. Gas giants are primarily made of hydrogen and helium, but these gases disappear from the disk in relatively short order. So a gas giant’s formation has to occur quickly and early, before the needed hydrogen and helium are blown off by radiation from the young star or consumed by it.

Gas giants around M-dwarfs may be one route to follow here, for core accretion seems to operate more slowly in M-dwarf systems, and we almost have to call in relatively short-order clumping – this is the disk instability model championed by Alan Boss – to explain how such worlds could form. So one observational path into gas giant formation is to look for such worlds around smaller stars, where their presence would indicate a model other than core accretion at work. But Bonsor and team have chosen an ingenious alternate route: They’re probing the atmospheres of white dwarf stars.

Bonsor points out that “[s]ome white dwarfs are amazing laboratories, because their thin atmospheres are almost like celestial graveyards.” Exactly so, because so-called ‘polluted’ white dwarfs show clear signs of heavy elements like magnesium, iron and calcium in their atmospheres, and the assumption is that these elements must be the result of small bodies within the stellar system falling into the parent star. So the method here is to use spectroscopic observations to probe the composition of asteroids that are long gone, but whose traces help us chart the conditions of their formation.

Studying the atmospheres of more than 200 polluted white dwarfs, the researchers found that the elements there can only be explained by the infall of asteroids that have undergone differentiation. In other words, they have gone through the process of melting, with iron sinking into their core while lighter elements rise to the surface. Amy Bonsor explains:

“The cause of the melting can only be attributed to very short-lived radioactive elements, which existed in the earliest stages of the planetary system but decay away in just a million years. In other words, if these asteroids were melted by something which only exists for a very brief time at the dawn of the planetary system, then the process of planet formation must kick off very quickly.”

Image: This is Figure 3 from the paper. Caption: The core- or mantle-rich materials in the atmospheres of white dwarfs are the collision fragments of planetesimals that formed earlier than ?1 Myr, when large-scale melting was fueled by the decay of 26Al. Alternatively, in the most massive, close-in, highly excited, planetesimal belts, catastrophic collisions between Pluto-sized bodies (anything with D > 1, 400 km) could supply most smaller planetesimals. Gravitational potential energy during accretion can fuel large-scale melting and core formation in these large bodies, such that almost all planetary bodies in the belt are the collision fragments of core–mantle differentiated bodies. tMS , tGB and tWD refer to the star’s main-sequence, giant branch lifetimes and the start of the white dwarf phase. Credit: Bonsor et al.

The researchers’ simulations of planetesimal and collisional evolution show that short-lived radioactive nuclides like Aluminium-26 (26Al) are the most likely heat source to explain the accreted iron core or mantle material. From the paper:

The need for enhanced abundances of 26Al to explain core- or mantle-rich white dwarf spectra provides distinct evidence for the early formation of planetesimals in exoplanetary systems contemporaneously with star formation. Rapid planetesimal formation offers an explanation for the difference in mass budgets between Class 0, I and II discs [6]. Our findings point to the growth of large, > 10 km-sized planetesimals, potentially even planetary cores, rather than just the coagulation of pebbles. The earlier planetary cores form, the more likely they are to grow to the pebble isolation mass and the more likely giant planet formation is to occur early-on, which can provide an explanation for substructures commonly observed with ALMA.

Early planet formation helps explain how gas giant planets form and seems to put pressure on gravitational instability models, although there may be multiple routes to the same result. But it is striking that the researchers, who include scientists at Oxford, the Ludwig-Maximilians-Universität in Munich, the University of Groningen and the Max Planck Institute for Solar System Research, Gottingen, find evidence for the early formation of planetesimals “contemporaneously with star formation.”

Thus star and planet formation begin concurrently, under this model, with planets evolving during the collapse of the circumstellar disk. I’ve always found white dwarfs fascinating, but that we might probe the origins of stellar systems by analyzing the composition of their atmospheres is remarkable. It points to the continuing vitality of work on this class of star for understanding both planetary and stellar evolution.

The paper is Bonsor et al. “Rapid formation of exoplanetesimals revealed by white dwarfs,”’ Nature Astronomy 14 November 2022. Abstract.