by Paul Gilster | Oct 8, 2021 | Missions |
The ongoing Interstellar Probe study at the Johns Hopkins University Applied Physics Laboratory reminds us of the great contribution of the Voyager spacecraft, but also of the need to develop their successors. Interstellar flight is a dazzling goal considered in the long term, but present technologies develop incrementally and missions to other stars are a multi-generational goal. But as we continue that essential effort with projects like Interstellar Probe, we can also make plans to explore objects from other stellar systems (ISOs) closer to home.
I refer of course to the appearance in the last three years of two such objects, 1I/’Oumuamua and 2I/Borisov, the ‘I’ in their names referencing the exciting fact that these are interstellar in nature, passing briefly through our system before moving on. Papers have begun to appear to examine missions to one or the other of these objects, or to plan how, with sufficiently early discovery, we could get a spacecraft to the next one. And keep in mind the ESA’s Comet Interceptor mission, which sets its sights on a long-period comet but could be used for an ISO.
Are missions to interstellar objects possible with near-term technology? A new paper from lead author Andreas Hein (Initiative for Interstellar Studies) and an international team of researchers answers the question in the affirmative. The paper characterizes such missions by the resources required to perform them, which in turn relates to the ISO’s trajectory. Unbound ISOs — those that pass through our system only once — can be contrasted with bound objects that have remained in the Solar System after their entry. If the ISO is unbound, a mission launched before perihelion would have the best chance of producing data and perhaps sample return.
Image: An artist’s impression of 2I/Borisov, an interstellar comet. Credit: NRAO/AUI/NSF, S. Dagnello.
In previous papers, Hein and team have considered chemical propulsion complemented by a reverse gravity assist at Jupiter and a Solar Oberth Maneuver to reach 1I/’Oumuamua, although they have also looked at thermal nuclear propulsion with gravity assist at Jupiter. Uncertainties in the object’s orbit are challenging but, the authors believe, surmountable through the use of a telescope like that of New Horizons (LORRI) or, a highly speculative idea, a swarm of chipsats that could be launched ahead of the probe to refine navigational data. This approach goes well beyond existing technology, though, as the authors acknowledge by citing the work on Breakthrough Starshot’s laser architecture, which is a long way from realization.
I’m also concerned about that notion of a Solar Oberth Maneuver, given what we’ve learned recently in connection with the research on Interstellar Probe, for the kind of spacecraft described here to intercept 1I/’Oumuamua would carry the needed upper stage kick engine, along with the heat-shield technology Interstellar Probe has been investigating. All this adds to mass. The authors believe Falcon Heavy (or, unlikely, a future SLS) would be up to the challenge, but I think the proposed Solar Oberth Maneuver at 6 solar radii is a problematic goal in the near-term.
The authors echo these sentiments in terms of the perihelion burn itself as well as the navigation issues to reach the ISO which will ensue. A propulsive burn at perihelion for a probe trying to intercept an interstellar object is a long way from proven technology, particularly when we’re hoping to deliver a substantial instrument package to the ISO for science return. The authors call for developing nuclear thermal propulsion in order to make a wider range of ISOs reachable without relying on the Oberth maneuver.
The paper usefully offers a taxonomy of interstellar objects, matched to their associated science and conceivable mission types. Objects with low inclinations, low hyperbolic escape velocity (v?), and those discovered well before perihelion are the most reachable targets. Of course, this survey of options for reaching an ISO isn’t intended to be specific to a given object but applicable to many, suggesting what is possible with present and near-term technologies. In the discussion of a mission to 1I/’Oumuamua, the authors also note the wide range of details that need to be considered:
Our brief analysis (and its attendant caveats) should not be regarded as exhaustive. Other issues that we have not delineated include the difficulties posed by long CCD exposure times (11 hours in our scenario) such as the cumulative impact of cosmic rays and the necessity of accounting for parallax motion of the object during this period. Obstacles with respect to measuring the position of the object, calculating offsets, and relaying it to the spacecraft may also arise. Hence, we acknowledge that there are significant (but not necessarily insurmountable) and outstanding challenges that are not tackled herein, as they fall outside the scope of this particular paper.
In any event, 1I/’Oumuamua may be quite a tricky object to catch at this juncture even for this kind of fast flyby. Objects detected earlier in their entry into our system should present a much more workable challenge, and with the Vera Rubin Observatory coming into play, we are probably going to be finding many more of them, some well before perihelion. Hence the need to know what is possible for future operations at ISOs, ensuring we have a plan and resources available to fly when we next have the opportunity.
A rendezvous mission may one day be in the cards, with the authors relying on electric or magnetic sail propulsion schemes to allow the spacecraft to slow down and study the target at close hand. But it may be more reasonable to consider rendezvous with captured interstellar objects in bound elliptical orbits. These are missions which are examined here in relation to two potential ISOs (not yet confirmed as such), (514107) Ka’epaoka’awela, a Jupiter co-orbital in retrograde orbit, and the Centaur 2008 KV42. The paper examines rendezvous strategies and provides trajectories for multiple years. 2008 KV42, for example, should be reachable for rendezvous with launch in 2029 and a flight duration of 15 years.
Finally, nuclear thermal technologies should allow sample return from some interstellar objects using a pre-positioned interceptor at the Sun/Earth L2 point. The paper considers an interceptor mission to comet C/2020 N1, serving as a surrogate for particular types of ISOs. The spacecraft, using nuclear thermal or solar electric propulsion, would deploy an impactor on approach to the object and travel through the plume, perhaps using swarm subprobes to return samples to the main craft depending on whether or not the plume is thought likely to be hazardous.
Even without nuclear thermal capability, though, missions can be flown to some types of interstellar objects with technologies that are currently in use. From the paper:
Our results indicate that most mission types elucidated herein, except for sample return, could be realized with existing technologies or modified versions of existing technologies, such as chemical propulsion and a Parker Solar Probe-type heat shield (Hein et al., 2019; Hibberd et al., 2020). Collisions with dust, gas, and cosmic rays and spacecraft charging in the interplanetary or interstellar medium will engender deflection of the spacecraft trajectory and cause material damage to it, but both effects are likely minimal even at high speeds (Hoang et al., 2017; Hoang & Loeb, 2017; Lingam & Loeb, 2020, 2021), and the former can be corrected by onboard thrusters.
So we learn that missions to interstellar objects are feasible, with some fast flyby scenarios capable of being accomplished with today’s technologies. Rendezvous and sample return missions await the maturation of solar electric and nuclear thermal propulsion. Here the concept ‘near-term’ is speculative. When will we have nuclear thermal engines available for this kind of mission? I am speaking in a practical sense — we know a great deal about nuclear thermal methods, but when will we deploy workable engines at a high enough Technology Readiness Level to use?
There is much we could learn from an ISO intercept, whether a flyby, a rendezvous or a sample return. Given that we are a long way from being able to sample interstellar objects in other stellar systems (I doubt seriously we’ll have this capability in a century’s time), ISOs represent our best bet to discover the structure and composition of extrasolar objects. This and the capability of doing interplanetary dust and plasma science along the way should be enough to keep such missions under active study as our new generation telescopes come online.
The paper is Hein et al., “Interstellar Now! Missions to Explore Nearby Interstellar Objects,” in press at Advances in Space Research (abstract / preprint).
by Paul Gilster | Oct 6, 2021 | Missions |
I notice that the question of ‘when to launch’ has surfaced in comments to my first piece on Interstellar Probe, the APL study to design a spacecraft that would be, in effect, the successor to Voyager. It’s a natural question, because if a craft takes 50 years to reach 1000 AU, there will likely be faster spacecraft designed later that will pass it in flight. I’m going to come down on the side of launching as soon as possible rather than anticipating future developments.
Two reasons: The research effort involved in stretching what we can do today to reach as high a velocity as possible inevitably moves the ball forward. We learn as we go, and ideas arise in the effort that can hasten the day of faster spacecraft. The second reason is that a vehicle like Interstellar Probe is hardly passive. It does science all along its route. By the time it reaches 1000 AU, it has returned massive amounts of information about the interstellar medium, our Sun’s passage through it, and the heliosphere that protects the Solar System.
All of that is germane to follow-on missions, and we have useful science data all the way. So I’m much in favor of pushing current technology into stretch missions even as we examine how to go faster still with the next iteration, the one that would succeed Interstellar Probe.
Getting Up to Speed
How fast can we travel now, as compared to 1977, when we launched Voyagers 1 and 2? We know we can reach 17 kilometers per second with 1977 technology because that is what Voyager 1 is doing right now. Interstellar Probe advocates would like to see something in the range of 95 kilometers per second as a way of making the 1000 AU journey in 50 years. That’s still, I suppose, within the lifetime of a researcher, but not by much, and it’s heartening to me that we’re extending the boundaries into a frank admission of the fact that some missions may be launched by one generation, maintained by another, and brought home by a third.
I always assumed we had an ace up our sleeves when it came to ramping up Voyager speed levels. Moving close to the Sun and making a propulsive burn at just the right moment seemed a sure way to exploit that deep gravity well and fling a probe outward at high velocity. The idea first appeared in Hermann Oberth’s Wege zur Raumschiffahrt (Paths to Spaceflight), which was published in 1929 in Germany. At the time, Oberth was also working as a consultant on the Fritz Lang film Frau im Mond (The Woman in the Moon), which would popularize the idea of rocketry and space travel. In fact, Oberth would dedicate Wege zur Raumschiffahrt to Lang and actress and screenwriter Thea von Harbou.
The authors of the Interstellar Probe 2019 report note in their extremely useful appendices that Oberth’s thinking on the maneuver that would be named after him anticipated in many ways the idea of using a gravity assist that was developed in the 1960s by Michael Minovitch. His thought experiment involved an astronaut on an asteroid 900 AU from the Sun. The astronaut, apparently quite long-lived, wants to go to a star some 1015 kilometers away (roughly the distance of Regulus). His asteroid has an orbital speed of 1 km/s and an orbital period of 27,000 years.
I won’t go into this in huge detail because it’s laid out so well in the report’s appendix (available here). But Oberth’s setup is that the target star is in the orbital plane of the asteroid, and he assumes the astronaut has a rocket that can produce a velocity change of 6 km/s. Sun, asteroid and target star are in a line in that order. He asks: What is the fastest way to reach the star?
Using the rocket alone reaches it in 5,555,000 years. Waiting for 20,000 years to add the asteroid’s orbital velocity to the velocity of the ship reduces that to 4,760,000 years. But Oberth realizes that the best answer is to use the rocket to move opposite to the asteroid’s motion, falling in toward the Sun to reach 500 km/s at perihelion, then using the remaining rocket fuel to boost the speed a bit further. He ultimately gets 70.9 km/s moving out of the Solar System, and his transit time is now reduced to 470,000 years. Thus the ‘Oberth maneuver’ enters the literature.
A spacecraft launched from Earth has to lose the heliocentric angular momentum of Earth’s orbit to fall toward the Sun in order to make the Oberth maneuver possible, the most efficient method being a direct trajectory from Earth to Jupiter, a retrograde gravity assist at Jupiter, and a long fall back to perihelion, at which point a kick-stage provides the further propulsive burn. All of this, including of course the thermal issues raised by putting the payload into such proximity to the Sun, has to be weighed against a straight gravity assist at Jupiter, with no close solar pass, when contemplating how best to accelerate the Interstellar Probe for the journey.
Image: This is an image of Parker Solar Probe as envisioned by Goddard Media Studios at NASA’s Goddard Space Flight Center in Maryland. It’s the closest I could come to what a close solar pass would look like, though it lacks the propulsive element of the Oberth maneuver. Credit: NASA GSFC.
Oberth in Today’s Terms
When I contacted Interstellar Probe principal investigator Ralph McNutt (JHU/APL) about these issues, he pointed out that the Mission Concept Report for the entire project would be made available on the probe website in the first week of December. Putting what the report will describe as the Solar Oberth Maneuver (SOM) through the severe filter of engineering capabilities with today’s technologies is a major priority of this report, and the results McNutt conveyed make it clear that my enthusiasm for the concept has been unjustified.
Unjustified, that is, in terms of a spacecraft being designed, as this one must be, around current technologies. Remember that we’re talking about a mission with a specific timeframe, one with a launch in the early 2030s, meaning that the materials and techniques to build and fly it have to be within range today. The Oberth maneuver at the Sun may have possibilities for us down the road. But today’s engineering constraints make the issues stark. As McNutt told me in an email:
…after a very careful look and relying on the same people, including the mission system engineer, who worked the thermal protection system (TSA) for Parker Solar Probe (PSP) we have concluded (1) the SOM offers no advantage over prograde gravity assists in rapid escape from the solar system for a “technology horizon” in the 2030’s and (2) there is no obvious “path” to changing this conclusion for the foreseeable future.
Image: Ralph L. McNutt Jr., chief scientist for Space Science at the Johns Hopkins University Applied Physics Laboratory and principal investigator for Interstellar Probe. Credit: Johns Hopkins University.
In other words, going to Jupiter straightaway, with no Oberth maneuver, is just as workable, and as we’ll see, avoids a series of thorny problems. One issue is the need for thermal protection, another the demand of launching a payload sufficiently large, one that would incorporate not only the propulsive stage for operations at perihelion preceding the long cruise, but would also include the science instrument package and the necessary high gain antenna that would be needed for data downlink at the distances the probe is envisioned to reach. We have to work within the constraints of present-day launch systems as well as existing engines for the kick.
On thermal issues, the Interstellar Probe team worked with Advanced Ceramic Fibers, an Idaho-based company, on ultra-high temperature material studies, the question being how one could take existing thermal protection as found on the current Parker Solar Probe mission and extend it into the range needed for the Solar Oberth Maneuver. But shield mass, said McNutt, is only one consideration. A ‘ballast’ mass is also required to keep the center of gravity moving along the engine centerline as the propellant burns down during the maneuver.
These issues of mass are critical. Let me quote McNutt again:
The real problem is the mass of the thermal shield assembly – multiple shields plus the supporting structure – to shield just the kick stage itself, even with no Interstellar Probe spacecraft. We’ve adopted solid rocket motors (SRMs) with specific impulses approaching 300s with masses of up to ~4,000 kg (Orion 50XL). In that case, we have an engineering solution that closes on paper, has all of the design margins included, would require specialized design work (> ~10’s of millions and multiple years of dedicated effort) and ends up with about the same performance (flight distance after 50 years) as a prograde Jupiter gravity assist, but with significantly more inherent risk, both in development and in the actual execution of the burn at the Sun itself. Bottom line: it may be doable with an investment of significantly more time and money, but it offers no advantage, and, therefore, we have concluded it would be a poor trade.
Within the upcoming report will be the 181 staging scenarios the team examined by way of reaching its conclusions about the Solar Oberth Maneuver. It becomes clear from the synopsis that McNutt gave me that existing technologies are simply not up to speed to realize the potential of the SOM, and even extending the technologies forward to nuclear rocket engines and greatly enhancing the performance of today’s launch vehicles would not change this fact. To make the Oberth maneuver at the Sun into a viable option, it appears, would take decades of work and demand billions of dollars in new investment. Best to shelve Oberth’s concept for this mission, though I suspect that future technologies will keep the concept in play.
Where to next with Interstellar Probe? If we rule out Oberth, then the two scenarios involving a Jupiter gravity assist remain, the team having considered other options including solar sails and finding them not ready within the needed timeframe. The first is a ‘passive’ flyby, in which every rocket stage is fired in an optimized launch sequence. The second is a powered gravity assist, in which a final kick-stage is reserved for use at Jupiter. We will see what the upcoming report has to say about these options, balancing among outbound speed, complexity, and mass.
by Paul Gilster | Oct 5, 2021 | Missions |
Our doughty Voyager 1 and 2, their operations enabled by radioisotope power systems that convert heat produced by the decay of plutonium-238 into electricity, have been pushing outward through and beyond the Solar System since 1977. Designed for a four and a half year mission, we now have, more or less by accident and good fortune, our first active probes of nearby interstellar space. But not for long. At some point before the end of this decade, both craft will lack the power to keep any of their scientific instruments functioning, and one great chapter in exploration will close.
What will the successor to the Voyagers look like? The Johns Hopkins University Applied Physics Laboratory (JHU/APL) has been working on a probe of the local interstellar medium. We’re talking about a robotic venture that would be humanity’s first dedicated mission to push into regions that future, longer-range interstellar craft will have to cross as they move far beyond the Sun. If it flies, Interstellar Probe would be our first mission designed from the start to be an interstellar craft.
Pontus Brandt is an Interstellar Probe Concept Study project scientist, in addition to being principal investigator for two instruments aboard the European Space Agency’s Jupiter Icy Moon Explorer (JUICE) Mission. Brandt puts the ongoing work in context in a recent email:
Interstellar Probe would represent Humanity’s first deliberate step into interstellar space and go farther and faster than any spacecraft before. By using conventional propulsion, Interstellar Probe would travel through the boundaries of the protective heliosphere into the unknown interstellar cloud for the first time. Within its lifetime, it would push far beyond the Voyager mission to explore the heliospheric boundary and interstellar space so that we can ultimately understand where our home came from, and where we are going.
Image: A possible operation scenario, divided into phases and indicating science goals along the way. Credit: JHU/APL, from the Interstellar Probe 2019 Report.
The nature of the interstellar cloud Brandt refers to is significant. But before examining it, a bit of background. APL’s role in Interstellar Probe has roots in principal investigator Ralph McNutt’s tireless advocacy of what was once called Innovative Interstellar Explorer, a report originally funded by NASA in 2003 and often discussed in these pages. The current study began in 2018 and will continue through early 2022, examining the technologies that would make Interstellar Probe possible, with an eye on the coming Decadal Survey within NASA’s Heliophysics Science Division. Bear in mind as well that the space community has been discussing what we can call ‘interstellar precursor’ missions all the way back to the 1960s — an interesting story in itself! — and the Interstellar Probe concept appeared in the 2003 and 2013 Heliophysics Decadal Surveys.
About those Decadals: Every ten years, Decadal Surveys appear for the four NASA science mission divisions: Planetary Science, Astrophysics, Heliophysics and Earth Science, the idea being to provide guidance for the agency’s science program going forward. So the immediate context of the current effort at APL is that it is being conducted to provide technical input that can feed into the next Heliophysics Decadal Survey, which will cover the years 2023 to 2032. But the implications for science across all four divisions are part of APL’s remit, affecting specific targets and payloads.
What can realistically be done within the 2023-2032 time frame? And what kind of science could a mission like this, launching perhaps in 2030, hope to accomplish? Workshops began in June of 2018 and continue to refine science goals and support engineering trade studies in support of what the team calls “a ‘pragmatic’ interstellar probe mission.” The most recent of these, the fourth, just concluded on October 1. You can see its agenda here.
A launch in the early 2030s demands not futuristic technologies now in their infancy but proven methods that can be pushed hard in new directions. This is, you might say, ‘Voyager Plus’ rather than the starship Enterprise, but you build interstellar capability incrementally absent unexpected breakthroughs. That calls for a certain brute force determination to keep pushing boundaries, something Ralph McNutt and team have been doing at APL, to their great credit , for many years now. A spacecraft like this would be a flagship mission (now known as a Large Strategic Science Mission) — these are the most ambitious missions the agency will fly, a class that has included the Voyagers themselves, Cassini, Hubble and the James Webb Space Telescope.
A variety of methods for reaching beyond the heliosphere in the shortest possible time have been under consideration, including an “Oberth maneuver” (named after scientist Hermann Oberth, who documented it in 1929), where a propulsive burn is performed during a close solar pass that has itself been enabled by a retrograde Jupiter gravity assist. Other Jupiter flyby options, with or without a propulsive burn via a possible upper stage, remain on the table. The plan is to drive the probe out of the Solar System at speeds sufficient to reach the heliopause in 15 years. The participating scientists talk in terms of a flyout speed of 20 AU/year, which translates to 95 kilometers per second. Voyager 1, by comparison, is currently moving at roughly 17.1 kilometers per second.
The Voyagers own our current distance records, with Voyager 1 currently at 154 AU and Voyager 2 at 128 AU. Interstellar Probe would still be returning science at 1000 AU, meaning it would be capable of looking back and seeing not just the Earth in the context of the Solar System, as in Voyager’s ‘pale blue dot’ image, but also taking measurements of the heliosphere from well outside it, helping us understand both the interstellar medium and the effect of our stellar system as it moves through it.
There is much to be learned about the protective magnetic bubble called the heliosphere in which the entire Solar System is embedded. We have to understand that it is anything but static, as Pontus Brandt explains:
During its evolutionary journey around the galaxy, [the Sun] has plowed through widely different environments, witnessed supernova explosions on its path, that have all shaped the system that we live in today. The vast differences in interstellar densities, speeds, charge fractions have been responsible for an extreme range of sizes and shapes of the global heliosphere throughout its history – from many times bigger than today, to a tiny heliosphere below even the orbit of Earth. This, in turn, has had dramatic consequences for the penetration of the primordial soup of interstellar material that have affected several crucial aspects of elemental and isotopic abundances, atmospheric evolution, conditions for habitability and perhaps even biological evolution. Only some 60, 000 years ago, the Sun entered the vast Local Interstellar Cloud (some 30 light years across), and in just a few thousand years the solar system will enter a completely different interstellar cloud that will continue to shape its evolution.
Image: The Sun is on the way to exiting the Local Interstellar Cloud and entering another unexplored interstellar region. Credit: NASA/Goddard/Adler/U. Chicago/Wesleyan.
An interstellar precursor mission can examine energetic neutral atoms (ENAs) to provide data on the overall shape of the heliosphere. Major issues include how plasma from the Sun’s solar wind interacts with interstellar dust to form and continue to shape the heliosphere.
But a mission like this also shapes our views of time, as the Voyagers have done as we have watched their progress through the Solar System, the heliosphere and beyond. Mission scientists turned the 4.5 year mission into a surprising 45 year one solely on the strength of their design and the quality of their components, not to mention the unflagging efforts of the team that operates them. A mission designed from the start for 50 years, as Interstellar Probe would be, will likely have a lifetime far beyond that. Its components are meant to be functional when our grandchildren are in their dotage. Most of its controllers in 2080 have yet to be born.
So this is a multi-generational challenge, a reach beyond individual lifetimes. Let me quote from the Interstellar Probe Study 2019 Report, which is now available online.
It is important to note that the study does not purport to center on “the one and only” interstellar probe but rather on this mission as a first step to more advanced missions and capabilities… In addition to promising historically groundbreaking discoveries, the Interstellar Probe necessitates a transformation in the programmatics needed to accommodate lifetime, reliability, and funding requirements for this new type of multigenerational, multi-decade operational mission. Paving the way for longer journeys utilizing future propulsion technologies, such as those not invoked here, the Interstellar Probe is the first explicit step we take today on the much longer path to the stars.
Principal investigator Ralph McNutt tells me that the Interstellar Probe team is finishing up a Mission Concept Report for NASA on the progress thus far, incorporating results of the recent workshop. This report should be available on the Interstellar Probe website in early December, with a number of items clarifying aspects of the currently available 2019 report. We need to dig into some of the issues that will appear there, for the concept is changing as new studies emerge. In particular, let’s look next time at the ‘Oberth maneuver’ idea, what it means, and whether it is in fact a practical option. I’m surprised at what’s emerging on this.
by Paul Gilster | Oct 1, 2021 | Exoplanetary Science |
I was interested in yesterday’s story about the two super-Earths around nearby M-dwarfs — TOI-1634b and TOI-1685b — partly because of the research that follows. In both cases there is the question of atmospheres. The two TESS planets are so numbingly close to their host stars that they may have lost their original hydrogen/helium atmospheres in favor of an atmosphere sustained by emissions from within. Hearteningly, we should be able to find out more with the James Webb Space Telescope, on which ride the hopes of so many exoplanet researchers.
Today’s system is the intriguing L 98-59, only 35 light years from Earth and possessed of at least four planets, with a fifth as yet unconfirmed. Here we have two rocky inner worlds, a possible ocean planet (L 98-59 d) and another likely rocky world to the inside of the habitable zone boundary. Perhaps within the habitable zone, if it exists, is L 98-59f, so this is a system to keep an eye on, an obvious candidate as a JWST target.
At UC Riverside, Daria Pidhorodetska wants to know whether small, rocky planets orbiting M-dwarfs like this one have atmospheres. The paper is devoted to the question of whether either Hubble or JWST, perhaps working in tandem, could detect atmospheres in this system. The authors proceeded to model four different types of atmospheres to answer the question with regard to the three inner planets.
Image: This is an infographic from the European Southern Observatory showing a comparison between the L 98-59 exoplanet system (top) with part of the inner Solar System (Mercury, Venus and Earth), highlighting the similarities between the two. L 98-59 contains four confirmed rocky planets (marked in color in the top panel), orbiting a red-dwarf star 35 light-years away. The planet closest to the star is around half the mass of Venus, making it the lightest exoplanet ever detected using the radial velocity technique. Up to 30% of the third planet’s mass could be water, making it an ocean world. The existence of the fourth planet has been confirmed, but scientists don’t yet know its mass and radius (its possible size is indicated by a dotted line). The team also found hints of a potential fifth planet, the furthest from the star, though the team knows little about it. If confirmed, it would sit in the system’s habitable zone where liquid water could exist on its surface. Credit: ESO/L. Calçada/M. Kornmesser (Acknowledgment: O. Demangeon). [Note: The distances from the stars and between the planets in the infographic are not to scale. The diagram has been scaled to make the habitable zone in both the Solar System and in L 98-59 coincide].
A major problem for M-dwarf planets, to go along with tidal lock, is the fact that during their formation, they are bathed in intense ultraviolet radiation. Enough so that the potential is there to cause any water at the surface to evaporate, while their atmospheres would be under a fierce barrage and might not survive. The question for Pidhorodetska and team is, then, whether the two inner rocky planets have lost their atmospheres completely, if they had one, or if they have been able to replenish them.
The range of atmospheric scenarios takes in planets with atmospheres dominated by water, hydrogen, carbon dioxide, or oxygen and ozone (remaining after loss of hydrogen). The authors argue that an oxygen-dominated atmosphere is the most likely. For each of these scenarios, the authors simulated transmission spectroscopy. L 98-59’s proximity to Earth as well as the fast orbits (less than a week) of its planets speeds up the process of discovery. In fact, says Edward Schwieterman (UC Riverside):
“It would only take a few transits with Hubble to detect or rule out a hydrogen- or steam-dominated atmosphere without clouds. With as few as 20 transits, Webb would allow us to characterize gases in heavy carbon dioxide or oxygen-dominated atmospheres.”
I’m interested, though, specifically in that question of atmosphere loss, with hydrogen escape leaving oxygen and ozone behind. The paper explains:
Highly irradiated planets such as those of the L 98-59 system could have a desiccated atmospheric composition, such as one that is dominated by O2, as a result of major ocean loss during an extended runaway greenhouse phase. A desiccated planet that is rich in abiotic O2 would be expected to form O3 from the photochemical processing of O2, meaning that the direct detection of O3 absorption could be another key indicator of this planetary state.
And the authors point out in their conclusion that we can learn a great deal about the evolution of these planets depending on whether we detect water in their atmospheres. An atmosphere high in oxygen due to the loss of hydrogen during the star’s pre-main sequence phase — in other words, an atmosphere that survives utter desiccation — should have no oceans to detect. Water, or the lack of it, is another marker for this early stage of planetary evolution, and our instruments should be able to make the call.
The paper is Pidhorodetska et al., “L 98-59: A Benchmark System of Small Planets for Future Atmospheric Characterization,” Astronomical Journal Vol. 162, No. 4 (29 September 2021), 169 (full text).
