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A Path to Planet Formation in Binary Systems

How planets grow in double-star systems has always held a particular fascination for me. The reason is probably obvious: In my younger days, when no exoplanets had been discovered, the question of what kind of planetary systems were possible around multiple stars was wide open. And there was Alpha Centauri in our southern skies, taunting us by its very presence. Could a life-laden planet be right next door?

What Kedron Silsbee and Roman Rafikov have been working on extends well beyond Alpha Centauri, usefully enough, and helps us look into how binaries like Centauri A and B form planets. Says Rafikov (University of Cambridge), “A system like this would be the equivalent of a second Sun where Uranus is, which would have made our own solar system look very different.” How true. In fact, imagining how different our system would work if we had a star among the outer planets raises wonderful questions.

Could we have a habitable world around each star in such a binary? And if so, wouldn’t the incentive to develop spaceflight take hold early among the denizens of such a world? We used to imagine a habitable Mars, by stretching Percival Lowell’s observations of what Giovanni Schiaparelli described as ‘canali’ (‘channels’) to their limit. How much more would a green and blue world with clouds and oceans beckon?

Image: Artist’s impression of a hypothetical planet around Alpha Centauri B. Credit: ESO/L. Calçada/N. Risinger.

But back to Rafikov, whose paper with Silsbee (Max Planck Institute for Extraterrestrial Physics) has been accepted at Astronomy & Astrophysics. The two researchers have refined binary star planet formation through a series of simulations, with Alpha Centauri in mind as well as the tight binary Gamma Cephei, a K-class star with red dwarf companion and a planet orbiting the primary. Silsbee explains the problem they were trying to solve: How does the companion star affect the existing protoplanetary disk of the other? He adds:

“In a system with a single star the particles in the disc are moving at low velocities, so they easily stick together when they collide, allowing them to grow. But because of the gravitational ‘eggbeater’ effect of the companion star in a binary system, the solid particles there collide with each other at much higher velocity. So, when they collide, they destroy each other.”

Gamma Cephei is a case in point: The system yields planetesimal collision velocities of several kilometers per second at the 2 AU distance of the system’s known planet, which should be enough, the authors note, to destroy even planetesimals as large as hundreds of kilometers in size. This problem appears in the literature as the fragmentation barrier, and it looms large, even when taking into account the aerodynamic drag induced by the gases of the protoplanetary disk. We can expect high collision velocities here.

And there go the planetesimals, which should, according to core accretion theory, grow out of dust particles as they gradually begin to bulk up into larger solid bodies. Given that we now know about numerous exoplanets in binary systems, how did they emerge? Were they all ‘rogue’ planets that ambled into the gravitational influence of the binary pair? And if that idea seems unlikely, how then do we explain their growth?

Rafikov and Silsbee show through their simulations that given realistic processes and the mathematics to describe them, such worlds will emerge. Incorporated in the resulting model is a new look at the question of gas drag and its effects. They find that drag in the disk — Silsbee likens it to a kind of wind — can indeed alter planetesimal dynamics and can offset the gravitational influence of the nearby stellar companion.

For although a number of earlier studies included gas drag in their models, their calculations ignored the effect of disk gravity, which according to the authors changes the dynamics of the population of planetesimals. They are able to identify quiet zones in the disk in which planetesimals can grow into planets. And they believe their model fully accounts for planetesimal dynamics throughout the young system. Among their conclusions:

The gravitational effect of the protoplanetary disk plays the key role in lowering the minimum initial planetesimal size necessary for sustained growth by a factor of four. This reduction can be achieved in protoplanetary disks apsidally aligned with the binary, in which a dynamically quiet zone appears within the disk provided that the mass-weighted mean disk eccentricity ≲ 0.05…

And this:

For most disk parameters considered in this paper, planet formation in binaries such as γ Cephei can successfully occur provided that the initial planetesimal size is ≳ 10 km; however, for favorable disk parameters, this minimum initial size can go down to ≲ 1 km.

We should expect, then, that planets could form in systems like Alpha Centauri, where the hunt for worlds around the Centauri A and B pair continues. This can occur if the planetesimals can reach this minimum size, and it assumes a protoplanetary disk that is close to circular. Given those parameters, planetesimal relative velocities are slow enough in certain parts of the disk to allow planet formation to take place.

How to get planetesimals to the minimum size needed? The streaming instability model of planetesimal formation may be operational here, in which the planetesimals grow rapidly. In this model, drag in the disk slows solid particles and leads to their swift agglomeration into clumps that can gravitationally collapse. Streaming instability is a rapid alternative to the alternate theory of planetesimals growing steadily through coagulation alone. In fact, the paper cites a timescale of tens of local orbital periods, rapidly producing a population of ‘seed’ planetesimals.

Whether or not streaming instability does offer a pathway to planets is a question that is still unresolved, though the theory has implications for planet formation around single stars as well. It certainly eases formation in the binaries considered here.

The paper is Silsbee & Rafikov, “Planet Formation in Stellar Binaries: Global Simulations of Planetesimal Growth,” accepted at Astronomy and Astrophysics (abstract / preprint).

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The Case of PDS 70 and a Moon-forming Disk

The things we look for around other stars do not necessarily surprise us. I think most astronomers were thinking we’d find planets around a lot of stars when the Kepler mission began its work. The question was how many — Kepler was to give us a statistical measurement on the planet population within its field of stars, and it succeeded brilliantly. These days it seems clear that we can find planets around most stars, in all kinds of sizes and orbits, as we continue to seek an Earth 2.0..

The continuing news about the star PDS 70, a young T Tauri star about 400 light years away in Centaurus, fits the same mold. Here we’re talking not just about planets but their moons. No exomoons have been confirmed, but there seems no reason to assume we won’t begin to find them — surely the process of forming moons is as universal as that of planet formation. The interest is in the observation, how it is made, and what it implies about our ability to move forward in characterizing planetary systems.

The process takes time, and results can be ambiguous. Back in 2019, PDS 70 was the subject of work performed at Monash University (Australia), led by Valentin Christiaens. The story received an exomoon splash in the press: The researchers believed they were looking at a circumplanetary disk around one of two gas giants forming in this system (see Exoplanet Moons in Formation?).

Everything pointed to a moon-forming disk around one of two young gas giants in the system, though the conclusion could only be considered tentative. I want to mention this because work from the European Southern Observatory that we’ll discuss today also finds a circumplanetary disk at PDS 70, though not around the same still-forming planet examined in the Christiaens et al. study. What an intriguing system this is!

While Christiaens and team looked at PDS 70b, the ESO work examines new high-resolution images of the second gas giant, PDS 70c, using data obtained through the Atacama Large Millimetre/submillimetre Array (ALMA). Led by Myriam Benisty (University of Grenoble and University of Chile), the international team now declares the detection of a circumplanetary disk — though not yet a moon — unambiguous. Says Benisty:

“Our work presents a clear detection of a disk in which satellites could be forming. Our ALMA observations were obtained at such exquisite resolution that we could clearly identify that the disk is associated with the planet and we are able to constrain its size for the first time.”

Image: This image shows wide (left) and close-up (right) views of the moon-forming disk surrounding PDS 70c, a young Jupiter-like planet nearly 400 light-years away. The close-up view shows PDS 70c and its circumplanetary disk center-front, with the larger circumstellar ring-like disk taking up most of the right-hand side of the image. The star PDS 70 is at the center of the wide-view image on the left. Two planets have been found in the system, PDS 70c and PDS 70b, the latter not being visible in this image. They have carved a cavity in the circumstellar disk as they gobbled up material from the disk itself, growing in size. In this process, PDS 70c acquired its own circumplanetary disk, which contributes to the growth of the planet and where moons can form. This disk is as large as the Sun-Earth distance and has enough mass to form up to three satellites the size of the Moon. Credit: ALMA (ESO/NAOJ/NRAO)/Benisty et al.

The high-resolution data allow Benisty and team to state that the circumplanetary disk has a diameter of about 1 AU, with enough mass to form up to three moons the size of our own Moon. The planetary system forming around this star is reminiscent of the Jupiter and Saturn configuration in our own Solar System, though notice the size differential. The disk around PDS 70c is 500 times larger than Saturn’s rings. The two planets are also at much larger distances from the host star, and appear to be migrating inward. We are seeing the system in the process of formation, which should offer insights into how not just moons but planets themselves form around infant stars.

Interestingly, the second world here, PDS 70b, does not show evidence of a circumplanetary disk in the ALMA data. One supposition is that it is being starved of dusty material by PDS 70c, although other mechanisms are possible. Here’s a bit more on this from the paper, noting an apparent transport mechanism between disk and forming planet:

These ALMA observations shed new light on the origin of the mm emission close to planet b. The emission is diffuse with a low surface brightness and is suggestive of a streamer of material connecting the planets to the inner disk, providing insights into the transport of material through a cavity generated by two massive planets.

And as to PDS 70b:

The non-detection of a point source around PDS 70 b indicates a smaller and/or less massive CPD [circumplanetary disk] around planet b as compared to planet c, due to the filtering of dust grains by planet c preventing large amount of dust to leak through the cavity, or that the nature of the two CPDs differ. We also detect a faint inner disk emission that could be reproduced with small 1 µm dust grains, and resolve the outer disk into two substructures (a bright ring and an inner shoulder).

The Monash University team in Australia was able to image PDS 70b in the infrared and, like the ESO astronomers, was able to find a spiral arm seeming to feed a circumplanetary disk, while making the case for PDS 70b as the world with the disk. Remember that the two teams were working with different instrumentation and at different wavelengths — the Monash researchers operated at infrared wavelengths to analyze the spectrum of the planet produced by SINFONI (Spectrograph for INtegral Field Observations in the Near Infrared) at the Very Large Telescope in Chile. The ESO team used data from ALMA.

So do we have one or two circumplanetary disks in this system? We’ll see how this is resolved as the investigation of the planets around PDS 70 continues through a variety of instruments. For the importance of the system is clear, as the Benisty paper argues:

Detailed studies of the circumplanetary disks, and of the leakage of material through the cavity, will provide strong constraints on the formation of satellites around gas giants, and on the ability to provide the mass reservoir needed to form terrestrial planets in the inner regions of the disk. Upcoming studies of the gas kinematics and chemistry of PDS 70 will complement the view provided by this work, serving as a benchmark for models of satellite formation, planet-disk interactions and delivery of chemically enriched material to planetary atmospheres.

The paper is Benisty et al., “A Circumplanetary Disk around PDS70c,” Astrophysical Journal Letters Vol. 916, No. 1 (22 July 2021). Abstract.

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Milan M. Ćirković’s work has been frequently discussed on Centauri Dreams, as a glance in the archives will show. My own fascination with SETI and the implications of what has been called ‘the Fermi question’ led me early on to his papers, which explore the theoretical, cultural and philosophical space in which SETI proceeds. And there are few books in which I have put more annotations than his 2018 title The Great Silence: The Science and Philosophy of Fermi’s Paradox (Oxford University Press). Today Dr. Ćirković celebrates Stanislaw Lem, an author I first discovered way back in grad school and continue to admire today. A research professor at the Astronomical Observatory of Belgrade, (Serbia), Ćirković obtained his PhD at the Dept. of Physics, State University of New York in Stony Brook in 2000 with a thesis in astrophysical cosmology. He tells me his primary research interests are in the fields of astrobiology (habitable zones, habitability of galaxies, SETI studies), philosophy of science (futures studies, philosophy of cosmology), and risk analysis (global catastrophes, observation selection effects and the epistemology of risk). He co-edited the widely-cited anthology Global Catastrophic Risks (Oxford University Press, 2008) with Nick Bostrom, has published three research monographs and four popular science/general nonfiction books, and has authored about 200 research and professional papers.

by Milan Ćirković

This year we celebrate a centennial of the birth of a truly great author and thinker who is still, unfortunately, insufficiently well-known and read. Stanislaw Lem was born in 1921 in then Lwów, Poland (now Lviv, Ukraine). That was the year Čapek’s revolutionary drama R.U.R. premiered in Prague’s National Theatre and defined the word “robot”, Albert Einstein was awarded the Nobel Prize in physics for his work on the photoelectric effect in the course of which he effectively discovered photons, and one Adolf Hitler became the leader of a small far-right political party in Weimar Germany.

All three of these central-European developments have exerted a strong influence on Lem’s life and career. His studies of medicine, inspired by both his father’s distinguished medical career and his early-acquired mechanistic view of human beings, have been interrupted three times due to the chaos of WW2 and post-war changes. He narrowly escaped being executed by German authorities during the war for his resistance work. Finally, when he was on the verge of acquiring a diploma at the famous Jagiellonian University of Krakow, in 1949, he abandoned the pursuit in order to avoid the compulsory draft to which physicians were susceptible in the new communist Poland. He did some practical medical work in a maternity ward, but very quickly left medicine for good and became a full-time writer.

The apex of Lem’s creative career spans about three decades, from The Investigation published in 1958, to the publication of Fiasco and Peace on Earth in 1987. During that period, he published his greatest novels, in particular Solaris (1961), The Invincible (1966), His Master’s Voice (1968), and The Chain of Chance (1976), along with numerous short story anthologies, the most important being The Cyberiad (1965), as well as the Ijon Tichy and Pilot Pirx story cycles.

Image: Polish science fiction writer Stanislaw Lem. Credit: Wojciech Zemek.

Finally, several works in the Borgesian meta-genre of imaginary forewords, introductions, and book reviews, notably The Perfect Vacuum of 1971. This has been complemented by very extensive non-fiction writing, mainly in several fields of philosophy of science, futures studies, and literary criticism. The last two decades of Lem’s life were characterized by essayistic and publishing activity, as well as receiving innumerable prizes and awards, but no original fiction writing. Lem passed away peacefully on March 27, 2006, at the age of 84 in his home in Krakow.

Lem was obssessed by the theme of Contact: from his very first science-fiction novel, The Astronauts in 1951 (which he himself denounced as “childish”) to the last, great and deeply disturbing Fiasco, which is a kind of literary and philosophical testament. Nowhere, however, is his thought more in touch with the practical aspects of our SETI/search for technosignatures projects as in His Master’s Voice (originally published in 1968, that is only 8 years after the original Ozma Project! Translated into English by Michael Kandel only in 1983).

It is a brilliant work, perhaps the best novel ever written about SETI, but also a dense tract indeed. So, instead of many examples, I shall concentrate upon this one as a case study for the tremendous usefulness of reading Lem for anyone interested in astrobiology/SETI studies.

The study of the motives and ideas relevant for these fields would require a book-length treatment, as is obvious from the list of auxiliary topics Lem masterfully weaves into the narrative: from the ontological status of mathematical objects to the psyche of the Holocaust survivors, from preconditions for abiogenesis to the origin of the arrow of time. It is a challenging text in more than one sense; there is almost no dialogue and no manifest action beyond the recounting of a SETI project that not only failed but was never truly comprehended in the first place.

Image: A 1983 English edition of His Master’s Voice from Harcourt Brace Jovanovich, one of many editions available worldwide.

And this is a book whose plot should not be spoilt, since it is not as widely read as it should be half a century later. Without revealing too much, His Master’s Voice is set at a time when neutrino astrophysics is advanced enough to be able to detect possible modulations (imagined to have occurred near the end of the 20th century in the continued Cold War world). A neutrino signal repeating every 416 hours is discovered from a point in the sky within 1.5° of Alpha Canis Minoris. An eponymous top-secret project is then formed in order to decrypt the extraterrestrial signal, burdened by all the Cold-War paranoia and heavy-handed bureaucracy of the second half of the twentieth century. The project has its ups and downs, including some quite dramatic and literally threatening the survival of human civilization, but it is—obviously—mostly unsuccessful. The protagonist, a mathematical genius and cynic named Peter Hogarth, is neither a hero nor a villain; the SETI plot ends in anticlimactic uncertainty.

An intriguing consequence of Lem’s scenario is a realization that, while detectability generally increases with the progress of our astronomical detector technology, it does so very unevenly, in jumps or bursts. Although the powerful source of the “message” in the novel (presumably an alien beacon) had been present for a billion years or more, it became detectable only after a sophisticated neutrino-detecting hardware was developed. And even then, the detection of the signal happened serendipitously. Thus, in a rational approach to SETI—not often followed in practice, alas—the issue of detectability should be entirely decoupled from the issue of synchronization (the extent to which other intelligent species are contemporary to us).

Fermi’s paradox does not figure explicitly in His Master’s Voice (in contrast to many other of Lem’s works, especially his late and in my opinion equally magnificent Fiasco), and for an apparently obvious reason: “the starry letter” has always been here, or at least long enough on geological timescales. Detectability is, at least in part, a function of historical human development.

And there is a very real possibility, in the context of the plot, that “the letter” does not originate with intentional beings at all. The fulcrum of the book is reached when three radical hypotheses are presented to weary researchers, including the one attributing the signal to purely natural astrophysical processes! But even in this revisionist case, there are other problems, especially in light of the fact that the signal manifests “biophilic” properties: it helps complex biochemical reactions, and scientists in the novel speculate about whether it helped the abiogenesis on Earth. If it did so, the same necessarily occurred on many other planets in the Galaxy, so even if we abstract the mysterious Senders, it is natural to ask: where are our peers? This leads to more severe versions of Fermi’s paradox. In the same time, it makes us think about the various forms directed panspermia could, in fact, take when we reject our anthropocentric thinking.

There is another key lesson. While the discovery of even a single extraterrestrial artefact (and Lem’s neutrino message can surely be regarded as an artefact in the sense of the contemporary search for technosignatures), would be a great step forward, it would not, at least not immediately, resolve the problem. If one could conclude, as some of the protagonists of His Master’s Voice do, that there exist just two civilizations in the Galaxy, us and the mysterious Senders, that would still require explanation. Two is, in this particular context, sufficiently close if not equal to one.

And this shows, finally, the true gift of Lem’s thought to astrobiology and SETI studies: a capacity to go one step beyond in strangeness, to kick us sufficiently strongly out of the grooves of conventional thinking, to disturb us—and offend us, if necessary—and make us reject the comfortable and usual and mundane. In a general sense, all philosophy should do the same for us; that it usually does not is indeed discouraging and depressing. From time to time, however, a thinker passes with a bright torch illuminating the path and indicating how clueless we in fact are.

Lem was just such a figure. Reading him is indeed the highest form of celebration of reason and wisdom.

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Radial Velocity: NEID Spectrograph Goes to Work

The NEID spectrograph has passed the Operational Readiness Review necessary for final acceptance and regular operations. Developed by NASA and the National Science Foundation’s NN-EXPLORE exoplanet science program, it has been put through a lengthy commissioning process in the five years since the radial velocity planet hunter design was selected. NEID is mounted on the WIYN 3.5m telescope at Kitt Peak National Observatory in Arizona, and we now have word that its scientific mission has begun.

Image: Sunset over Kitt Peak National Observatory during NEID commissioning in January 2020. Credit: Paul Robertson.

As a radial velocity instrument, NEID is all about the tugs one or more planets exert on the host star, as measured radially — toward Earth, then away from it — during the planets’ orbits. The Doppler shift in the star’s light contains the information. That these are exquisitely tiny measurements should be obvious. Jupiter induces a 13 meter per second wobble on our star, but the Earth only manages to induce a wobble of 9 centimeters per second. NEID’s single measurement precision is already better than 25 centimeters per second, making it an excellent addition to the toolkit for finding new worlds.

Image: A schematic of the Doppler effect: as the star wobbles under the gravitational influence of its planets, NEID measures the resulting wavelength shifts in its spectrum. Credit: NEID team.

It’s a tribute to the effort behind NEID that the team had to work through COVID-related shutdowns, in essence forcing them to start the commissioning process over, and as the NEID blog notes, for two winters in a row, they had to endure 12-hour nights of observing for up to a week at a time to get the job done. Nice work!

From December of 2020 to April of 2021, a series of experiments tested the reliability of the instrument, its precision and its limitations, making measurements of Doppler-stable stars to analyze the spectrometer’s limiting velocity measurement precision, as recounted in this blog entry by team members, from which this:

What we learned is that across a wide variety of targets, and in a wide variety of conditions, NEID offers radial velocity measurement precision that rivals the best facilities in the world. Our measurements of stable stars consistently show variability less than 1 meter per second. This on-sky stability reflects a combination of noise sources, including the instrument, statistical fluctuations (so-called “photon noise”), and the star’s inherent atmospheric variability. Thus, while it is hard to pin down an exact number, we are assured that NEID’s instrument-limited measurement precision is significantly better than 1 meter per second.

NEID seems on course to complement other high precision spectrographs like HARPS (High Accuracy Radial Velocity Planet Searcher), which is installed at the European Southern Observatory’s 3.6m telescope at La Silla Observatory in Chile, and its successor ESPRESSO (Echelle Spectrograph for Rocky Exoplanet- and Stable Spectroscopic Observations). How far can these instruments push into the centimeters-per-second range, so crucial for finding Earth-class planets?

Image: NEID radial velocity measurements of the quiet star tau Ceti. Our on-sky measurements are stable to better than 50 centimeters per second, which indicates the instrument itself is even more stable. Credit: NEID Team.

The NEID team also plans to use a smaller solar telescope in combination with NEID during the day, its express purpose to gather data to produce better machine learning algorithms with which to separate the signal of planets from ‘starspots’ on the target star. These can confound detection efforts by mimicking a planet’s signature. The solar observations will be released publicly to help scientists address the problem, with data processing coordinated by the NASA Exoplanet Science Institute (NExScI) at Caltech/IPAC. The data will be made available through the NEID science archive.

For one direction NEID goes next, we can turn to Andrea Lin (Penn State), who designed and built the solar telescope. Lin explains her own choice of targets:

“The solar telescope was a fun project to work on. I look forward to using NEID for my doctoral dissertation research. One of my planned projects with NEID is to look for planets around K-dwarfs. These stars line up incredibly well with NEID’s capabilities, and the radial velocity method in general, so I’m hoping to discover some small—hopefully terrestrial!—planets around nearby K-stars.”

For further background on NEID, see the Centauri Dreams article New Entry in High Precision Spectroscopy.

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Can Life Survive a Star’s Red Giant Phase?

If we ever find life on a planet orbiting a white dwarf star, it will be life that has emerged only after the red giant phase has passed and the white dwarf has emerged as a stellar relic. That’s the conclusion of a study being discussed today at the National Astronomy Meeting of Britain’s Royal Astronomical Society, which convened online due to COVID concerns. The work is also recently published in Monthly Notices of the Royal Astronomical Society.

At issue is the damage caused by powerful stellar winds that occur as a star makes the transition from red giant to white dwarf stage. This is the scenario that awaits our own Sun, which should swell to red giant status in roughly five billion years, eventually becoming a dense white dwarf about the size of the Earth. We’ve speculated in these pages about life surviving this phase of stellar evolution, but the study, in the hands of Dimitri Veras (Warwick University) concludes that this is all but impossible.

We know that the Earth is protected by a magnetosphere that thwarts atmospheric depletion by channeling harmful particles along magnetic field lines. You would think that a magnetosphere would ease this kind of erosion in the far future for those planets that have one (Mars, for example, does not) but the stellar winds of the evolving star will be far stronger than the Sun’s today. The authors modeled the winds from eleven different kinds of stars in a range of masses. They find this:

The plot shows that an exo-Jovian analogue would just reach the threshold for hosting a magnetopause at some point during giant branch evolution. However, much higher fields would be required to maintain any magnetopause throughout these giant branch phases. For terrestrial and potentially habitable planets, any protection previously afforded by the magnetosphere would effectively disappear. This lack of protection, compounded with orbital expansion and varying stellar luminosities, suggest that life would be challenged to survive throughout the giant branch phases of stellar evolution.

Could scenarios emerge in which moons around the gas giants maintain life under an ice crust? It’s hard to see how. Veras, working with Aline Vidotto (Trinity College, Dublin) points out that a habitable zone supporting liquid water would move from some 150 million kilometers from the Sun to up to 6 billion kilometers, pushing it beyond the orbit of Neptune. Planets can migrate during this phase, but the paper argues that the habitable zone moves outward faster than the planet, a likely fatal threat. Thus life around a white dwarf will need to start over.

Image: An illustration of material being ejected from the Sun (left) interacting with the magnetosphere of the Earth (right). When the Sun evolves to become a red giant star, the Earth may be swallowed by our star’s atmosphere, and with a much more unstable solar wind, even the resilient and protective magnetospheres of the giant outer planets may be stripped away. MSFC / NASA. Licence type Attribution (CC BY 4.0).

Thus the movement of the habitable zone outward and the difficulty in maintaining a magnetosphere throughout this phase of stellar evolution make preserving habitability extremely unlikely. The authors’ model shows that the strong stellar wind combines with the expanding orbits of surviving planets to first shrink and then expand the magnetosphere of a planet over time. It would take a magnetic field 100 times stronger than Jupiter’s to maintain a stable magnetosphere all the way through the transition of red giant to white dwarf:

“We find that a planetary magnetosphere will always be quashed at some point during the giant branch phases, unless the planet’s magnetic field strength is at least two orders of magnitude higher than Jupiter’s current value.”

And afterwards? White dwarfs do not emit stellar winds, so that threat disappears. Any life we find around a white dwarf will doubtless have developed during the white dwarf phase. If such exists, we may be able to detect its biomarkers through future space missions — recall that white dwarfs are roughly the size of the Earth, and a transiting planet would produce profound transit depth and would seemingly be an ideal target for transmission spectroscopy, in which we analyze the components of a planetary atmosphere as starlight passes through it.

Most of the exoplanets we know about orbit main sequence stars, but about 100 are known to orbit red giants, and at least four have been found orbiting white dwarf stars. These worlds are survivors of stellar evolution and thus useful as benchmarks in tracing the lifetime of their systems. Two of the white dwarf planets, says Veras, are close to their star’s habitable zone, an indication of planet migration showing that an Earth-sized planet could exist in such an orbit. And he adds:

“These examples show that giant planets can approach very close to the habitable zone. The habitable zone for a white dwarf is very close to the star because they emit much less light than a Sun-like star. However, white dwarfs are also very steady stars as they have no winds. A planet that’s parked in the white dwarf habitable zone could remain there for billions of years, allowing time for life to develop provided that the conditions are suitable.”

The paper is Veras & Vidotto, “Planetary magnetosphere evolution around post-main-sequence stars,” Monthly Notices of the Royal Astronomical Society Vol. 506, Issue 2 (September 2021), pp. 1697-1703. Abstract / Preprint.

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The Io Trigger: Radio Waves at Jupiter

Our recent discussion about Europa (Europa: Below the Impact Zone) has me thinking about those tempting Galilean moons and the problems they present for exploration. With a magnetic field 20,000 times stronger than Earth’s, Jupiter is a radiation generator. Worlds like Europa may well have a sanctuary for life beneath the ice, but exploring the surface will demand powerful radiation shielding for sensitive equipment, not to mention the problem of trying to protect a fragile human in that environment.

Radiation at Europa’s surface is about 5.4 Sv (540 rem), although to be sure it seems to vary, with the highest radiation areas being found near the equator, lessening toward the poles. In human terms, that’s 1800 times the average annual sea-level dose. Europa is clearly a place for robotic exploration rather than astronaut boots on the ground.

Jupiter offers up an environment where the solar wind, hurling electrically charged particles at ever-shifting velocities, interacts with the powerful magnetosphere, stretching it out almost 1000 kilometers away from the Sun. It’s essential that we learn more about the behavior of the magnetic fields generated by gas giants, and on that score new work out of Goddard Space Flight Center offers some insight. GSFC’s Yasmina Martos and team have been using the inner Galilean moon, Io, as a probe, studying what sets off one type of radio emissions known to emanate from Jupiter.

Io’s volcanoes have been observed since the first Voyager flyby, driven by internal heat as the moon experiences the gravitational pull not only of Jupiter but neighboring large moons. Gas and particles released by this activity are ionized and swiftly captured by Jupiter’s magnetic field, being accelerated along the field toward the Jovian poles.

Out of this we get decametric radio emissions (DAM) as electrons spiral in the magnetic field, waves that the Juno spacecraft’s Juno Waves Instrument has been detecting. Jupiter also produces radio waves at centimeter and decimeter wavelengths, caused by atmospheric phenomenon as well as activity in the magnetosphere apart from the Io interactions. The planet is, in fact, the noisiest radio emitter in the Solar System apart from the Sun. Homing in on the Io emissions, the GSFC work deploys a new magnetic field model with higher accuracy near the moon and targets the particular geometric configurations of planet and moon needed for Juno to detect the emissions.

Studying the radio emissions mediated by Io doesn’t help us cope with the radiation problem, but it does offer clues about this particular magnetosphere, a phenomenon we’ll come to know much better as future missions arrive. The researchers, reporting in the Journal of Geophysical Research: Planets, found that the decameter radio waves are controlled by not just the strength but the shape of Jupiter’s magnetic field. They emerge from a cone-like space thus formed, so that the spacecraft can only receive the radio signal when Jupiter’s rotation moves that cone across the instrument. The effect is similar to a lighthouse beacon sweeping out to sea.

Image: The multicolored lines in this conceptual image represent the magnetic field lines that link Io’s orbit with Jupiter’s atmosphere. Radio waves emerge from the source and propagate along the walls of a hollow cone (gray area). Juno, its orbit represented by the white line crossing the cone, receives the signal when Jupiter’s rotation sweeps that cone over the spacecraft. Credit: NASA/GSFC/Jay Friedlander.

I can remember trying to pick up radio emissions from Jupiter with my first shortwave receiving set — they’ve been a known phenomenon since 1955, detectable from Earth at between 10 and 40 MHz. What we get thanks to the Juno observations is a clarification about why decametric radio waves originating in the northern hemisphere seem more abundant than those from the southern. The paper explains for the first time the particular geometric configurations producing these effects. The authors offer up a plain language summary to go along with the paper’s abstract, from which this:

Thanks to Juno, the geometry of the magnetic field has been better constrained as waves and magnetic field data have been continuously collected within the Jovian environment since July 2016. In this study, we estimate where the radio waves generate and the energy of the electrons that generate these waves, which is up to 23 times higher than previously proposed. We ultimately demonstrate that the geometry of Jupiter’s magnetic field is a primary controller for the higher observation likelihood of radio wave groups originating in the northern hemisphere relative to those originating in the southern hemisphere.

Video: The decametric radio emissions triggered by the interaction of Io with Jupiter’s magnetic field. The Waves instrument on Juno detects radio signals whenever Juno’s trajectory crosses into the beam which is a cone-shaped pattern. This beam pattern is similar to a flashlight that is only emitting a ring of light rather than a full beam. Juno scientists then translate the radio emission detected to a frequency within the audible range of the human ear. Credit: University of Iowa/SwRI/NASA.

What a fascinating place the Jovian system is. Learning the precise locations within the magnetosphere where the decametric emissions originate helps to pin down the needed magnetic field strength and electron density to fit the Juno data. “The radio emission is likely constant,” says Martos, “but Juno has to be in the right spot to listen.”

The paper is Martos et al, “Juno Reveals New Insights Into Io‐Related Decameter Radio Emissions,” Journal of Geophysical Research: Planets (18 June 2020). Abstract.

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Huge Comet Found to be Active

An interstellar freebie like ‘Oumuamua or 2I/Borisov is priceless. We don’t need to travel light years to see it because it comes to us. Although we’re expecting to find a lot more such objects as instruments like the Vera Rubin Observatory come online, right now only two are known to have passed through our system. But only slightly less inaccessible places like the Oort Cloud also bring gifts in the form of long-period comets, and I don’t want the advent of C/2014 UN271 Bernardinelli-Bernstein to go unnoticed in these pages, given its startling size and already detected activity.

Pedro Bernardinelli (University of Pennsylvania), who along with colleague Gary Bernstein discovered the comet, estimates its nucleus as being between 100 and 200 kilometers (62 and 125 miles) long. This dwarfs Hale-Bopp, and Colin Snodgrass (University of Edinburgh) is quoted in the New York Times as saying: “With a reasonable degree of certainty, it’s the biggest comet that we’ve ever seen.”

The discovery involved reprocessing data from the Dark Energy Survey, data that had been acquired via the 4-meter Blanco instrument at Cerro Tololo in Chile between 2013 and 2019. Bernardinelli and Bernstein announced the find in June of this year.

C/2014 UN271 was at roughly the distance of Neptune when first acquired in the data, although it is now about 20 AU out, or twice the distance of Saturn. While there was no statement about activity on the comet when the discovery was announced, astronomers at Las Cumbres Observatory quickly turned their network of telescopes on the object.

Las Cumbres offers a globally distributed network of telescopes with 24-hour robotic operation that is finely tuned to detect transients. Its instruments have now revealed that C/2014 UN271 is active, with the 1-meter LCO telescope at the South African Astronomical Observatory returning images that, in keeping with the international flavor of the observing effort, drew the attention of astronomers in New Zealand.

Thus Michele Bannister (New Zealand’s University of Canterbury):

“Since we’re a team based all around the world, it just happened that it was my afternoon, while the other folks were asleep. The first image had the comet obscured by a satellite streak and my heart sank. But then the others were clear enough and gosh: there it was, definitely a beautiful little fuzzy dot, not at all crisp like its neighbouring stars!”

Comet C/2014 UN271 was indeed active, that fuzzy dot indicating a coma. Volatile ices, likely carbon dioxide and carbon monoxide, were already reacting to the scant sunlight available. The coma was active even with the comet almost three billion kilometers from the Sun. No comet has ever been observed to be active this far from Sol.

Image: Comet C/2014 UN271 (Bernardinelli-Bernstein), as seen in a synthetic color composite image made with the Las Cumbres Observatory 1-meter telescope at Sutherland, South Africa, on 22 June 2021. The diffuse cloud is the comet’s coma. Credit: LOOK/LCO.

This is one useful object, particularly since it was discovered early in its entry into the realm of the planets. It should offer astronomers over a decade of observing time. Perihelion is not expected until 2031, although when it comes, the comet will still be well beyond Saturn. Its orbit is now projected to take three million years to complete.

Image: An orbital diagram showing the path of Comet C/2014 UN271 (Bernardinelli-Bernstein) through the Solar System. The comets’ path is shown in gray when it is below the plane of the planets and in bold white when it is above the plane. Credit: NASA.

The discovery of activity on C/2014 UN271 draws attention to the LOOK Project now active at Las Cumbres. LOOK stands for LCO Outbursting Objects Key, an effort that investigates unexpected brightening of comets through burst activity and other aspects of comet evolution. Out of all this we should get a better understanding of comet outbursts, of which C/2014 UN271 is offering such a tantalizing sample.

These findings should also feed into future space missions like the European Space Agency’s Comet Interceptor, using data from this and other wide field surveys. Las Cumbres staff scientist Tim Lister explains:

“There are now a large number of surveys, such as the Zwicky Transient Facility and the upcoming Vera C. Rubin Observatory, that are monitoring parts of the sky every night. These surveys can provide alerts if one of the comets changes brightness suddenly and then we can trigger the robotic telescopes of LCO to get us more detailed data and a longer look at the changing comet while the survey moves onto other areas of the sky. The robotic telescopes and sophisticated software of LCO allow us to get images of a new event within 15 minutes of an alert. This lets us really study these outbursts as they evolve.”

That’s good news for the next interstellar object that wanders by as well. One of these days our growing network of instruments will be able to spot something like ‘Oumuamua in time for a spacecraft to pay it a visit.

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300 light years from Earth in the constellation Musca, the gas giant TYC 8998-760-1 b, along with a companion planet, orbits an infant K-class star about 17 million years old. We’re probably looking at a brown dwarf here rather than a gas giant like Jupiter, for TYC 8998-760-1 b is about 14 times Jupiter’s mass, nudging into brown dwarf territory, and it appears to be roughly three times as large, unusual for brown dwarfs. The planet’s separation from its host star is pegged at 160 AU.

An inflated atmosphere due to processes still unknown? We don’t know, but both this and the companion planet have been directly imaged. Now TYC 8998-760-1 b resurfaces through work with the European Southern Observatory’s Very Large Telescope, as reported in the latest issue of Nature. Led by first author Yapeng Zhang (Leiden University, The Netherlands), the team of astronomers detected carbon isotopes in the object’s atmosphere, showing higher than expected carbon-13 content.

Here is the image, first released in 2020, of the TYC 8998-760-1 system, showing the two young planets. One of the co-authors of the Zhang paper, Alexander Bohn (also at Leiden), worked on the earlier imaging as well. The caption is the original one from ESO accompanying the image and does not describe the later work by Zhang’s team.

Image: This image, captured by the SPHERE instrument on ESO’s Very Large Telescope, shows the star TYC 8998-760-1 accompanied by two giant exoplanets, TYC 8998-760-1b and TYC 8998-760-1c. The two planets are visible as two bright dots in the centre (TYC 8998-760-1b) and bottom right (TYC 8998-760-1c) of the frame, noted by arrows. Other bright dots, which are background stars, are visible in the image as well. By taking different images at different times, the team were able to distinguish the planets from the background stars. The image was captured by blocking the light from the young, Sun-like star (top-left of centre) using a coronagraph, which allows for the fainter planets to be detected. The bright and dark rings we see on the star’s image are optical artefacts. Credit: ESO/Bohn et al.

Different forms of the same atom, isotopes vary in the number of neutrons housed in the nucleus, so that while carbon-12 has six neutrons to go along with its six protons, carbon-13 has seven neutrons and carbon-14 has eight. The distinctions are useful because while chemical properties remain largely the same, isotopes can be distinguished as they react to different conditions and are formed in different ways.

Zhang’s team drew on the distinction between carbon-13 and carbon-12 as marked by the way each absorbs radiation at slightly different colors, using ESO’s Spectrograph for Integral Field Observations in the Near Infrared (SINFONI), mounted on the Unit 3 instrument of the VLT. Expecting about one in 70 carbon atoms to be carbon-13, they found twice the number. The planet’s formation seems to be implicated, according to co-author Paul Mollière (Max Planck Institute for Astronomy, Heidelberg):

“The planet is more than one hundred and fifty times further away from its parent star than our Earth is from our Sun. At such a great distance, ices have possibly formed with more carbon-13, causing the higher fraction of this isotope in the planet’s atmosphere today.”

Image: This is Figure 3 from the paper. Caption: The two planets inside the CO snowline denote Jupiter and Neptune at their current locations, whereas TYC 8998 b is formed far outside this regime, where most carbon is expected to have been locked up in CO ice and formed the main reservoir of carbon in the planet. We postulate that this far outside the CO snowline, the ice was 13CO-rich or 13C-rich through carbon fractionation, resulting in the observed 13CO-rich atmosphere of the planet. A similar mechanism has been invoked to explain the trend in D/H within the Solar System. Future isotopologue measurements in exoplanet atmospheres can provide unique constraints on where, when and how planets are formed. Credit: Zhang et al.

The study of isotope abundance ratios has proven significant in studying not only interstellar chemistry and star formation but also the evolution of the Solar System. While its uses in analyzing exoplanet atmospheres are in their infancy, the hope is that future work on a range of exoplanets will offer clues to their formation.

The paper is Zhang et al., “The 13CO-rich atmosphere of a young accreting super-Jupiter,” Nature 595 (14 July 2021), 370-372. Abstract.

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NEA Scout: Sail Mission to an Asteroid

Near-Earth Asteroid Scout (NEA Scout) is a CubeSat mission designed and developed at NASA’s Marshall Space Flight Center in Huntsville and the Jet Propulsion Laboratory in Pasadena. I’m always interested in miniaturization, allowing us to get more out of a given payload mass, but this CubeSat also demands attention because it is a solar sail, the trajectory of whose development has been a constant theme on Centauri Dreams.

And while NASA has launched solar sails before (NanoSail-D was deployed in 2010), NEA Scout moves the ball forward by going beyond sail demonstrator stage to performing scientific investigations of an asteroid. As Japan did with its IKAROS sail, the technology goes interplanetary. Les Johnson (MSFC) is principal technology investigator for the mission:

“NEA Scout will be America’s first interplanetary mission using solar sail propulsion. There have been several sail tests in Earth orbit, and we are now ready to show we can use this new type of spacecraft propulsion to go new places and perform important science. This type of propulsion is especially useful for small, lightweight spacecraft that cannot carry large amounts of conventional rocket propellant.”

Image: Engineers prepare NEA Scout for integration and shipping at NASA’s Marshall Space Flight Center in Huntsville, Alabama. Credit: NASA.

The spacecraft, one of several secondary payloads, has been moved inside the Space Launch System (SLS) rocket that will take it into space on the Artemis 1 mission, an uncrewed test flight. Artemis 1 will be the first time the SLS and Orion spacecraft have flown together (the previous launch was via a Delta IV Heavy). NEA Scout, which will deploy after Orion separates, has been packaged and attached to an adapter ring connecting the SLS rocket and Orion.

Once separated from the launch vehicle, NEA Scout will deploy a thin aluminized polymer sail measuring 85 square meters (910 square feet). In terms of sail deployment, we can think of the mission as part of a continuum leading to Solar Cruiser, which will feature a sail 16 times larger when it launches in 2025. Deployment will be via stainless steel alloy booms. Near the Moon, the spacecraft will perform imaging instrument calibration and use cold gas thrusters to adjust its trajectory for a Near-Earth Asteroid. The solar sail will provide extended propulsion during the approximately two year cruise to destination. The final target asteroid has yet to be selected.

Image: NASA’s NEA Scout spacecraft in Gravity Off-load Fixture, System Test configuration at NASA’s Marshall Space Flight Center in Huntsville, AL. Credit: NASA.

The pace of innovation in miniaturization is heartening. I note this from a 2019 conference paper describing the final design and the challenges in perfecting the hardware (citation below):

The figurative explosion in CubeSat components for low earth orbital (LEO) missions proved that spacecraft components could be made small enough to accomplish missions with real and demanding science and engineering objectives. Unfortunately, these almost-off-the-shelf LEO components were not readily usable or extensible to the more demanding deep space environment. However, they served as an existence proof and allowed the NEA Scout spacecraft engineering team to innovate ways to reduce the size, mass, and cost of deep space spacecraft components and systems for use in a CubeSat form factor.

Image: Illustration of NEA Scout with the solar sail deployed as it flies by its asteroid destination. Credit: NASA.

At destination, NEA Scout is to perform a sail-enabled low-velocity flyby at less than 30 meters per second, with imaging down to less than 10 centimeters per pixel, which should enlarge our datasets on small asteroids, those measuring less than 100 meters across. Says principal science investigator Julie Castillo-Rogez (JPL):

“The images gathered by NEA Scout will provide critical information on the asteroid’s physical properties such as orbit, shape, volume, rotation, the dust and debris field surrounding it, plus its surface properties.”

The more we learn about small asteroids, the better, given our need to track trajectories and potentially change them if we ever find an object on course to a possible impact on Earth.

The presentation on NEA-Scout is Lockett et al., “Lessons Learned from the Flight Unit Testing of the Near Earth Asteroid Scout Flight System,” available here.

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Europa: Below the Impact Zone

Yesterday we looked at the behavior of ice on Enceladus, a key to making long range plans for a lander there. But as we saw with Kira Olsen and team’s work, learning about the nature of ice on worlds with interior oceans has implications for other ice giant moons. This morning we look at the hellish surface environment of Europa, as high-energy radiation sleets down inside Jupiter’s magnetic field.

Europa’s surface radiation will complicate operations there and demand extensive shielding for any lander. But below the ice, that interior ocean should be shielded and warm enough to offer the possibility of life. With Europa Clipper on pace for a 2024 launch, we need to ask how the surface ice has been shaped and where we might find biosignatures that could have been churned up from below.

Tidal stresses on the ice leading to fracture are one way to force material up, but small impacts from above — debris in the Jovian system — also roil the surface. If we’re looking for potential biosignatures, we have to consider this surface churn and the effects of radiation upon what it produces. This is the subject of new work from a team led by Emily Costello (University of Hawaiʻi at Manoa), which has been studying the effects of electron radiation accelerated by Jupiter on complex molecules.

Image: The University of Hawaiʻi at Manoa’s Costello. Credit: UH Manoa.

The operative term in this paper, just published in Nature Astronomy, is impact gardening. The authors estimate through their modeling that the surface of Europa has been affected up to an average depth of 30 centimeters (about 12 inches). Over millions of years, the impacts add up even as surface material mixes with the subsurface, all bathed by radiation.

“If we hope to find pristine, chemical biosignatures, we will have to look below the zone where impacts have been gardening,” says Costello. “Chemical biosignatures in areas shallower than that zone may have been exposed to destructive radiation.”

Image: In this zoomed-in area (Figure 2) of Europa’s surface, an inset to Figure 1, a cliff runs across the middle of the image, revealing the interiors of the ridges leading up to it. The thin, bright layer at the top of the cliff is at least 20 to 40 feet (6 to 12 meters) thick. This thin surface layer, and possibly layers like it elsewhere over Europa’s surface, is where a process called “impact gardening” is thought to occur. Impact gardening is the small-scale mixing of the surface by space debris, such as asteroids and comets. Scientists are studying the cumulative effects of small impacts on Europa’s surface as NASA prepares to explore the moon with the upcoming Europa Clipper mission. New research and modeling estimate that the surface of Europa has been churned by small impacts to an average depth of about 12 inches (30 centimeters), within the layer of the surface that is visible here. Credit: NASA/JPL-Caltech.

The study looks not only at surface impacts but goes on to consider secondary impacts when debris returns to the surface after the initial strike. We learn there is a case for a particular zone on Europa — the moon’s mid- to high-latitudes — that would be less affected by radiation. In any case, a robotic lander may need to probe at least 30 centimeters down to find material unaffected by the ongoing impact gardening.

Rebecca Ghent (Planetary Science Institute, Tucson) is a co-author on the study:

“The work in this paper could provide guidance for design of instruments or missions seeking biomolecules; it also provides a framework for future investigation using higher-resolution images from upcoming missions, which would help to generate more precise estimates on the depth of gardening in various specific regions. The key parameters in this study are the impact flux and cratering rates. With better estimates of these parameters, and higher-resolution imaging resulting from upcoming missions, it will be possible to better predict the depths to which gardening has affected the shallow ice in specific regions.”

As a side note, I found when looking through Costello’s other papers that she and Ghent have done work on impact gardening at Ceres as well as Mercury and our own Moon. The paper on Ceres argues that the phenomenon is orders of magnitude less intense on Ceres than on the Moon, involving a much thinner regolith and leaving surface ice to be affected primarily by sublimation rather than impacts. It seems clear that our work on icy gas giant moons will need to take impact gardening into consideration, just as we monitor the movement of crustal ice.

The paper is Costello et al., “Impact gardening on Europa and repercussions for possible biosignatures,” Nature Astronomy 12 July 2021 (abstract). The paper on Ceres is Costello et al., “Impact Gardening on Ceres,” Geophysical Research Letters 11 April 2021 (abstract).

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