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Back into the Clouds of Venus

We’re a long way from knowing what is going on in terms of possible life in the clouds of Venus, but one thing is already clear: The phosphine signature, as well as its implications, is going to be thrashed out in the journals, as witness a new study from Rakesh Mogul (Cal Poly Pomona, Pomona, CA) and colleagues that looks at data from the Pioneer-Venus Large Probe Neutral Mass Spectrometer (LNMS), dating back to the Pioneer Venus Multiprobe mission in 1978. These data seem to support the presence of phosphine, while leaving its origin unknown.

But Clara Sousa-Silva (Harvard-Smithsonian Center for Astrophysics), who was involved in the earlier phosphine work led by Jane Greaves at Cardiff University (see What Phosphine Means on Venus), subsequently examined data collected in 2015 at Mauna Kea and found no sign of phosphine. And now we have another paper, this one submitted to Science by Ignas Snellen and team (Leiden University), that carries its message in the title: “Re-analysis of the 267-GHz ALMA observations of Venus: No statistically significant detection of phosphine.”

Finally, let me mention a study led by Arijit Manna (Midnapore City College, West Bengal, India) reporting on a possible detection of the amino acid glycine in the Venusian clouds. I’ve given citations for all of these papers below.

There are all kinds of reasons for data discrepancies on phosphine depending on its possible distribution in Venus’ atmosphere, so until we get further information, we’re left to speculate. But let’s welcome Venus back into the spotlight. I’m glad to see the re-emergence of public interest, and the fortunes of Venus in terms of future mission desirability are obviously on the rise, something the Venus science community must welcome as parched desert-crossers welcome an unexpected flowing spring.

Image: Venus from the perspective of the Japanese Akatsuki probe. Credit: JAXA/ISAS/DARTS/Damia Bouic.

Some kind of biology in the Venusian clouds might even implicate Earth and the possibility of biological spread through rocky debris. While speculation continues, I’m interested in the orbital movements of the Mercury-bound BepiColombo probe, which happened to be approaching Venus in October, using the planet to bleed off velocity as it nudges into the innermost system. It will hardly resolve the matter, but Venus and BepiColombo are intimately connected not only gravitationally but thanks to the opportunity Venus offers to check out key onboard systems.

BepiColombo is actually a combination mission, including the Mercury Planetary Orbiter (MPO), constructed by the European Space Agency, and the Mercury Magnetospheric Orbiter (MMO), a product of the Japan Aerospace Exploration Agency (JAXA). The duo are currently joined but will separate into individual orbits once Mercury is attained.

Two Venus flybys are in the works, as we recently saw in these pages, and the first of these, which took place on October 15, brought the craft within 11,000 kilometers of the planet. BepiColombo is swapping some of its kinetic energy to Venus as it in turn reduces speed, with the second Venus flyby planned for August of 2021 and six close Mercury flybys before the craft enters orbit around the planet at the end of 2025. We now get the chance to test BepiColombo’s MErcury Radiometer and Thermal Infrared Spectrometer (MERTIS), which has already been tested in an earlier Earth/Moon flyby that took place in our COVID spring.

Designed to measure the spectra of rock-forming materials on Mercury’s surface, MERTIS can likewise use its infrared sensors to probe the Venusian atmosphere, with a closer approach pending in the second flyby. According to German aerospace center DLR, MERTIS is sensitive to wavelengths of 7 to 14 and 7 to 40 micrometers respectively in its two uncooled radiation sensors. Both sensors were used during the approach to Venus and approximately 100,000 individual images are expected. At the same time, the Japanese Venus orbiter Akatsuki conducted its own observations, along with Earth-based instruments both professional and amateur.

Image: BepiColombo on the long journey to Mercury. Credit: ESA/ATG Medialab.

Phosphine is a short-lived molecule, suggesting that a source on Venus or in its atmosphere is replenishing it, and scientists are still trying to find out if abiotic factors could be in play here, including the possibility of volcanism or reactions following meteorite strikes, even lightning discharges. MERTIS and five other activated instruments on the Mercury Planet Orbiter will not be able to detect phosphine from the flyby distance, but the BepiColombo team has reasons for making these observations that do not involve the gas, says Gisbert Peter, MERTIS project manager at the DLR Institute of Optical Sensor Systems, where the instrument was built:

“During the Earth flyby, we studied the Moon, characterising MERTIS in flight for the first time under real experimental conditions. We achieved good results. Now we are pointing MERTIS towards a planet for the first time. This will allow us to make comparisons with measurements taken prior to the launch of BepiColombo, to optimise operation and data processing, and to gain experience for the design of future experiments.”

Peter Wurz is project leader on STROFIO, which is a mass spectrometer designed at the University of Bern to record the atmosphere of Mercury and examine its composition. Wurz anticipates the results at Venus using not just data from STROFIO but also the spacecraft’s MIPA and PICAM instruments, likewise developed at the university.

“We are expecting data from the ionized particles in Venus’ atmosphere from these two instruments, which are switched on during the Venus flyby. The amount of particle loss and its composition can be determined using the two instruments.”

All of which is an excellent workout for BepiColombo, but MERTIS can also be expected to examine sulphur dioxide concentrations, a reduction of which was recorded about ten years ago. Meanwhile, scientists can study Venus’ atmospheric composition, with the instruments aboard the Japanese Mercury Magnetospheric Orbiter tracking its structure and dynamics. Expect no answers to the astrobiological riddle, but helpful data about Venus otherwise.

And isn’t it interesting seeing how tricky it is to get into the inner system? All those flybys point back to Mariner 10, which used an initial Mercury flyby to enable additional close passes at the planet — these calculations came from Giuseppe ‘Bepi’ Colombo, the Italian physicist for whom the current mission is named. The recent maneuver reduced the craft’s relative speed compared to Mercury to 1.84 kilometers per second, the goal being to orbit the Sun at close to Mercury’s speed and eventually become captured by the gravity of the small world.

The paper referenced in the first paragraph is Mogul et al., “Is Phosphine in the Mass Spectra from Venus’ Clouds?” available as a preprint. The Sousa-Silva paper is “A stringent upper limit of the PH3 abundance at the cloud top of Venus,” in press at Astronomy & Astrophysics (abstract). The paper on glycine is Manna et al., “Detection of simplest amino acid glycine in the atmosphere of the Venus,” submitted to Science (preprint). The “Snellen paper is “Re-analysis of the 267-GHz ALMA observations of Venus: No statistically significant detection of phosphine,” submitted to Science (abstract).

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OSIRIS-REx: Sample Collection at Asteroid Bennu

A spacecraft about the size of an SUV continues operations at an asteroid the size of a mountain. The spacecraft is OSIRIS-REx, the asteroid Bennu, and yesterday’s successful touchdown and sample collection attempt elicits nothing but admiration for the science team that offered up the SUV comparison. They’re collecting materials with a robotic device 321 million kilometers from home. Yesterday’s operations seem to have gone off without a hitch, the only lingering question being whether the sample is sufficient, or whether further sampling in January will be needed.

If all goes well, we will acquire the largest surface sample from another world since Apollo. TAGSAM is the Touch-And-Go Sample Acquisition Mechanism aboard the craft, a 3.35-meter sampling arm extended from the spacecraft as OSIRIS-REx was descending roughly 800 meters to the surface. The ‘Checkpoint’ burn occurred at 125 meters as the craft maneuvered to reach the sample collection site, dubbed ‘Nightingale.’ The ‘Matchpoint’ burn followed ‘Checkpoint’ by 10 minutes to match Bennu’s rotation at point of contact. A coast past the ‘Mount Doom’ boulder was followed by touchdown in a crater relatively free of rocks.

This is dramatic stuff. The image below is actually from August during a rehearsal for the sample collection (images of yesterday’s touchdown are to be downlinked to Earth later today), but it’s an animated view that gets across the excitement of the event. Mission principal investigator Dante Lauretta (University of Arizona. Tucson) had plenty of good things to say about the result:

“After over a decade of planning, the team is overjoyed at the success of today’s sampling attempt. Even though we have some work ahead of us to determine the outcome of the event – the successful contact, the TAGSAM gas firing, and back-away from Bennu are major accomplishments for the team. I look forward to analyzing the data to determine the mass of sample collected.”

Image: Captured on Aug. 11, 2020 during the second rehearsal of the OSIRIS-REx mission’s sample collection event, this series of images shows the SamCam imager’s field of view as the NASA spacecraft approaches asteroid Bennu’s surface. The rehearsal brought the spacecraft through the first three maneuvers of the sampling sequence to a point approximately 40 meters above the surface, after which the spacecraft performed a back-away burn. Credit: NASA/Goddard/University of Arizona.

The goal is 60 grams of material, with the first indication of sample size being new images of the surface to see how much material was disturbed by the TAGSAM activities. Michael Moreau (NASA GSFC) is OSIRIS-REx deputy project manager:

“Our first indication of whether we were successful in collecting a sample will come on October 21 when we downlink the back-away movie from the spacecraft. If TAG made a significant disturbance of the surface, we likely collected a lot of material.”

Images of the TAGSAM head, taken with the camera known as SamCam, should provide evidence of dust and rock in the collector, with some possibility of seeing inside the head to look for evidence of the sample within. Beyond imagery, controllers will try to determine the spacecraft’s moment of inertia by extending the TAGSAM arm and spinning the spacecraft about an axis perpendicular to the arm. Comparison to data from a similar maneuver before the sampling should allow engineers to measure the change in the mass of the collection head.

Between the imagery and the mass measurement, we should learn whether at least 60 grams of surface material have been collected. Once this has been verified, the sample collector head can be placed into the Sample Return Capsule (SRC) and the sample arm retracted as controllers look to a departure from Bennu in March of 2021. If necessary, a second maneuver, at the landing backup site called ‘Osprey,’ could take place on January 12, 2021.

Image: These images show the OSIRIS-REx Touch-and-Go Sample Acquisition Mechanism (TAGSAM) sampling head extended from the spacecraft at the end of the TAGSAM arm. The spacecraft’s SamCam camera captured the images on Nov. 14, 2018 as part of a visual checkout of the TAGSAM system, which was developed by Lockheed Martin Space to acquire a sample of asteroid material in a low-gravity environment. The imaging was a rehearsal for a series of observations that will be taken at Bennu directly after sample collection. Credit: NASA/Goddard/University of Arizona.

Sample return is scheduled for September 24, 2023, with the Sample Return Capsule descending by parachute into the western desert of Utah. So far so good, and congratulations all around to the OSIRIS-REx team!

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TOI-1266: Confirming Two Planets around a Red Dwarf

The SAINT-EX telescope, operated by NCCR PlanetS, produces a nice resonance as I write this morning. The latter acronym stands for the National Centre of Competence in Research PlanetS, operated jointly by the University of Bern and the University of Geneva. The former, SAINT-EX, identifies a project called Search And characterIsatioN of Transiting EXoplanets, and the team involved explicitly states that they shaped their acronym to invoke Antoine de Saint-Exupéry, legendary aviator and author of, among others, Wind, Sand and Stars (1939), Night Flight (1931) and Flight to Arras (1942).

I’ve talked about Saint-Exupéry now and again throughout the history of Centauri Dreams, not only because he was an inspiration for my own foray into flying, but also because for our interstellar purposes he is credited with this inspirational thought:

“If you want to build a ship, don’t drum up the men to gather wood, divide the work and give orders. Instead, teach them to yearn for the vast and endless sea.”

I say Saint-Exupéry is ‘credited’ with this because it seems to be a condensation of a longer passage that appeared in his 1948 title La Citadelle and I’ve never been able to track the oft-quoted short version down to an original in this specific form. No matter, it’s a grand thought and it plays to our passions as explorers and mappers of new worlds.

So good for NCCR PlanetS for the nod to a favorite writer, and thanks for its recent work at TOI-1266, a red dwarf now known to host at least two planets. The confirmation was achieved with the SAINT-EX robotic 1-meter telescope that the project operates in Mexico; the paper was recently published in Astronomy & Astrophysics. TOI identifies a TESS Object of Interest, meaning the object had been considered promising for follow-up work to validate the candidate signatures as planets. The paper on TOI-1266 provides the needed confirmation.

Here’s what we know: TOI-1266 b is an apparent sub-Neptune about two and a half times the diameter of the Earth that orbits the primary in 11 days. TOI-1266 c is closer in size to a ‘super-Earth’ in being about one and a half times the size of our planet in a 19-day orbit.

The orbits are circular, co-planar and stable. Referring to the more than 3000 transiting planets thus far identified, which includes 499 with more than a single transiting world, the authors discuss distinctive planetary populations. Both planets at TOI-1266 are found at what lead author Brice-Olivier Demory calls the ‘radius valley,’ for reasons explained in the paper:

This large sample of transiting exoplanets allows for in-depth exploration of the distinct exoplanet populations. One such study by Fulton et al. (2017) identified a bi-modal distribution for the sizes of super-Earth and sub-Neptune Kepler exoplanets, with a peak at ∼1.3 R and another at ∼2.4 R. The interval between the two peaks is called the radius valley and it is typically attributed to the stellar irradiation received by the planets, with more irradiated planets being smaller due to the loss of their gaseous envelopes.

Image: Size of TOI-1266 system compared to the inner solar system at a scale of one astronomical unit, the distance between the Earth and the Sun. The orbital distances of the two exoplanets discovered around the star TOI-1266, which is half of the size of the Sun, are smaller than Mercury’s orbital distance. TOI-1266 b, the closest planet to the star at a distance of 0.07 astronomical units, has a diameter of 2.37 times that of Earth’s and is therefore considered a sub-Neptune. TOI-1266 c, at 0.01 astronomical units from its star and with 1.56 times the Earth’s diameter, is considered a super-Earth. For each planetary system, the star’s diameter and the orbital distances to its planets are shown in scale. The relative diameter of all planets of both systems are on scale, being TOI-1266 b the largest planet and Mercury the smallest. The zoom-in to TOI-1266, in the lower part of the image, shows that the irradiation received by TOI-1266 c from its star is 21% larger than the irradiation received by Venus from the Sun in the upper part of the image. © Institute of Astronomy, UNAM/ Juan Carlos Yustis.

It’s useful to have a super-Earth and a sub-Neptune in the same system because this allows for tighter constraints on formation models. The paper argues that this will be “a key system to better understand the nature of the radius valley around early to mid-M dwarfs.”

It’s also interesting that TOI-1266’s small size and relative proximity (about 117 light years) make it a possible target for atmospheric studies with future instruments. The authors show that the James Webb Space Telescope should be able to operate well above the observatory’s noise floor level in performing transmission spectroscopy here (i.e., viewing changes in the star’s light as the either planet moves first in front of and then is eclipsed by the star). The TOI-1266 planets compare favorably to TRAPPIST-1b on this score, and the host star shows no evidence of significant starspots that might distort the signal of the exoplanet atmospheres.

Image: Direct comparison among the planets of TOI-1266 system with the interior planets of the solar system. © Institute of Astronomy, UNAM / Juan Carlos Yustis.

The paper is Brice-Olivier Demory et al., “A super-Earth and a sub-Neptune orbiting the bright, quiet M3 dwarf TOI-1266,” Astronomy & Astrophysics Volume 642 (2 October 2020). Abstract.

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On John Barrow (1952-2020)

Peter Coles, who is a professor of theoretical physics at Maynooth University in Ireland, tells an anecdote about John Barrow, who died recently at the age of 67. Barrow had been Coles’ thesis supervisor and a profound influence on his work as well as a good friend. As Coles tells it in his In the Dark blog, Barrow had an engaging and sometimes slightly morbid sense of humor, dry enough to tease out the ironies abundant in life’s accomplishments. Thus his reaction to being made a Fellow of the Royal Society, which was to point out in an email that his joy was tempered by having received as his first communication from the Society not only a fat bill for his subscription, but also a form upon which to enter the details of his future obituary.

How saddening that Barrow’s obituary materials had to be put to use so soon. The man was 67, felled by cancer. As Coles notes, he was “one of cosmology’s brightest lights.” I can glance across my office to the nearest of many bookshelves where I see various Barrow titles, including of course The Anthropic Cosmological Principle (Oxford University Press, 1986) and The Left Hand of Creation (Oxford, 1983), the latter of which was my introduction to Barrow and the reason I snapped up the former. Along the way, Barrow produced numerous popular science titles and, of course, hundreds of papers in the journals.

Image: Cosmologist, mathematician and physicist John D. Barrow, who died on September 26. Credit: Tom Powell.

You can learn an extraordinary amount of physics from reading (and re-reading) The Anthropic Cosmological Principle, which Barrow wrote with Frank Tipler, and while theoretical astrophysics dominates the tome, you’ll come out of it steeped not only in cosmology but also philosophy and the history of ideas, from Johann Fichte to Henri Bergson, with a healthy dose of Teilhard de Chardin. None of that means you have to buy into anthropic cosmological principles in the way Barrow and Tipler framed their consequences, although the case is made with vigor and verve. The book is a treasure house of accumulated ideas; the reader can move among them learning, questioning, arguing. I wish I had had the chance to talk to Barrow, preferably over a leisurely dinner, for he was both a raconteur and a wit who spoke with a polymath’s elegance.

As to the anthropic cosmological principle, let’s say that parts of it are self-evident though sometimes productive. Drawing on Brandon Carter, the authors laid out the fact that the physical constants we study in the cosmos have to be such that life can exist within the universe they support. But let me quote Ethan Siegel’s recent piece on anthropic matters, as he’ll say it better. Two forms of the principle emerge in The Anthropic Cosmological Principle:

1) The observed values of all physical and cosmological quantities are not equally probable but they take on values restricted by the requirement that there exists sites where carbon-based life can evolve and by the requirement that the Universe be old enough for it to have already done so.

2) The Universe must have those properties which allow life to develop within it at some stage in history.

All of which seems self-evident until you absorb the implication in the second item: A universe without life is not allowed. Now we’re in the realm of teleology, making this take on the principle controversial, to say the least. To push the pedal down all the way, Barrow and Tipler introduce a Final Anthropic Principle (FAP) which points to the necessity for information processing to emerge and insists that once present, it will never disappear. Tipler would take these ideas and run with them in his own The Physics of Immortality (Doubleday, 1994). Martin Gardner was less impressed, calling the notion of an ‘Omega Point’ the Completely Ridiculous Anthropic Principle (CRAP). I would say it is entertaining metaphysics and leave it at that.

But the weak anthropic principle still resonates. Paul Davies calls it “a familiar part of the theorist’s arsenal” and Siegel discusses its use in Stephen Weinberg’s 1987 calculation of the energy density of empty space, which must be at least 118 orders of magnitude smaller than what scientists had derived from quantum field theory or we would not be here to calculate it (measurements now indicate the actual figure is 120 orders of magnitude smaller). How it can be used and sometimes abused is, in fact, the subject of the Siegel essay I referenced above. I’m not comfortable enough going through the intellectual thickets here to say more. I always wanted to ask Barrow about his current thinking on the matter to satisfy my questions, though as fate would have it, I never had the chance to meet the man.

What appealed about the dinner idea was Barrow’s interest in wine and cuisine. In the course of my checkered career, I labored as a wine critic and restaurant reviewer for a local paper, and the thought of engaging this wide-ranging intellect on issues ranging from early music to the fine structure constant while sipping, say, a Vosne-Romanée (I would have pulled out all the stops for Barrow!) was enticing. But I have to read the reminiscences of others to reinforce my sense of the man. Paul Davies’ has noted his sartorial elegance (all those trips to Italy), his quick wit, his grasp of literature and the visual arts. Much of which I had more or less inferred from his books.

I mention Italy because Barrow loved the country and I learned from Davies that his stage play Infinities saw its premiere in Milan (it went on to win the Italian Premi Ubu award for the best play in Italian theatre in 2002), and it was consoling to find out that Barrow and his wife Elizabeth had made one last trip there no more than weeks before he died. While we of course remember him here as a scientist, and an extraordinarily influential one at that, style manifests itself in many ways, from the elegance of a new suit to the figures on a physicist’s chalkboard. On his defining intellectual style, let’s turn back to Davies:

More recently, Barrow was interested in the possibility that the fine-structure constant—an unexplained number that describes the strength of the electromagnetic force—might not be constant at all but rather vary over cosmological scales. He produced a theoretical basis for incorporating such a phenomenon in physical law while also remaining open-minded on the observational evidence. His adventurous choices of research problems typified Barrow’s intellectual style, which was to challenge the hidden assumptions underpinning cherished mainstream theories. Fundamental problems in physics and cosmology may appear intractable, he reasoned, because we are thinking about them the wrong way. It was a mode of thought that resonated with many colleagues, this writer included, who are drawn to reflect on the deepest questions of existence.

Indeed, Davies’ many books display the kind of intellectual bandwidth that Barrow tapped so effortlessly, and which is on display in Barrow’s numerous books. He wrote, both in his professional work and his popular science titles, with a crisp, unaffected clarity. I appreciated, as I do with Davies, his willingness to engage with the public, to whom he communicated an obvious joy in the acquiring of knowledge. That’s quite a range, to move from analysis of the early universe and the nature of its physical constants to the reader on the underground, riding home from a harried day with book in lap explaining how mathematics gives shape to the cosmos. How cosmologists slide from the micro to the macro scale will always astonish me.

But practicality has played a hand in this. Go back to the early 1980s and consider, as Barrow often did, the changes that have occurred in how we communicate science. Public discourse on their research and its consequences was relatively rare among researchers until more recent times, when university science departments began to determinedly reach out not only for new students but for the visibility that attracts good faculty. Barrow would win the Royal Society’s Michael Faraday Prize for excellence in communicating science to UK audiences, a subject he further embraced through his involvement as director of the Millennium Mathematics Project, which seeks to spread mathematical skills broadly outside the boundaries of the academy. The latter seems to have inspired his 100 Essential Things You Didn’t Know You Didn’t Know About Maths and the Arts (Bodley Head, 2014), a series of short riffs on the subject.

Image: John Barrow receiving the Queen’s Anniversary Prize on behalf of the Millennium Mathematics Project. Credit: Centre for Theoretical Cosmology News.

Barrow could never have been accused of being a blinkered academic. Even so, he would write over 500 research papers in astrophysics and cosmology, much of the time asking how astronomical observations could inform our understanding of fundamental physics. I’ve mentioned the fine-structure constant and the possibility of variation in earlier cosmological epochs. But he also explored cosmic inflation and probed extremes of nature, the singularities suggested by the nature of the Big Bang. His honors and awards are too numerous to list in the space of this article. His over 20 books have been translated into 28 languages.

What a fine spirit he inspired in those around him. I caught that sense of quest and possibility early, from The Left Hand of Creation, which Barrow wrote with Joseph Silk. The book’s final paragraph is worth quoting:

Could there be any shortcuts to the answers to the cosmological questions? There are some who foolishly desire contact with advanced extraterrestrials in order that we might painlessly discover the secrets of the universe secondhand and prematurely extend our understanding. Such a civilization would surely resemble a child who receives as a gift a collection of completed crossword puzzles. The human search for the structure of the universe is more important than finding it because it motivates the creative power of the human imagination. About 50 years ago a group of eminent cosmologists were asked what single question they would ask of an infallible oracle who could answer them with only yes or no. When his opportunity came, Georges Lemaître made the wisest choice. He said, “I would ask the Oracle not to answer in order that a subsequent generation would not be deprived of the pleasure of searching for and finding the solution.”

Dinner with Barrow would have been just the ticket. A toast, then, to the man who said “A universe simple enough to be understood is too simple to produce a mind capable of understanding it.” Here’s to a universe of complexity so rich that it could inspire the life of such a man.

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The ‘Cold Jupiter’ Factor

Gas giant planets in orbits similar to Jupiter’s are a tough catch for exoplanet hunters. They’re far enough from the star (5 AU in the case of Jupiter) that radial velocity methods are far less sensitive than they would be for star-hugging ‘hot Jupiters.’ A transit search can spot a Jupiter analogue, but the multi-year wait for the proper alignment is obviously problematic.

Still, we’d like to know more, because the gravitational influence of Jupiter may have a crucial role in deflecting asteroids and comets from the inner system, thus protecting terrestrial worlds like ours. If this is the case, then we need to ask whether we just lucked out by having Jupiter where it is, or whether there is some mechanism that makes the presence of a gas giant in a kind of ‘protective’ outer orbit likely when rocky worlds inhabit the inner system.

Enter the computer simulations run by Martin Schlecker at the Max Planck Institute for Astronomy (MPIA) in Heidelberg. Working with scientists at the University of Bern and the University of Arizona, Schlecker has run simulations of 1000 planetary systems evolving around Sun-like stars. In this case, ‘Sun-like’ means solar-type stars with the same mass as our own Sun. The simulated planetary systems are derived through a model known as the Generation III Bern global model of planetary formation and evolution, and the team has used the results to make a statistical comparison with a sample of observed exoplanet systems.

Schlecker calls gas giants in more distant orbits ‘cold Jupiters.’ They’re beyond the snowline, so that water can exist in the form of ice. The paper homes in on ‘super-Earths’ more massive than our own planet but far less massive than gas giants, which are estimated to orbit as many as 50% of stars in class F, G and K. In question is how cold Jupiters affect the formation of these inner super-Earths, and if in fact they make it easier for super-Earths to survive. Some recent studies have found a positive correlation between super-Earths and cold Jupiters. Is it sound?

Image: Artistic impression of a planetary system with two super-Earths and one Jupiter in orbit around a Sun-like star. Simulations show that massive protoplanetary disks in addition to rocky super-Earths with small amounts of ice and gas often form a cold Jupiter in the outer regions of the planetary systems. Credit: MPIA graphics department.

The team’s goal is a “population synthesis of multi-planet systems from initial conditions representative of protoplanetary disks in star forming regions.” The computer model tracks how systems starting from random initial conditions evolve over several billion years as planets coalesce and interact with other objects in the system, with the possibility of collisions or ejections as orbits change due to the gravitational interactions factored in. You can see that with the difficulty in observing actual cold Jupiters in such settings, computer simulations are the way to proceed as the tools of observation are tuned up for future detections.

A hot Jupiter would be likely to disrupt planetary orbits, perhaps preventing the formation of some inner worlds altogether, but a cold Jupiter is far enough from the star that we can expect systems with inner super-Earths to exist, and indeed, some studies suggest that systems with a cold Jupiter often contain a super-Earth. But the simulations of Schlecker’s team indicate that only about a third of cold Jupiters occur in systems accompanied by at least one super-Earth. The correlation of inner super-Earths and cold Jupiters is there, but somewhat weaker than has been found through actual observation.

A possible explanation for the discrepancy between observation and simulation: Gas giant migration involving ‘warm Jupiters’ on intermediate orbits, causing super-Earth collisions or ejections. Here we can look to the parameters of the simulations. If the tendency of the simulated gas giants to migrate is slightly lowered, more inner super-Earths remain in numbers that are compatible with what we have recorded with our telescopes and spectrographs.

Here the limited number of actually observed systems with gas giant and super-Earths becomes an issue. Whereas the study’s simulations find a tendency to produce systems with both a cold Jupiter and at least one dry super-Earth (“with little water or ice, and a thin atmosphere at most,” as Schlecker notes), we only have a small number of systems (24) out of the 3200 planetary systems currently known that follow this configuration. Schlecker also notes that the Earth is comparatively dry — “…the Earth is, despite the enormous oceans and the polar regions, with a volume fraction for water of only 0.12% altogether a dry planet.” What we don’t find observationally are ice-rich super-Earths with a cold Jupiter in the same system.

Image: Schematic diagram of the scenarios of how according to the analyzed simulations icy super-Earths (a) or rocky (ice-poor) super-Earths form together with a cold Jupiter (b). The mass of the protoplanetary disk determines the result. Credit: Schlencker et al./ MPIA.

The mass of the protoplanetary disk may be the key. A massive enough disk can form inner rocky planets with cold gas giants beyond the snowline, with the rocky planets being poor in ice and gas. Outer super-Earths that form ice-rich would be unable to migrate inward because of the giant planet. In disks of medium mass, by contrast, material to produce warm super-Earths is limited, and gas giants beyond the snowline give way to super-Earths abundant in ice.

In the authors’ words:

We find a difference in the bulk composition of inner super-Earths with and without cold Jupiters. High-density super-Earths point to the existence of outer giant planets in the same system. Conversely, a present cold Jupiter gives rise to rocky, volatile-depleted inner super-Earths. Birth environments that produce such dry planet cores in the inner system are also favorable for the formation of outer giants, which obstruct inward migration of icy planets that form on distant orbits. This predicted correlation can be tested observationally.

and…

…low-mass solid disks tend to produce only super-Earths but no giant planets. Intermediate-mass disks may produce both super-Earths and cold Jupiters. High-mass disks lead to the destruction of super-Earths and only giants remain.

The mass of the protoplanetary disk is thus crucial, with super-Earth occurrence in combination with cold Jupiters rising with increasing disk mass, then dropping for disks massive enough to become dominated by giant planets. Interestingly, the study indicates that host stars of high-metallicity tend to have planetary systems with warm giant planets that can wreak havoc on inner planets in the system.

We’re in that region of the discovery space where we’re building theories that can be tested against future observations. Thus the next generation of space telescopes like the James Webb Space Telescope, as well as the Extremely Large Telescope from the European Southern Observatory. Do the simulations of Schlecker and team hold up? Do they show a disposition for planets of Earth size, rather than the larger super-Earths, to exist in systems with cold Jupiters?

We won’t know, the scientist acknowledges, until we have the ability to detect planets like ours in large numbers. For now, the only numbers we can begin to run are those of the larger super-Earth population, and even here, we’re still building the statistical sample. What the new instrumentation shows in coming years will help us fine-tune these simulated results.

The paper is Schlecker et al., “The New Generation Planetary Population Synthesis (NGPPS). III. Warm super-Earths and cold Jupiters: A weak occurrence correlation, but with a strong architecture-composition link,” in process at Astronomy & Astrophysics (abstract).

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IRS 63: How Quickly Do Planets Form?

I’m startled by the findings in a new paper from Dominique Segura-Cox (Max Planck Institute for Extraterrestrial Physics), who argues that based on the evidence of one infant system, we may have planet formation all wrong, at least in terms of when it occurs. The natural assumption is that the star appears first, the planets then accruing mass from within the circumstellar disk. But Segura-Cox and team have found a system in which planet and star seem to be forming all but simultaneously. IRS 63 is a protostar about 470 light years out that is less than half a million years old. Swathed in gas and dust, the star is still gathering mass, but evidence from the disk suggests that the planets have already begun to form.

One reason for the surprise factor here is that we’ve looked at many young stellar systems and their disks, most of them at least one million years old, and the assumption has been that the stars were well along in their own formation process before the planets began to build. Ian Stephens (Center for Astrophysics | Harvard & Smithsonian), a co-author on this paper, is quite clear on the matter: “Traditionally it was thought that a star does most of its formation before the planets form, but our observations showed that they form simultaneously.”

Image: The dense L1709 region of the Ophiuchus Molecular Cloud, mapped by the Herschel Space Telescope, which surrounds and feeds material to the much smaller IRS 63 proto-star and planet-forming disk (location marked by the black cross). Credit: MPE/D. Segura-Cox; Herschel data from ESA/Herschel/SPIRE/PACS/D. Arzoumanian.

Planets and host star as siblings? It takes a bit of getting used to, but the Segura-Cox team is finding gaps and rings, the rings accumulating dust suitable for planet formation, within the IRS 63 disk. The paper summarizes earlier disk observations at other stars, where the gaps and rings found in the disks have generally appeared in so-called Class II disks with ages of one million years or more — ALMA (Atacama Large Millimeter/submillimeter Array) has turned up more than 35 of these — with gaps showing little or no dust emission, a fact usually explained by a planet-mass object shaping the dust into rings.

Fitting IRS 63 into this picture, we get this:

These results indicate that planet formation must be both efficient and well underway by the class II phase. Recent dust mass measurements of class II disks also indicate that observed dust depletion could be explained if substantial mass is locked into planetesimals on timescales less than 0.1–1 Myr, placing early planet formation into the younger class I phase and possibly even earlier. In comparison to the rings and gaps found in the older disks of class II objects, the annular substructures in the younger disk of IRS 63 have dust emission even in the G1 and G2 gaps and are wider and have lower contrast. In IRS 63, it is unclear whether or not sizeable planetary-mass bodies are creating these gaps.

Stephens estimates that planets start to form as early as the first 150,000 years of a system’s formation, at the earliest stages of star growth. If each of the gaps at IRS 63 is opened by a single planet, the authors estimate the first gap (G1) would require a planet of 0.47 Jupiter mass, while the second gap (G2) demands 0.31 Jupiter mass, both numbers being upper limits.

Image: The ALMA image of the young planet-forming dust rings surrounding the IRS 63 proto-star, which is less than 500,000 years old. Credit: MPE/D. Segura-Cox.

The paper notes that Jupiter is an interesting world in this kind of scenario. The disk at IRS 63 is similar in size to our own Solar System, and the mass of the proto-star not far off that of the Sun. Such early formation implies a distant new planet migrating into its present position:

For a planetary embryo made of solids to trigger runaway accretion of gaseous material—required in the core-accretion model of gas-giant planet formation—the critical mass of 10MEarth must be met by the solid core. This condition is readily met in the young IRS 63 disk, even for a somewhat low efficiency (<10%) of core formation out of the available dust grains. The large dust mass in the outer dust disk—combined with the 27 and 51 au radii of the rings of IRS 63—is consistent with evidence that Jupiter’s core could have formed beyond 30 au in our own Solar System and later migrated inward. Giant-planet cores may often form in the exterior regions of large disks starting from the early phases of star formation.

Image: The rings and gaps in the IRS 63 dust disk compared to a sketch of orbits in our own Solar System at the same scale and orientation of the IRS 63 disk. The locations of the rings are similar to the locations of objects in our Solar System, with the inner ring about the size of the Neptune orbit and the outer ring a little larger than the Pluto orbit. Credit: MPE/D. Segura-Cox.

At IRS 63, the researchers find that about 0.5 Jupiter masses of dust are available at distances beyond 20 AU, along with unspecified amounts of gas. Planet formation at large distances from the star might involve solid material totalling at least 0.03 Jupiter masses, forming the kind of planetary core that can accrete gas and swell into a gas giant planet.

The paper is Segura-Cox, et al., “Four annular structures in a protostellar disk less than 500,000 years old,” Nature Vol. 586 (7 October 2020), pp. 228–231 (abstract).

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M-dwarf Superflares and Habitability

We could use a lot more information about flare activity on M-dwarf stars, which can impact planetary atmospheres and surfaces and thus potential habitability. Thus far much has been said on the subject, but what has been lacking are details about the kinds of flares in question. It’s a serious issue given that, in order to be in the liquid water habitable zone, an M-dwarf planet has to orbit in breathtaking proximity to the host star.

Flares occur through a star’s magnetic field re-connection, which releases radiation across the electromagnetic spectrum. While flares can erode atmospheres and bathe the surface in UV flux, too few flares could actually be detrimental as well, providing as a new paper on the matter suggests, “insufficient surface radiation to power prebiotic chemistry due to the inherent faintness of M-dwarfs in the UV.”

The paper is out of the University of North Carolina, measuring a large sample of superflares in search of a clearer picture of their effect. Flares on the Sun are common enough, as witness the image below, shot in 2012, which shows the kind of coronal mass eruption (CME) associated with stellar flaring. At young M-dwarfs, we can get ‘superflares’ in energy ranges 10 to 1,000 times larger than our Sun provides, bathing a nearby planet in intense ultraviolet light.

“We found planets orbiting young stars may experience life-prohibiting levels of UV radiation, although some micro-organisms might survive,” says lead study author Ward S. Howard.

Or could flares be a driver of evolution? Just how much is too much when it comes to flaring?

Image: A beautiful prominence eruption producing a coronal mass ejection (CME) shot off the east limb (left side) of the sun on April 16, 2012. Such eruptions are often associated with solar flares, and in this case an M1 class (medium-sized) flare occurred at the same time, peaking at 1:45 PM EDT. The CME was not aimed toward Earth. The eruption was captured by NASA’s Solar Dynamics Observatory in the 304 Angstrom wavelength, which is typically colored in red. Credit: NASA/SDO/AIA.

Howard’s team used data from TESS, the Transiting Exoplanet Survey Satellite, coupled with observations from the university’s Evryscope telescope array, to make the study. Previous flare work has homed in on a relatively small number of stars in terms of flare temperatures and radiation flux. Using the largest sample of superflares ever studied in terms of temperature, the new study finds it predictive of the amount of radiation that is likely to reach a planetary surface. A statistical relationship emerges between the size of a superflare and its temperature.

Superflares are not long-lasting events, emitting most of their UV in a rapid peak that may last between 5 and 15 minutes. The TESS data, taken simultaneously with the Evryscope observations, were obtained at a two-minute cadence for 42 superflares from 27 K5-M5 dwarfs. The work extends the range of our observations, while simultaneously playing into the potential target list for the James Webb Space Telescope.

The authors note that flare emissions have usually been approximated by a 9000 K blackbody (a surface that absorbs all radiant energy falling on it). But if superflares are hotter than this, the UV emission may surge by a factor of 10 higher than would otherwise be predicted through optical observation. Only a handful of multi-wavelength observations over short periods of time have been performed, and the TESS/Evryscope work doubles the total number found in the literature, helping us understand how temperature evolves in M-dwarf superflares. 43% of the superflares studied emit above 14,000 K, 23% above 20,000 K and 5% above 30,000 K, with the hottest one observed briefly reaching an incandescent 42,000 K.

We’re dealing, in other words, with a lot of UV flux here, particularly the dangerous ultraviolet C (UV-C) wavelength. From the paper, which takes note of a 2016 superflare observed at Proxima Centauri:

If HZ planets orbiting <200 Myr stars typically receive ∼120 W m−2 and often up to 103 W m−2 during superflares, then significant photo-dissociation of planetary atmospheres may occur (Ribas et al. 2016; Tilley et al. 2019). As a point of comparison, the likely water loss of Proxima b is due to the long-term effects of a time-averaged XUV flux (including flares) of less than 1 W m−2 (Ribas et al. 2016). The median value from the flares observed in YMGs [young moving group stars] is comparable to the ∼100 W m−2 of UV-C flux estimated at the distance of Proxima b during the Howard et al. (2018) Proxima superflare. While Abrevaya et al. (2020) found 10−4 microorganisms would have survived the Proxima superflare, it is presently unclear what effects a 10× increase in UVC flux would have on the evolution and survival of life prior to 200 Myr; it is possible such high rates of UV radiation could drive pre-biotic chemistry (Ranjan et al. 2017; Rimmer et al. 2018), suppress the origin of life on worlds orbiting young M-dwarfs (Paudel et al. 2019), or not impact astrobiology at all if the timescale for life to emerge is longer than 200 Myr (Dodd et al. 2017; Paudel et al. 2019).

Addendum: Alex Tolley makes a solid point about this material in the context I gave it. Let me quote him:

“The extract starting with:

“If HZ planets orbiting <200 Myr stars typically receive
~120 W m-2”

does not make it clear that the flux is for UV, not total output. Only the low values (compared to Earth) and the later inclusion of “UV” allows the reader to infer that this is only the UV flux. In the paper, this section is prefaced by some text that talks about UV and makes the next paragraph clearer. For readers like me, not steeped in stellar knowledge, this could be clearer.”

Exactly so, and I appreciate the note, Alex. — PG

Three of the stars targeted in this study, the aforementioned Proxima Centauri, LTT 1445 and RR Cac AB are already known to host planets. And what of the age difference between young M-dwarfs and their presumably more sedate elder cousins? Should we preferentially target older M-dwarfs? The authors argue that it’s too early to make a call:

Although higher-mass young M-dwarfs may emit more biologically-relevant UV flux as a consequence of frequent superflares than do lower-mass young M-dwarfs, we do not confirm that more UV-C flux from early M-dwarf superflares consistently reaches the HZ. The relative habitability of early versus mid M-dwarf planets is a topic for future work. In particular, the shorter active lifetimes of early M-dwarfs may allow planetary atmospheres to recover as the star ages via degassing (Moore & Cowan 2020).

So we have a useful survey of optical temperature evolution in M-dwarf superflares, demonstrating the predictive quality of flare energy and impulse. The authors intend to continue observations of future flare activity at the same 2 minute cadence, aided by the re-observation of the Evryscope flare targets at its own 20 second cadence.

The paper is Howard et al., “EvryFlare III: Temperature Evolution and Habitability Impacts of Dozens of Superflares Observed Simultaneously by Evryscope and TESS,” in process at the Astrophysical Journal (preprint).

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Thoughts on Immensity

The Hubble Deep Field images of 1995 and 1998 gave us an unprecedented look at a small patch of sky with few nearby bright objects, a region about one-tenth the diameter of the full Moon in the constellation Fornax. The ensuing Hubble Ultra Deep Field, released in 2004, contains as many as 10,000 individual galaxies. Hubble’s Advanced Camera for Surveys (ACS) was deployed for this, as well as its Near Infrared Camera and Multi-object Spectrometer (NICMOS), producing a stunning harvest of elliptical and spiral galaxies, as well as oddly shaped collections of stars from the chaotic early days of the universe.

Image: The original NASA release image of the Hubble Ultra Deep Field, containing galaxies of various ages, sizes, shapes, and colors. The smallest, reddest galaxies, of which there are approximately 10,000, are some of the most distant galaxies to have been imaged by an optical telescope, probably existing shortly after the Big Bang. Credit: NASA/ESA.

I immediately thought of a Brian Aldiss title when I first saw the HUDF: Galaxies Like Grains of Sand came out in 1960, a collection of previously published stories later linked into a kind of future history. The grains of sand analogy always captures the imagination — most of us have walked on beaches. But here all sense of scale seems to drop away.

Sara Seager commented in her powerful The Smallest Lights in the Universe (2020) that with the original Hubble Deep Field, Robert Williams, then director of the Space Telescope Science Institute, had “…revealed three thousand previously unseen points of light. Not three thousand new stars. Three thousand new galaxies. Bob Williams almost single-handedly discovered millions of billions of possible worlds.”

How many planets are there around stars in the visible universe? How many galaxies? Astronomers and mathematicians work routinely with the kind of numbers involved here, but for us civilians, I don’t think it’s possible to emotionally comprehend such immensity. The Hubble Deep UV (HDUV) Legacy Survey image, released in 2015, ups the catch to 15,000 galaxies.

Tiny lights in the sky dwarf us whether they are stars or galaxies. It was imagery from TESS, our Transiting Exoplanet Survey Satellite, that brought on these reflections. Have a look at the starfield below. TESS has discovered 74 planets at this point, with another 1200 awaiting confirmation in an ongoing mission to examine nearby stars. The field of view is 400 times larger than what Kepler could cover in its tight stare in the direction of Cygnus and Lyra, but both missions rely on the transit method, detecting the presence of a planet crossing in front of its star in the stellar lightcurve. We’re going to get a lot of planets out of TESS, but among the targeted, nearby stars, probably around 1200 to 1500.

Stars like grains of sand. Here we’re looking toward Cygnus, back in Kepler country, and the sky is a glory of objects even if TESS will have a gap in coverage in the northern hemisphere, given an attempt by the science team to minimize the effects of scattered light from the Earth and the Moon. A mosaic of the southern coverage takes in more sky.

Image: This detail of the TESS northern panorama features a region in the constellation Cygnus. At center, the sprawling dark nebula Le Gentil 3, a vast cloud of interstellar dust, obscures the light of more distant stars. A prominent tendril extending to the lower left points toward the bright North America Nebula, glowing gas so named for its resemblance to the continent. Credit: NASA/MIT/TESS and Ethan Kruse (USRA).

The TESS numbers are already robust, with each of 13 northern hemisphere sectors imaged for nearly a month with four cameras, each of the latter with 16 charge-coupled devices (CCD). That adds up to 38,000 full science images for each of the CCDs, a total of 40 terabytes of data. It’s exhilarating to realize that these numbers are about to jump as TESS goes into its extended mission, revisiting the southern sky for another year, while relying upon improvements in data collection and processing that will return full-sector images every 10 minutes, as opposed to the earlier 30. TESS measures tens of thousands of stars, says NASA, every two minutes.

“These changes promise to make TESS’s extended mission even more fruitful,” said Padi Boyd, the mission’s project scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “Making high-precision measurements of stellar brightness at these frequencies makes TESS an extraordinary new resource for studying flaring and pulsating stars and other transient phenomena, as well as for exploring the science of transiting exoplanets.”

Image: This mosaic of the northern sky incorporates 208 images taken by NASA’s Transiting Exoplanet Survey Satellite (TESS) during its second year of science operations, completed in July 2020. The mission split the northern sky into 13 sectors, each of which was imaged for nearly a month by the spacecraft’s four cameras. Among the many notable celestial objects visible: the glowing arc and obscuring dust clouds of the Milky Way (left), our home galaxy seen edgewise; the Andromeda galaxy (oval, center left), our nearest large galactic neighbor located 2.5 million light-years away; and the North America Nebula (lower left), part of a stellar factory complex 1,700 light-years away. The prominent dark lines are gaps between the detectors in TESS’s camera system. Credit: NASA/MIT/TESS and Ethan Kruse (USRA).

Does anyone ever get jaded with the sheer numbers we talk about as we delve into the cosmos? I suppose a kind of workaday numbness may occasionally settle in, and I suppose it would happen as well to geologists, for example, when trying to wrap their heads around the deep time suggested by the varied strata that first gave scientists a glimpse of how old the Earth really was. I just finished reading Hugh Raffles’ extraordinary The Book of Unconformities, a startling look not only into deep time but the landscape of loss as Raffles confronts twin deaths in his family that came seemingly out of nowhere. All of this against the background of stone considered at geologically significant sites. A taste of this:

At Siccar Point on the east coast of Scotland, a path leads precipitously down the cliff to Hutton’s Unconformity, the line of contact between the two rocks that James Hutton showed the mathematician John Playfair and the experimental geologist Sir James Hall on that memorable June day in 1788. It’s the physical proof of a gap in time, in this case the gap between the Silurian graywackes that formed 440 million years ago on the seafloor at the margins of the Iapetus Ocean (ancient ocean of the Southern Hemisphere) and were forced upward as the vast water body closed, and the Devonian Old Red Sandstone that was laid down on their eroded surface sixty-five million years later by rivers flowing into what was then most likely a tropical floodplain.

Thus modern geology was born. Hutton found in Siccar Point layered sandstone pierced by gray metamorphic rock that would confirm his belief that the Earth was far older than European natural scientists had ever expected. Playfair would later write: “The mind seemed to grow giddy by looking so far back into the abyss of time; and whilst we listened with earnestness and admiration to the philosopher who was now unfolding to us the order and series of these wonderful events.”

There should be a word that signifies the mind all but buckling with the wild surmise of the scale of things. In the late 18th Century, a notion of the ‘sublime’ began to re-emerge in European thought, one suggested by the grandeur of jagged alpine landscapes, for example, but today’s usage of the word doesn’t take in the aspect of dislocation, even disorientation, that such landscapes were freighted with back then, mingling with profundity. Go back to that earlier usage, though, and I’ll maybe opt for ‘sublime’ as the right word to suggest my own response to such imagery.

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Kuiper Belt Oddity? Explaining Arrokoth’s Shape

Now and then people mention that our Pioneers and Voyagers made it through the Kuiper Belt on their long journey toward system’s edge, though unfortunately without operational cameras to record what they saw. A missed opportunity? Not really. Think about how long it took to find a Kuiper Belt Object like Arrokoth, the first one ever seen close up thanks to the mighty work of the New Horizons team. Without a major search to find a target, a craft passing through the Kuiper Belt is almost certainly going to encounter no objects whatsoever within range to record the details.

For now, Arrokoth, the object once known as Ultima Thule before running afoul of our times, has to serve as our example of what can emerge in this distant region, and an odd object it is. When its shape is compared to a flattened snowman, as it often is, the real story is in the word ‘flattened.’ How does this roughly 30-kilometer object emerge in the shape it’s in, and under what conditions was it spawned out there in its 298 year orbit around the Sun?

Two connected lobes are involved, as the famous image below makes clear, making this an evident contact binary, where two formerly discrete objects nudged into each other at low velocity. That part of the origin story is easy, and in fact the bi-lobed shape is not unique. We can find it in certain comets as well — consider 67P/Churyumov–Gerasimenko.

But both lobes of Arrokoth are flattened, not just one. A new paper in Nature Astronomy goes to work on the question of how this happened. Ladislav Rezac (Max Planck Institute for Solar System Research), one of the two first authors of the paper, comments:

“We like to think of the Kuiper Belt as a region where time has more or less stood still since the birth of the Solar System. There is as yet no explanation as to how a body as flat as Arrokoth could emerge from this process.”

Exactly so, because four billion kilometers out from the Sun, KBOs should have remained largely unchanged, frozen into their form in ways that give us information about that era. Yet here we have Arrokoth, whose surface seems smooth, uncratered, and again, flat. Is this shape primordial, or are we actually looking at an evolution of a cold classical Kuiper Belt Object that should have formed where we observe it today?

Image: Arrokoth’s flattened shape can only be seen from a certain perspective. The first images returned by NASA’s New Horizons spacecraft gave the impression of a “normal” snowman-shaped object. Arrokoth’s surface is surprisingly smooth and displays only few craters. Credit: © NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.

To get from there to here, Rezac and co-author Yuhui Zhao (Purple Mountain Observatory, Chinese Academy of Sciences) ran a ‘mass-loss-driven shape evolution model’ called MONET that suggests a process driving the answer, one developed in another paper earlier this year (citation below).

At the heart of the discussion is Arraokoth’s rotational axis, which is close to alignment with the orbital plane; the obliquity is about 99°. One polar region faces the Sun while the other faces away, while equatorial regions and the lower latitudes are dominated by daily variations in flux. The poles heat and release frozen gases, to produce a loss of mass which, the authors argue, produces the observed flattening over time. Thus “…the polar regions reach higher peak temperatures than the equator, due to the fact that the thermal timescales are longer than the rotational timescales, and the poles experience more sublimation than the equatorial regions.”

We wind up with symmetric erosion between northern and southern hemispheres. The process is thought to have occurred early on, perhaps in 1 to 100 million years, as volatile ices sublimated. The authors assume methane as the driving volatile for the evolution of Arrakoth’s precursor body, but add that N2, CO and CH4 ices all show active sublimation and condensation on large KBOs.

The escape of volatiles early in the formation process would not be particularly surprising, in comparison to what New Horizons found at Pluto, where larger size and stronger gravity allowed the dwarf planet to retain carbon monoxide, nitrogen and methane gases to the present era. Small bodies like Arrokoth would hardly have the gravitational attraction to hold on to these volatiles. Moreover, this may be a common part of the evolution of Kuiper Belt Objects:

This process most likely occurred early in the evolution history of the body, during the presence of supervolatile ices in the near subsurface layers. This could be the dominant process shaping the structure of KBOs if there were no catastrophic collision reshaping the body in their later history. Furthermore, while cold classical KBOs reserve their shape sculptured by early out-gassing, the structure of Centaurs and Jupiter-family comets will be further modified by the same scenario once they enter their current orbit configuration from the Kuiper belt, under sublima- tion of different volatile species. We suggest that this mechanism should be taken into account in models studying planetesimal formation and the shape evolution of KBOs, as well as other small icy bodies in regions where supervolatile ices are expected to be present.

Image: Snapshots from numerical simulation of shape evolution of Arrokoth’s analogue due to sublimation driven mass loss. The bottom most shape is a digital terrain model derived from New Horizon observations. The color represents single orbit averaged temperatures. Red stands for warm and blue for cooler regions. Credit: © PMO/MPS.

The paper is Zhao, Rezac et al., “Sublimation as an effective mechanism for flattened lobes of (486958) Arrokoth,” Nature Astronomy 5 October 2020 (abstract). The earlier paper referenced above is Zhao et al., “The phenomenon of shape evolution from solar-driven outgassing for analogues of small Kuiper belt objects,” Monthly Notices of the Royal Astronomical Society 492, 5152–5166 (2020). Abstract.

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The Best of All Possible Worlds

I’ve always loved the notion of ‘superhabitability,’ which forces us to ask whether, in our search for planets like the Earth, we may in our anthropocentric way be assuming that our own planet is a kind of ideal. Some scientists have been asking for years whether it is possible that the Earth is not as ‘habitable’ as it might be (see What Makes a Planet ‘Superhabitable’?). The question then becomes: What factors would make a planet a better place for life than our own?

Now Dirk Schulze-Makuch (Washington State University), working with René Heller (Max Planck Institute for Solar System Research, Göttingen) and Edward Guinan (Villanova University) runs through the characteristics of superhabitability, which take in planets that are a bit warmer than ours, a bit larger, and somewhat wetter, not to mention those that circle stars that live longer than our G-class Sun. 24 interesting planets emerge, all more than 100 light years out, but none of those so far identified meet all the specifications for superhabitability enumerated in the paper, nor could they, given the limitations on our current observing methods.

As we dig into this, let’s first remember the significance of target selection as we continue to refine our telescopes. With interesting new resources coming, space observatories like the James Webb Space Telescope and the European Space Agency’s PLATO, we will delve ever more deeply into exoplanet atmospheres.The goal here is to consider the star systems we will want to investigate as we learn how habitability works and how it might be improved upon. Choosing the list carefully is paramount, for time on these observatories will be precious.

We’ve talked before about the virtues of K-class stars, which are less massive and less luminous than the Sun, while also offering a more capacious lifetime, at minimum twice that of a typical G-class star, and in some cases quite a bit longer than that. Given that life on Earth will likely be rendered impossible within a billion years or so due to changes on the Sun, how much more useful to have tens of billions of years longer to allow life to grow and advance. Various other factors likewise come into play, among them a planet’s geothermal heat and magnetic field. The paper runs through the possibilities (and you might also want to have a look at Orange Dwarfs: ‘Goldilocks’ Stars for Life?, which contains several further citations). From the paper:

…studies of solar proxies of our Sun have shown that young dG [dwarfs of spectral class G] stars rotate > 10 times faster than dG stars near the age of our Sun, and have correspondingly high levels of magnetic dynamo-driven activity and very intense coronal X-ray and chromospheric FUV emissions (Guinan et al., 2005), which makes the origin and early evolution of life challenging. Heller and Armstrong (2014) argued that the increased life span of stars with masses lower than one solar mass may allow inhabited planets to build up a higher biodiversity and possibly even a more complex ecosystem…. This argument would lift K- and M-dwarf stars into the realm of superhabitable planet host stars.

But as the authors point out, moving the range of superhabitability into the realm of M-dwarfs is going too far — here we confront problems of tidal locking, flare activity and atmospheric loss due to the proximity of the planet to the star and exposure to a strong stellar wind. Avi Loeb and Manasvi Lingam have also written on the likelihood that K-dwarfs offer the most stable long-lived environment conducive to superhabitability, and it is worth remembering that we have a K-dwarf in the closest stellar system to our own, Centauri B in Alpha Centauri.

What would a superhabitable planet look like? A planet a bit larger than Earth — one with, say, 1.5 times Earth’s mass — would retain internal heating from radioactive decay longer, while the stronger gravity would allow the planet’s atmosphere to be retained for a longer period. A planet between 5 and 8 billion years old seems to be optimum, an age at which geothermal heat and protective magnetic fields persist. Temperature is also an interesting factor:

…higher temperatures than currently existing on Earth seem to be more favorable. The caveat is that the necessary moisture has to be available as well because inland deserts with low biomass and biodiversity are also common on our planet. One example is the early Carboniferous period, which was warmer and wetter (Raymond, 1985; Bardossy, 1994) on our planet than today, with so much biomass produced that we still harvest the organic deposits in the form of coal, oil, and natural gas from it. Thus, a slightly higher temperature, perhaps by 5°C—similar to that of the early Carboniferous time period—would provide more habitable conditions until some optimum is reached. However, this will depend on the biochemistry and physiology of the inhabiting organisms and the amount of water present.

Image: This is Figure 1 from the paper. Caption: Star-planet distances (along the abscissa) and mass of the host star (along the ordinate) of roughly 4500 extrasolar planet and extrasolar planet candidates. The temperatures of the stars are indicated with symbol colors (see color bar). Planetary radii are encoded in the symbol sizes (see size scale at the bottom). The conservative habitable zone, defined by the moist-greenhouse and the maximum greenhouse limits (Kopparapu et al., 2013) is outlined with black solid lines. Stellar luminosities required for the parameterization of these limits were taken from Baraffe et al. (2015) as a function of mass as shown along the ordinate of the diagram. The dashed box refers to the region shown in Fig. 2. Data from exoplanets.org as of May 20, 2019. Color images are available online.

Here is the complete list of factors that lead to superhabitability as listed in the paper. 9 of the 24 planets identified orbit K stars, while 16 of them are between 5 and 8 billion years old. None of these worlds meet all the criteria, in any case. To be fully identified as superhabitable, a planet would meet the following benchmarks:

  • In orbit around a K dwarf star
  • About 5–8 billion years old
  • Up to1.5 more massive than Earth and about 10% larger than Earth
  • Mean surface temperature about 5°C higher than on Earth
  • Moist atmosphere with 25–30% O2 levels, the rest mostly inert gases (e.g., N2)
  • Scattered land/water distributed with lots of shallow water areas and archipelagos
  • Large moon (1–10% of the planetary mass) at moderate distance (10–100 planetary radii)
  • Has plate tectonics or similar geological/geochemical recycling mechanism as well as a strong protective geomagnetic field

As I mentioned, none of the planets of interest identified in this paper is known to have all these characteristics, but we have to keep in mind “the uncertainties in our mostly qualitative model and…the uncertainties in the observed parameters” as we examine the list. Moreover, only two of these — Kepler 1126 b and Kepler-69c — have been validated; the others continue to be planet candidates (Kepler Objects of Interest, or KOIs), and some may turn out to be false positives, while one of the confirmed worlds, Kepler-69c is, at almost 2000 light years, too far out to be a promising candidate for JWST, or even possible successors like LUVOIR.

The near-term question arising from this initial cut at superhabitability is whether we can use these criteria to find worlds closer than 100 light years, and thus accessible for high-quality observations from the TESS mission. The list of criteria presented here should help to establish observing priority for any worlds meeting a large number of these standards that can be identified in this distance range, making them high-value targets for investigating biosignatures.

The paper is Schulze-Makuch, Heller & Guinan, “In Search for a Planet Better than Earth: Top Contenders for a Superhabitable World,” Astrobiology 18 September 2020 (full text). The Lingam & Loeb paper is “Physical Constraints on the likelihood of life on exoplanets,” International Journal of Astrobiology Vol. 17, No. 2 (April, 2018), 116-126 (abstract), but see also the same authors’ later paper “Is life most likely around Sun-like stars?,” Journal of Cosmology and Astroparticle Physics Vol. 2018, May 2018 (abstract). Citations to earlier work from René Heller and colleagues on the question of superhabitability can be found in the Centauri Dreams stories I linked to above.

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