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Simultaneous Growth of Star and Planet?

The evolving system known as [BHB2007] 1 is a part of the Pipe Nebula (also called Barnard 59), about 600 light years away in the constellation Ophiuchus. It is part of a binary star system in formation that has been studied with the Atacama Large Millimeter Array (ALMA). Both protostars show disks in formation around them, surrounded by filaments of gas and dust drawn from the larger disk that are being referred to as ‘feeding filaments.’ Paola Caselli (Max Planck Institute for Extraterrestrial Physics, Germany), who made that reference in 2019, is co-author of new work on the stellar object, which gives us an unusual look at early system formation.

Image: This false-color image shows the filaments of accretion around the protostar [BHB2007] 1. The large structures are inflows of molecular gas (CO) nurturing the disk surrounding the protostar. The inset shows the dust emission from the disk, which is seen edge-on. The “holes” in the dust map represent an enormous ringed cavity seen sideways in the disk structure. Credit © MPE.

This is apparently a case of star and planet growing in tandem. The young stellar object [BHB2007] 1 is thought to be no older than 1 million years, with most of the surrounding envelope having dissipated and clear emission visible from the circumstellar disk. The authors note the clean, wide gap in the dust, adding that it is surprising for such a young system, for accretion from the molecular cloud seems to be continuing, as the filament activity makes clear.

Most circumstellar disks in this age range are fully formed as planet formation begins. Felipe Alves (MPE) is lead author on the paper, which has just appeared in Astrophysical Journal Letters:

“We were quite surprised to observe such prominent accretion filaments falling into the disk. The accretion filament activity demonstrates that the disk is still growing while simultaneously nurturing the protostar.”

Supplementing the ALMA data with observations at radio frequencies from the Very Large Array, the researchers make the case for a young giant planet or perhaps a brown dwarf present within a 70 AU cavity inside the disk, a zone filled with hot molecular gas. Planetary accretion of an object between 4 and 70 Jupiter masses would produce the disk gap.

Image: Two different observations of the protoplanetary disk show signatures of the formation of a companion to the protostar . The grey scale represents the dust thermal emission from the disk, same as in the inset of Fig. 1. The red/blue contours show the molecular CO brightness emission levels from the northern/southern side of the dust cavity observed with ALMA. The brighter CO emission from the south indicates that the gas is hotter there. This location coincides with a zone of non-thermal emission tracing ionised gas (green contours) observed with the VLA (middle), which is observed in addition to the protostar (centre of the image). The team proposes that both the ionised gas and the hot molecular gas are due to the presence of a protoplanet or a brown dwarf in the cavity. The configuration of such a system is shown in the sketch on the right. Credit: © MPE; illustration: Gabriel A. P. Franco.

MPE’s Caselli explains the significance of the finding:

“We present a new case of star and planet formation happening in tandem. Our observations strongly indicate that protoplanetary disks keep accreting material also after planet formation has started. This is important because the fresh material falling onto the disk will affect both the chemical composition of the future planetary system and the dynamical evolution of the whole disk.”

Studying star and planet formation together gives us interesting constraints for how young systems evolve out of the original cloud surrounding them. [BHB2007] 1 gives us the kind of gap in a circumstellar disk we’ve observed in other young systems, but in this case it’s a disk still being actively fed by filaments from the surrounding molecular cloud. In at least one case, then, we see the possible formation of planets before the disk itself has fully formed. The existence of an object in formation is supported by VLA’s data on radio emission within the dust gap between the inner and outer disk, giving heft to the assertion that simultaneous formation is occurring.

The paper is Alves et al., “A case of simultaneous star and planet formation,” Astrophysical Journal Letters Vol. 904, No. 1 (19 November 2020). Abstract / Preprint.


Arecibo in Petition and Poetry

I’m tracking an online petition conceived by Jorge Santiago Ortiz that challenges the National Science Foundation: Repair the Arecibo Observatory, do not decommission it. Given Friday’s news of the planned shutdown due to problems with support cables and the dangers of possible repairs, it’s good to see an effort being made to explore the possible. Ortiz points out that the observatory employs more than 120 people, is visited by some 200 scientists every year working on research projects, and draws 100,000 visitors yearly from the general population.

I notice the petition is approaching 6,000 signatures this morning as people react to the Arecibo news. It is possible there is a path toward keeping the observatory alive? Also noted by Centauri Dreams reader Jeff Brandt, himself a resident of Puerto Rico, is an attempt to free the facility from National Science Foundation funding and repair the structure.

Brandt notes that Jenniffer González-Colón, Puerto Rico’s representative in the US Congress, has sent a letter to the House & Senate Appropriations Committee requesting funding to stabilize the structure, as you can see below. We’ll keep an eye on both efforts.

It seems apropos in discussing Arecibo to give a nod to the Cornell connection, given the university’s involvement in its inception and subsequent management. The radio telescope was conceived by Cornell professor William E. Gordon, while its early scientific investigations were coordinated by Thomas Gold, who created the Cornell Center for Radiophysics and Space Research. Looking back, the university’s Jonathan Lunine comments on its significance:

“Arecibo has been an incredibly productive facility for nearly 60 years. For the Cornell scientists and engineers who took a daring dream and realized it, for the scientists who made new discoveries with this uniquely powerful radio telescope and planetary radar, and for all the young people who were inspired to become scientists by the sight of this enormous telescope in the middle of the island of Puerto Rico, Arecibo’s end is an inestimable loss.”

Thoughts Upon Hearing the Arecibo Radio Observatory was About to be Closed

Which brings me to Henry Cordova, who has graced these pages before with his SETI Reality Check, and who now looks back at a visit to Arecibo in the observatory’s early days in an essay that was written, I hasten to add, before the recent news of the site’s decommissioning. Trained as an astronomer and mathematician, Henry’s interest in the ways in which we observe the stars continues unabated. Arecibo clearly called out the poetry in him, as it did in me.

by Henry Cordova

I visited Arecibo Observatory in 1971, I was in Puerto Rico on business, and I took a Sunday off to visit the place. It’s a two hour drive from San Juan, and nestled in some pretty spectacular jungle-covered Karst topography: a very beautiful drive into an isolated and haunted countryside.

When I arrived the place was deserted. There was a small building, similar to a motel, where I supposed visiting researchers were quartered; but nobody was home. The permanent staff probably had houses in town (Arecibo proper is about a half-hour drive further north, on the coast). Next door, the control room was visible; through the locked glass doors I could see electronic equipment, powered up, but no one was there. Only my car was in the parking area. At the edge of the lot was a little observation platform where you could walk right up to the edge of the dish itself. It spread before me, filling a vast natural depression. The feeling was very much like standing at the edge of Meteor Crater in Arizona, except I could see suspended above me, on huge white towers, the receivers placed at the focus of the parabola.

The silence, the isolation, the grandeur of it all really affected me. The sheer audacity of the structure, the combination of natural beauty and technological brilliance was almost overpowering. I imagine it would be very similar to be standing alone at Stonehenge on a sunny windy day, accompanied only by ghosts.

Observatories are holy places. They are as impressive and beautiful as a medieval cathedral and by necessity are usually located in lonely and desolate landscapes. Like cathedrals, they are temples to the ineffable, to the incredibly remote, and to our faith in being able to connect with it–places of worship, in a way, sacred places. I know it’s sentimental and impractical of me, but if this site is to be abandoned, let it not be replaced with a farm or village or reservoir or some other practical symbol of the economy. Let it naturally decay into ruins, as a monument to our boldness, and to our stupidity. Centuries from now, men will stand in that place and say ‘we once explored the stars from here’.


On Losing Arecibo

I always wanted to get to Arecibo, the magnificent 305-meter telescope that has for so long been a locus for radio astronomy research, but I was never able to make it to Puerto Rico. Now I’ve run out of time. The National Science Foundation doesn’t make these decisions lightly but multiple engineering companies have delivered assessments that point to catastrophic failure of the telescope structure as a real possibility. Too dangerous to repair, and faced with stability issues even if it could be repaired, the Arecibo Observatory will be decommissioned.

The breakdown in the vast structure has been ongoing, bits and pieces of news that added further dismay to an already dismal 2020. A support cable detached in August, resulting in an evaluation from the University of Central Florida, which manages the site. Replacement auxiliary cables were then on the way, temporary cables available, but on November 6 another main cable broke. The stresses on the second cable evidently told the story, making it clear that to proceed with repair would be to push against acceptable standards of safety.

Ralph Gaume, director of NSF’s Division of Astronomical Sciences, sums up the situation:

“Leadership at Arecibo Observatory and UCF did a commendable job addressing this situation, acting quickly and pursuing every possible option to save this incredible instrument. Until these assessments came in, our question was not if the observatory should be repaired but how. But in the end, a preponderance of data showed that we simply could not do this safely. And that is a line we cannot cross.”

Image: Arecibo Observatory’s 305-meter telescope in November of 2020. Credit: University of Central Florida.

It’s going to take some time for me to get my head around losing Arecibo, which has been since the early 1960s a part of my mental landscape when contemplating our civilization and its context in the cosmos. I had gotten to thinking in terms of ‘Arecibos’ of transmitting power, meaning that a dish like Arecibo could pick up an installation of comparable power over a span of 1,000 light years, a volume in which there are more than 10,000,000 stars. Ideas like that fueled my interest in SETI, which grew into a general passion for exoplanet research.

Remember, although we sometimes hear 51 Pegasi b referred to as the first exoplanet discovered, the honor actually belongs to the planets at the pulsar PSR B1257+12, which were found three years earlier in 1992 by Aleksander Wolszczan and Dale Frail using Arecibo data (51 Pegasi b was the first exoplanet found around a main sequence star, a valid distinction since pulsar planets are an unusual extreme as we contemplate the conditions extant in planetary systems). Speaking of pulsars, the first binary one was uncovered in Arecibo data in 1974 by Russell Hulse and Joseph H. Taylor, Jr., a find that earned the duo the Nobel Prize for Physics in 1993.

Arecibo has produced radar maps of Venus and Mercury and has charted near-Earth asteroids. As far back as 1965, it uncovered the actual rotation rate of Mercury (59 days as opposed to the previously believed 88). It analyzed the pulsar from the Crab Nebula supernova remnant in 1968 and performed the first radar ranging to an Earth-crossing asteroid (1862 Apollo) in 1980. Its planetary radar found evidence for hydrocarbon lakes on Titan and the observatory was used to study frequencies in the 1,000 MHz to 3,000 MHz range as part of the SETI effort.

Indeed, the list of accomplishments is far too long to list here, so I’ll direct you to this summary page. I do want to mention in the SETI context (although it was a matter of transmitting rather than listening) the 1974 message sent toward the globular cluster M13 by Frank Drake, primarily performed, I’m told, as a way of demonstrating what newly installed equipment could do. Even so, that transmission is an oft-cited marker in our thinking about our own place in the universe and the possibility of other technological civilizations we might encounter.

Invariably I think of Jill Tarter in terms of SETI, in this context because of her involvement in Project Phoenix, which moved to Arecibo in 1998 after stints at Parkes (Australia) and Green Bank (WV). As soon as I learned of Arecibo’s decommissioning, I wrote to ask for a comment. My own reflections on Arecibo hardly match the poetic depth of Dr. Tarter’s response:

I’ve been going to Arecibo since 1978. Over the decades, we’ve built a lot of Arecibo-specific hardware, written a lot of software, and bent the telescope control system into modes it was never designed for.

Arecibo was an impressive feat of engineering, a scientific workhorse, and it never lost that aura of being slightly exotic, no matter how many times I visited there; the constant croaking of the coquis [a frog common to Puerto Rico], the perfumes of the tropical forest, the local Ron del Barrelito [a rum said to be the best on the island], the Gregorian dome with its unmistakable compressor cadence, the jogging track underneath the dish ringed with small orchids, Orion rising over the treetops as seen from the balcony of the VSQ [Visiting Scientists Quarters], before heading off to my midnight shift of Project Phoenix observations, and the absolutely best view on the island from atop the platform.

But most of all I remember the staff and the resident scientists who were very close knit, offered us superb technical support, and threw wonderful parties with lots of dancing. It is very sad to witness the passing of this scientific Queen. She withstood powerful hurricanes, but age appears to have gotten the upper hand.

Arecibo’s demise also led me to touch base with Greg Matloff (New York City College of Technology (NYCCT)), whose work on interstellar propulsion was what originally drew me to the field (his Starflight Handbook, written with Eugene Mallove, was a frequently consulted text and provoked the research that led to my Centauri Dreams book in 2004). We had discussed Arecibo’s role in planetary protection in many conversations. Said Matloff:

“The loss of Arecibo is heartbreaking. This observatory has contributed so much to the study of Earth’s upper atmosphere, Solar System and deep sky objects, and SETI. But its greatest significance today is its service as planetary radar. Much of what we know about Near Earth Asteroids that might someday impact the Earth is due to the imaging capabilities of this instrument. It is my hope that an upgraded Arecibo can be constructed at the same site to continue this work of Earth Defense.”

That’s a hope many of us share, and we will see what comes of it. I do notice in the National Science Foundation’s materials on the decommissioning that NSF intends “to restore operations at assets such as the Arecibo Observatory LIDAR facility — a valuable geospace research tool — as well as at the visitor center and offsite Culebra facility, which analyzes cloud cover and precipitation data. NSF would also seek to explore possibilities for expanding the educational capacities of the learning center.”

What may emerge following telescope decommissioning is worth pondering.


DESTINY+: Mission to 3200 Phaethon

With successful operations at Ryugu (Hayabusa2) and Bennu (OSIRIS-REx), asteroid exploration seems to be moving full tilt, with the prospect of surface samples on the way. We can also look ahead to 16 Psyche, the object of interest for a NASA mission planned to launch in 2022, and the Lucy mission to Jupiter’s trojan asteroids, with launch now scheduled for 2021. The latest asteroid entry comes in the form of an interesting collaboration between the Japan Aerospace Exploration Agency (JAXA) and the German Aerospace Center (DLR) targeting asteroid 3200 Phaethon, a flyby mission designed to launch no earlier than 2024.

DESTINY+ is its name, the acronym standing for Demonstration and Experiment of Space Technology for INterplanetary voYage with Phaethon fLyby and dUst Science (try to say all that quickly before you’ve had your morning coffee). The agreement for the bilateral mission was signed on November 11 as part of a joint strategy dialogue between the two space agencies.

The new mission continues German and Japanese cooperation in space. You may recall that DLR and the French space agency CNES (National Center for Space Studies) developed the MASCOT rover that was deployed on Ryugu in 2018, with several papers already published on its findings. The BepiColombo mission to Mercury grows out of ESA and Japanese collaboration, while the proposed Martian Moons eXploration (MMX) mission, developed by JAXA, will include a rover developed by DLR and CNES, along with scientific instrumentation from NASA.

At Phaethon, we’re looking at an interesting object, an Apollo asteroid that appears to be the parent body of the Geminids meteor shower. The Apollo asteroids are the largest group of near-Earth objects, numbering over 10,000, but among them, Phaethon has a special claim to fame. Carsten Henselowsky is DESTINY+ project manager at DLR:

“With a minimum approach distance of approximately 21 million kilometres, Phaethon gets closer to the Sun than the planet Mercury. In the process, its surface heats up to a temperature of over 700 degrees Celsius, causing the celestial body to release more dust particles. The aim of the DESTINY+ mission is to investigate such cosmic dust particles and to determine whether the arrival of extraterrestrial dust particles on Earth may have played a role in the creation of life on our planet.”

Image: Animation of Phaethon’s orbit. Credit: Phoenix7777. Data source: JPL HORIZONS. CC BY-SA 4.0.

The flyby will take the spacecraft to within 500 kilometers of Phaethon at a time in its orbit when the asteroid is approximately 150 million kilometers from the Sun. Key to the success of the mission is the DDA dust analyzer built by DLR (DESTINY+ Dust Analyzer). The plan is to study the dust environment through the entire four-year cruise to Phaethon, with projected flyby in 2028. The DDA is, according to this recent presentation at the Europlanet Science Congress, “the technological successor to the Cosmic Dust Analyzer (CDA) aboard Cassini-Huygens, which prominently investigated the dust environment of the Saturnian system.”

With the goal of determining the origin of each dust particle, mission scientists will be focusing on the proportion of organic matter given the possible delivery of organics to the early Earth by such particles. The asteroid is on a highly eccentric orbit resembling that of a comet more than an asteroid, crossing the orbits of Mercury, Venus, Earth and Mars. Dust tails have been observed coming from the object, likely the result of solar heat creating surface fractures.

Launch will be aboard an Epsilon S vehicle from the Uchinoura Space Center in Japan. The spacecraft will use solar-electric propulsion, with a 1.5-year period after launch in which it will raise its orbit. A series of lunar flybys will then accelerate the probe into an interplanetary trajectory. The scientific payload will, in addition to the dust analyzer, include two cameras: The Telescopic Camera for Phaethon (TCAP) and the Multi-band Camera for Phaethon, MCAP.

By chance, I ran across a passage this morning in Oliver Morton’s book The Moon: A History for the Future (The Economist, 2019) that explains why it takes so long to get to objects that, like NEAs, sound as though they should be nearby. Such serendipity coaxes me into quoting it:

…what matters in space is delta-v, not distance. To get from the Moon to the Earth requires only about a fifth of the delta-v that is needed to make the same journey in the opposite direction. Another demonstration of this proposition is that the amount of delta-v it takes to get a spacecraft to the surface of the Moon can also get it to destinations much farther afield. “Near-Earth asteroids” (NEA) are only near inasmuch as they have orbits that very occasionally bring them moderately close to the Earth. At any given time a typical NEA will be 100 or 200 times as far away as the Moon. But in terms of delta-v, quite a lot of these asteroids are just as easy to reach as the Moon is; it is just a matter of getting on to the right trajectory and waiting. Indeed, it takes no more delta-v to reach the little moons of Mars than it does the great Moon of Earth (the surface of Mars is another matter).

Image: Asteroid explorer DESTINY+. Credit: JAXA/Kashikagaku.


Exoplanet Atmospheres: Keeping Up with ARIEL

How is a planet’s composition related to its host star? The European Space Agency’s ARIEL mission (Atmospheric Remote-sensing Infrared Exoplanet Large-survey) is designed to probe the question, examining planetary atmospheres to determine the composition, temperature and chemical processes at work in a large sample of planetary systems.

Transmission spectroscopy is the method, examining spectra as known exoplanets pass in front of, then behind their host stars. Researchers will use light filtering through the atmospheres to unlock the chemical processes within each. ARIEL will survey about 1,000 planetary systems in both visible and infrared wavelengths, probing not just chemistry but the thermal conditions that affect their composition. The mission’s focus is on super-Earths to gas giants, all with temperatures greater than 320 Celsius.

I suspect that principal investigator Giovanna Tinetti (University College London) has been asked about the choice of targets to the point of exhaustion, but one reason for focusing on planets in this size and temperature range is our need to build up a catalog of atmospheres that will inform the entire field, so that when we drill down to small, rocky worlds using future instruments, we’ll have a context in which to place what we see. A high temperature atmosphere is helpful here because it remains in continuous circulation, without the obscuring clouds that make characterization difficult.

Image: Giovanna Tinetti (University College London), principal investigator for ARIEL.

ARIEL will be positioned around L2, the second Sun-Earth Lagrange point, 1.5 million kilometres directly ‘behind’ Earth as viewed from the Sun. A just released ESA report explains the issues the mission will investigate, noting the wide range of planets and stars:

This large and unbiased survey will contribute to answering the first of the four ambitious topics listed in the ESA’s Cosmic Vision: “What are the conditions for planet formation and the emergence of life?”. Thousands of exoplanets have now been discovered with a huge range of masses, sizes and orbits: from rocky Earth-like planets to large gas giants grazing the surface of their host star. There is no known, discernible pattern linking the presence, size, or orbital parameters of a planet to the nature of its parent star. We have little idea whether the chemistry of a planet’s surface and atmosphere is linked to its formation environment, or whether the type of host star drives the physics and chemistry of the planet’s birth and evolution.

Remember, too, the statistical nature of the inquiry. The sample population is large, so that we move from the small number of atmospheres currently characterized to hundreds. Our understanding of the early stages of planet and atmosphere formation during the first few million years in an infant system as it emerges from the nebular phase should help us relate the chemistry in exoplanets to their other parameters and to the chemical environment of the star.

Image: An example spectrum Ariel could measure from light passing through an exoplanet’s atmosphere. Credit: ESA/STFC RAL Space/UCL/UK Space Agency/ATG Medialab.

Key components — water vapor, carbon dioxide, methane — of planetary atmospheres will be detected, but also more unusual metallic compounds that define the chemical environment within each system studied. For a smaller number of target planets, the spacecraft will perform a survey of cloud systems and atmospheric changes at a seasonal and daily level. Says ARIEL project scientist Theresa Lueftinger: “Our chemical census of hundreds of solar systems will help us understand each planet in context of the chemical environment and composition of the host star, in turn helping us to better understand our own cosmic neighbourhood.”

ESA has just announced that ARIEL has moved from study to implementation phase, the step before negotiations begin with industrial contractors to build the spacecraft, which is scheduled for launch from Kourou (French Guiana) in 2029. Bids on spacecraft hardware will be requested within months, with the prime contractor chosen by the summer of 2021. 50 institutes from 17 European countries are involved, as is NASA, in the payload module, which will have at its heart a one-meter class cryogenic telescope along with associated science instruments.

Three ESA missions with an exoplanet charter are thus framed within a ten-year window, with ARIEL joined by CHEOPS (CHaracterising ExOPlanet Satellite), launched in December 2019, and PLATO (PLAnetary Transits and Oscillations of stars), to be launched in 2026. The latter emphasizes rocky planets in the habitable zone of Sun-like stars.

To keep up with ARIEL, you may want to follow @ArielTelescope on Twitter. You’ll find background information in the recently published ARIEL Definition Study Report, available here.


A New Source for Plumes on Europa

How salty should we expect the ice on Europa’s surface to be? It would be helpful to know, because the salinity of the surface will be a factor in how transparent the ice shell is to radar waves. Europa Clipper will fly with an instrument called REASON — Radar for Europa Assessment and Sounding: Ocean to Near-surface — which will be investigating both the surface ice and the ocean beneath. Recent research, in which its principal investigator, Don Blankenship (University of Texas), is involved is offering insights into the salinity of the ice.

Here’s a bit of background on REASON from a NASA page on Europa Clipper:

Depending on their wavelength, radio waves can either bounce off or penetrate different materials. REASON will use high frequency (HF) and very high frequency (VHF) radio signals to penetrate up to 18 miles (30 kilometers) into Europa’s ice to look for the moon’s suspected ocean, measure ice thickness, and better understand the icy shell’s structure. The instrument will also study the elevation, composition, and roughness of Europa’s surface, and will search Europa’s upper atmosphere for signs of plume activity.

Image: Europa Clipper, scheduled for launch in 2024. Credit: NASA/JPL-Caltech.

So plumes are in play for Europa Clipper, and the new research finds evidence of a process that can produce them. The paper in Geophysical Research Letters focuses on what the authors call brine migration, in which small pockets of salty water migrate within the ice to warmer icy areas. We get into questions of salinity in this work because migration of icy brines, analyzed here through study of the impact crater Manannán, may have resulted in the creation of a plume. Imaging data from the Galileo mission allowed the team to study the resulting surface feature and to calculate that Europa’s ocean is about a fifth as salty as Earth’s.

But let’s back up to that plume. If eruptions from the ocean below periodically break through the ice, we might have a way to sample ocean materials without ever trying to drill through the surface shell. The new work, led by Gregor Steinbrügge (Stanford University) homes in on Manannán, a 30-kilometer wide crater on Europa that is the result of an impact some tens of millions of years ago. The paper models the melting and refreezing of water following the event. And it suggests that not all plumes carry materials from the ocean below.

Manannán is located on Europa’s trailing hemisphere at 3°0’N and 120°50’E. The crater was imaged by the Galileo spacecraft’s Near-Infrared Mapping Spectrometer. Subsequent geological mapping of Manannán has shown what appear to be impact melt materials filling the crater floor, along with ejecta deposits that would have been brought up from below at the time of impact.

According to the researchers, as water transformed back into ice following the impact, pockets of water with higher salt content were created in the surface, migrating sideways through the shell by melting adjacent areas of less salty ice. “We developed a way that a water pocket can move laterally – and that’s very important,” says Steinbrügge. “It can move along thermal gradients, from cold to warm, and not only in the down direction as pulled by gravity.”

Image: This illustration of Jupiter’s icy moon Europa depicts a cryovolcanic eruption in which brine from within the icy shell could blast into space. A new model proposing this process may also shed light on plumes on other icy bodies. Credit: Justice Wainwright.

Pockets of brine moving about within the surface become saltier as they move through less salty water around them and eventually erupt, according to this model, which shows freezing and pressurization as the factors leading to a cryovolcanic event. At Manannán crater, a migrating brine pocket finally freezing at the center generated a plume estimated to be 1-2 kilometers high, leaving a surface feature, roughly spider-like in shape, that turned up in the Galileo data. The spider-shaped fractures consist of 17 segments, surrounded by a series of concentric faults. This would have been a relatively small plume, and the Manannán findings do not explain what may be larger plumes hypothesized based on Hubble and Galileo data.

Image: The ‘spider’ feature within Manannán impact crater. Credit: NASA.

The point is that brine pocket migration is a surface phenomenon that can generate plumes, implying that such plumes do not require a connection with the ocean beneath. Whether there are other, larger plumes that do make such connections has not yet been determined.

Oceanic origin or not, all of this makes Europa’s surface an even more dynamic place than we’ve been considering, while somewhat tempering our expectations for astrobiology in plume activity even as we continue to observe the moon for future, perhaps much larger plumes. The team’s modeling of how melting and subsequent freezing of a water pocket within the icy shell would produce an eruption should have implications for other icy bodies within the Solar System. Robert Pappalardo (JPL) is a project scientist on the Europa Clipper mission:

“The work is exciting, because it supports the growing body of research showing there could be multiple kinds of plumes on Europa. Understanding plumes and their possible sources strongly contributes to Europa Clipper’s goal to investigate Europa’s habitability.”

The paper is Steinbrügge et al. ,”Brine Migration and Impact‐Induced Cryovolcanism on Europa,” Geophysical Research Letters 5 November 2020 (abstract).


Radioactive Elements and Planetary Habitability

A planetary dynamo may be a key factor in creating the conditions needed for life. And creating that dynamo seems to depend on the radioactive decay of thorium and uranium, generating internal heating and driving plate tectonics. Let’s carry this line of thought further, though, as the authors of a new paper out of UC-Santa Cruz do, and point out that these heavy elements are necessary to create a magnetic field like Earth’s, which protects us from damaging radiation.

In the rocky planets, magnetic fields are generated by convection in a metallic core, which in turn is driven by heat extracted into the mantle (Nimmo 2015; Labrosse 2015; Boujibar et al. 2020). Since mantle radiogenic heat production controls how much heat is extracted from the core, it will also influence the presence or absence of a dynamo. Similarly, heat production will control the mantle temperature and thus the rate of silicate melting and volcanism.

That quote is from the paper, whose lead author is Francis Nimmo, and it points to the key role of radiogenic heat in the mantle. Within the Earth, there are enough radioactive elements to generate a persistent ‘geodynamo,’ in which molten iron and nickel in motion in the planet’s outer core produce electric currents, forming a magnetic field as the Earth rotates. The authors find that Earth “had just enough radiogenic heating to maintain a persistent dynamo.” And that has implications for the atmosphere.

UC-Santa Cruz’ Natalie Batalha is director of the university’s Astrobiology Initiative, whose multidisciplinary work is designed to produce the kind of widespread collaboration found in this paper. Says Batalha:

“It has long been speculated that internal heating drives plate tectonics, which creates carbon cycling and geological activity like volcanism, which produces an atmosphere. And the ability to retain an atmosphere is related to the magnetic field, which is also driven by internal heating.”

To ponder how all these effects come together, we have only to look at Mars, where the lack of a magnetic field can be tied to the gradual loss of an atmosphere that was once thick enough to support water on the surface and, potentially, a living environment. The planet’s low gravity was one factor in the gradual thinning of the atmosphere, but the solar wind of charged particles from the Sun is likewise a problem, for without a protective magnetic field, the atmosphere erodes.

We have an angle here into habitability on planets around other stars, for the amount of radioactive material is going to vary from one stellar system to another. The so-called r-process elements — created by nuclear reactions that are responsible for producing roughly half of the atomic nuclei heavier than iron — owe their origin to supernova events and the mergers of neutron star remnants, events that will be stochastic and result in planets with varying degrees of radiogenic materials. Back to the paper on this point:

From the point of view of long-term radiogenic heating, the most important elements are potassium, thorium, and uranium. Th and U are both r-process elements, most likely produced in neutron star mergers (NSMs) (Kasen et al. 2017; Cowan et al. 2019), with possibly significant contributions also from massive star collapse explosions (Siegel et al. 2019; Macias & Ramirez-Ruiz 2019). Because such events are rare (an occurrence rate of tens/Myr in our Galaxy) and produce large quantities of r-process elements, the resulting concentration of r-process elements should vary considerably. This is especially true of low-metallicity stars, because turbulent mixing of the interstellar medium is inefficient in the early Galaxy (Shen et al. 2015; Naiman et al. 2018). This expectation is in agreement with measurements of r-process europium in low-metallicity stars (e.g., Macias & Ramirez-Ruiz 2018) and with r-process abundances in dwarf galaxies (e.g., Ji et al. 2016).

Lead author Francis Nimmo realized that if planets vary in the amount of radioactive elements, we should then expect geological activity and magnetic fields to vary as well. The UC-Santa Cruz scientists essentially adjusted radiogenic heat production in their models to see how this factor affected planetary habitability. As Nimmo puts it:

“We took a model of the Earth and dialed the amount of internal radiogenic heat production up and down to see what happens. Just by changing this one variable, you sweep through these different scenarios, from geologically dead to Earth-like to extremely volcanic without a dynamo.”

When radiogenic heating is dialed up higher than the Earth’s, no dynamo is produced. With most of the thorium and uranium in the mantle, the excessive heat becomes an insulator, so that the loss of heat from the molten core is slowed. The result: Heat loss from the core cannot generate convection strong enough to produce a magnetic field, with all that implies about the survival of an atmosphere and radiation flux on the surface.

Given all that, the associated volcanic activity produced by greater levels of radiogenic internal heating is almost lost in the shuffle, but it could obviously be a factor in mass extinction events. And while that implies that high levels of radiogenic heating are a key factor in suppressing a habitable environment, the study also shows that too little heat results in no volcanism whatsoever. We wind up with a planet that, like Mars, appears geologically dead.

A new study identifies internal heating from radioactive decay as a critical factor in a planet’s ability to generate a magnetic field and retain an atmosphere. Credit: Illustration by Melissa Weiss for UC Santa Cruz

Image: These illustrations show three versions of a rocky planet with different amounts of internal heating from radioactive elements. The middle planet is Earth-like, with plate tectonics and an internal dynamo generating a magnetic field. The top planet, with more radiogenic heating, has extreme volcanism but no dynamo or magnetic field. The bottom planet, with less radiogenic heating, is geologically “dead,” with no volcanism. Caption credit: UC-Santa Cruz.

So we are working with a single variable, one that can be studied via spectroscopy to measure the abundance of the various elements in stars, the assumption being that planets will have the same composition as the stars they orbit. Here the rare earth element europium comes into play, readily observed in stellar spectra, and created by the same processes that produce thorium and uranium. In this work, europium is the tracer for studying radiogenic abundance.

We already have europium measurements for many nearby stars, which allowed Nimmo and team to set a range of inputs for their models of radiogenic heating, with the Sun’s composition establishing the middle of the range. Europium thus becomes a measurement that can flag radiogenic elements in exoplanetary systems as we look forward to the deployment of the James Webb Space Telescope. The europium measurement may well show us which stellar systems are likely to produce planets with the internal composition life would need to survive. From the paper:

…higher mantle heating rates increase volcanic activity and shut off the dynamo, both likely deleterious for habitability. In contrast, reduced mantle heating rates will at some point stop melt production. A potential consequence is that plate tectonics will then cease (Louren¸co et al. 2018), and with it, the dynamo, as at Venus (Nimmo 2002). It is tempting to speculate that the Earth is habitable in part because it possesses a “Goldilocks” concentration of radiogenic elements: high enough to permit long-lived dynamo activity and plate tectonics, but not so much that extreme volcanism and dynamo shutoff occur. As yet, however, our understanding of the complicated feedbacks involved is too rudimentary to make such a definitive conclusion; more detailed calculations, and better characterization of the radiogenic element abundances in planet-hosting stars, are both required.

The paper is Nimmo et al., “Radiogenic Heating and Its Influence on Rocky Planet Dynamos and Habitability,” Astrophysical Journal Letters Vol. 903, No. 2 (10 November 2020). Abstract / Preprint.


An Unusual (and Promising) Brown Dwarf Detection

The naming of names is quite a project when it comes to new astronomical objects, and given the sheer numbers — 300 million habitable planets around G- and K-class stars, for example — we might do better to stick with simple identifiers. On the other hand, it’s a bit charming that a new brown dwarf known by its identifier as BDR J1750+3809 has been dubbed ‘Elegast’ by the discovery team. This is the first substellar object found through radio observations.

The name is both appropriate and specific to the discovery space. Elegast appears in a poem in Middle Dutch (12th or 13th Century) called ‘Karel ende Elegast,’ with the character Elegast being a vassal of Charlemagne who seems to be king of the elves (Wikipedia to the rescue, vindicating once again my decision to send them a monthly donation).

The Dutch connection is that the radio work comes out of LOFAR (Low-Frequency Array), which is currently the largest radio telescope operating at the lowest frequencies that we can observe from Earth. Based in the Netherlands, LOFAR is an international effort with field stations in eight countries.

Elegast breaks from the norm in not being discovered from the kind of infrared sky surveys through which most other brown dwarfs have been found. You might recall our discussions of the WISE mission and the possibility, not realized, of finding a brown dwarf closer than the Alpha Centauri stars. But it’s also true that infrared surveys can’t detect objects that are sufficiently cold and faint, whereas we now have radio wavelengths to fill the gap.

Is there still some faint possibility of a closer brown dwarf? T-class dwarfs are defined as being less than 1500 Kelvin, but WISE identified Y-dwarfs with temperatures as cool as 25 °C (298 K), which gets us down to where my thermostat is set as I write this. WISE 1828+2650, I subsequently learned, is so cold that it emits no visible light at all and is more or less indistinguishable from a free-floating planet. Get any colder and infrared surveys won’t find such worlds, which is why the news about Elegast is so encouraging.

Given that the population of so-called ‘rogue’ planets may be vast, it’s wonderful to see a new method becoming available with which to study nearby examples. Brown dwarfs may not be able to create the kind of fusion reactions found in stars like the Sun, but they emit at radio wavelengths just as gas giant planets do. Jupiter’s emissions are well studied, the result of powerful magnetic fields around the planet accelerating charged particles to produce radio waves as well as aurorae. Brown dwarfs are faint at these wavelengths but detectable.

Image: Artist’s impression of Elegast. The blue loops depict the magnetic field lines. Charged particles moving along these lines emit radio waves that LOFAR detected. Some particles eventually reach the poles and generate aurorae similar to the northern lights on Earth. Credit: ASTRON / Danielle Futselaar.

The LOFAR observations had to contend with background galaxies when investigating potential brown dwarfs. The key turned out to be identifying signals that were circularly polarized, which distinguishes planets and brown dwarfs from the background ‘noise.’ What this kind of polarization means is that the electromagnetic field of the radio waves rotates in a characteristic circular pattern. Says LOFAR project scientist Tim Shimwell: “We could not have picked out Elegast in our standard radio images from among the crowd of millions of galaxies, but Elegast immediately stood out when we made circularly polarised images.”

In the case of Elegast, the Gemini-North Telescope had produced archival imagery used to confirm the discovery, and the NASA Infrared Telescope Facility (IRTF) deployed an infrared spectrometer known as SpeX, recently upgraded, to obtain a spectrum of Elegast. The signature of methane, common in brown dwarfs and gas giant planets, was revealed.

Beyond its possibilities at identifying the faintest brown dwarfs and free-floating planets, radio astronomy may also be used to study exoplanet magnetic fields, testing theories on magnetic field strength on objects that have enough similarities to gas giants to make the investigation worthwhile. The team is now making follow-up observations on Elegast to measure its magnetic field even as they use LOFAR to look for more such objects. Harish Vedantham, an astronomer at the Netherlands Institute for Radio Astronomy (ASTRON) is lead author of the study:

“Our ultimate goal is to understand magnetism in exoplanets and how it impacts their ability to host life. Because magnetic phenomena of cold brown dwarfs like Elegast are so similar to what is seen on solar system planets, we expect our work to provide a vital datapoint to test theoretical models that predict the magnetic fields of extrasolar bodies.”

The paper is Vedantham et al., “Direct radio discovery of a cold brown dwarf,” Astrophysical Journal Letters Vol. 903, No. 2 (9 November 2020). Abstract / Preprint.


Europa: Night-time Glow a New Tool for Analysis

When it comes to Europa, it’s the surface that counts as we try to learn more about the ocean beneath. Maybe one day we’ll be able to get some kind of probe through the ice, but for now we have to think about things like upwellings of water that percolate up through cracks in the frozen landscape, and unusual areas like Europa’s ‘chaos’ terrain. Here, fractures and evident ‘rafts’ of ice show disruptions where the icy surface of the moon experiences Jupiter’s tidal effects.

Image: The surface of Jupiter’s moon Europa features a widely varied landscape, including ridges, bands, small rounded domes and disrupted spaces that geologists called “chaos terrain.” This newly reprocessed image, along with two others along the same longitude, was taken by NASA’s Galileo spacecraft on Sept. 26, 1998, and reveals details of diverse surface features on Europa. Credit: NASA/JPL-Caltech/SETI Institute.

What kind of materials might we find frozen into the cracks and grooves of such terrain? Europa Clipper will doubtless tell us more as it updates Galileo’s two-decade old data. Meanwhile, we recently learned about another way to study the moon’s surface composition. At the Jet Propulsion Laboratory, Murthy Gudipati and team have been doing laboratory work that informs our understanding of how Europa reacts to the inflow of high-energy radiation from Jupiter. The high flux of charged particles occurs because of interactions with Jupiter’s magnetic field.

The key to the work is the fact that different salty compounds produce their own unique signature as they respond to this radiation bath. A glow emerges — sometimes green to the eye, sometimes edging into blue or white — depending on the materials in question. Gudipati’s team used a spectrometer, as we would expect, to study the surface, but here the observations are taking place not with reflected sunlight on the dayside but Europa’s own glow at night. The paper on this work, which appeared in Nature Astronomy, shows us how much data could be gathered by a method that seems to have surprised the scientists involved.

JPL co-author Bryana Henderson explains:

“[W]e never imagined that we would see what we ended up seeing. When we tried new ice compositions, the glow looked different. And we all just stared at it for a while and then said, ‘This is new, right? This is definitely a different glow?’ So we pointed a spectrometer at it, and each type of ice had a different spectrum.”

Image: This illustration of Jupiter’s moon Europa shows how the icy surface may glow on its nightside, the side facing away from the Sun. New lab experiments re-created the environment of Europa and find that the icy moon shines, even on its nightside, due to an ice glow. As Jupiter bombards Europa with radiation, the electrons penetrate the surface, energizing the molecules underneath. When those molecules relax, they release energy as visible light. Variations in the glow and the color of the glow itself could reveal information about the composition of ice on Europa’s surface. Different salty compounds react differently to the radiation and emit their own unique glimmer. Color will vary based on the real composition of Europa’s surface. Credit: NASA/JPL-Caltech.

The process seems straightforward, but sometimes it takes replicating known conditions as closely as possible in a lab to discover the consequences. Start with ice mixed with salts like magnesium sulfate and sodium chloride. Bathe the mix in radiation. The glow is a natural result, its variations linked to different compositions of the ice. Those variations are what gives this work implications for missions like Europa Clipper.

Sodium chloride brine, for example, turned out to produce a lower level of glow in the team’s setup, which used an instrument built at JPL known delightfully as ICE-HEART (Ice Chamber for Europa’s High-Energy Electron and Radiation Environment Testing). The experiments were run at a high-energy electron beam facility in Gaithersburg, Maryland once ICE-HEART had been taken there, the original plan being to study organic material under the Europan ice. The continuous glow on Europa’s night side now emerges as a source of future data, one that can be compared with these laboratory results to identify salty components on the surface.

“It’s not often that you’re in a lab and say, ‘We might find this when we get there,'” Gudipati said. “Usually it’s the other way around – you go there and find something and try to explain it in the lab. But our prediction goes back to a simple observation, and that’s what science is about.”

So Europa Clipper can use night-time flybys to delve into the chemical composition below, a method that may turn out to be helpful on other Galilean moons, or wherever high doses of ionizing radiation sleet down upon a frozen surface. Specific to Europa Clipper’s instruments, we learn the following:

Observation of night-time, high-energy, radiation-induced ice glow on the trailing hemisphere could thus provide more constraints on the chemical composition of non-ice material, revealing whether sulfates or chlorides are present, what their counterions could be, and whether pure water-ice patches are detectable. Dark regions could imply sodium- and chloride-dominant surfaces, whereas brighter regions could imply magnesium- and sulfate-dominant surfaces in the absence of water ice. The presence or absence of water ice could unambiguously be determined by the MISE [Mapping Imaging Spectrometer for Europa] instrument, designed to span a wavelength range of 0.8 μm to 5 μm, covering ice absorption features in the daytime.

In addition, the glow of night-time ice can be measured against daylight observations from Europa Clipper’s Wide Angle Camera and the MISE spectrometer to dig deeper into the chemical composition below. Some ultraviolet features of the night-time spectra will allow mission scientists to flag temperature anomalies on the surface for future observations.

The paper is Gudipati et al., “Laboratory predictions for the night-side surface ice glow of Europa,” Nature Astronomy 9 November 2020 (abstract).


On 300 Million Habitable Zone Planets

We’ve talked about the Drake Equation a good deal over the years, but I may not have mentioned before that when Frank Drake introduced it in 1961, it was for the purpose of stimulating discussion at a meeting at the National Radio Astronomy Observatory in Green Bank, West Virginia that was convening to discuss the nascent field of SETI.

This was in the era of Drake’s Project Ozma and the terms of the SETI debate were hardly codified. Moreover, as Nadia Drake recounts in this absorbing look back at her father’s work in that era, Drake had spent the time immediately before the meeting trying to line up Champagne for UC-Berkeley biochemist Melvin Calvin, who was about to win the Nobel Prize.

So there was a certain ad hoc flavor to the equation, one that Drake assembled more or less on the fly to clarify the factors to be considered in looking for other civilizations. How Drake did all this while trying to locate a sufficient quantity of good Champagne in the rural West Virginia of 1961 is beyond me and adds to his mystique.

Image: Astronomer Frank Drake speaking at Cornell University in Schwartz Auditorium, 19 October 2017. Credit: Wikimedia Commons CC BY-SA 4.0.

Sparkling wine aside, the Drake Equation in various forms has continued to inform discussion. The likelihood of detecting alien civilizations could be approached by multiplying the seven factors Drake came up with, which are shown in the figure below. The number of detectable alien civilizations is N. The Drake Equation obviously relied on guesswork at the time, given that we knew little about the factors involved other than the rate of star formation.

Image: The Drake Equation. Credit: Ming Hsu (UC-Berkeley).

There’s still a lot of play in these numbers, of course, but it’s fascinating to watch the progress of exoplanetary science as we begin to fill in the numbers through actual observation. Notice in particular ne the number of planets, per star, that could support life. This value is what gets play in the recently released (on arXiv) paper from Steve Bryson and a large number of colleagues at the SETI Institute, NASA and a variety of other organizations.

What catches the eye is the figure of 300 million, which is the number the researchers give for potentially habitable planets in the Milky Way. Let’s drill into this a bit: The researchers are computing the occurrence of rocky worlds, defined here as planets within a certain range of radius (0.5 R ≤ r ≤ 1.5 R), orbiting stars with effective temperatures of 4,800-6,300 K. The host stars cover main-sequence dwarf stars from Kepler’s DR25 planet candidate catalog as well as stars in data compiled by the European Space Agency’s Gaia mission. As the authors note: “We base our occurrence rates on differential population models dependent on radius, instellation flux and host star effective temperature.”

This is a change of pace from the norm, so let’s turn to the paper:

Most of the existing literature on habitable zone occurrence rates are in terms of orbital period, where a single period range is adopted to represent the bounds of the habitable zone for the entire stellar population considered. However, no single period range covers the habitable zone for a wide variety of stars…While these period ranges cover much of the habitable zone for G stars, they miss significant portions of the habitable zones of K and F stars, and include regions outside the habitable zone even when restricted to G stars. This will be true for any fixed choice of orbital period range for the range of stellar effective temperatures required for good statistical analysis. Such coverage will not lead to accurate occurrence rates of planets in the habitable zone.

Hence the decision to work with instellation flux, which measures the photon flux on each planet as received from its host star. The authors say that this is the first paper on occurrence rates for habitable zone planets that operates on star-dependent photon output. In terms of effective temperature, G-class stars like the Sun are in the range of 5,200–6,000 K. F-class is 6,000–7,500 K, but as the paper notes, the paucity of F stars in the sampled data leads to the authors setting the temperature limits lower. K-class stars show up at effective temperatures of 3,700–5,200 K. The range used in this paper — 4,800-6,300 K — also excludes M-dwarfs, whose effective temperatures range from 2,400–3,700 K.

Leaving out M-dwarfs could substantially under-count habitable zone worlds, but we also have enough concerns about tidal lock, stellar flare activity and atmospheric loss that we can’t assume M-dwarf planets are habitable. In any case, the authors have other reasons for the decision, including a very practical matter of future observation. After all, an analysis like this may well be useful as we ponder our target lists, and we also have to remember the limits of transit observation Kepler had to deal with:

The reason for limiting to Teff > 4800 K is two fold: (1) The inner working angle (IWA, the smallest angle on the sky at which a direct imaging telescope can reach its designed ratio of planet to star flux) for the LUVOIR coronagraph instrument ECLIPS falls off below 48 milliarc sec at 1 micron (3λ/D) for a planet at 10 pc for Teff ≤ 4800 K, and (2) Planets are likely tidal-locked or synchronously rotating below 4800 K that could potentially alter the inner HZ limit significantly…The upper limit of 6300 K is a result of planets in the HZs having longer orbital periods around early F-stars, where Kepler is not capable of detecting these planets…

So bear this in mind: Excluding what could be vast numbers of habitable planets in M-dwarf orbits, we still wind up with 300 million possibilities in the broad range of K-class through G-class stars. Co-author Jeff Coughlin is director of Kepler’s Science Office:

“This is the first time that all of the pieces have been put together to provide a reliable measurement of the number of potentially habitable planets in the galaxy. This is a key term of the Drake Equation, used to estimate the number of communicable civilizations — we’re one step closer on the long road to finding out if we’re alone in the cosmos.”

Image: This illustration depicts Kepler-186f, the first validated Earth-size planet to orbit a distant star in the habitable zone. Credit: NASA Ames/JPL-Caltech/T. Pyle.

When you go through this paper, bear in mind what Centauri Dreams associate editor Alex Tolley pointed out to me — The Drake ne factor refers to the number of planets per star that can support life. What the Bryson et al. paper takes as its starting point is the number of rocky planets in the habitable zone, and this could mean that the figure of 300 million ‘habitable’ worlds takes in planets that resemble Venus more than Earth. It may also include water worlds, where the likelihood of technological civilization is unknown.

So Drake’s term ne is not the same value as taken up in the new paper. Nonetheless, let’s return to that dazzling figure of 300 million, because when we’re dealing with that many planets of interest, we can afford to lose a number that turn out to be uninhabitable and still consider ourselves overwhelmed with possibilities for life.

Numbers like these have implications for stars relatively near the Sun. The authors look at both the conservative and optimistic habitable zone, with the narrower ‘conservative habitable zone’ bounded by the ‘moist greenhouse’ and ‘maximum greenhouse’ limits, and the wider ‘optimistic habitable zone’ bounded by the ‘current Venus’ and ‘early Mars’ limits. I’m drawing this descrtiption from the Planetary Habitability Laboratory’s summary of work by Ravi kumar Kopparapu and colleagues (citation below).

Image: Habitable Zone of around main sequence FGKM stars. The warm ‘habitable’ zone is divided into a ‘conservative habitable zone’ (light green) and an ‘optimistic habitable zone’ (dark green). Earth is at the inner edge of the ‘conservative habitable zone.’ Credit: PHL.

Filtering their results using calculations for the conservative habitable zone, the authors maintain they can say with 95 percent confidence that the nearest rocky habitable zone planet around either a G- or K-class star is within 6 parsecs (roughly 20 light years). There could be four habitable zone rocky planets around G- and K-dwarfs within 10 parsecs of the Sun.

How to build our small planet catalog to reduce uncertainties in the calculations? The answer is clearly more space-based observations even as new ground-based telescopes come online. Let’s also remember what we lost because of Kepler’s mechanical problems. While we did get a K2 extended mission, the original Kepler extended mission was meant to continue the ‘long stare’ at the original starfield, adding four more years of precision photometric data. The number of small planets in the habitable zone would have been significantly extended.

…by definition, Kepler planet candidates must have at least three observed transits. The longest orbital period with three transits that can be observed in the four years of Kepler data is 710 days (assuming fortuitous timing in when the transits occur). Given that the habitable zone of many F and late G stars require orbital periods longer than 710 days, Kepler is not capable of detecting all habitable-zone planets around these stars.

Given that upcoming missions like PLATO do not include such long stares on a single field of stars (PLATO plans no more than 3 years of continuous observation of a single field), we will need future missions to achieve what the original Kepler extended mission might have done, which would have been a doubling of the DR25 dataset and a large yield of small habitable zone planets.

The paper is Bryson et al., “The Occurrence of Rocky Habitable Zone Planets Around Solar-Like Stars from Kepler Data,” in process at the Astronomical Journal (abstract). The Kopparapu et al. paper is “Habitable Zones Around Main-Sequence Stars: New Estimates,” Astrophysical Journal Vol. 765, No. 2 (26 February 2013). Abstract.