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Seeing Titan Globally

When I think about mapping new places, I remember Vincent Van Gogh, who once said “To look at the stars always makes me dream, as simply as I dream over the black dots of a map representing towns and villages. Why, I ask myself, should the shining dots of the sky not be as accessible as the black dots on the map of France?”

Why not indeed? The exciting process of mapping new worlds continues to take place as we pursue our reconnaissance of the Solar System, now pushing well into the Kuiper Belt. Mapping Saturn’s giant moon Titan is particularly satisfying, because as you’ll recall, Voyager 1 was diverted from its original trajectory because Titan was just too interesting a target to miss. No Titan mapping then, though we did have useful scientific results, because Voyager 1 saw a world shrouded in orange smog. Cassini changed the game, of course, and now we’re looking deep beneath the clouds.

Image: The colorful globe of Saturn’s largest moon, Titan, passes in front of the planet and its rings in this true color snapshot from NASA’s Cassini spacecraft. The north polar hood can be seen on Titan (5,150 kilometers across) and appears as a detached layer at the top of the moon here. This view looks toward the northern, sunlit side of the rings from just above the ring plane. Images taken using red, green and blue spectral filters were combined to create this natural color view. The images were obtained with the Cassini spacecraft narrow-angle camera on May 21, 2011, at a distance of approximately 2.3 million kilometers from Titan. Image scale is 14 kilometers per pixel on Titan. Credit: NASA/JPL-Caltech/Space Science Institute.

Cassini made something like 120 flybys of Titan in the course of its long campaign at Saturn between 2004 and 2017, using its radar imager to penetrate the atmosphere of nitrogen and methane, while supplementing these views with data from visible and infrared instruments that could make out the larger geological features of the Mercury-sized world. Now Rosaly Lopes (JPL) has led a team of researchers in producing the first map that shows Titan’s global geology, pulling together all of these data to reveal a wide range of terrains. Says Lopes:

“This study is an example of using combined data sets and instruments. Although we did not have global coverage with synthetic aperture radar (SAR), we used data from other instruments and other modes from radar to correlate characteristics of the different terrain units, so we could infer what the terrains are even in areas where we don’t have SAR coverage.”

Image: The first global geological map of Saturn’s largest moon, Titan, is based on radar and visible and infrared images from NASA’s Cassini mission, which orbited Saturn from 2004 to 2017. Labels point to several of the named surface features. Also located is the landing site of the European Space Agency’s Huygens Probe, part of NASA’s Cassini mission. Credit: NASA/JPL-CalTech/ASU.

When we turn Titan into a map, we begin to sense how many features on the surface resemble similar features on Earth, evidencing similar geological processes at work, with methane/ethane being the operative fluid here. We’re seeing a hydrologic cycle active on a surface rich in hydrocarbon rain, a place segmented by rivers and punctuated by lakes and seas. It’s interesting to see, by the way, that David Williams (Arizona State), who was a major player in extrapolating radar images into areas not covered by radar, is also the man who will make the first global geologic map of (16) Psyche on a mission scheduled for a 2022 launch. For more on that one, see Psyche Mission Moved Up, a 2017 Centauri Dreams article.

The paper is Lopes et al., “A global geomorphologic map of Saturn’s moon Titan,” Nature Astronomy 18 November 2019 (abstract).

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The Electric Sail and Its Uses

The electric sail is an intriguing propulsion concept that Pekka Janhunen at the Finnish Meteorological Institute has been championing for some years. It’s currently the subject of a NASA Phase II study and continues to draw attention despite the fact that we’re in the early stages of turning what looks like sound physical theory into engineering. What captures the imagination here is the same thing that is so attractive about solar sails — in both cases, we are talking about carrying no propellant, but instead relying on natural sources to do the work.

Here we have to be careful about terminology, because it’s all too easy to refer to solar photons as a kind of ‘wind,’ especially since the predominant metaphor is sailing. So let’s draw the lines sharply. There is indeed a ‘solar wind’ in today’s parlance, but it refers not to light but to the stream of particles, plasma and magnetic fields flowing out from the Sun into the heliosphere. An electric sail will ride this solar wind to achieve interplanetary velocities. A solar sail, on the other hand, will use solar photons, which carry no mass but do convey momentum.

Two entirely different concepts, even if both have resemblance to traditional nautical sails. Then we have the other terminological complication: A sail designed to be pushed not just by sunlight but rather by a laser or microwave beam is sometimes called a ‘light sail,’ which is how I have always referred to it, but it still uses photons for propulsion, even if they don’t come from the Sun. Maybe Manasvi Lingam and Avi Loeb have it right in their new paper to refer to photon-pushed sails of any kind as ‘light sails,’ distinguishing these from both electric and magnetic sails (magsails) that use the ‘solar wind’ as their driver. Thus:

Light sails — solar sails and those driven by beamed arrays — use electromagnetic radiation and the momentum transfer of photons. Electric sails use the particle stream of the solar wind.

The electric sail that Janhunen continues to study is the subject of Lingam and Loeb’s new paper, which has been submitted to Acta Astronautica. At the Florida Institute of Technology and Harvard University respectively, the two scientists have calculated performance possibilities for a spinning spacecraft that deploys a number of long wires to which an electrostatic charge has been induced. Solar wind protons (not photons!) reflect off these wires to produce thrust. The wires are kilometers long, and with that slight positive bias, the spacecraft carries an electron gun to manage the charge, retaining the bias against ambient solar wind electrons.

Image: The electric sail is a space propulsion concept that uses the momentum of the solar wind to produce thrust. Credit: Alexandre Szames.

Light sails, to use the Lingam and Loeb terminology, have been considered for interstellar missions for decades now (hats off to the early work of Robert Forward, Gregory Matloff and Geoff Landis, among others), but electric sails are new enough that we need information on how well an electric sail might do for this purpose. Could this technology get us to another star?

For a species like ours, anxious to see missions completed within a few human lifetimes, the answer is no. While a huge laser array like the one contemplated by Breakthrough Starshot could send a small light sail at relativistic speeds to another star, the electric sail cannot achieve the needed velocities.

A species with a different attitude toward time might fare better. The paper explains, for example, how electric sails could leverage the stellar winds of red dwarf stars, which are by far the most common kind of star in the galaxy. Because the interstellar medium itself can decelerate the sail, turning off the electron gun in deep space is essential. Careful maneuvering from star to star over millennia then allows relativistic speeds. From the paper:

…a series of repeated encounters with low-mass stars, and taking advantage of their winds, will enable the electric sail to achieve progressively higher speeds. We showed that sampling ∼ 104 stars could enable electric sails to achieve relativistic speeds of ∼ 0.2 c and that this mechanism would require ∼ 1 Myr. While this constitutes a long timescale by human standards, it is not particularly long in comparison to many astronomical and geological timescales. The ensuing relativistic spacecraft would be well-suited for tackling interstellar and even intergalactic exploration.

This is an eye-opener. We can’t rule out the possibility that species capable of operating in this time frame might deploy electric sails, but the time involved precludes their use as the primary propulsion method for interstellar missions by us. The authors note as well that because an electric sail will have a low cross-sectional area, its presence would be all but undetectable, whereas a light sail driven by a laser would demand huge amounts of energy and would be theoretically detectable at interstellar distances. So for a civilization hoping to explore in ‘stealth’ mode, an electric sail would have its advantages. These are not good SETI targets.

Returning to M-dwarf stars, the authors show that if stars are small enough (less than about 0.2 solar masses), the pressure of the stellar wind dominates over photon pressure, Speeds in the range of 500 kilometers per second seem feasible for electric sails near late-type M-dwarfs. Indeed, for F-, G- and K-class stars, electric sails fare better as propulsion systems in the vicinity of the home star than light sails.

So we are looking at a technology that, if it can be properly engineered, could play a role in shaping an interplanetary infrastructure, while yielding to faster methods for missions to other stars, unless we humans somehow attain an all but geological patience.

The paper is Lingam and Loeb, “Electric sails are potentially more effective than light sails near most stars,” in process at Acta Astronautica (preprint). For Pekka Janhunen’s concept of the electric sail as a fast interplanetary probe, see Electric Sails: Fast Probe to Uranus.

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Surveying Multiple-Star Exoplanetary Systems

While the majority of exoplanet-hosting stars discovered so far are single, we do have multiple star systems in various configurations with planetary companions. This is fertile ground for study, and not just because the nearest stellar system, Alpha Centauri, contains a tight binary pair that is being closely investigated for planets. The third star here is, of course, Proxima Centauri, around which we already know of the existence of a planet in the habitable zone. The much broader question is, how likely are multiple star systems to host planets?

Tackling this question in a new study is Markus Mugrauer (Friedrich Schiller University, Jena), who has been investigating how the existence of multiple stars in a system affects the formation and development of planets. Mugrauer has been working with the second data release from the European Space Agency’s Gaia mission (made available in April of last year). This release contains data collected by Gaia during the first 22 months of its mission. Mugrauer’s survey searched for stellar companions of 1367 exoplanet host stars within about 500 parsecs of the Sun, or roughly 1600 light years, as listed in the Extrasolar Planets Encyclopedia.

Image: These images show some of the exoplanet host stars with companion stars (B, C) that were found during the project. The images are RGB composite images taken with the Panoramic Survey Telescope and Rapid Response System (PanSTARRS) in the y- (960 nm), i- (760 nm), and g-band (480 nm). The image in the middle shows a hierarchical triple star system. Credit: Markus Mugrauer, PanSTARRS.

207 companion stars turn up in this work, varying strongly in mass, temperature and stage of stellar evolution. Of these, 176 are found to be binaries, 27 triple star systems, and one is a quadruple star system, which produces a multiplicity rate of about 15 percent. In each case, Mugrauer uses the Gaia data to demonstrate that the host stars are equidistant and share a common proper motion.

The heaviest of the companion stars weighs 1.4 solar masses; the lightest is a scant 8 percent of the Sun’s mass. As might be expected from their ubiquity, most companion stars turn out to be low-mass, cool red dwarf stars, although it’s interesting to note that eight white dwarfs are also found among the faint stellar companions. The projected separation between these exoplanet hosting systems ranges from 20 AU to 9100 AU, with the highest frequency being within 1000 AU (by comparison, the mean separation of Centauri A and B is 23 AU).

Image; A triple star system approx. 800 light years from the Earth in the Leo constellation with the planetary host star K2-27 (bright star on the left). The image is an RGB composite image taken with PanSTARRS in the y- (960 nm), i- (760 nm), and g-band (480 nm). To the right of it, the first companion star (A) can be clearly distinguished. Just below K2-27 is the second companion star (C) that glows faintly red. Credit: Markus Mugrauer, PanSTARRS.

The results point to what is likely the disruptive influence of several stars in a system where planets are forming, for Mugrauer’s 15 percent incidence of multiple star systems contrasts with the frequency of multiple systems in general. From the paper;

In order to compare the companion star fraction found in this study among exoplanet host stars to that of the solar like stars in general one has to consider only the range of projected separation of the companions detected here, i.e. 19 up to the applied search radius of 10 000 au. This requires the determination of the orbital periods of the detected companions, following the procedure, as described by Raghavan et al. (2010) for wide companions…

Working these calculations, Mugrauer comes up with the following:

The range of separation 19 up to 10 000 au corresponds to orbital periods log(P[d]) = 4.59–8.67. According to the period distribution of companions of solar like stars for this range of period one expect[s] a companion star fraction of 30 ± 2  per cent, which is about twice as large as the fraction found in the study, presented here, among exoplanet host stars.

In other words, the frequency of exoplanet-hosting multiple star systems is about half what we would expect for solar-like stars in general, and the distances between companion and primary star in exoplanet systems are roughly five times greater than in ordinary systems. We may be looking at the gravitational influence of the companion on the gas and dust disk out of which planets emerge. Says Mugrauer: “These two factors taken together could indicate that the influence of several stars in a star system disrupts the process of planet formation as well as the further development of their orbits.”

Mugrauer’s work is continuing via an international observing campaign being conducted at the Paranal Observatory of the European Southern Observatory in Chile, which will apply data from Gaia to more precisely characterize newly discovered planetary host stars and their companions.

Image: HIP116454 is a planetary host star in the Pisces constellation and it is approx. 200 light years from the Earth. The star is accompanied by a significantly fainter white dwarf (B). The image is an RGB composite image composed of images taken in the i- (760 nm), r- (620 nm), and g-band (480 nm) as part of the Sloan Digital Sky Survey (SDSS). Credit: Markus Mugrauer, SDSS.

The paper is Mugrauer, “Search for stellar companions of exoplanet host stars by exploring the second ESA-Gaia data release,” Monthly Notices of the Royal Astronomical Society 13 November 2019 (full text).

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Hayabusa2: Commencing the Return

We’re seeing our final images of asteroid Ryugu as the Hayabusa2 spacecraft leaves its orbit some 300 million kilometers from Earth. The Japanese Aerospace Exploration Agency (JAXA) intends to keep taking images of the receding Ryugu for several more days, after which it will be necessary to perform an attitude control maneuver to orient the craft for proper operation of its ion engines. An ion engine test period will culminate in cruise operations on December 3 to return the spacecraft to Earth.

Image: Asteroid Ryugu captured with the Optical Navigation Camera – Telescopic (ONC-T) immediately after departure. Image time is November 13 10:15 JST (onboard time), 2019. Credit: JAXA, Chiba Institute of Technology, University of Tokyo, Kochi University, Rikkyo University, Nagoya University, Meiji University, University of Aizu, AIST.

Happily, we are asked to join in JAXA’s ‘Goodbye Ryugu’ campaign by sending a #Sayonara_Ryugu tweet (https://twitter.com/haya2e_jaxa), although the agency also encourages old-fashioned cards and letters (as a collector of vintage fountain pens, I rather appreciate this). The address: Hayabusa2 Project, JAXA Institute of Space & Astronautical Science (ISAS), 3-1-1 Yoshinodai, Chuo-ku, Sagamihara City, Kanagawa Prefecture, 252-5210, Japan.

Says JAXA: “5 years after launch and at the conclusion of about a year and a half of missions at Ryugu, we would love to hear your thoughts on Hayabusa2 and asteroid Ryugu.”

Departure images will be available here. As always, it’s exhilarating to see the crew at a spacecraft command center cheering, as they did after project manager Yuichi Tsuda confirmed the departure.

Now we think ahead to the sample return, scheduled for the end of 2020 when the Hayabusa2 re-entry capsule will be recovered at the Woomera Prohibited Area (WPA) located in the outback desert of South Australia. It’s a long way from there to here, but have a look at the candidate recovery site (image credit: JAXA). Final details are still being negotiated with the Australian government, according to Tsuda.

Remember that Hayabusa2 made two touchdowns on the asteroid, the first in February of this year, the second in July. We have surface material as well as underground samples to look forward to, and may learn valuable lessons about the distribution of carbon, organic matter and water in the Solar System. The underground materials, unaffected by radiation to which the surface was exposed, should offer insights into the formation of the Solar System some 4.6 billion years ago.

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Red Dwarf Planets and Habitability

The question of habitability on planets around M-dwarfs is compelling, and has been a preoccupation of mine ever since I began working on Centauri Dreams. After all, these dim red stars make up perhaps 75 percent of the stars in the galaxy (percentages vary, but the preponderance of M-dwarfs is clear). The problems of tidal lock, keeping one side of a planet always facing its star, and the potentially extreme radiation environment around young, flaring M-dwarfs have fueled an active debate about whether life could ever emerge here.

At Northwestern University, a team led by Howard Chen, in collaboration with researchers at the University of Colorado Boulder, NASA’s Virtual Planet Laboratory and the Massachusetts Institute of Technology, is tackling the problem by combining 3D climate modeling with atmospheric chemistry. The paper on this work, in press at the Astrophysical Journal, examines how general circulation models (GCM) have been able to simulate the large-scale circulation and climate system feedbacks on planets around red dwarfs, but these models have not accounted for atmospheric chemistry-driven interactions that the authors believe are critical for habitability. Thus so-called coupled chemistry-climate models (CCM) are needed to factor in how an atmosphere responds to the star’s radiation.

The study takes both ultraviolet radiation (UV) from the star and the rotation of the planet into consideration, noting how UV affects gases like water vapor and ozone. Says Chen:

“3D photochemistry plays a huge role because it provides heating or cooling, which can affect the thermodynamics and perhaps the atmospheric composition of a planetary system. These kinds of models have not really been used at all in the exoplanet literature studying rocky planets because they are so computationally expensive. Other photochemical models studying much larger planets, such as gas giants and hot Jupiters, already show that one cannot neglect chemistry when investigating climate.”

Image: An artist’s conception shows a hypothetical planet with two moons orbiting within the habitable zone of a red dwarf star. Credit: NASA/Harvard-Smithsonian Center for Astrophysics/D. Aguilar.

The researchers simulate the atmospheres of synchronously-rotating planets (i.e., with one side always facing the star) at the inner edge of the habitable zones of both K- and M-class stars. using numerical simulations of climate coupled with photochemistry and atmospheric chemistry through their 3D CCM. They find that the thin ozone layers produced on planets around active stars can render an otherwise habitable planet (in terms of surface temperatures) hazardous for complex life, as there is insufficient ozone to block UV radiation from reaching the surface.

Active photochemistry is a crucial issue, for according to Chen and team, planets can also lose significant amounts of water due to vaporization. Added to the ozone issue, we find boundaries beyond which a planet habitable in terms of liquid water on the surface is rendered lifeless. Understanding stellar activity becomes a predictive tool for gauging which M-dwarfs are most likely to merit precious telescope time for future missions looking for biosignatures. More active M-dwarfs appear far less likely to host life-bearing planets. From the paper:

…we find that only climates around active M-dwarfs enter the classical moist greenhouse regime, wherein hydrogen mixing ratios are sufficiently high such that water loss could evaporate the surface ocean within 5 Gyrs. For those around quiescent M-dwarfs, hydrogen mixing ratios do not exceed that of water vapor. As a consequence, we find that planets orbiting quiescent stars have much longer ocean survival timescales than those around active M-dwarfs. Thus, our results suggest that improved constraints on the UV activity of low-mass stars will be critical in understanding the long-term habitability of future discovered exoplanets (e.g., in the TESS sample…)

The effects of stellar UV radiation become a useful predictive tool as we narrow the target list. Vertical and horizontal winds in the upper atmosphere are strengthened as UV flux goes up. Moreover, the global distribution of ozone and hydrogen depends upon all these processes, which can affect the contrast between the dayside and nightside conditions under varying UV flux. The authors believe that only by bringing atmospheric chemistry into the picture of 3D modeling can we gauge whether a planet can attain true habitability and maintain it. Usefully, using their results, they show that both water vapor and ozone features could be detectable by instruments aboard the James Webb Space Telescope if we choose our targets carefully.

The paper is Chen et al., “Habitability and Spectroscopic Observability of Warm M-dwarf Exoplanets Evaluated with a 3D Chemistry-Climate Model,” in press at the Astrophysical JournaL (preprint).

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ARIEL Emerging

It’s good to see the European Space Agency’s ARIEL mission getting a bit more attention in the media. The Atmospheric Remote-sensing Infrared Exoplanet Large-survey was selected earlier this year as an ESA science mission, scheduled for launch in 2028. Here the goal is to cull a statistically large sample of exoplanets to examine their evolution in the context of their parent stars. Giovanna Tinetti (University College London) is principal investigator.

I would urge seeing ARIEL in the context of a different kind of evolution, that being the gradual growth in our technologies as we continue getting closer to studying the atmospheres of terrestrial-class worlds. For while ARIEL cannot achieve this feat — its focus is on exoplanets of Jupiter-mass down to super-Earths, all on close orbits, with temperatures greater than 320 Celsius — it leverages the fact that high temperature atmospheres keep their various interesting molecules in continual circulation, rather than letting them sink into obscuring layers of cloud. They are thus more easily detected and provide fodder for future work.

Image: Giovanna Tinetti (UCL), principal investigator for ARIEL.

The goal is to study hundreds of transiting exoplanets, looking at the spectra of their atmospheres as they pass in front of their host stars, allowing starlight to filter through the gaseous envelope for analysis. The light emitted by these atmospheres will also be analyzed just before and after the planets pass behind their primaries. Such transmission spectroscopy allows scientists to unlock the composition, temperature and chemical processes at work. No other spacecraft has been so tightly devoted to atmospheric analysis as ARIEL, and here we will be working with a large sample population in search of commonalities and differences. We go from just a few characterized atmospheres to hundreds.

I see that NASA is contributing fine guidance sensors in two photometric bands in an instrument called CASE — Contribution to ARIEL Spectroscopy of Exoplanets — which will observe clouds and hazes at near-infrared as well as visible wavelengths, complementing ARIEL’s other instrument, an infrared spectrometer that operates at longer wavelengths. It will be CASE that measures planetary albedo while examining how clouds influence the composition and other properties of the atmospheres under study. ARIEL should provide abundant insights into how future telescopes can home in on worlds much more like our own.

Image: This artist’s concept shows the European Space Agency’s ARIEL spacecraft on its way to Lagrange Point 2 (L2) – a gravitationally stable, Sun-centric orbit – where it will be shielded from the Sun and have a clear view of the sky. NASA’s JPL will manage the mission’s CASE instrument. Credit: ESA/STFC RAL Space/UCL/Europlanet-Science Office.

Remember that while we await the launch of the James Webb Space Telescope, JWST is by no means a dedicated exoplanet mission, though it will work with a small sample of exoplanets for detailed study as it shares observing time with other investigations. The ARIEL team should be able to draw from JWST’s experience as it homes in on a final target list. Keep in mind as well that ESA’s PLATO mission — PLAnetary Transits and Oscillations of stars — is also in the pipeline, slated for a 2026 launch. As I say, the tools are evolving as our focus sharpens.

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Finding Alpha Centauri

It’s always breathtaking to see the band of the Milky Way under good viewing conditions. I remember so well the night I saw it best, about 20 years ago on a cold, absolutely clear night from a boat in the middle of Lake George. This is up in New York’s Adirondacks, and when I glanced up as we crossed the lake heading back to our hotel, I was simply stunned by the vista. When you contemplate what you’re looking at and think of yourself within that ghostly band, you feel somehow a deep connection to all the myriad processes that put us here as observing beings.

Now we have another fine view of the Milky Way, this time from TESS. The scientists working data from the Transiting Exoplanet Survey Satellite have just given us a composite drawn from 208 TESS images taken during the mission’s first year of science operations, which ended July 18. Have a look at the southern sky, and realize what while TESS has found 29 exoplanets thus far, another 1,000 or so are in candidate stage and being investigated.

Image: This mosaic of the southern sky was assembled from 13 images taken by NASA’s Transiting Exoplanet Survey Satellite (TESS) during its first year of science operations, completed in July 2019. The mission divided the southern sky into 13 sectors, each of which was imaged for nearly a month by the spacecraft’s four cameras. Credit: NASA/MIT/TESS.

Lots of good things to see here. TESS has divided the southern sky into 13 sectors, each of which received almost a month’s worth of imaging by the four cameras aboard. The Milky Way’s band is easily recognized, but look in the center to see the Large Magellanic Cloud, and at the top of the image, you should be able to identify the Orion Nebula, a birthing place for stars.

Can you find Alpha Centauri in this image? Here’s a second image, one showing the confirmed TESS planets to date. I’ve inserted an arrow to identify our nearest star(s).

Image: The host stars of the 29 TESS planet discoveries to date are shown on this version of the southern sky mosaic. Credit: NASA/MIT/TESS and Ethan Kruse (USRA).

TESS is doing excellent work, capturing a full sector of the sky every 30 minutes as it hunts for exoplanet transits. In the first year of operations, its CCDs captured 15,347 30-minute science images. These make up a part of the more than 20 terabytes of southern sky data returned thus far. The TESS survey of the northern sky is now underway.

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Latest Findings from Voyager 2

It’s heartening to consider that the two Voyager spacecraft, though built for a 4 ½ year mission, have continued to function ten times longer than that. This fact, and data from other missions, will help us get a handle on longevity in spacecraft systems as we contemplate pushing out beyond the heliosphere with a spacecraft specifically designed for the job. Mission longevity is mysterious for it often seems to surprise even the designers, who would like to have a more concrete sense of how to ensure operations continue for decades.

Voyager 2 broke Pioneer 6’s record of 12,758 days of operation way back in 2012, but we can also consider spacecraft like Landsat 5, launched in 1984 and carrying two instruments, the Multispectral Scanner System (MSS) and the Thematic Mapper (TM). Managed by the U.S. Geological Survey (USGS), Landsat 5 completed over 150,000 Earth orbits and sent back more than 2.5 million images of Earth’s surface, with operations lasting almost three decades. Design life for Landsat 5 was estimated at three years, but it became, as Guinness World Records labels it, the ‘longest-operating Earth observation satellite.’

While the Landsat accomplishment is significant, the two Voyagers have actually taken us into a new realm, with Voyager 2 joining Voyager 1 beyond the heliosphere on November 5, 2018. Outside the protective ‘bubble’ blown by the stream of particles and magnetic fields from the Sun known as the solar wind, these craft are now the subject of five new research papers in Nature Astronomy describing the data Voyager 2 has returned since the crossing. Have a look at the relative position of the two spacecraft.

Image: This illustration shows the position of NASA’s Voyager 1 and Voyager 2 probes, outside of the heliosphere, a protective bubble created by the Sun that extends well past the orbit of Pluto. Voyager 1 crossed the heliopause, or the edge of the heliosphere, in August 2012. Heading in a different direction, Voyager 2 crossed another part of the heliopause in November 2018. Credit: NASA/JPL-Caltech.

Note the plasma flow lines both inside and outside the heliopause. Plasma is a gas composed of charged particles, a ‘wind’ that differs in direction depending on whether its source is the Sun or the interstellar medium itself. The Voyagers have two instruments returning data on plasma at the borderline between the Sun’s domain and interstellar space. The data show hot and sparse plasma inside the heliosphere, while interstellar plasma is colder and denser. We learned from Voyager 1 that the heliosphere protects the Solar System from about 70 percent of the incoming cosmic ray radiation, which is made up of particles accelerated by exploding stars.

While Voyager 1 showed higher than expected plasma density just outside the heliosphere (an indication, researchers say, of compression), Voyager 2’s findings demonstrated slightly warmer plasma than expected, while confirming the compression at the edge of the heliosphere. Meanwhile, the spacecraft’s particle instruments (two of the five still operating instruments can detect particles in different energy ranges) showed some particles slipping across the boundary into interstellar space, indicating a more porous boundary in Voyager 2’s location outside the ‘flank’ of the heliosphere, as opposed to Voyager 1’s exit at its front.

Magnetic field issues still raise questions. Voyager 1 had shown that the magnetic field just beyond the heliopause is parallel to the magnetic field inside the heliosphere. Voyager 2’s magnetometer confirms this finding of field alignment. Ed Stone (Caltech) is the all but legendary project scientist for Voyager:

“The Voyager probes are showing us how our Sun interacts with the stuff that fills most of the space between stars in the Milky Way galaxy. Without this new data from Voyager 2, we wouldn’t know if what we were seeing with Voyager 1 was characteristic of the entire heliosphere or specific just to the location and time when it crossed.”

Having two spacecraft leaving the heliosphere has been a tremendous boon for science. Voyager 1 and Voyager 2 exited the heliosphere at different locations as well as at different times in the 11-year solar cycle. The latter keeps the solar wind mutable and frothing, something to be borne in mind when we consider spacecraft designs that could ‘sail’ on this wind, and affects the shape of the heliosphere itself, whose boundaries vary with solar changes. We learn from the new papers that neither Voyager is in undisturbed interstellar space, but rather in a churning transitional region, outside the heliosphere but still affected by its presence.

The papers, all of them in Nature Astronomy’s October 2019 issue, are Richardson et al., “Voyager 2 plasma observations of the heliopause and interstellar medium”; Krimigis et al., “Energetic charged particle measurements from Voyager 2 at the heliopause and beyond”; Stone et al., “Cosmic ray measurements from Voyager 2 as it crossed into interstellar space”; Burlaga et al., “Magnetic field and particle measurements made by Voyager 2 at and near the heliopause”; and Gurnett & Kurth, “Plasma densities near and beyond the heliopause from the Voyager 1 and 2 plasma wave instruments.”

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We often think of Jupiter as a mitigating influence on asteroid or comet strikes in the inner system, its gravity changing the trajectories of potential impactors. That would make gas giants a powerful determinant of the survivability of Earth analogues, at least in terms of habitability. While we continue to investigate the question, it’s interesting to consider the damage a gas giant on an elliptical orbit might do to habitable zone planets. Stephen Kane (UC-Riverside), working with Caltech astronomer Sarah Blunt, decided to find out what would happen if, in their modeling, they introduced an elliptical gas giant into the system of an Earth twin.

You may remember Kane’s work earlier this year combining radial velocity with direct imaging methods to find three gas giants that had been previously unobserved (citation below). The monitoring of ten target stars continues even as this new work is published. We’re beginning to find more planets at ever larger distances from their stars as radial velocity and direct imaging methods improve, allowing us to better understand how the architecture of our own Solar System measures up to systems around other stars. Kane and Blunt’s paper implies that a gas giant on an elliptical orbit does not necessarily preclude a habitable planet’s survival.

The planetary system at HR 5183 is a little over 100 light years away in Virgo, home to an eccentric gas giant in a 75 year orbit that the researchers used in their modeling. The primary here is a G-class star. Its planet has one of the longest orbital periods currently known among exoplanets. The eccentricity of this world is e = 0.84, where e = 0 would be perfectly circular, and e = 1 would be a line segment. To find out whether such a world really would be a ‘wrecking ball’ for its neighbors, the researchers introduced a habitable zone terrestrial world as a test case to study extreme system architectures and their effects on habitability.

Image: Comparison of HR 5183b’s eccentric orbit to the more circular orbits of the planets in our own solar system. Credit: W. M. Keck Observatory/Adam Makarenko.

The dynamical simulations here involved the exploration of 200 evenly spaced semi-major axes between 1.0 and 3.0 AU, intended to encompass the range of the optimistic habitable zone around such a star. Kane and Blunt then placed an Earth-mass planet at randomized starting positions and propagated the effects of the eccentric gas giant over time. Says Kane:

“In these simulations, the giant planet often had a catastrophic effect on the Earth twin, in many cases throwing it out of the solar system entirely. But in certain parts of the planetary system, the gravitational effect of the giant planet is remarkably small enough to allow the Earth-like planet to remain in a stable orbit.”

This being the case, we’re called upon to imagine the view from the surface of a habitable zone planet in this system. The gas giant is on a 75 year orbit, something akin to Halley’s Comet in our own system. Kane says that when the gas giant makes its closest approach to the terrestrial planet during that orbit, it would appear 15 times brighter than Venus, a spectacular object that would dominate the night sky before receding once again into the outer reaches.

Here’s a clip from the paper talking about the significance of these findings. Note that the Milankovitch cycles discussed below are cyclical movements — eccentricity, axial tilt, and precession — related to a planet’s orbit around a star. From the paper:

The importance of such systems from a planetary habitability perspective arises from a thorough investigation of the dynamical stability of terrestrial planetary orbits, such as the one presented here. The careful analysis of the dynamical integrations demonstrates that planets can survive within a narrow range of locations in the HZ of such systems, even in the presence of a wrecking ball whose orbital origin is likely a chaotic event involving vast exchanges of angular momentum.

So we have planetary survival in certain locations, but habitability is severely challenged:

…the case of the HR 5183 system also shows that the presence of an eccentric planet will often have a profound effect on the Milankovitch cycles of the HZ terrestrial planetary orbits, causing significant orbital oscillatory behavior. The implications for the climate effects on such worlds may rule out temperate surface conditions, although the stabilizing effects of surface liquid water oceans can also potentially prevent a climate catastrophe.

In other words, our terrestrial world in its habitable zone orbit in a system with a highly eccentric gas giant is in a dangerous position indeed, though not one that completely rules out life. This seems to represent a slight widening of habitable zone possibilities as we examine exoplanetary systems, though the ‘wrecking ball’ hypothesis still seems the most likely outcome.

The paper is Kane & Blunt, “In the Presence of a Wrecking Ball: Orbital Stability in the HR 5183 System,” Astronomical Journal Vol. 158, No. 5 (31 October 2019). Abstract / Preprint. The paper on gas giant detection is Kane et al., “Detection of Planetary and Stellar Companions to Neighboring Stars via a Combination of Radial Velocity and Direct Imaging Techniques,” accepted at the Astronomical Journal. Preprint.

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Benefits of a ‘Snow Line’ Neptune

The formation of planets like Neptune under the core accretion model involves a protoplanetary core that reaches around 10 Earth masses before beginning to pull in surrounding gas, the latter being a runaway process that quickly builds the atmosphere around the object. Core accretion is most efficient at doing this just outside the snow line, but if we want to understand and test the theory, we need to know a lot more about how planets are distributed in this region.

And that’s a problem, because recent microlensing surveys have found that planets like Neptune are most abundant much more distant from their host stars. Outward migration can account for such worlds, but we know little about exoplanets that form at the snow line, which is where the condensation of ices can factor into the emergence of a new world.

Is this just an artifact of our still evolving microlensing detection techniques? Perhaps, and exceptions to the rule can therefore be helpful. Recent work that began with a discovery by a Japanese amateur astronomer has now blossomed into a full-scale study of a snow line Neptune around a star that, unlike most viewed by microlensing, is actually fairly close. The amateur, Tadashi Kojima in Gunma Prefecture, Japan, found the object in Taurus, the beginning of observations from numerous observatories that uncovered the microlensing behind the discovery.

The planet Kojima-1Lb orbits a star 1600 light years away, while the star it passed in front of is some 2600 light years out. Remember that the curvature of spacetime in the presence of massive objects accounts for this phenomenon, as warped space around the nearby star acts as a lens that focuses the light from the background star. Within this brightening, a transient but useful phenomenon, changes in intensity can reveal a planet orbiting the foreground star, as happened here. This discovery is unusual because most microlensed planets have been observed toward galactic center, which makes sense given the sheer abundance of stars there. This one is found close and toward the galactic anticenter.

Image: Diagram illustrating the microlensing event studied in this research. Red dots indicate previous exoplanet systems discovered by microlensing. Inset: Artist’s conception of the exoplanet and its host star. Credit: The University of Tokyo.

76 days of observation by a team led by Akihiko Fukui at the University of Tokyo took advantage of 13 telescopes around the world, including two at the National Astronomical Observatory of Japan’s Okayama Astrophysical Observatory. The work, as revealed in a paper just published in the Astronomical Journal, show a Neptune-class planet orbiting a star on the border line between K and M-class dwarf status. The planet is about 20 Earth masses and orbits at 1.08 AU, snow line distance for this system.

So we’ve got a helpful Neptune at the snow line. The paper draws an interesting but highly tentative conclusion from this detection:

The orbit of Kojima-1Lb is a few times closer to the host star than the other microlensing planets around the same type of star and is likely comparable to the snow-line distance at its youth. We have estimated that the detection efficiency of this planet in this event is ∼35%, which may imply that Neptunes are common around the snow line.

In other words, Fukui and colleagues calculate the a priori detection probability of this kind of planet at 35 percent, making this chance detection a possible indication of an abundance of such worlds around the snow line of other stars. The paper goes on to point out that the host star here is the brightest among all those studied in microlensed systems, offering the opportunity to do follow-up spectroscopic analysis to characterize the host star and to refine both mass and orbit of the planet through radial velocity studies.

The paper is Fukui et al. “Kojima-1Lb is a Mildly Cold Neptune around the Brightest Microlensing Host Star,” Astronomical Journal Vol. 158, No. 5 (November 1, 2019). Abstract / preprint.

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