Centauri Dreams
Imagining and Planning Interstellar Exploration
Titan and Astrobiology
Night launches are spectacular, that’s for sure, especially with a rocket as muscular as the SpaceX Falcon Heavy. Less spectacular, at least at this point in my life, is staying up until 0230, but delays are part of the rocket business, and what counts is a launch successful in everything but the return of the center booster (both side boosters landed upright at Cape Canaveral). Prox-1, the carrier vehicle for The Planetary Society’s LightSail 2, was released at 720 kilometers, with deployment of the sail itself scheduled for July 2.
While we wait for LightSail developments and also follow the fortunes of NASA’s Deep Space Atomic Clock, launched as one of 24 satellites deployed by this bird, the 2019 Astrobiology Science Conference in Seattle draws attention this morning with new information about Saturn’s tantalizing moon Titan. I’m still having to adapt to not having Cassini in Saturn space, but without its presence scientists are proceeding with laboratory studies that re-create conditions like those on the surface of Titan. Among their findings: Compounds and minerals not found on Earth, including a ‘co-crystal’ composed of solid acetylene and butane.
I had to look this one up. A co-crystal (often written without the hyphen) is a molecular crystal with multiple components. An evidently authoritative source (G.P. Stahly, writing in a journal called Crystal Growth & Design in 2007), says that co-crystals “consist of two or more components that form a unique crystalline structure having unique properties.” In the case of the work presented by JPL’s Morgan Cable at the Astrobiology Science Conference, both acetylene and butane appear as gases on Earth, but on Titan, they are solid and combine to form crystals.
The mineral Cable’s team identified in the lab may account for a surface feature found in Cassini imagery in the form of orange shadings around the edges of Titan’s seas and lakes. We know that some of these areas show signs of evaporated material left behind as the liquid receded. These bright features show up in data from the spacecraft’s Visual and Infrared Mapping Spectrometer (VIMS) and Synthetic Aperture Radar (SAR) around lakes in the north polar region. The thinking is that organic molecules generated in the atmosphere and deposited on the surface dissolve in liquid methane and ethane and precipitate when it evaporates.
Image: A false-color, near infrared view of Titan’s northern hemisphere collected by NASA’s Cassini spacecraft shows the moon’s seas and lakes. Orange areas near some of them may be deposits of organic evaporite minerals left behind by receding liquid hydrocarbon. Credit: NASA / JPL-Caltech / Space Science Institute.
To investigate, the scientists used a cryostat filled with liquid nitrogen, gradually warming the chamber until the nitrogen turned to gas, and then adding methane and ethane along with other carbon-containing molecules. Benzene crystals emerged from the mix, creating a co-crystal with ethane molecules. The acetylene and butane co-crystal, likely more common on Titan given the moon’s composition, was then discovered. Other undiscovered hydrocarbon crystals may exist, say the researchers, although finding them will require a mission to Titan’s shorelines.
At surface temperatures in the range of 90 K, the authors believe, the acetylene-butane co-crystal “…may be ubiquitous in these regions of Titan’s surface.” Cable and colleagues are interested in chemical gradients that could potentially be exploited by life, as their presentation notes:
The catalytic hydrogenation of acetylene has been proposed as a possible energy-yielding reaction for metabolism (Schulze-Makuch et al. 2005; McKay et al. 2005; Tokano et al. 2009). It is possible that acetylene-based co-crystals might be a mechanism for storing acetylene, in a manner similar to how carbon dioxide is stored in carbonate deposits on Earth, where it might be more readily accessible to a putative microbial community.
What a complex place we’re dealing with, one whose atmosphere undergoes chemical reactions that produce organic molecules that wind up on the surface, where chemical reactions may occur as the landscape is manipulated by frigid winds and rain. A liquid water ocean beneath the surface may exchange materials with the surface over long timeframes. Being discussed at the conference is the question of whether conditions on Titan could produce the chemical complexity and particularly the energy life would demand. We’re a long way from the answer.
The presentation is Cable et al., “The Acetylene-Butane Co-Crystal: A Potentially Abundant Molecular Mineral on Titan,” with abstract here.
Into the Uranian Rings
Both DSAC (the Deep Space Atomic Clock) and LightSail 2 are on the line when a SpaceX Falcon Heavy launches on Monday evening. Both missions portend interesting developments in our push to deep space, with DSAC testing our ability to extend navigational autonomy, and LightSail 2 a solar sail that will use the power of solar photons to raise its orbit. You can follow the launch (now scheduled for 2330 Eastern (0330 UTC) on NASA Live. Also lifting off with the Falcon Heavy from the Kennedy Space Center will be almost two dozen other satellites, a nod both to the Falcon Heavy’s capabilities but also to increasing spacecraft miniaturization.
And speaking of interesting missions, here’s something good about one whose anniversary we’re about to celebrate. My friend Al Jackson, who served as astronaut trainer on the Lunar Module Simulator in the Apollo days, passed along a link to the Air-to-Ground Loop and the Flight Director’s Loop from Apollo 11. Give yourself 20 minutes or so and don’t miss this:
https://www.firstmenonthemoon.com/
On to today’s topic.
Return to the Planet of Doubt
I first mentioned Stanley Weinbaum’s story “The Planet of Doubt” back in 2011, certainly the most memorable monicker Uranus has ever received, even if Weinbaum’s Uranus doesn’t hold a candle to later imaginings from Geoff Landis and, in particular, Gerald Nordley (everyone should read Landis’ “Into the Blue Abyss” and Nordley’s brilliant “Into the Miranda Rift.”). And then there is Kim Stanley Robinson’s The Memory of Whiteness (1985), a vivid reading memory from a trip with my oldest son to Maine, where I couldn’t put it down. This is how it begins:
Dear Reader, two whitsuns orbit the planet Uranus; one is called Puck, the other, Bottom. They burn just above the swirling clouds of that giant planet, and with the help of the planet’s soft green light they illuminate all that dark corner of the solar system. Basking in the green glow of this trio are a host of worlds — little worlds, to be sure, worlds no bigger (and many smaller) than the asteroid Vesta — but worlds, nevertheless, each of them encased in a clear sphere of air like little villages in glass paperweights, and each of them a culture and society unto itself. These worlds orbit in ellipses just outside the narrow white bands of Uranus’s rings; you might say that the band of worlds forms a new ring in the planet’s old girdle: the first dozen made of ice chunks held in smooth planes, the newest made of an irregular string of soap bubbles, filled with life. And what holds all these various worlds together, what is their lingua franca? Music.
That’s quite a start, and it certainly had me hooked that summer up near Blue Hill. The Memory of Whiteness is one of the ultra-rare pieces of fiction in which the Uranian rings appear, which is why I quote it, because we’re learning more about the rings through the work of a team of scientists from UC-Berkeley and the University of Leicester in the UK, who have released updated views of the system. Here we get a glimpse of the thermal glow of these dim structures, a tough catch in visible light as well as near-infrared for any but the largest telescopes. The instruments in play are the Atacama Large Millimeter/submillimeter Array (ALMA) as well as the Very Large Telescope (VLT).
Results from both sets of observations appeared this week in The Astronomical Journal; note that the VLT work was done in 2017 using the VISIR instrument, a spectrometer working at infrared wavelengths that is now, in upgraded form, in use by the NEAR project in the search for planets around Centauri A and B. This is the first time that the temperature of these rings has been measured, at 77 Kelvin, which is the boiling temperature of liquid nitrogen. Prior images have all shown the rings in reflected sunlight. The new work offers millimeter (ALMA) and mid-infrared imaging (VLT/VISIR) to produce what you see below.
Image: Composite image of Uranus’s atmosphere and rings at radio wavelengths, taken with the ALMA array in December 2017. The image shows thermal emission, or heat, from the rings of Uranus for the first time, enabling scientists to determine their temperature: a frigid 77 Kelvin (-320 F). Dark bands in Uranus’s atmosphere at these wavelengths show the presence of molecules that absorb radio waves, in particular hydrogen sulfide gas. Bright regions like the north polar spot (yellow spot at right, because Uranus is tipped on its side) contain very few of these molecules. Credit: UC Berkeley. Image by Edward Molter and Imke de Pater.
We also learn a good deal about the composition of the rings, which differs from what we see elsewhere in the Solar system. Imke de Pater (UC-Berkeley), a co-author of the paper on this work, points to the range of particle sizes from meters down to micron-size in Saturn’s innermost D ring up to tens of meters in the main rings. At Uranus, the smaller size particles are missing in the epsilon ring, the brightest and densest of the Uranian rings, which appears to be made up of particles no smaller than golf balls. A dust component does show up between the main rings.
Graduate student Edward Molter is lead author:
“We already know that the epsilon ring is a bit weird, because we don’t see the smaller stuff. Something has been sweeping the smaller stuff out, or it’s all glomming together. We just don’t know. This is a step toward understanding their composition and whether all of the rings came from the same source material, or are different for each ring.”
These rings are also dark as charcoal, as well as being quite narrow compared to the rings of Saturn. The epsilon ring is the widest, varying from 20 to 100 kilometers in width, while Saturn’s rings can range from hundreds to tens of thousands of kilometers wide. 13 rings have thus far been found around Uranus, which Voyager flew past in 1986, likewise finding a scarcity of dust-sized particles. This sets Uranus apart from the other ice giant, for Neptune’s rings are largely dust, while gas giant Jupiter’s are mostly made up of particles of micron size.
Image: The Uranian ring system captured at different wavelengths by the ALMA and VLT telescopes. The planet itself is masked since it is very bright compared to the rings. Credit: Edward Molter, Imke de Pater, Michael Roman and Leigh Fletcher, 2019.
While the observations here are not sensitive to dust down to micron-size, the results are consistent with studies of the rings in optical and near-infrared reflected light that indicate no dust of this size is present. Here’s the paper’s conclusion:
The consensus between our millimeter and mid-infrared observations and literature visible-wavelength observations shows that the properties of the main rings remain the same at any observed wavelength despite the fact that our observations are not sensitive to micron-sized dust. This finding confirms the hypothesis, proposed based on radio occultation results (Gresh et al. 1989), that the main rings are composed of centimeter-sized or larger particles. A simple thermal model similar to the NEATM model for asteroids was applied to determine that the ring particles display roughly black-body behavior at millimeter/mid-infrared wavelengths at a temperature of 77 ± 2 K, suggesting the particles’ thermal inertia may be small enough and rotation rate slow enough, to induce longitudinal temperature differences between their dayside and nightside.
The image below, taken from earlier work, offers a near-infrared view of the rings that shows some of the interleaved dust between them, with the epsilon ring at the bottom.
Image: Near-infrared image of the Uranian ring system taken with the adaptive optics system on the 10-meter Keck telescope in Hawaii in July 2004. The image shows reflected sunlight. In between the main rings, which are composed of centimeter-sized or larger particles, sheets of dust can be seen. The epsilon ring seen in new thermal images is at the bottom. Credit: UC Berkeley image by Imke de Pater, Seran Gibbard and Heidi Hammel, 2006.
The paper is Molter et al., “Thermal Emission from the Uranian Ring System,” accepted at The Astrophysical Journal. Preprint.
Ring Imagery from Cassini’s Deep Dive
Cassini’s productivity at Saturn continues to provide fodder for scientific papers and encouragement for the builders of complex missions, who have seen enough data gathered by this one to guarantee continuing insights into the ringed planet for years to come. The June 14 issue of Science offers up four papers (citations below) that show results from four of the spacecraft’s instruments, including startling views of the main rings.
The data examined in the Science papers were gathered during Cassini’s ring-grazing orbits from December, 2016 to April of 2017 as well as during the ‘Grand Finale’ between April and September of 2017, when Cassini flew closer than ever before to the giant planet’s cloud tops. Consider the image below, showing an infrared view as captured by the spacecraft’s Visible and Infrared Mapping Spectrometer (VIMS), with (at the left) the natural color view taken as a composite by Cassini’s Imaging Science Subsystem.
Imaging the rings in visible and near-infrared wavelengths, VIMS surprised scientists by finding weak water ice bands in the outer parts of the A ring, a seeming contradiction given the highly reflective nature of this area, which has been taken to indicate less contaminated ice and stronger water ice bands. While water ice is dominant in the rings, the spectral map shows no ammonia or methane ice, nor does it see organic compounds. This is an oddity given that organics have been found flowing out of the D ring into Saturn’s atmosphere.
“If organics were there in large amounts — at least in the main A, B and C rings — we’d see them,” says Phil Nicholson, Cassini VIMS scientist of Cornell University in Ithaca, New York. “I’m not convinced yet that they are a major component of the main rings.”
Image: The false-color image at right shows spectral mapping of Saturn’s A, B and C rings, captured by Cassini’s Visible and Infrared Mapping Spectrometer (VIMS). It displays an infrared view of the rings, rather than an image in visible light. The blue-green areas are the regions with the purest water ice and/or largest grain size (primarily the A and B rings), while the reddish color indicates increasing amounts of non-icy material and/or smaller grain sizes (primarily in the C ring and Cassini Division). At left, the same image is overlaid on a natural-color mosaic of Saturn taken by Cassini’s Imaging Science Subsystem. Credit: NASA/JPL-Caltech/University of Arizona/CNRS/LPG-Nantes. Saturn image credit: NASA/JPL-Caltech/Space Science Institute/G. Ugarkovic.
The rings, now believed to have formed much later than the planet itself, continue to deliver complexities that will feed into future models of ring evolution. The last phase of Cassini’s mission is, in the words of project scientist Linda Spilker (JPL), author of one of the papers in Science, “like turning the power up one more notch on what we could see in the rings,” a higher resolution that even as it answers some questions raises still more.
At the outer edge of the main rings, for example, impact-generated streaks appear in the F ring that are of approximately equal length and similar orientation, suggesting a string of impactors striking at the same time. The implication: The F ring, at least, is shaped by materials already orbiting Saturn rather than by materials the planet encounters in its orbit of the Sun. Small moons embedded in the rings can be considered in that sense to be sculpting them, according to Cassini scientist Matt Tiscareno (SETI Institute). Here we might see useful analogs in young exoplanet systems, where stellar systems form out of circumstellar disks that are in turn shaped by the emerging planets within them, processes out of which our own Solar System formed.
We also see how remarkably ring textures can differ even in segments close to each other, as seen in the image below. Teasing out the interactions that shaped the patterns found here will take us deeply into chemistry and temperature changes as mapped by Cassini.
Image: New images of Saturn’s rings show how textures differ even in close proximity of one another. The image on the right has been filtered so that the newly visible straw-like textures and clumps are more visible. Credit: NASA/JPL-Caltech/Space Science Institute.
Now have a look at Daphnis, one of the embedded moons, which is here shown creating three waves in the outer edge of the Keeler Gap, at the outer edge of the A ring. The spacecraft was at a shallow 15 degree angle above the rings when the image was taken. The images here were taken in visible light using Cassini’s narrow-angle camera at a distance of about 28,000 kilometers from Daphnis and at a Sun-Daphnis-spacecraft phase angle of 71 degrees.
Image: This enhanced-color image mosaic shows Daphnis, one of the moons embedded in Saturn’s rings, in the Keeler Gap on the sunlit side of the rings. Daphnis is seen kicking up three waves in the gap’s outer edge. Three wave crests of diminishing sizes trail the moon. In each successive crest, the shape of the wave changes as the ring particles within the crest interact and collide with each other. A thin strand of ring material to the lower left of Daphnis is newly visible in this image, and there are intricate features that also hadn’t been previously observed in the third wave crest downstream. Credit: NASA/JPL-Caltech/Space Science Institute / Tilmann Denk (Freie Universität, Berlin).
And finally, this image showing the third wave crest, which displays multiple strands of ring material, an indication of the effects of Daphnis on the ring. Here we are again in visible light, now at a distance of 23,000 kilometers and at a phase angle (Sun-Daphnis-spacecraft) of 94 degrees, with an image scale of 160 meters per pixel.
Image: A closeup of the third wave crest (toward the far left in the color image) focusing on detail scientists hadn’t seen before, with multiple strands of ring material visible. The image shows how the ring material behaves after losing the structure Daphnis triggered and goes back to interacting with itself. The ribbon of material that appears to be protruding into the gap in the right-hand portion of the image is probably actually soaring above the ring plane. Toward the left-hand part of the image, that same ribbon of material dives below the ring plane and becomes obscured by the main part of the rings, which is why it disappears from view. Credit: NASA/JPL-Caltech/Space Science Institute.
I had never thought of Saturn as providing a kind of astrophysical laboratory that could illuminate processes at work in the early Solar System, but this is what the four papers in Science suggest, for here we see the kinds of interactions that take place in disk structures that contain larger accreted objects. The Cassini imagery and data will always symbolize the era of early Solar System reconnaissance on which we are embarked. Imagine what we might do with a next-generation craft, a Cassini follow-up around Uranus or Neptune.
The four papers on the Cassini ‘Grand Finale’ all appear in the June 14 issue of Science, Vol. 364, Issue 6445, and can be accessed here. They are Tiscareno et al., “Close-range remote sensing of Saturn’s rings during Cassini’s ring-grazing orbits and Grand Finale”; Buratti et al., “Close Cassini flybys of Saturn’s ring moons Pan, Daphnis, Atlas, Pandora, and Epimetheus”; Militzer et al., “Measurement and implications of Saturn’s gravity field and ring mass”; and Spilker, “Cassini-Huygens’ exploration of the Saturn system: 13 years of discovery.”
Can We Catch the Next ‘Oumuamua?
Ever since the passage of interstellar interloper ‘Oumuamua, we’ve become aware of the opportunities presented by objects entering our system from interstellar space, at the same time wishing we had the resources at hand to investigate them close-up. Andreas Hein and colleagues at the Initiative for Interstellar Studies have examined the possibilities for reaching ‘Oumuamua through Project Lyra (see Project Lyra: Sending a Spacecraft to 1I/’Oumuamua), a study that also takes in the kind of future infrastructure that could allow us to react to the next such object.
Now comes the interesting news that the European Space Agency is developing a mission called Comet Interceptor, one capable of visiting a long-period comet coming into the inner system from the Oort Cloud, but just as capable of reaching an interstellar visitor. The idea revolves around not a single spacecraft, but a combination of three. The composite vehicle would be capable of orbiting the L2 Lagrange point 1.5 million kilometers from Earth until it finds a suitable target. At that point, it would journey to the object and separate into three modules.
Image: Comet Interceptor has been selected as ESA’s new fast-class mission. It will be the first spacecraft to visit a truly pristine comet or other interstellar object that is only just starting its journey into the inner Solar System. The spacecraft will wait at the Sun-Earth Lagrange point L2, which is 1.5 million kilometres ‘behind’ Earth as viewed from the Sun. It will travel to an as-yet undiscovered comet, making a flyby of the chosen target when it is on the approach to Earth’s orbit. The mission comprises three spacecraft that will perform simultaneous observations from multiple points around the comet. Credit: ESA.
Each module will be equipped with a science payload that complements the instrumentation on the others, offering insights into cometary gas and dust and the plasma environment near the object through a mass spectrometer along with dust, field and plasma instruments. Thus we get ‘multi-point’ measurements offering insights into cometary interactions with the solar wind, the stream of plasma from the Sun that itself is constantly changing in velocity and intensity.
This is a fundamentally different concept from previous missions like Giotto and Rosetta. Giotto flew within 600 kilometers of Comet 1P/Halley in 1986, with another pass by Comet Grigg-Skjellerup in 1992. Rosetta targeted Comet 67P/Churyumov-Gerasimenko in a highly successful mission in 2014. Both comets are short-period objects with periods of less than 200 years, with 67P/Churyumov-Gerasimenko orbiting every 6.5 years and Halley every 76.
In both cases, the comet’s frequent passage into the inner system has meant changes to the surface. What Comet Interceptor is looking for is a first-time visitor, one whose materials should be relatively unprocessed since the earliest days of the system. But ESA is also thinking about interstellar objects like ‘Oumuamua as potential destinations, for the mission has the luxury of being able to choose its target from its stable vantage point at L2. Given the success of the Pan-STARRS effort at finding new comets and the construction of the Large Synoptic Survey Telescope in Chile, slated to reach first light in 2020, we should have no shortage of targets.
ESA director of science Günther Hasinger describes the mission in context:
“Pristine or dynamically new comets are entirely uncharted and make compelling targets for close-range spacecraft exploration to better understand the diversity and evolution of comets. The huge scientific achievements of Giotto and Rosetta – our legacy missions to comets – are unrivalled, but now it is time to build upon their successes and visit a pristine comet, or be ready for the next ‘Oumuamua-like interstellar object.”
Image: Kuiper Belt and Oort Cloud in context. Credit: ESA.
In official terminology, Comet Interceptor is an F-class mission, the ‘F’ standing for ‘fast,’ as in ‘fast implementation’ — the total development time from selection of the mission to readiness to launch is to be eight years. But we might also consider it in terms of ‘fast response,’ just what is needed to reach objects that appear with no prior warning. This category of mission will have a launch mass of less than 1,000 kilograms. Comet Interceptor is now seen going into space along with exoplanet hunter ARIEL in 2028, both missions being delivered to L2.
Breakthrough Listen: SETI Data Release
On Monday I was talking about the rise of open access scientific journals, using the European Space Agency’s Acta Futura as just one example. The phenomenal arXiv service, not itself a journal but a repository for preprints of upcoming papers, is already well known in these pages. Now we have the largest public release of SETI data in the history of the field, a heartening follow-through on a trend that broadens the audience for scientific research.
Breakthrough Listen is presenting two publications in the scientific literature (available as full text, citation below) describing the results of three years of radio and optical observations, along with the availability of a petabyte of data from its work at the Green Bank instrument in West Virginia and the Parkes Radio Telescope in Australia. This covers a sample of 1327 nearby stars (within 160 light years from Earth) and builds on the team’s results on 692 stars as presented in 2017.
No signs of extraterrestrial civilizations turn up in the analysis, says Parkes project scientist Danny Price, who emphasizes that the search will continue:
“This data release is a tremendous milestone for the Breakthrough Listen team. We scoured thousands of hours of observations of nearby stars, across billions of frequency channels. We found no evidence of artificial signals from beyond Earth, but this doesn’t mean there isn’t intelligent life out there: we may just not have looked in the right place yet, or peered deep enough to detect faint signals.”
Image: The Green Bank site in West Virginia, where Breakthrough Listen observations continue. Credit: NRAO/AUI.
There are reasons why making such data public benefits the SETI effort. Both within the public and the astronomical community, those interested can now download the results of these observations and examine them independently. Those with programming skills may well develop algorithms for the detection of signals and filtering out of background noise that improve on the current model. And there may be information within the datasets that will prove useful in the investigation of unrelated astrophysical phenomena.
The existing tools developed by the Breakthrough Listen science team at the Berkeley SETI Research Center (BSRC) include both radio frequency searches as well as optical scans and algorithms designed to flag unexplained astrophysical phenomena. Go to this UC-Berkeley page for the overview, including the two just released papers. Likewise available to the public are software tools used in the analysis such as blimpy (for loading raw format data files), and turboSETI (for running Doppler drift searches). The datasets are examined in the analysis paper by Dr. Price and made available at the Breakthrough Listen Open Data Archive and via BSRC (more search options available at the latter).
For those wanting to get into data crunching themselves, the second paper (lead author Matt Lebofsky at Berkeley) goes into the intricacies of the current analysis, the tools used, the data formats and the archival systems now in play. “While we have been making smaller subsets of data public before in varying forms and contexts,” says Lebofsky, “we are excited and proud to offer this first cohesive collection along with an instruction manual, so everybody can dig in and help us search. And we’re just getting started – there’s much more to come!”
Considering the complexities involved in creating a search ‘pipeline’ that can scan through billions of radio channels, the more eyes on search algorithms and filtering techniques, the better. Thus far the detected signals have come from human technologies, with the Breakthrough Listen team filtering for narrow-band signals showing a Doppler drift, meaning they change in frequency with time because of their motion with respect to the telescope.
A second filter in the pipeline removes signals that do not appear to originate from a fixed point on the sky. The application of both techniques reduces millions of signals down to a comparative few, all of which have been examined and found to be human-generated frequency interference. From the Price paper, which notes that in its search for narrowband signals showing Doppler drift, 51 million hits emerged, with 6154 that cleared the automated filtering process, leading to a final round of manual inspection and cross-referencing against known sources of interference:
…these observations constitute the most comprehensive survey for radio evidence of advanced life around nearby stars ever undertaken, improving on the results of Enriquez et al. (2017) in both sensitivity and number of stars. Together with other recent work from the resurgent SETI community, we are beginning to put rigorous and clearly defined limits on the behavior of advanced life in the universe. We note that significant additional observational and theoretical work remains to be done before we are able to make general statements about the prevalence of technologically capable species.
Be aware that the archive also includes data from Breakthrough Listen observations of the first repeating fast radio burst ever detected, FRB 121102, as well as scans of the ‘Oumuamua object, along with optical data from the Automated Planet Finder at Lick Observatory. The search of nearby stars continues while also being expanded into the galactic disk at Parkes, and a one-million star sample with the MeerKAT telescope in South Africa is forthcoming.
The papers are Price et al., “The Breakthrough Listen Search for Intelligent Life: Observations of 1327 Nearby Stars over 1.1-3.4 GHz,” submitted to The Astrophysical Journal (preprint) and Lebofsky et al., “The Breakthrough Listen Search for Intelligent Life: Public Data, Formats, Reduction and Archiving,” submitted to Publications of the Astronomical Society of the Pacific (preprint).
CARMENES: Two Habitable Zone Planets around a Nearby Red Dwarf
We rarely talk about Teegarden’s Star when mentioning interesting objects near the Solar System, probably because the star was only discovered in 2003 and until now had not been known to host planets. Today we learn, however, that an international team led by the University of Göttingen has found two planets close to Earth mass in what it considers to be the habitable zone around the tiny star. Interestingly, from where the system is located, any local astronomers would be able to see the planets of our Solar System in transit across the face of the Sun, about which more in a moment.
One of the reasons that this comparatively nearby star has been so late to be discovered is its size. We are dealing with an M-class red dwarf, this one in the constellation Aries, and no more than 12.5 light years from us. It took three years of patient radial velocity monitoring to track down planets around a star that is only about 2700 degrees Celsius in temperature, and fully 10 times lighter than the Sun. What we now have are two planet candidates, each with a minimum mass 1.1 times that of Earth, with orbital periods of 4.91 and 11.4 days respectively.
These are, according to the paper, “…the first Earth-mass planets around an ultra-cool dwarf for which the masses have been determined using radial velocities.”
Because no transits have been detected, the scientists have no information on planetary radii, and therefore estimated them based on various possible compositions, from rocky to gaseous mini-Neptune, finding that the resulting radii differ by a factor of about three. The other stellar and planetary parameters were plugged into the Earth Similarity Index (ESI), which compares key parameters to those of Earth. If these worlds are not mini-Neptunes, we get this (from the paper):
Except for the case of a mini-Neptune composition, the two planets have a high ESI. For a potentially rocky composition, the ESI value is 0.94 and 0.8 for planets b and c, respectively. This makes Teegarden’s Star b the planet with currently the highest ESI value. However, the ESI is only an estimate, and different weighting of the parameters may lead to changing ESIs. This ESI definition, for example, does not take into account the stellar spectral energy distribution and the resulting planetary atmospheric composition, which very likely have an effect on habitability.
So we push at the boundaries of what we still don’t know. Lead author Matthias Zechmeister (University of Göttingen) noted the resemblance between these two worlds and the inner planets of our Solar System, saying “They are only slightly heavier than Earth and are located in the so-called habitable zone, where water can be present in liquid form.” I asked Dr. Zechmeister to amplify on the habitability zone finding, to which he replied:
You may have noted that PHL (http://phl.upr.edu/press-releases/pr_draft_tee4321) ranks Teegarden b now as the exoplanet most similar to Earth. The conditions are good for liquid water on the surface, given a similar insolation and mass as Earth’s. Still, we cannot be sure to 100%. We have measured “only” the mass (which is a minimum mass, but true masses are statistically only ~16% higher). So we do not know the true chemical composition, though a rocky composition is probable (see Fig. 12 in the paper for other compositions).
Dr. Zechmeister also made note of the fact that Teegarden’s Star is about 8 billion years old, roughly twice the age of the Sun, allowing plenty of time for interesting things to develop if life ever took hold there. And he raised a caution re the habitability issue, noting that the diminutive star is what he refers to as ‘an extreme host,’ a type of star about which we still have a great deal to learn.
So small is Teegarden’s Star that it is not far above the upper size limit for brown dwarfs, often considered to be somewhere between 60 and 90 Jupiter masses, and at magnitude 15, it demands a large telescope to see it at all. In fact, it was actually discovered in 2003 via stored data in the Near-Earth Asteroid Tracking (NEAT) program, and had been logged in our data earlier, turning up on photographic plates from the Palomar Sky Survey taken in 1951.
Image: Comparison habitable zone in Teff [effective temperaeture] – HZ diagram. Credit: C. Harman.
Now ponder this: Radial velocity detections have produced more than 800 exoplanets, but few have been found around old, cool M-dwarfs. In fact, we have only two other planet hosts with effective temperatures cooler than 3,000 K, and one of these is Proxima Centauri, while the other is TRAPPIST-1, around which fully seven transiting planets are known to exist. The authors of the paper on the Teegarden’s Star work consider the lack of planet detections around very late-type stars the result of observational bias owing to the faintness of the objects at visible wavelengths.
From the paper:
Both planets have a minimum mass close to one Earth mass, and given a rocky, partially iron, or water composition, they are expected to have Earth-like radii. Additionally, they are close to or within the conservative HZ, or in other words, they are potentially habitable. Our age estimate of 8 Gyr implies that these planets are about twice as old as the solar system. Interestingly, our solar system currently is within the transit zone as seen from Teegarden’s Star. For any potential Teegardians, the Earth will be observable as a transiting planet from 2044 until 2496.
A later note from co-author Guillem Anglada-Escudé, the discoverer of Proxima Centauri b, unpacks this further. The transit that would be visible from this system in 2044 would have occurred in our Solar System in 2032, factoring in the 12 year light travel time. There have been SETI discussions regarding the possibility of conducting communication attempts of stars whose planets would see our own world in transit around the Sun, so any such attempt would need to take place in 2032 or later, with the earliest potential response expected around 2056.
Image: Top 19 potentially habitable exoplanets, sorted by distance from Earth. Credit: A. Mendez (PHL).
The team behind this work used data from CARMENES (Calar Alto high-Resolution search for M dwarfs with Exoearths with Near-infrared and optical Échelle Spectrographs), an effort using two separate spectrographs located at the 3.5m telescope at the Calar Alto Observatory in Almeria, Spain. The goal of the project, conducted by a consortium of German and Spanish institutions, is to carry out a survey of approximately 300 late-type main-sequence stars with the goal of detecting low-mass planets in their habitable zones.
We have not, in other words, heard the last from CARMENES.
The paper is Zechmeister et al. “The CARMENES search for exoplanets around M dwarfs – Two temperate Earth-mass planet candidates around Teegarden’s Star,” accepted at Astronomy & Astrophysics 2019 (abstract).
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