Centauri Dreams
Imagining and Planning Interstellar Exploration
Exoplanetology Beyond Kepler
Useful synergies continue to emerge among our instruments as we ponder the future of exoplanet studies. Consider the European Space Agency’s PLATO mission (PLAnetary Transits and Oscillations of stars). Operating from the L2 Lagrangian point, PLATO will use 34 telescopes and cameras on a field of view that includes a million stars, using transit photometry, as Kepler did, to find planetary signatures. Working at optical wavelengths, PLATO will look for nearby Earth-sized and ‘super-Earth’ planets in the habitable zone of their stars.
This mission is scheduled to be launched in 2024, an interesting date because it’s also the year that the European Extremely Large Telescope (E-ELT) is scheduled to see first light. Huge new installations like these, although ground-based, are so powerful that they should be able, with the help of adaptive optics, to study planetary atmospheres on the PLATO-discovered planets. Thus we get the best of both worlds, with repairable and upgradable ground telescopes fleshing out the data gathered by our space instruments, just as today we can use Kepler data to find planet candidates and then confirm them using radial velocity studies from the ground.
The TESS mission (Transiting Exoplanet Survey Satellite) launches earlier (probably in 2018) but offers the same kind of synergies with other instruments. Both TESS and PLATO, for example, will hand off data to the James Webb Space Telescope, scheduled for a 2018 launch. Here again we can look for deepened studies of the targets other missions have found. And in the process, we can be assured that we’ll enrichen our catalog of extrasolar worlds.
Just what we might find is the subject of new work by Michael Hippke (Institute for Data Analysis, Neukirchen-Vluyn, Germany) and Daniel Angerhausen, a postdoc at NASA GSFC. Writing in The Astrophysical Journal, the duo explain in one recent paper that while planets with sizes and orbits similar to Mars or Mercury will be out of reach (around solar-class stars, at any rate), planets the size of Venus or Earth should show up readily for TESS and PLATO. The optimum target for life-hunters, of course, is an Earth-class world in an Earth-like orbit, and both instruments are believed to be capable of finding these. From the paper:
In this work, we have shown that future photometry will be able to detect Earth- and Venus-analogues when transiting G-dwarfs like our Sun… Larger sized planets (> 2R?) will be detected in a single transit around G-dwarfs, in low stellar noise cases, and assuming one can find them in the first place. The search techniques for such single transits will require further research and validation, and will likely be performed remotely, due to the large storage requirements.
But Hippke and Angerhausen’s interests extend beyond planets. They believe that a large planet like Jupiter that has several large moons should produce a characteristic signature, allowing its ‘exomoons’ to be detected. The detection would be marginal, as Angerhausen explains: “We wouldn’t have a clear detection, and we wouldn’t be able to say whether the planet had a single large moon or a set of small ones, but the observation would provide a strong moon candidate for follow-up by other future facilities.”
Let me drop back quickly to one of Hippke’s papers from early 2015, which explored exomoon candidates from the Kepler data, using what is known as the orbital sampling effect, which stacks numerous planet transits and tries to extract an exomoon signature. In this paper, Hippke used numerical simulations to inject exomoon signals into real Kepler data. This is useful because it shows us that there is a size limit to what we can find, one that TESS and PLATO should be able to improve on. The paper finds that for suitable planets with orbits between 35 and 80 days, an exomoon’s detectable radius is approximately 2120 kilometers, or about a third the radius of Earth, while for longer period planets, even larger moons are the minimum.
Such moons go beyond what we generally see in our system, but as the paper notes:
…our solar system might not be the norm – we have no Hot Jupiters, warm Neptunes, or Super-Earths in our solar system, and thus no reference for typical moons around such planets. Also, there is a strong selection bias, based on the detection limits…, and in addition the simple fact that the strongest dips are most significant. The first moons to be found will likely be at the long (large/massive) end of exomoon distribution, as was the case for exoplanets.
Hippke is surely right that the first moons found will be at the larger end of the size range, just as the first exoplanets we detected were massive worlds in close orbits that were the easiest to see with our instruments. For more on Hippke’s work and the methods he employs, see the article he wrote for Centauri Dreams, Exomoons: A Data Search for the Orbital Sampling Effect and the Scatter Peak.
But back to the Hippke and Angerhausen paper I started with. It notes that while the detection of moons will remain problematic for planets analogous to those in our own system, moons around planets orbiting quiet M-dwarf stars should be easier to detect. This paper, “Photometry’s bright future: Detecting Solar System analogues with future space telescopes,” focuses in directly on the capabilities of instruments like TESS and PLATO in offering datasets beyond Kepler’s.
Here again the authors deploy the Orbital Sampling Effect:
The OSE can be used to detect a significant flux loss before and after the actual transit (if present), which might be indicative of an exomoon in transit. The basic idea is that at any given transit the moon(s) must be somewhere: They might transit before the planet, after the planet, or not at all – depending on the orbit configuration. But by stacking many such transits, one gets, on average, a flux loss before and a flux loss after the exoplanet transit.
And bear in mind that moons are only one of the things we might expect to extract from TESS and PLATO data. Right now we have one detected ring system, around the planet J1407b, a massive ring more than 200 times larger than Saturn’s. The authors show that a transiting planet with a ring system produces a definitive signal. Even Trojan asteroids, which lead and follow a planet by 60 degrees in its orbit, should be in range for detection. In a third paper, the authors use Kepler data, injecting synthetic Trojan light curves to search for the limits of detectability. From the paper:
Our result gives an upper limit to the average Trojan transiting area (per planet) corresponding to one body of radius < 460km at 2? confidence. We find a significant Trojan-like signal in a sub-sample for planets with more (or larger) Trojans for periods >60 days.
The authors call these results tentative and suggest that improved data from TESS and PLATO should help us refine them. “As good as the Kepler data are, we’re really pushing them to the limit, so this is a very preliminary result,” adds Hippke in this NASA news release. “We’ve shown somewhat cautiously that it’s possible to detect Trojan asteroids, but we’ll have to wait for better data from TESS, PLATO and other missions to really nail that down.”
All of which tells us that we have much to expect from TESS and PLATO and the instruments that will subsequently home in on the targets they have provided. The papers are Hippke, “On the detection of Exomoons: A search in Kepler data for the orbital sampling effect and the scatter peak,” The Astrophysical Journal Vol. 806, No. 1 (abstract / preprint); Hippke and Angerhausen, “Photometry’s bright future: Detecting Solar System analogues with future space telescopes,” accepted at The Astrophysical Journal (preprint); and Hippke and Angerhausen, “A statistical search for a population of Exo-Trojans in the Kepler dataset,” accepted at The Astrophysical Journal (preprint).
Voyager Update: Probing the Boundary
I always feel that my day starts right when a story involving the Voyagers crosses my desk. The scope, the sheer audacity of these missions in their day cheers me up, and the fact that they are still communicating with us is a continual cause for celebration. With Voyager 1 now moving beyond the heliosphere, we’ve got an interstellar craft on our hands, one that’s telling us a good deal about the perturbed regions through which it moves. Every day that the Voyagers stay alive is a triumph for an inquisitive and exploring species, and one day we’ll be launching their successor, targeting the local interstellar medium with instruments designed for the task.
Image: This artist’s concept shows NASA’s Voyager spacecraft against a backdrop of stars. Credit: NASA/JPL-Caltech.
The heliosphere is that ‘bubble’ blown by the particles of the Sun’s solar wind in surrounding interstellar space. As such, it’s a moving and malleable thing, flexing, flowing, contracting here, expanding there according to the stream of particles filling it from our star. Now we’re finding, not surprisingly, I think, that the magnetic field just outside the heliosphere is perturbed. Voyager 1 data show that the magnetic field here is more than forty degrees at variance from observations of the interstellar magnetic field that have been produced by other spacecraft.
Remember that while Voyager 1 is the first spacecraft to reach such distances from the Sun, we’ve had other missions exploring the outer regions of the system even though they operate well within it. The Interstellar Boundary Explorer (IBEX) is the outstanding example. Launched in 2009, IBEX is in Earth orbit (apogee 322,000 kilometers, perigee 16,000 kilometers), using instruments that track the solar wind’s interactions with ionized interstellar material. Like the Ulysses spacecraft before it, IBEX is also measuring inflowing neutral particles that penetrate the heliosphere, in the process creating a map of the Solar System’s elastic boundaries.
Image: An artist’s rendition of a portion of our heliosphere, with the solar wind streaming out past the planets and forming a boundary as it interacts with the material between the stars. Credit: Adler Planetarium/IBEX team.
IBEX has already told us a lot, including the fact that our Sun is located in a region of space where dust and gas are much more dispersed than they were when the Sun was formed. With IBEX data, scientists can map the distribution of elements like hydrogen, helium, neon and oxygen as they enter the heliosphere, and can use the neon to oxygen ratio in the Sun to trace element ratios in the distant past. We see less oxygen than expected in the interstellar medium today, indicating the changes to the medium since solar formation.
We also see that our Sun is close to the boundary of a local cloud of gas and dust, but still within it — this work challenges earlier Ulysses findings that found the Sun to exist between two clouds, though close to the boundary of the ‘Local Cloud.’ Within a few thousand years, we will be moving out of the Local Cloud and into a somewhat different galactic environment.
IBEX, then, is helping us understand the general region of space through which we move, while Voyager 1 is reporting on conditions just outside the heliosphere boundary. In 2009, IBEX data showed what principal investigator David McComas (SwRI) called a ‘very narrow ribbon that is two to three times brighter than anything else in the sky’ at the interstellar boundary. While this circular arc is still under study, the current view is that it is produced by neutral hydrogen atoms from the solar wind that were reionized in interstellar space and then became neutral again by picking up an additional electron. (For more on IBEX, see IBEX: The Heliosphere in Motion).
This is where the recent Voyager findings come in. Nathan Schwadron (University of New Hampshire) and colleagues have reanalyzed magnetic field data from Voyager 1, discovering that the direction of the magnetic field has been turning ever since the craft crossed into interstellar space. The work, published in Astrophysical Journal Letters confirms that the magnetic field direction at the center of the IBEX ‘ribbon’ is aligned with the magnetic field in the interstellar medium. Voyager is, in other words, now moving through a distorted region. By 2025, the magnetic field around it should align with the field direction found by IBEX.
At that point, we’ll be able to say that Voyager 1 has reached a more settled part of the interstellar medium, less perturbed by the ‘churn’ of the heliosphere. “This study provides very strong evidence that Voyager 1 is in a region where the magnetic field is being deflected by the solar wind,” says Schwadron in this JPL news release. A Voyager follow-up mission will be built from the outset as an interstellar probe, carrying instruments optimized for exploring this active boundary. We can hope that one day even more ambitious missions will use the data thus gathered to chart the regions through which they’ll fly on their way to another star.
The paper is Schwadron et al., “Triangulation of the Interstellar Magnetic Field,” Astrophysical Journal Letters Vol. 813, No. 1, L20. (abstract).
Science Fiction and the Symposium
Science fiction is much on my mind this morning, having just been to a second viewing of The Martian (this time in 3D, which I didn’t much care for), and having just read a new paper on wormholes that suggests a bizarre form of communication using them. More about both of these in a moment, but the third reason for the SF-slant is where I’ll start. The 100 Year Starship organization’s fourth annual symposium is now going on in Santa Clara (CA), among whose events is the awarding of the first Canopus Awards for Interstellar Writing.
A team of science fiction writers will anchor what the organization is calling Science Fiction Stories Night on Halloween Eve. Among the writers there, I’m familiar with the work of Pat Murphy, whose novel The Falling Woman (Tor, 1986) caught my eye soon after publication. I remember reading this tale of an archaeological dig in Central America and the ‘ghosts’ it evokes with fascination, although it’s been long enough back that I don’t recall the details. Joining Murphy will be short story writer Juliette Wade, novelist Brenda Cooper and publisher Jacob Weisman, whose Tachyon publishing is a well-known independent press.
As to the Canopus Awards, they’re to be an annual feature of the 100 Year Starship initiative aimed at highlighting “the importance of great story telling to propel the interstellar movement” (I’m quoting here from their press materials). In case you’re looking for some reading ideas, here are the Canopus finalists going into the event.
In the category of “Previously Published Long-Form Fiction” (40,000 words or more):
Other Systems by Elizabeth Guizzetti
The Creative Fire (Ruby’s Song) by Brenda Cooper
InterstellarNet: Enigma (Volume 3) by Edward M. Lerner
Aurora by Kim Stanley Robinson
Coming Home by Jack McDevitt
——-
In the category of “Previously Published Short-Form Fiction” (between 1,000 and 40,000 words):
“Race for Arcadia” by Alex Shvartsman
“Stars that Make Dark Heaven Light” by Sharon Joss
“Homesick” by Debbie Urbanski
“Twenty Lights to the Land of Snow” by Michael Bishop
“Planet Lion” by Catherynne M. Valente
“The Waves” by Ken Liu
“Dreamboat” by Robin Wyatt Dunn
——-
In the category of “Original Fiction” (1,000-5,000 words):
“Landfall” by Jon F. Zeigler
“Project Fermi” by Michael Turgeon
“Everett’s Awakening” by Ry Yelcho
“Groundwork” by G. M. Nair
“His Holiness John XXIV about Father Angelo Baymasecchi’s Diary” by Óscar Garrido González
“The Disease of Time” by Joseph Schmidt
——-
In the category of “Original Non-Fiction” (1,000-5,000 words):
“Why Interstellar Travel?” by Jeffrey Nosanov
“Finding Earth 2.0 from the Focus of the Solar Gravitational Lens” by Louis D. Friedman and Slava Turyshev
Of Martians and Wormholes
This will be the first 100YSS symposium I’ve missed and I’ll regret missing the chance to meet Pat Murphy and see Mae Jemison, Lou Friedman, Jill Tarter and many others who have made past events so enjoyable. I imagine Jack McDevitt will be there as well; he usually goes to these. His Canopus Award entry Coming Home (Ace, 2014) is another in the Alex Benedict series, featuring a future antiques dealer among whose many artifacts are ‘antiques’ that were crafted far in our own future. I mention Jack because I admire him, have read all the Alex Benedict novels, and thought Coming Home was one of his best.
As to The Martian, it’s a movie I loved for its attention to detail and the sheer bravura of its proceedings. For people who remember Apollo, the idea of a Mars exploration program of similar audacity is a wonderful morale-booster, a reminder that the spirit that took us to the Moon is still alive. It also makes for a jolting comparison between those days and today’s public apathy and budgetary dilemmas, all of which make Mars a target that always seems to be, like fusion, somewhere in the future. Movies like The Martian could do something to reach younger generations, and perhaps ignite interest in both government and private attempts to get to the Red Planet. But be aware that the regular version offers far more verisimilitude than the 3D, whose effects seem contrived and often distracting.
I don’t have time to dig deeply into Luke Butcher’s new paper on wormholes, but I do at least want to mention this effort as one that has caught the interest of wormhole specialist Matt Visser, and should intrigue science fiction authors for its plot possibilities. Working at the University of Cambridge, Butcher has been studying how to keep wormhole mouths open, the problem being that although people like Kip Thorne have speculated on using negative energy to do the trick, wormholes appear to be utterly unstable, closing before they can be used.
If wormholes do exist and we could find a way to use them, we might have a way to cross huge distances without contradicting Einstein’s limits on travel faster than light, using the wormhole’s ability to shortcut its way through spacetime itself. Butcher looks at negative energy in terms of Casimir’s parallel plates sitting close together in a vacuum. What if a wormhole’s own shape could generate such Casimir energies, thus holding it open long enough to use?
Image: Imagining a wormhole. Here we see a simulated traversable wormhole that connects the square in front of the physical institutes of University of Tübingen with the sand dunes near Boulogne sur Mer in the north of France. The image is calculated with 4D raytracing in a Morris-Thorne wormhole metric, but the gravitational effects on the wavelength of light have not been simulated. Credit: Wikimedia Commons.
Butcher can’t find ways to keep wormholes open for long, but he does offer the theoretical possibility that we might be able to keep one open long enough to get a beam of light into it. Get the picture? Communications moving through the wormhole, with the same effect as if they were moving faster than light, with all the interesting causal issues that raises. From the paper:
…the negative Casimir energy does allow the wormhole to collapse extremely slowly, its lifetime growing without bound as the throat-length is increased. We find that the throat closes slowly enough that its central region can be safely traversed by a pulse of light.
So there you are, science fiction writers, another plot possibility involving communications between starships or, for that matter, between planets in, say, a galaxy-spanning civilization of the far future. Make of it what you will. The delight of science fiction is that it can take purely theoretical constructs like this one and run down the endless chain of possibilities. In our era of deep space probes, astrobiology and exoplanet research, science fiction has truly moved out of the literary ghetto in which it once saw itself enclosed. Canopus Award winners take note: You’re starting to go mainstream.
The paper is Butcher, “Casimir Energy of a Long Wormhole Throat,” submitted to Physical Review D (preprint).
Where We Might Sample Europa’s Ocean
No one interested in the prospects for life on other worlds should take his or her eyes off Europa for long. We know that its icy surface is geologically active, and that beneath it is a global ocean. While water ice is prominent on the surface, the terrain is also marked by materials produced by impacts or by irradiation. Keep in mind the presence of Io, which ejects material like ionized sulfur and oxygen that, having been swept up in Jupiter’s magnetosphere, eventually reaches Europa. Irradiation can break molecular bonds to produce sulfur dioxide, oxygen and sulfuric acid. And we’re learning that local materials can be revealed by geology.
A case in point is a new paper that looks at infrared data obtained with the adaptive optics system at the Keck Observatory. The work of Mike Brown, Kevin Hand and Patrick Fischer (all at Caltech, where Fischer is a graduate student), suggests that the best place to look for compounds indicative of life would be in the jumbled areas of Europa called chaos terrain. Here we may have materials brought up from the ocean below.
“We have known for a long time that Europa’s fresh icy surface, which is covered with cracks and ridges and transform faults, is the external signature of a vast internal salty ocean,” says Brown, and our imagery of these areas taken by Galileo shows us a shattered landscape, with great ‘rafts’ of ice that have broken, moved and later refrozen. The clear implication is that water from the internal ocean may have risen to the surface as these chaos areas shifted and cracked. And while a direct sampling of Europa’s ocean would be optimal, our best bet for studying its composition for now may well be a lander that can sample frozen deposits.
Image: On Europa, “chaos terrains” are regions where the icy surface appears to have been broken apart , moved around, and frozen back together. Observations by Caltech graduate student Patrick Fischer and colleagues show that these regions have a composition distinct from the rest of the surface which seems to reflect the composition of the vast ocean under the crust of Europa. Credit: NASA/JPL-Caltech.
Brown and team, whose work has been accepted at The Astrophysical Journal, examined data taken in 2011 using the OSIRIS spectrograph at Keck, which measures spectra at infrared wavelengths. Keck is also able to bring adaptive optics into play to sharply reduce distortions produced by Earth’s atmosphere. Spectra were produced for 1600 different locations on the surface of Europa, then sorted into major groupings using algorithms developed by Fischer. The results were mapped onto surface data produced by the Galileo mission.
The result: Three categories of spectra showing distinct compositions on Europa’s surface. From the paper:
The first component dominates the trailing hemisphere bullseye and the second component dominates the leading hemisphere upper latitudes, consistent with regions previously found to be dominated by irradiation products and water ice, respectively. The third component is geographically associated with large geologic units of chaos, suggesting an endogenous identity. This is the first time that the endogenous hydrate species has been mapped at a global scale.
We knew about Europa’s abundant water ice, and we also expected to find chemicals formed from irradiation. The third grouping, though, being particularly associated with chaos terrain, is intriguing. Here the chemical indicators did not identify any of the salt materials thought to be on Europa. The paper continues:
The spectrum of component 3 is not consistent with linear mixtures of the current spectral library. In particular, the hydrated sulfate minerals previously favored possess distinct spectral features that are not present in the spectrum of component 3, and thus cannot be abundant at large scale. One alternative composition is chloride evaporite deposits, possibly indicating an ocean solute composition dominated by the Na+ and Cl? ions.
Image: Mapping the composition of the surface of Europa has shown that a few large areas have large concentrations of what are thought to be salts. These salts are systematically located in the recently resurfaced “chaos regions,” which are outlined in black. One such region, named Western Powys Regio, has the highest concentration of these materials presumably derived from the internal ocean, and would make an ideal landing location for a Europa surface probe.
Credit: M.E. Brown and P.D. Fischer/Caltech , K.P. Hand/JPL.
The association with chaos areas is significant. Because these spectra map to areas with recent geological activity, they are likely to be native to Europa, and conceivably material related to the internal ocean. In this Caltech news release, Brown speculates that a large amount of ocean water flowing out onto the surface and then evaporating could leave behind salts. As in the Earth’s desert areas, the composition of the salt can tell us about the materials that were dissolved in the water before it evaporated. Brown adds:
“If you had to suggest an area on Europa where ocean water had recently melted through and dumped its chemicals on the surface, this would be it. If we can someday sample and catalog the chemistry found there, we may learn something of what’s happening on the ocean floor of Europa and maybe even find organic compounds, and that would be very exciting.”
So we’re learning where a Europa lander should be able to do the most productive science in relation to astrobiology and the ocean beneath the ice. Keep your eye on the western portion of the area known as Powys Regio, where the Caltech team found the strongest concentrations of local salts. Powys Regio is just south of what appears to be an old impact feature called Tyre. The image below, with the concentric rings of Tyre clearly visible, reminds us that an ocean under a mantle of ice is vulnerable to surface activity and external strikes that would break through the ice and deposit ocean materials within reach of the right kind of lander.
Image: The feature called Tyre, showing signs of an ancient Europan impact. Credit: NASA/JPL-Caltech.
The paper is Fischer, Brown & Hand, “Spatially Resolved Spectroscopy of Europa: The Distinct Spectrum of Large-scale Chaos,” accepted at The Astrophysical Journal (preprint).
Catching Up with the Outer System
We now pivot from Dysonian SETI to the ongoing exploration of our own system, where lately there have been few dull moments. Today the Cassini Saturn orbiter will make its deepest dive ever into the plume of ice, water vapor and organic molecules streaming out of four major fractures (the ‘Tiger Stripes’) at Enceladus’ south polar region. The plume is thought to come from the ocean beneath the moon’s surface ice, and while Cassini is not able to detect life, it is able to study molecular hydrogen levels and more massive molecules including organics. Understanding the hydrothermal activity taking place on Enceladus helps us explore the possible habitability of the ocean for simple forms of life.
Image: This artist’s rendering showing a cutaway view into the interior of Saturn’s moon Enceladus. NASA’s Cassini spacecraft discovered the moon has a global ocean and likely hydrothermal activity. A plume of ice particles, water vapor and organic molecules sprays from fractures in the moon’s south polar region. Credit: NASA/JPL-Caltech.
Cassini’s cosmic dust analyzer (CDA) instrument can detect up to 10,000 particles per second, telling us how much material the plume is spraying into space from the internal ocean. Another key measurement in the flyby will be the detection of molecular hydrogen by the spacecraft’s INMS (ion and neutral mass spectrometer) instrument, says Hunter Waite (SwRI):
“Confirmation of molecular hydrogen in the plume would be an independent line of evidence that hydrothermal activity is taking place in the Enceladus ocean, on the seafloor. The amount of hydrogen would reveal how much hydrothermal activity is going on.”
We’re also going to get a better picture of the plume’s structure — individual jets or ‘curtain’ eruptions — that may clarify how material is making its way to the surface from the ocean below. Cassini will move through the plume at an altitude of 48 kilometers, which is about the distance between Baltimore and Washington, DC. “We go screaming by all this at speeds in excess of 19,000 miles per hour,” says mission designer Brent Buffington. “We’re flying the deepest we’ve ever been through this plume, and these instruments will be sensing the gases and looking at the particles that make it up.” Cassini has just one Enceladus flyby left, on December 19.
NASA’s online toolkit for the final Enceladus flybys can be found here.
Meanwhile, in the Kuiper Belt…
Although its mission at Pluto/Charon has been accomplished, there are a lot of reasons to be excited about New Horizons beyond the data streaming back from the spacecraft. On October 25th, mission controllers directed a targeting maneuver using the craft’s hydrazine thrusters, one that lasted about 25 minutes and was the largest propulsive maneuver ever conducted by New Horizons. The burn, the second in a series of four, adjusts the spacecraft’s trajectory toward Kuiper Belt object 2014 MU69.
If all goes well (and assuming NASA signs off on the extended mission), the encounter will occur on January 1, 2019. The science team intends to bring New Horizons closer to MU69 than the 12500 kilometers that separated it from Pluto on closest approach. Two more targeting maneuvers are planned, one for October 28, the other for November 4. Every indication from data received at the Johns Hopkins University Applied Physics Laboratory is that the Sunday burn was successful. We now have a new destination a billion miles further out than Pluto.
Image: Path of NASA’s New Horizons spacecraft toward its next potential target, the Kuiper Belt object 2014 MU69. Although NASA has selected 2014 MU69 as the target, as part of its normal review process the agency will conduct a detailed assessment before officially approving the mission extension to conduct additional science. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Alex Parker.
Remember that this is a spacecraft that was designed for operations in the Kuiper Belt, one that carried the extra hydrazine necessary for the extended mission, and one whose communications system was designed to operate at these distances. The science instruments aboard New Horizons were also designed to operate in much lower light levels than the spacecraft experienced at Pluto/Charon, and we should have enough power for many years.
Principal investigator Alan Stern commented on the choice of this particular Kuiper Belt object (also being called PT1 for ‘Potential Target 1’) at the end of the summer, noting its advantages:
“2014 MU69 is a great choice because it is just the kind of ancient KBO, formed where it orbits now, that the Decadal Survey desired us to fly by. Moreover, this KBO costs less fuel to reach [than other candidate targets], leaving more fuel for the flyby, for ancillary science, and greater fuel reserves to protect against the unforeseen.”
What we know about 2014 MU69 is that it is about 45 kilometers across, about ten times larger than the average comet, and 1000 times more massive. Even so, it’s only between 0.5 and 1 percent of the size of Pluto, and 1/10,000th as massive. Researchers consider objects like these the building blocks of dwarf worlds like Pluto. While we’ve visited asteroids before, Kuiper Belt objects like this one are thought to be well preserved samples of the early Solar System, having never experienced the solar heating that asteroids in the inner system encounter.
We also have a newly received image of Pluto’s tiny moon Kerberos, which turns out to be smaller and much more reflective than expected, with a double-lobed shape. The larger lobe is about eight kilometers across, the smaller approximately 5 kilometers. Mission scientists speculate that the moon was formed from the merger of two smaller objects. Its reflectivity indicates that it is coated with water ice. Earlier measurements of the gravitational influence of Kerberos on the other nearby moons were evidently incorrect — scientists had expected to find it darker and larger than it turns out to be, another intriguing surprise from the Pluto system.
Image: This image of Kerberos was created by combining four individual Long Range Reconnaissance Imager (LORRI) pictures taken on July 14, 2015, approximately seven hours before New Horizons’ closest approach to Pluto, at a range of 396,100 km from Kerberos. The image was deconvolved to recover the highest possible spatial resolution and oversampled by a factor of eight to reduce pixilation effects. Kerberos appears to have a double-lobed shape, approximately 12 kilometers across in its long dimension and 4.5 kilometers in its shortest dimension. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.
Why SETI Keeps Looking
How do you feel about a universe that shows no signs of intelligent life? Let’s suppose that we pursue various forms of SETI for the next century or two and at the end of that time, find no evidence whatsoever for extraterrestrial civilizations. Would scientists of that era be disappointed or simply perplexed? Would they, for that matter, keep on looking?
I suspect the latter is the case, not because extraterrestrial civilizations would demonstrate that we’re not alone, but because in matters of great scientific interest, it’s the truth we’re after, not just the results we want to see. In my view, learning that there was no other civilization within our galaxy — at least, not one we can detect — would be a profoundly interesting result. It might imply that life itself is rare, or even more to the point, that any civilizations that do arise are short-lived. This is that tricky term in the Drake equation that refers to the lifespan of a technological civilization, and if that lifetime is short, then our own position is tenuous.
The anomalous light curve in the Kepler data from KIC 8462852 focuses this issue because on the one hand I’m hearing from critics that SETI researchers simply want to see extraterrestrials in their data, and thus misinterpret natural phenomena. An equally vocal group asks why people like me are so keen on looking for natural explanations when the laws of physics do not rule out other civilizations. All I can say is that we need to be dispassionate in the SETI search, looking for interesting signals (or objects) while learning how to distinguish their probable causes.
In other words, I don’t have a horse in this race. The universe is what it is, and the great quest is to learn as much as we can about it. I am not going to lose sleep if we discover a natural cause for the KIC 8462852 light curves because whatever is going on there is astrophysically interesting, and will help us as we deepen our transit studies of other stars. The recent paper from Wright et al. discusses how transiting megastructures could be distinguished from exoplanets, and goes on to describe the natural sources that could produce such signatures. The ongoing discussion is fascinating in its own right and sharpens our observational skills.
Image: The Kepler field of view, containing portions of the constellations Cygnus, Lyra, and Draco. Credit: NASA.
Yesterday’s post looked at ‘gravity darkening’ as a possible explanation for what we see at KIC 8462852, with reference to conversations we’ve been having in the comments section here. Gravity darkening appears in the Wright paper, though not with reference to KIC 8462852, and is also under study in other systems, particularly the one called PTFO 8-8695. But its prospects seem to be dimming when it comes to KIC 8462852, as Wright explained in a tweet.
@centauri_dreams @steinly0 This "solution" doesn't work: planets don't block 22% of stellar disk. Star spins fast, but not THAT fast.
— Jason Wright (@Astro_Wright) October 26, 2015
He went on to elaborate in yesterday’s comments section:
Gravity darkening might be a small part of the puzzle, but it does not explain the features of this star. Tabby’s star does not rotate fast enough to experience significant gravity darkening. That post also suggests that planets could be responsible, but planets are not large enough to produce the observed events, and there are too many events to explain with planets or stars.
The Wright paper lists nine natural causes of anomalous light curves in addition to gravity darkening, including planet/planet interactions, ring systems and debris fields, and starspots. Exomoons, the subject of continuing work by David Kipping and colleagues at the Hunt for Exomoons with Kepler project, also can play a role, with a sufficiently large moon producing its own transit events and leaving a signature in transit timing and duration variations.
We have examples of objects whose anomalies have been investigated and found to be natural, including the interesting CoRoT-29b, in which gravity darkening is likewise rejected. From the paper:
CoRoT-29b shows an unexplained, persistent, asymmetric transit — the amount of oblateness and gravity darkening required to explain the asymmetry appears to be inconsistent with the measured rotational velocity of the star (Cabrera et al. 2015). Cabrera et al. explore each of the natural confounders in Table 2.3 for such an anomaly, and find that none of them is satisfactory. Except for the radial velocity measurements of this system, which are consistent with CoRoT-29b having planetary mass, CoRoT-29b would be a fascinating candidate for an alien megastructure.
We can also assign a natural explanation to KIC 1255b, an interesting find because its transit depths vary widely even between consecutive transits, and its transit light curves show an asymmetry between ingress and egress. What we are apparently looking at here is a small planet that is disintegrating, creating a thick, comet-like coma and tail that is producing the asymmetries in the transit light curves. This is an intriguing situation, as the Wright paper notes, with the planet likely pared of 70 percent of its mass and reduced to an iron-nickel core.
We may well find a natural explanation that takes care of KIC 8462852 as well, and the large scope of the challenge will ensure that the object remains under intense scrutiny. Both CoRoT-29b and KIC 1255b are useful case studies because they show us how unusual transit signatures can be identified and explained. We also have to keep in mind that such signatures may not be immediately found because Kepler data assessment techniques are not tuned for them, as the paper notes:
…in some cases of highly non-standard transit signatures, it may be that only a model-free approach — such as a human-based, star-by-star light curve examination — would turn them up. Indeed, KIC 8462852 was discovered in exactly this manner. KIC 8462852 shows transit signatures consistent with a swarm of artificial objects, and we strongly encourage intense SETI efforts on it, in addition to conventional astronomical efforts to find more such objects (since, if it is natural, it is both very interesting in its own right and unlikely to be unique).
Thus we leave the KIC 8462852 story for now, although I would encourage anyone interested in Dysonian SETI to read through the Wright paper to get a sense of the range of transiting signatures that draw SETI interest. The paper is Wright et al., “The ? Search for Extraterrestrial Civilizations with Large Energy Supplies. IV. The Signatures and Information Content of Transiting Megastructures,” submitted to The Astrophysical Journal (preprint).
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