Fast Radio Bursts: SETI Implications?

by Paul Gilster on April 1, 2015

With SETI on my mind after last week’s series on Dysonian methods, it seems a good time to discuss Fast Radio Bursts, which have become prominent this week following the appearance of a new paper. A New Scientist piece titled Is this ET? Mystery of strange radio bursts from space is also circulating, pointing out that these powerful bursts of radio waves lasting for milliseconds, each covering a broad range of radio frequencies, are still unexplained, and that they seem to follow a mathematical pattern.

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Image: The 64-metre Parkes radio telescope in New South Wales (Australia), where Fast Radio Bursts of unknown origin have been detected. Credit: CSIRO Parkes Observatory.

Eleven bursts have been detected so far, dating back to 2001. The paper, by Michael Hippke (Institute for Data Analysis, Neukirchen-Vluyn, Germany), Wilfried Domainko (Max-Planck-Institut fur Kernphysik, Heidelberg) and John Learned (University of Hawaii, Manoa), points out that the pulses have dispersion measures higher than we would expect from sources inside the galaxy. But for reasons the paper goes on to explain, they’re probably not of extragalactic origin.

To untangle all this, let’s pause on the term ‘dispersion measure.’ It comes from the study of radio pulsars, where it has been observed that a pulse can be delayed depending upon its radio frequency as it sweeps through charged particles.

To clarify this, let me quote from a useful online discussion on pulsar research that Ryan Lynch (now at McGill University) published on the matter. Lynch points out that the dispersion measure (DM), which is a characteristic of a pulsar signature, doesn’t have anything to do with the pulsar itself. It is, instead, a marker that tells us something about the space between Earth and the pulsar, or as in the case above, a Fast Radio Burst. That space may contain various charged particles, but the delay they produce is inversely proportional to the mass of the particles — in other words, the amount of dispersion is dominated by electrons. According to Lynch:

These electrons disperse the pulsar’s signal… causing lower observing frequencies to arrive later than higher observing frequencies. The electrons can also scatter the signal much the same way smoke scatters visible light. The dispersion measure is a way of telling us how many electrons the signal encountered on its way to Earth. The larger the dispersion measure, the more electrons the signal encountered. This could happen for two reasons — either the pulsar is very far away, or the density of electrons in the space between Earth and the pulsar is relatively high. Both will cause an increase in the dispersion measure.

Michael Hippke and team want to consider what the dispersion measures of the detected Fast Radio Bursts tell us about their nature, hence the title of the paper, “Discrete Steps in Dispersion Measures of Fast Radio Bursts.” It’s an interesting astronomical question because it could be that FRBs could be used as ‘standard candles’ that could help us understand more about dark energy. The researchers wanted to know whether a clustering in the dispersion measure values could show which came from within our galaxy and which from without.

What turns up is what the authors describe as a ‘potential discrete spacing in DM of the FRBs.’ The bursts’ dispersion measures are integer multiples of the number 187.5, which the paper argues makes an extragalactic origin unlikely. The thinking here is that intergalactic dust would randomize the DM values, and thus we are more likely dealing with a source within the Milky Way. Just what the source could be makes for interesting speculation. From the paper:

A more likely option could be a galactic source producing quantized chirped signals, but this seems most surprising. If both of these options could be excluded, only an artificial source (human or non-human) must be considered, particularly since most bursts have been observed in only one location (Parkes radio telescope). A re-assessment of man-made phenomena, such as perytons (Burke-Spolaor et al. 2011), would then be required. Failing some observational bias, the suggestive correlation with terrestrial time standards seems to nearly clinch the case for human association of these peculiar phenomena.

That latter point has to do with the fact that FRBs seem to arrive at close to the full integer second, suggesting a man-made signal. As to the Burke-Spolaor paper cited above, it examines a series of pulses likewise detected at the Parkes instrument that are, as the abstract says, of ‘clearly terrestrial origin,’ and hence an example of our need to tune up radio-pulse survey techniques to rule out terrestrial signals. I also note the citation’s last sentence, which makes the case for a human origin. But as the Hippke paper points out, ‘In the end we only claim interesting features which further data will verify or refute.’

FRBs merit our attention because while we may have a human origin for these bursts, it’s conceivable that an extraterrestrial beacon could be in play. New Scientist quotes Jill Tarter, former director of the SETI Institute, as saying “I’m intrigued. Stay tuned.” Which we’ll all do, though mindful of the fact that with such a small number of bursts, we are working with little data. A hitherto undiscovered astronomical phenomenon would be a useful discovery, but we need to see how the pattern holds up as more FRBs are detected.

Back in 1967, The Byrds produced a song called “CTA-102,” a musical reflection of the discussion of the quasar by the same name, whose emissions had excited worldwide interest. Nikolai Kardashev himself wondered in 1963 whether a Type II or Type III civilization could be behind the then unidentified source, and subsequent observations by Gennady Sholomitskii kept the idea in the public imagination. We found a natural explanation for CTA-102, but as with FRBs, checking our data against the possibility of extraterrestrial communication makes good sense and could in the process reveal a new type of astronomical object or, more likely, a nearby human explanation.

The paper is Hippke et al., “Discrete Steps in Dispersion Measures of Fast Radio Bursts” (abstract). The Burke-Spolaor paper is “Radio Bursts with Extragalactic Spectral Characteristics Show Terrestrial Origins,” Astrophysical Journal Vol. 727, No. 1 (2011), 18 (abstract).

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A Fresh Look at Rhea

by Paul Gilster on March 31, 2015

When it comes to Saturn, have you noticed what’s been missing lately? Well, actually for the last two years. While the Cassini orbiter has had high-profile encounters with Titan, it has been in a high-inclination orbit that has meant no recent flybys of other Saturnian moons. All that has now changed as Cassini returned to the planet’s equatorial plane this month, which means we can look forward to more interesting views like these mosaics of the planet’s second largest moon Rhea.

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Image: Two mosaics of Saturn’s icy moon Rhea, with constituent images taken about an hour and a half apart on February 9, 2015. Images taken using clear, green, infrared and ultraviolet spectral filters were combined to create these enhanced color views, which offer an expanded range of the colors visible to human eyes in order to highlight subtle color differences across Rhea’s surface. The moon’s surface is fairly uniform in natural color. Credit: JPL.

The Rhea imagery comes from a flyby of the moon on February 9, the first encounter other than Titan since early 2013. Resolution in the smaller mosaic is 450 meters per pixel, while the view on the right has a resolution of 300 meters per pixel. The images going into the mosaic on the right were acquired at a distance that ranged from 57,900 kilometers to 51,700 kilometers; those on the left from distances between 82,100 to 74,600 kilometers. Each mosaic is made up of multiple narrow-angle camera (NAC) images, with wide-angle camera data used where necessary to fill in the areas where the NAC data were not available.

Rhea is the ninth largest moon in the Solar System, and while it doesn’t turn up very often in discussions of astrobiology, I should note a 2006 paper from Hauke Hussmann (Universidade de São Paulo) and colleagues that investigates the possibility of sub-surface oceans in the medium-sized icy satellites as well as the largest trans-Neptunian objects (TNOs). The paper argues that assuming differentiation and an equilibrium between heat production in the rocky cores and heat loss through the ice shell, sub-surface oceans are possible on Rhea, Titania, Oberon, Triton, and Pluto and on the large TNO’s Eris, Sedna, and Orcus (2004 DW).

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Image: Cassini’s wide-angle camera took this image of Rhea from about 200 kilometers away from the moon’s surface in early 2011. Credit: JPL.

Back in 2011, Cassini completed a close flyby of Rhea, with closest approach achieved on January 11 of that year. The imagery above shows an old, cratered and apparently inert surface, with some evidence of straight faults that show up better in other Rhea imagery. Further study of Rhea via Cassini will help us determine how its extremely tenuous atmosphere of oxygen and CO2 interacts with particles in Saturn’s magnetosphere.

Next up for Cassini in its new, nearly equatorial orbit: A May 7 flyby of Titan, the first of four in 2015, as well as two encounters with Dione and three with the always interesting Enceladus. The Hussmann paper mentioned above is “Subsurface oceans and deep interiors of medium-sized outer planet satellites and large trans-neptunian objects,” Icarus Volume 185, Issue 1 (November 2006), pp. 258–273 (abstract).

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An Alpha Centauri Bb Transit Search

by Paul Gilster on March 30, 2015

Alpha Centauri continues to be a maddening and elusive subject for study. Two decades of radial velocity work on Centauri A and B have been able to constrain the possibilities — we’ve learned that there are no gas giants larger than Jupiter in orbits within 2 AU of either of the stars. But lower mass planets remain a possibility, and in 2012 we had the announcement of a planet slightly more massive than Earth in a tight orbit around Centauri B. It was an occasion for celebration (see Lee Billings’ essay Alpha Centauri and the New Astronomy for a glimpse of how that moment felt and how it fit into the larger world of exoplanet research).

But the candidate world, Centauri Bb, remains controversial, and for good reason. The work involved radial velocity methods at a level of precision that pushed our instruments to the limit. Andrew LePage explored the issues in Happy Anniversary α Centauri Bb?, where the question-mark tells the tale. Here he discusses the instrumentation involved in the 2012 work:

The first team to announce any results from their search was the European team using the HARPS (High Accuracy Radial Velocity Planetary Searcher) spectrometer on the 3.6-meter telescope at the European Southern Observatory in La Silla, Chile. They employed a new data processing technique to extract the 0.5 meter per second signal of α Centauri Bb out of 459 radial velocity measurements they obtained between February 2008 and July 2011. These radial velocity data had a measurement uncertainty of 0.8 meters per second and contained an estimated 1.5 meters per second of natural noise or “jitter” resulting from a range of activity on the surface of α Centauri B modulated by its 38-day period of rotation.

A planet with a 3.24 day orbital period was the result of an extremely low-amplitude signal, and subsequent analysis raised doubts about its validity, with Artie Hatzes (Thuringian State Observatory) finding that additional observations were needed to make sure we weren’t seeing noise in the data instead of a planet. Bear in mind that we also have Debra Fischer (Yale University) and team investigating Alpha Centauri Bb at Cerro Tololo Inter-American Observatory (CTIO) in Chile and a team at Mt. John University Observatory in New Zealand.

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Image: An artist’s impression of the still unconfirmed α Centauri Bb, whose discovery was announced on October 16, 2012. The planet is the subject of a new transit search discussed below. (Credit: ESO/L. Calçada/Nick Risinger)

Now comes Brice-Oliver Demory (University of Cambridge), whose team has gone after a different kind of detection, working with the Hubble Space Telescope on a transit search of Centauri B in hopes of finding the signature of the controversial planet. Transits depend upon the alignment of star, planet and observer, so a null result doesn’t demonstrate that the planet doesn’t exist, but using 40 hours of observation, the team was able to rule out a transit of Centauri Bb under the published orbital parameters with a confidence level of 96.6 percent.

The story does have an intriguing coda in the form of a single 2013 event, one that lasted longer than expected for a Centauri Bb transit. The team worked through the possibilities of instrument error and other factors, as the paper explains:

We explore in the following the possibility that the July 2013 transit pattern is due to stellar variability, instrumental systematics or caused by a background eclipsing binary. We do not find any temperature or HST orbital dependent parameter, nor X/Y spectral drifts to correlate with the transit pattern. The transit candidate duration of 3.8 hours is 2.4 times longer than the HST orbital period, making the transit pattern unlikely to be attributable to HST instrumental systematics. As the detector is consistently saturated during all of our observations, we also find it unlikely that saturation is the origin of the transit signal.

Another source of confusion could be activity on the star itself, but the researchers do not see it as a factor:

…the duration of the transit candidate (3.8-hr) is not consistent with the stellar rotational period of 36.2 days…, to enable a spot (or group of spots) to come in and out of view. In such a case, star spots would change the overall observed flux level and produce transit-shape signals, as is the case for stars having fast rotational periods…

We are left with the possibility that this may have been an actual planetary transit with a different orbital period than described in the Centauri Bb discovery paper. Is it a second possible planet around Centauri B, one with an orbital period in the vicinity of 20 days? It will take follow-up photometric observations of an extremely tricky stellar system to tell us more.

In this New Scientist article on the Hubble observations, Demory mentions the possibility of a low-cost, perhaps crowd-funded mission, a small satellite whose sole purpose would be the kind of intensive Alpha Centauri ‘stare’ that busy instruments like Hubble haven’t time for. It’s an interesting idea, and would make for a KickStarter project in the range of $2 million. Says Demory: “Anyone fancy chipping in to find our nearest neighbours?”

The paper is Demory et al., “Hubble Space Telescope search for the transit of the Earth-mass exoplanet Alpha Centauri B b,” accepted at Monthly Notices of the Royal Astronomical Society (preprint). For a thorough analysis of the data involved in this work, see Andrew LePage’s essay Has Another Planet Been Found Orbiting Alpha Centauri B?

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SETI Explores the Near-Infrared

by Paul Gilster on March 27, 2015

This has been a week devoted to extraterrestrial technologies and the hope that, if they exist, we can find them. Large constructions like Dyson spheres, and associated activities like asteroid mining on the scale an advanced civilization might use to make them, all factor into the mix, and as we’ve seen, so do starships imagined in a wide variety of propulsion systems and designs. Dysonian SETI, as it is called, takes us into the realm of the hugely speculative, but hopes through sifting our abundant astronomical data to find evidence of distant engineering.

This effort is visible in projects like the Glimpsing Heat from Alien Technologies (G-HAT) SETI program, which proceeds in the capable hands of Jason Wright and colleagues Steinn Sigurðsson and Matthew Povich at Penn State (see Wright’s Glimpsing Heat from Alien Technologies essay in these pages as well as his AstroWright blog). For those wanting to follow up these ideas, an excellent introduction is the paper “Dysonian Approach to SETI: A Fruitful Middle Ground?”, which ran in JBIS in 2011 (Vol. 64, pp. 156-165). It’s not, unfortunately, available online, though the British Interplanetary Society offers a print copy of the entire back issue here.

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Image: NGC 2403 in Camelopardalis. Dysonian SETI, not limited to relatively nearby stars, looks for signs of astroengineering not just in our own but in distant galaxies like this one, some ten million light years away.. Credit & Copyright: Martin Pugh.

Into the Infrared

The more conventional radio and optical SETI methods continue as well. I’ve written often in these pages about Frank Drake’s Project Ozma at the Green Bank (WV) site, and cited the classic 1959 paper “Searching for Interstellar Communications” by Giuseppe Cocconi and Philip Morrison, which more or less opened up the entire field. But equally significant is Charles Townes’ 1961 paper “Interstellar and Interplanetary Communication by Optical Masers,” which ran in Nature (Vol. 190, No. 4772, pp. 205-208), from which this quote:

We propose to examine the possibility of broadcasting an optical beam from a planet associated with a star some few or some tens of light-years away at sufficient power-levels to establish communications with the Earth. There is some chance that such broadcasts from another society approximately as advanced as we are could be adequately detected by present telescopes and spectrographs, and appropriate techniques now available for detection will be discussed. Communication between planets within our own stellar system by beams from optical masers appears a fortiori quite practical.

Townes, who died recently, built the first maser, which worked primarily in the microwave region of the spectrum. He was a major figure in the development of both maser and laser technologies, and a winner of the Nobel Prize in 1964. The field of optical SETI has not been as visible as the older radio SETI but its proponents are actively pursuing the search at sites like Lick Observatory, where the 1-meter Nickel Telescope has been equipped with a new pulse-detection system using three light detectors, an installation that allows what Frank Drake calls “…perhaps the most sensitive optical SETI search yet undertaken.”

The new instrument is called NIROSETI, which stands for near-infrared optical SETI. It promises to gather copious data by recording levels of light over time to look for patterns that might signify a distant civilization. The beauty of working at near-infrared wavelengths is that such light penetrates much farther through gas and dust than visible light, helping us widen the search to stars thousands of light years away. NIROSETI saw first light on March 15.

Unlike Dysonian SETI, optical SETI operates under the premise that an extraterrestrial civilization may be trying to communicate with us, beaming light explicitly at our Solar System. According to this news release from the SETI Institute, NIROSETI’s use of three light detectors will allow the team to separate the brief pulses of light they are looking for from false alarms of the sort that have troubled other optical SETI experiments using fewer detectors. Optical SETI ‘noise’ can consist of cosmic rays, incident starlight, muon showers and radioactive decay in the glass of the photomultiplier tubes of the detectors, all events to be screened out of the data.

Dan Werthimer, who along with Richard Treffers (UC-Berkeley) designed an earlier instrument for optical SETI, notes where NIROSETI departs from its predecessors:

“This is the first time Earthlings have looked at the universe at infrared wavelengths with nanosecond time scales. The instrument could discover new astrophysical phenomena, or perhaps answer the question of whether we are alone.”

Shelley Wright (UC-San Diego) led the development of the new instrument while at the University of Toronto, finally signing off on detectors sensitive enough to deploy on the telescope. In addition to Wright and Werthimer, the group also includes Geoff Marcy and Andrew Siemion (UC-Berkeley), Patrick Dorval and Elliot Meyer (University of Toronto) and pioneering SETI scientist Frank Drake, whose take on the investigation is determinedly optimistic:

“There is only one downside: the extraterrestrials would need to be transmitting their signals in our direction. If we get a signal from someone who’s aiming for us, it could mean there’s altruism in the universe. I like that idea. If they want to be friendly, that’s who we will find.”

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Image: Skies cleared for a successful first night for NIROSETI at Lick Observatory. The ghost image is Shelley Wright, pausing for a moment during this long exposure as the rest of her team continued to test the new instrument inside the dome. Credit: UC-San Diego.

We can hope that Frank Drake’s ideas come to pass. In any event, it’s clear that the definition of SETI is evolving as we continue to explore radio, optical and Dysonian strategies. In my view, the emergence of the Dysonian approach has been a genuine boon for our investigations. It reminds us how much astronomical data we have accumulated that can now be subjected to analysis in these terms. Will evidence of the existence of an extraterrestrial civilization come, if it does come, through a radio burst, an optical signal, or the observation of an anomaly in a distant galaxy?

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Starship Detection: The K2 Perspective

by Paul Gilster on March 26, 2015

‘Classical’ SETI, if I can use that term, is based on studying the electromagnetic spectrum primarily in the radio wavelengths thought most likely to be used for communication by an extraterrestrial civilization. SETI’s optical component is largely focused on searching for signals intended as communication. What is now being called Dysonian SETI is a different approach, one based on gathering observational evidence that may already be in our archives, data that demonstrate the existence of extraterrestrial activity far beyond our capability.

Just as a Dyson Sphere would reveal the workings of a civilization of Kardashev Type II — producing something like ten billion times the energy of a Type I culture — the detection of a starship would show us technology in action, even if the craft were, as Ulvi Yurtsever and Steven Wilkinson have speculated, a vehicle pushing up against light speed millions of light years away. As physicist Al Jackson has tackled starship detection in recent years, he has taken note of the work of D. R. J. Viewing and Robert Zubrin, which dealt with some but not all design and detection possibilities. Beamed propulsion, for example, does not turn up in these sources.

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Jackson also points to a caveat in such work: If we are hoping to detect a starship using many of the methods described in previous studies, we need to be inside the engine’s exhaust cone or the transmitter cone of the energy beam. We also know that the cone will be narrow. Even so, there are a number of ways to proceed, ranging from the craft’s interactions with the interstellar medium to detection of its own waste heat.

Image: Physicist Al Jackson. I can’t remember who took this (it may even have been me).

Imagine a highly advanced ship built by a Kardashev Type II civilization. Give it a gamma factor of 500, which translates to 0.999998 times the speed of light. Assume the ship is as hot as 5000 K (near the melting temperature of graphene). All these are extreme assumptions (see below) but we’re pushing the envelope here. This is, after all, K2.

Would we be able to detect such a craft? Waste heat can be modeled as isotropic radiation, says Jackson, in the rest frame of the ship, while to an observer in another inertial frame, this radiation appears beamed. We get this result:

Considering a ship of modest size and mass, a K2 ship accelerating at one gravity. For instance, if we have a ship 1000 meters long and 50 meters in diameter, generating 11402 terawatts in its rest frame, Doppler boosting will generate approximately 1.2×108 terawatts beamed into the forward direction. However… unless the ship is headed straight at the observer, it will be hard to see. Take into account the Doppler shifting of the characteristic wavelength, from near green in the rest frame to soft x-ray in the observer’s frame. One might look for small anomalies in data from a host of new astrophysical satellite observatories.

Not very encouraging, but then, detecting the signature of a starship is not going to be easy. One possible place to look is in the realm of what Jackson calls ‘gravitational machines,’ such as the massive binaries Freeman Dyson once suggested could be used as gravitational slingshots. We might consider not just white dwarf and neutron star binaries but even black hole binaries. A gravitational assist in such scenarios might reach as high as .006c.

On the other hand, wouldn’t a civilization that could already reach binaries like these have acquired capabilities greater than those it would gain by using the binaries in the first place? Perhaps better to consider black holes as a source of direction change for fast-moving starships. Jackson points out that a starship orbiting a black hole will have visible waste radiation. In fact, a close-orbiting ship will have fluctuating emissions peaked at those times that the ship, black hole and observer line up, a phenomenon that is the result of gravitational focusing.

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Other extreme astronomical objects may be worthy of investigation in these terms. Jackson points to SS 433, a neutron star or black hole orbited by a companion star, with material being drawn from the companion into an accretion disk. Jets of particles are being blown outward from the poles. While at SS 433 the particles in the jets are moving at 26 percent of the speed of light, jet material in configurations like these can reach 95 percent of lightspeed. Using such jets to propel magsails that reach .5 times the speed of light would allow a K2 civilization an abundant source of energy for repeated missions at a high percentage of c.

Image: Magsail ‘jet riding’. Credit: Doug Potter.

We don’t know what a K2 civilization will choose to do, but exploiting naturally occurring resources like these may be an attractive proposition. There may be interesting prospects not just for magsails but so-called ‘lightsails’ around extreme astronomical objects:

Consider a K2 civilization taking advantage of a Schwarzschild or Kerr black hole as a means of focusing radiation from a beaming station onto a sail. The advantage of this is the tremendous amount of amplification possible. One of the most promising modes of interstellar flight propulsion is the use of a sail, a transmitter, and maybe a ‘lens’ to focus a beam of laser light or microwaves. Extrapolate to a K2 civilization the use of a black hole as the focusing device. An approximate calculation for a Schwarzschild black hole shows that beamed radiation can be amplified by a factor 105 to 1015.

So-called ‘strong focusing’ is tricky to model and, as Jackson explains in some detail, the astronomical configuration — the location of a source and the best location for the sail — are topics that need much more work. But the idea that a K2 civilization would use the immense energies available in the area of black holes makes them a natural hunting ground for Dysonian SETI activity. Could a black hole in the vicinity of a starship’s destination be used for braking?

Robert Bussard’s 1960 paper on interstellar ramjets posited a spacecraft that could collect gas from the interstellar medium, compressing it to a plasma that could be brought to fusion temperatures. Carl Sagan would later suggest that a magnetic scoop would be the ideal way for this gas collection to proceed, but later work by Dana Andrews and Robert Zubrin revealed how much drag such a magnetic scoop would produce. The ‘magsail’ actually acts like a brake.

Why not, then, use these magsail properties, shedding energy and momentum as a spacecraft nears its destination? Craft moving at relativistic velocities might find this an efficient way to arrive, one that produces a ‘bow shock’ whose radiation ranges from the optical to the X-ray bands. “A starship will be much smaller than a neutron star,” writes Jackson, “but detection of the radiation signature of a starship’s bow shock could imply a very peculiar object.”

Figure 3

Image: Two examples of neutron star bow shock, the one on the right an artist’s concept. Credit: Wikimedia Commons.

Jackson’s paper is a work in progress, with an early version printed in Horizons, the AIAA bulletin for the organization’s Houston chapter. A journal submission is in the works as he refines the draft. It’s a fascinating discussion that reminds us how much we have to speculate about when we talk K2 civilizations. Jackson notes the major assumptions: Ships can run ‘hot,’ and that means as high as 5000 K; material structural strength limits have been overcome; extreme accelerations are allowable and dust/gas shielding issues resolved. We can argue about the limits here, but it’s clear that a K2 civilization will have capabilities far beyond our science, and it may be the random anomaly in astronomical data that flags its existence.

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Our View of a Decelerating Magsail

by Paul Gilster on March 25, 2015

Yesterday’s post looked at the question of starship detection. But the paper by Ulvi Yurtsever and Steven Wilkinson that I discussed actually focused on a highly specific subset of such observations, the case of an artificial object moving at such high gamma factors that the ship’s velocity was over 99 percent of the speed of light. It may be that such things become possible to sufficiently advanced civilizations, but if they do and we observe them, we will be doing something akin to what Richard Carrigan does when he looks for Dyson spheres. Hunting a relativistic starship between galaxies is a kind of interstellar archaeology.

What I mean is that if any of the researchers now looking for observational data of advanced civilizations turn something up in, say, M31, that construct will be so far away from us in both space and time that we might well be studying the ruins of an ancient culture. I made this case not long ago in an essay called Distant Ruins for Aeon magazine. This is a different kind of SETI, one in which communication is almost certainly not an issue, just as would be the case if we detected the signature of a starship in the intergalactic deep.

Science fiction that looks at starship detection usually takes a more active stance. In Arthur C. Clarke’s wonderful Rendezvous with Rama (1973), it is an early warning system to detect dangerous asteroids that initially notices the starship humans will name Rama. Further observations help nail down a trajectory that comes from interstellar space and will return there, while the detection of a rapid rotation and, later, photographs of the object as a perfect cylinder make it clear that this will be our species’ first encounter with an alien vessel.

And then there’s The Mote in God’s Eye by Larry Niven and Jerry Pournelle, published the following year, in which astronomers detect a starship in the form of a laser-beamed lightsail. It’s an older technology in the world of the novel, for in this future scenario humans have already produced a star drive that eclipses the much slower sail method. Observed changes in the brightness and color of a Sun-like star in a binary system (the star is called the Mote for reasons explained in the novel) turn out to have been caused by the operation of the laser system beaming the craft toward the Earth.

Here the scene is the bridge of a human starship as the crew discusses the observations that lead to the discovery that they are dealing with an alien technology, specifically its laser beaming system:

“…I checked with Commander Sinclair. He says his grandfather told him the Mote was once brighter than Murcheson’s Eye [the other star in the binary pair], and bright green. And the way Gavin’s describing that holo – well, sir, stars don’t radiate all one color. So -”

“All the more reason to think the holo was retouched. But it is funny, with that intruder coming straight out of the Mote…”

“Light,” Potter said firmly.

“Light sail!” Rod shouted in sudden realization…”

Niven and Pournelle had vetted their lightsail concept with Robert Forward at a time when the idea was just gaining traction thanks to the latter’s work in the journals.

In both novels, the starship detection has huge consequences for our species as the craft in question is entering human space. When I went back and looked at Robert Zubrin’s 1995 paper on starship detection, I remembered that he came up with interesting figures for different kinds of starships. An antimatter photon rocket would produce gamma ray emissions that would be undetectable at visual wavelengths, but Zubrin found that based on his assumptions on an arbitrarily chosen 1,000,000 ton craft, an exhaust with an effective irradiated power of 1,800,000 TW would be produced. He went on to describe its detectability:

Such an object at a distance of 1 light-year would be seen from Earth as a 17th magnitude light source, and could be detected on film by a first class amateur telescope. The 200 inch telescope on Mount Palomar could image it at 20 light years, and the Hubble Space Telescope at a distance of about 300 light years… Since at least for the upper-end telescopes considered, the number of stellar systems within range is significant (100,000 stars are within 200 light years of Earth) this approach offers some hope for a successful search. The light from the photon rocket could be distinguished from that of a dim star by the lack of hydrogen lines in the rocket’s emissions.

Here again we’re dealing with a vast volume of space but a distance of no more than 200 light years in any direction. But beyond the visual spectrum, Zubrin discusses a variety of scenarios, noting that radio waves may be emitted from a starship due to plasma interactions with the deceleration field of a magnetic sail, or the confinement field of a plasma drive engine. Now the detection distances grow greater. The plasma drive engine‘s electron and ion cyclotron radiation could be detected on the ground by radio telescopes. Magsails produce electron cyclotron radiation with frequencies of tens of kHz and ion cyclotron radiation with frequencies of tens of Hz. No ground detection here, but the magsail radiation would be apparent to receivers of sufficient size working outside the Earth’s atmosphere.

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Image: One configuration of a magsail as envisioned by Steve Bowers on the Orion’s Arm site. This design uses multiple superconducting loops for maximum braking effect against the interstellar medium. Credit: Steve Bowers.

The low frequency magsail radiation is made to order for large antennae in space, making it the easiest starship configuration to detect:

It can be seen that the magsail radiation of a characteristic fusion starship being decelerated from a cruise velocity of 0.1c could be detected by a 6 km orbiting antenna from a distance of 400 light years, while that emitted by a characteristic antimatter photon rocket in its deceleration phase could be seen as far away as 2,000 light years. There are about 100,000,000 stellar systems to be found within the latter distance. This extended range detection capability combined with magsail radiation’s unique time-dependent frequency spectrum appears to make a search for magsail radiation the most promising option for extraterrestrial starship detection.

As mentioned yesterday, Al Jackson has been considering the question of starship detectability for some time, and in mid-2014 published a preliminary paper on the matter describing his findings. It’s Jackson’s work that, even more than Yurtsever and Wilkinson, pushes the boundaries of speculation the furthest, or as Al puts it in the paper, “…the methods used to attain relativistic speed, using high-energy astrophysical processes, are far out in the tail of the distribution of speculation…” And he adds this tantalizing thought: “Is there a ‘Wow!’ signal lurking in the non-standard parts of the SETI electromagnetic spectrum? Starships braking in a dense interstellar region are attractive possible observations.”

Tomorrow I want to look at some of these speculative starship ideas and the kind of signatures they might throw, as per Jackson’s paper. For today, the Zubrin paper is “Detection of Extraterrestrial Civilizations via the Spectral Signature of Advanced Interstellar Spacecraft,” Progress in the Search for Extraterrestrial Life, ASP Conference Series Vol. 74 (1995). Available online.

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Starship Observational Signatures

by Paul Gilster on March 24, 2015

Now and again in relatively rarefied SETI discussions the topic of starship detection comes up. Specifically, if there were a starship moving through the interstellar medium in the general vicinity of our perch in the Orion Arm, would we be able to detect any sort of signature in our astronomical data? Centauri Dreams regular Al Jackson has looked into this for a variety of starship types (and discussed the matter at Starship Congress in 2013), and so has Gregory Benford, whose 2006 novelette “Bow Shock” describes the detection of an object whose synchrotron radiation fits the signature of the bow shock of a craft something like a Bussard ramjet.

We also have a 1995 paper from Robert Zubrin on the spectral signatures of starships and, back in 1977, a JBIS paper by D. R. J. Viewing and colleagues on relativistic spacecraft detection. Various detection methods come to mind, but Al Jackson has pointed out that the simplest would be finding the signature of waste heat (see SETI: Starship Radiation Signatures). A common theme in these discussions is to look for starships moving in the interstellar medium and try to identify interactions between that medium and the ship.

The choice makes sense, for we’ve looked on many occasions at what happens as we move swiftly between the stars. The designers of Project Daedalus knew that at 12 percent of the speed of light, some kind of shielding would be a good idea to protect the craft from interstellar dust particles and larger objects. And a new paper from Ulvi Yurtsever and Steven Wilkinson (Raytheon Company) gives us further information. Get your spacecraft moving fast enough and you’re dealing with collisions like those in particle accelerators but at much higher energies.

Into the Intergalactic Deep

manchu_starship

Relativistic effects can be described through the Lorentz factor, cited as γ (gamma) in the relevant equations. Gamma depends upon the speed at which an object is moving. We don’t see length contraction and time dilation in our everyday lives because we don’t move fast enough to notice them, but gamma increases exponentially as we approach the speed of light even though it is all but negligible at lower speeds. A spacecraft moving at 25 percent of lightspeed, for example, has a gamma of just 1.03, meaning time or length contraction of no more than 3 percent.

That gamma factor may be low, but even this speed is high enough to cause serious problems for a starship moving through interstellar dust, and as we go higher, the situation rapidly worsens. At a gamma factor of 2, our craft is moving at 86 percent of the speed of light. Now an object the size of a baseball with mass of 150 grams, note Yurtsever and Wilkinson, has an impact energy of 36 megatons. The authors define ‘extreme velocities’ as those with gamma greater than 9.1, which is well over 99 percent of the speed of light. Here we are in the range of the kind of proton/proton collisions used in particle accelerators to produce antiprotons.

The authors see no safe speeds in the interstellar medium greater than a gamma of 1.3, which is still more than 60 percent of the speed of light. Even the latter seems much too high: Shielding systems to protect a craft moving through the interstellar medium at 60 percent of c are well beyond anything I’ve ever seen discussed. But where the new paper really moves beyond earlier work is in its choice of destinations. What if an advanced civilization were to travel through not interstellar but intergalactic space? Here there is much less debris per unit volume and the chances of collision are sharply reduced. The interactions in question are with photons from the cosmic microwave background (CMB) as the craft moves close to c. From the paper:

The possibility of detecting radiation associated with distant relativistic spacecraft has been discussed in the literature before. These discussions mostly focus on detecting radiation from spacecraft engines or light from nearby stars reflecting off the spacecraft. Our approach is different in that we do not speculate on possible propulsion technologies but are interested in how a large relativistic object would interact with the interstellar/intergalactic medium and mainly with the CMB radiation. As a baryonic spacecraft travels at relativistic speeds it will interact with the CMB through scattering to cause a frequency shift that could be detectable on Earth with current technology.

Collisions with photons from the cosmic microwave background will appear in the spacecraft frame as highly energetic gamma rays. The paper assumes that a technology capable of reaching these speeds will be able to handle the effects of ionization and other interactions, but pair production is likely still an issue. Here we are looking at interactions between photons and matter in the hull of the spacecraft, with a single CMB photon creating an electron-positron pair when it collides with a nucleus in the hull, producing a characteristic signature. The paper looks at two special reference frames, the spacecraft frame and the rest frame of the cosmic microwave background, to understand how the CMB is distorted from these viewpoints.

The starship’s interactions with the CMB photons should produce a unique signature. The paper notes that the effect of CMB scattering from macroscopic relativistic objects like starships is similar to the Sunyaev-Zel’dovich effect in cosmology, in which low-energy CMB photons receive an energy boost when they collide with high energy electrons. The effects of this distortion of the cosmic microwave background have been used by cosmologists to study density perturbations in the universe and have been applied to the study of galactic clusters.

We don’t know if any civilization will learn how to travel at the kind of gamma factors discussed here, which take us well above 99 percent of the speed of light, but if a civilization is able to build spacecraft that can move this fast, they will have solved the problems of matter interactions that are much like those that occur in particle accelerators. The kind of technologies used would be well beyond our understanding but perhaps not beyond our detection. The paper concludes:

Our calculation for what an observer on earth could detect predicts a very unusual signature that is unlikely to be caused by any naturally occurring object in the known universe. This result is independent of propulsion technology, but the ability to detect the signal from Earth depends on available detector technologies. We are currently working to predict how far can we see this signature given our current capability.

The paper is Yurtsever and Wilkinson, “Limits and Signatures of Relativistic Spaceflight,” now available on the arXiv site.

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White Dwarfs and Dyson Spheres

by Paul Gilster on March 23, 2015

There is a wonderful moment in Larry Niven’s 1970 novel Ringworld when protagonist Louis Wu is first shown an image of an artificial ring completely encircling a star. These days the concept of a Dyson sphere is well established as a way for a civilization to capture as much energy as possible from the host star, but back then I had never heard of the concept. Dyson thought both a solid shell and a ring would be unstable and believed the best form for his concept was what he described as “…a loose collection or swarm of objects traveling on independent orbits around the star.” In that sense, Niven’s Ringworld wasn’t really Dysonian, but I found it staggering.

Ringworld

What a place! An engineered ring the diameter of Earth’s orbit fully 1.6 million kilometers wide, giving a habitable inner surface equal to about three million Earth-sized planets. A broader backdrop for science fiction adventure could scarcely be imagined unless it were a full-blown Dyson sphere. And indeed, Ringworld became the venue for a number of subsequent novels that tied into the collection of futuristic lore Niven called Known Space.

Speaking of science fiction authors reminds me that Dyson himself harked back to Olaf Stapledon, whose 1937 novel Star Maker seems to have given him his inspiration. In fact, Dyson has opined (I believe this was at the Starship Century symposium in San Diego) that ‘Stapledon sphere’ would be a more accurate name for the idea. Here’s Stapledon’s original read on Dyson spheres as a common outcome in a galaxy, viewed by the visionary protagonist as he witnesses events of the far future, when civilization “…began to avail itself of the energies of its stars upon a scale hitherto unimagined”:

“Not only was every solar system now surrounded by a gauze of light traps, which focused the escaping solar energy for intelligent use, so that the whole galaxy was dimmed, but many stars that were not suited to be suns were disintegrated, and rifled of their prodigious stores of sub-atomic energy.”

Addendum: Dyson’s talk in San Diego, which I had initially mis-reported as having been made in London, is available at the Starship Century site — click on the link to the videos.

Capturing all the power of a host star is a marker for a civilization that has reached Type II in Nikolai Kardashev’s classification of intelligent civilizations. Type III — this is what Stapledon depicts in the passage above — can tap the energy resources of its entire galaxy. Dyson Spheres would be vast indeed, but the various searches that have thus far been undertaken for them have relied on the amount of infrared radiation such an object would produce (see, among a number of articles in the archives here, Toward an Interstellar Archaeology, which talks about Richard Carrigan’s leading role in conceiving and mounting this search).

The White Dwarf Alternative

We have no Dyson sphere detections as of now, but speculative work on the concept continues, as revealed in a new paper from İbrahim Semiz and Salim Oğur (Boğaziçi University, Istanbul), forwarded to me recently by Adam Crowl. The duo posit a new take on Dyson spheres in which they are built around not G-class stars like our Sun but white dwarfs. Building around a white dwarf offers a number of possible solutions to Dyson sphere problems. And it turns out there are many of these.

Consider: If we somehow built a true sphere in the Solar System with a 1 AU radius, the gravity of the Sun would, according to the authors, be a miniscule 5 X 10-4 g. Inhabitants would be faced with microgravity. Rotating a Dyson Sphere would be one way out of the problem, but doing this would leave the equatorial regions as the truly usable surfaces, which would make it more sensible to build a ring-like structure in the first place rather than an all encompassing sphere that was largely useless. A system of independently rotating rings might be imagined but seems gratuitously complex.

The authors are aware of the instability that would plague both a Dyson sphere and a ring, but their paper isn’t about finding solutions to these problems but arguing for a different central object, which has advantages of its own. A white dwarf is much smaller than a G-class star, but applying Dysonian principles here would still give us a similar way of capturing energy while providing, by the authors’ estimate, 105 times the living area of a planet on the outside shell while not subjecting inhabitants to microgravity. A possible scenario is a civilization surviving the red giant stage of its star and returning to construct a Dyson Sphere around the white dwarf that eventually emerged.

From the paper:

…the red giant stage is also quite long, about a billion years for a solar-mass star (it decreases with mass), so it might seem that life/intelligence/civilization could also develop during that stage. However, conditions like radiative flux and stellar wind, even the mass of the star, are quite variable during the red giant phase, as opposed to the main-sequence period, when they are stable. Hence, while the astronomically-long-term outlook of an intelligent civilization is subject of conjecture, a civilization set to build a Dyson Sphere as discussed in this work would probably arise during the main sequence period of its star, and find ways to survive the red giant stage; maybe temporarily migrating to an orbit farther from its star, either to a planet, or failing/rejecting that, to orbital habitats a la O’Neill. In fact, such an effort would provide the experience needed before undertaking the construction of the Dyson Sphere around the eventual white dwarf.

We wind up with a 106 kilometer-scale Dyson Sphere with the inhabitants living on the outer shell as an alternative to the more common Dyson Sphere of fiction and speculation, one that would still be theoretically detectable through its infrared emissions but a much greater challenge because of its lower power. Its smaller size also means the white dwarf Dyson Sphere needs less construction material, with a 10-meter thick shell requiring approximately the mass of a single terrestrial planet. The required compressive strength of building materials for such a rigid sphere is also higher, demanding an extra means of support about which the authors choose not to speculate.

The paper is Semiz and Oğur, “Dyson Spheres around White Dwarfs,” available on the arXiv site as a preprint.

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Large Exomoons Shown to Be Detectable

by Paul Gilster on March 20, 2015

The search for sub-planetary scale features in other solar systems continues, with encouraging news from the Hunt for Exomoons with Kepler project. A moon around a distant exoplanet is a prize catch, but as we’ve also seen recently, scientists are weighing the possibilities in detecting exoplanetary ring systems (see Searching for Exoplanet Rings). Confirming either would be a major observational step, but exomoons carry the cachet of astrobiology. After all, a large moon around a gas giant in the habitable zone might well be a living world.

David Kipping (Harvard University) and colleagues at HEK have released a new study that tackles the question of how detectable exomoons really are. Published online today by the Astrophysical Journal, the paper examines 41 Kepler Objects of Interest, bringing the total number of KOIs surveyed by HEK thus far up to 57. The paper demonstrates that the process is beginning to move out of the realm of computer simulations and assumption-laden theory to actual data from Kepler. The paper’s goal is to determine how small a moon could be detected in each case given the kind of signatures that flag an exomoon’s presence.

kipping_chart

Image: After surveying nearly 60 exoplanets for moons, the HEK team have derived empirical limits for each world, demonstrating an ability to detect even the smallest moons capable of sustaining an Earth-like atmosphere (“Mini-Earths”) for 1 in 4 cases studied. Whilst a confirmed discovery remains elusive, the painstaking survey of 60 planets spanning several years reveals what is possible with current technology. Credit: The Hunt for Exomoons with Kepler (HEK) Project.

An examination of an exoplanet that does not turn up an exomoon thus leads to a statement of how massive a moon has been excluded by the current data, which means the team is learning much about the sensitivity of its methods. From the paper:

… based on empirical sensitivity limits, we show for the first time that the HEK project is sensitive to even the smallest moons capable of being Earthlike for 1 in 4 cases (after accounting for false-positives). In terms of planet-mass ratios, we find even that the Earth-Moon mass-ratio is detectable for 1 in 8 of cases, posing a challenge but not an insurmountable barrier. Mass ratios of ∼ 10−4, such as that of the Galilean satellites, have never been achieved. However, if Galilean-like satellites reside around lower-mass planets than Jupiter, of order ∼ 20 M, then we do find sensitivity, as demonstrated by the limit of 1.7 Ganymede masses achieved for Kepler-10c.

This is encouraging news, for the team can now make statements about the actual mass of a detectable exomoon. In 1 of 3 planets surveyed, an exomoon with Earth’s mass is detectable. Kipping believes that we can move down to the smallest moon thought capable of supporting an Earth-like atmosphere and still detect it in 1 of 4 of the cases studied. No exomoons have yet been detected but we are learning just what our capabilities are. Says Kipping:

“Here we report on our null results and the first estimate of empirical sensitivities. Ultimately, we would like to actually discover a clear signal and are pursuing some interesting candidates we hope will pan out. Either way though, I like to recall what the Nobel Prize winning American physicist Richard Feynman said about science: ‘Nature is there and she’s going to come out the way she is, and therefore when we go to investigate it we shouldn’t pre-decide what it is we’re trying to do except to find out more about it’.”

HEK_Sensitivity_MassRatios

Image: The Moon has about 1% the mass of the Earth posing a challenge for the HEK team, since such configurations are detectable for 1 in 8 planets surveyed. The much larger Pluto-Charon mass-ratio of 11.6% is much more detectable. Credit: Hunt for Exomoons with Kepler Project.

No exomoons turn up in the 41 KOIs surveyed in the study, with four, KOI-0092.01, KOI-0458.01, KOI-0722.01 and KOI-1808.01, showing up as false positives for an exomoon. Stellar activity is a likely cause, as the paper comments:

When dealing with a handful of transits, quasi-periodic distortions to the transit profile, such as those due to spots… can be well fitted by the flexible exomoon model. However, since an exomoon is not the underlying cause, this model lacks any predictive power and thus should fail F2a [a follow-up test described in the paper]. We therefore suggest that stellar activity is likely responsible for these four instances.

KOI-1808.01, in fact, passes the basic criteria for an exomoon detection, but the paper explains that the observed transit signal is distorted by the effects of star spots. Transit timing variations observed at KOI-0072.01 (Kepler-10c) seem to point to an additional planet in the system rather than an exomoon.

Thirteen of the KOIs produce some kind of spurious detection, assigned by the paper to effects like perturbations from unseen bodies, stellar activity or instrumental artifacts. Through the range of KOIs the project has studied thus far (57), 46 null detections are found from which upper limits on an exomoon’s mass can be derived. The paper reminds us that “…exomoons live in the regime where correlated noise is present and one must employ methods to guard against it when seeking such signals.”

The declared purpose of the Hunt for Exomoons with Kepler project is to ‘determine the occurrence rate of large moons around viable planet hosts,’ a task with implications for the abundance of life in the universe, for if habitable moons are common, there could be more of them than habitable planets, and conceivably more than one orbiting a single planet. An additional benefit of studying exomoons is that they can teach us about solar systems formation by showing us planet/moon systems in a variety of configurations.

The paper is Kipping et al., “The Hunt for Exomoons with Kepler (HEK): V. A Survey of 41 Planetary Candidates for Exomoons,” submitted to the Astrophysical Journal (preprint).

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Chariklo & Chiron: Centaurs with Possible Rings

by Paul Gilster on March 19, 2015

You may be forgiven if you aren’t familiar with the name Chariklo. Discovered in 1997, 10199 Chariklo is a ‘centaur,’ an outer system body with an orbit that moves between the orbits of Saturn and Uranus, just nudging the orbit of the latter. Its odd name (we’re big on names and their derivations here) comes from a nymph who in Greek mythology was the wife of Chiron and daughter of Apollo. No centaur is larger than Chariklo (estimated diameter 250 kilometers), and until just the other day, no other centaur was known to have what Chariklo did: A system of rings.

We’ve just learned, though, that the second largest centaur, 2060 Chiron, may have a set of rings of its own, although there are alternative ways of interpreting the data. Whether Chiron’s rings are confirmed or not, what was once thought to be an unusual phenomenon, a feature of Saturn alone, is now turning out to be far more common, with rings known to orbit Jupiter, Uranus and Neptune as well as Chariklo. So we have full-scale planets with rings of various size and density, and centaurs, which have been found to share the characteristics of both comets and asteroids. There may well be more ring systems out there, as there are estimated to be 44,000 centaurs in the Solar System with a diameter larger than 1 kilometer.

Artist’s impression of the rings around Chariklo

Image: An artist’s impression of the dense and narrow rings around Chariklo. The origin of these rings remains a mystery, but they may be the result of a collision that created a disc of debris. Data from a recent occultation implies that another centaur, Chiron, may also have a ring system, although other explanations are still being examined. Credit: European Southern Observatory.

The discovery of rings around Chariklo and the work on Chiron came about because of stellar occultations as the objects in question passed in front of a bright star, Chariklo in 2013 and Chiron in 2011. In both cases, the occultation produced a useful light signature for researchers studying the brief shadow. A disk of debris circling Chiron is one reading of the data from that centaur’s occultation, but we can’t rule out jets of material from the surface or even a shell of gas and dust enveloping the object. Amanda Bosh (MIT), a co-author of the paper on the discovery in the journal Icarus, calls that an intriguing result because of Chiron’s location, “…part of that middle section of the solar system, between Jupiter and Pluto, where we originally weren’t thinking things would be active, but it’s turning out things are quite active,”

Traces of activity on Chiron actually trace back to the early 1990s, when MIT’s James Elliot studied a similar stellar occultation by the centaur and was able to make estimates of its size. At that time, features in the data also implied the existence of jets of water and dust coming from the centaur’s surface. The new data, drawn from NASA’s Infrared Telescope Facility on Mauna Kea and the Las Cumbres Observatory Global Telescope Network at Haleakala, give us a more precise readings on an event that in its entirety lasted no more than a few minutes.

MIT’s Jessica Ruprecht, lead author of the paper, notes the range of possibilities involved in this work:

“If we want to make a strong case for rings around Chiron, we’ll need observations by multiple observers, distributed over a few hundred kilometers, so that we can map the ring geometry. But that alone doesn’t tell us if the rings are a temporary feature of Chiron, or a more permanent one. There’s a lot of work that needs to be done.”

According to this MIT news release, the two features observed in the data are each about 300 kilometers from the center of the object and are not dissimilar to what Elliot observed in the 1990s. These may be symmetrical jets of gas and dust rather than rings, perhaps the result of the centaur’s having moved inward from the Kuiper Belt, warming enough to turn frozen gases into jets that throw dust and other material off the surface. Ruprecht also notes that debris from a nearby object could conceivably be captured by a centaur like Chiron to produce rings. At the moment, then, we don’t know if we’re looking at a long-lasting feature or a transient event.

Back to Chariklo, whose stellar occultation in 2013 revealed the existence of two rings, one about 3 kilometers and the other about 7 kilometers wide, separated by about 9 kilometers. The find was startling because no previous ring systems around minor bodies had been discovered, and there were questions about how stable a ring system could be around such a small object. The two rings have received nicknames (Olapoque for the larger, Chui for the smaller) derived from the names of rivers in Brazil, but the IAU will at some point confer official names on both.

The Chiron paper is Ruprecht et al., “29 November 2011 stellar occultation by 2060 Chiron: Symmetric jet-like features,” Icarus Vol. 252 (15 May 2015), pp. 271-276 (abstract). For more on Chiron, see Ortiz et al., “Possible ring material around centaur (2060) Chiron,” Astronomy & Astrophysics 576 (2015) A18 (preprint). For the Chariklo work, see Braga-Ribas et al., “A ring system detected around the Centaur (10199) Chariklo,” Nature Vol. 508, Issue 7494 (2014), pp. 72-75 (preprint).

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