by Paul Gilster | Jul 14, 2017 | Astrobiology and SETI |
The SETI Institute’s just announced Laser SETI funding campaign intends to put into practice what SETI researchers have been anticipating for decades, an all-sky, all-the-time observing campaign. The Institute’s Eliot Gillum and Gerry Harp are behind the project, backed by an impressive list of advisors, with the intention of using optical SETI methods to look for signs of extraterrestrial civilizations. In doing so, they’re reminding us how we’ve done SETI, how we can surmount its current limitations, and what a SETI of the future will look like.
Think about how SETI has evolved since the days when Frank Drake created Project Ozma at the National Radio Astronomy Observatory at Green Bank (WV). Fresh with the insights of Giuseppe Cocconi and Philip Morrison, who examined radio methods and suggested a search for signals near the 21 centimeter wavelength of neutral hydrogen, Drake turned a 26-meter radio telescope to examine the nearby Sun-like stars Tau Ceti and Epsilon Eridani.
Would SETI be a matter of looking at specific stars at certain wavelengths? Immediately the list of questions began to grow. Once you have chosen a target (and there are 18 million stars within 1000 light years), how long do you need to examine it before moving on to another? For that matter, are radio wavelengths optimal? And if our method is to look at particular places at particular times, how would we detect periodic signals that are on no schedule we can hope to predict? What do we miss in between?
Image: The view toward Messier 24, the Sagittarius star cloud. We are looking for signals amidst immensity. Credit: Hubble Heritage Team (AURA/ STScI/ NASA).
All Sky All the Time
We started doing SETI at radio wavelengths at a time when there were no operational lasers, but today we can look at the experience of our own civilization to see how high-capacity fibers have changed the way we communicate. As we add laser methods to SETI, we begin looking for the kind of monochromatic optical signal that nature does not tend to produce. We also look on timescales of nanoseconds, for no natural sources produce nanosecond pulses.
Configured to serve as a beacon, the Helios laser at Lawrence Livermore National Laboratory could outshine our own Sun by a factor of 10,000. Couldn’t an ET civilization do the same?
But of course we have no way of making assumptions about what an alien civilization might do. We may not receive a beacon at all. We may find ourselves intercepting alien communications or activities like power beaming that produce optical signatures but are not intended as communications. For that matter, here on Earth we use powerful radars (think Arecibo) to examine near-Earth asteroids as we assess impact possibilities. The beams from these searches should be detectable, but would appear in an alien sky as a transient.
So we have to get away from the assumption that any extraterrestrial civilization will be ‘always on,’ just waiting for us to detect it. Searching with instruments pointed at specific targets and limited by short ‘dwell’ times (how long we remain on that target), we wouldn’t find the great bulk of transients that could be telltale evidence of other civilizations. We’re just now learning, for example, about Fast Radio Bursts (FRBs), millisecond radio pulses thought to occur in their thousands every day and still poorly understood. Because they appear as transients, we’ve only catalogued a handful. SETI sometimes detects radio transients that never, to our knowledge, reoccur. Or perhaps some do, but we aren’t looking then.
Current methods are, to use Paul Shuch’s phrase, looking at the sky through a soda straw. Laser SETI proposes to put an end to that limitation with new detectors that will become the basis of observatories that will one day provide global coverage of the entire sky.
Moreover, the detector Eliot Gillum talked about in last year’s Breakthrough Discuss meeting, is not hypothetical. The design has been turned into a prototype and tested with sky observations to validate the methods and analyze performance. The Laser SETI campaign on Indiegogo seeks to raise the funds needed to produce a minimum of two cameras, enough to localize targets on the sky and examine the algorithms used in signal detection.
Pushing Imaging Technologies in SETI’s Direction
Laser SETI’s technology involves cameras with a wide field of view that use ‘drift scanning’ methods (a fixed camera tracking the celestial scene as it passes above). The camera is a charged coupled device (CCD), a familiar technology widely used not only in astronomical observations but also in cell phone cameras and numerous scientific applications. Photons striking a CCD’s light-sensitive elements generate a charge that can be read by electronics within the device and turned into a digital copy of the light falling onto the surface.
But that just begins the story. Remember, we’re talking about the night sky moving across the camera’s fixed field of view. Laser SETI thus incorporates what is called Time-Delay Integration (TDI), a CCD readout technique used in many applications to capture images of fast-moving objects. TDI can preserve both sensitivity and image quality even when dealing with fast relative movement, with photo-charges constantly being shifted down the CCD detector from pixel to pixel as they record changes to the light pattern being observed.
In TDI work as it is normally used — this might be, for example, in factories for quality control — the shift rate within the CCD matches the rate of the target being imaged. But Laser SETI’s new technology uses TDI in an entirely novel way. Instead of producing a normal working image, the idea is to overclock the TDI so that the ‘scene’ — the sky above — becomes smeared out and spread over the entire CCD. The beauty of this is that while the background sky loses definition, any transient pulses that appear show up as a single point.
Reading out the camera at over 1000 times per second, the detector also employs a transmission grating that spreads each point source into two spectra, allowing a single color of light to be distinguished from other kinds of sources. Gillum and Harp’s methods demand two cameras, the second looking at the same field of view but turned 90 degrees sideways. The coordination between the two instruments allows the software to recover accurate sky coordinates for any transient being observed. Putting four cameras at a given site, coupled the same way (two per field of view), allows the entire night sky to be covered.
Image: The first detector. The CCD camera is at the base, the lens atop it, and the transmission grating at the top beneath the hood. Two of the major three components here are off-the-shelf, keeping costs low. Credit: SETI Institute/Eliot Gillum.
Moving Laser SETI Worldwide
You can see what all this is building toward. The Indiegogo project’s intent is to raise the money for two cameras, but further funding takes us toward multiple site operations. The entire sky can’t be seen from any one part of the globe, but six observatories could cover all of it, with eventual secondary observatories adding valuable statistical validation, and also necessary sky coverage during times when the weather is inclement at any one site.
Have a look at the Laser SETI campaign for further background. The low cost of these detectors is significant. And bear this in mind. When we look at particular points in the sky, we have to be lucky enough that the signal we seek is available just then, just there. Looking everywhere all the time, we see the brightest signals in the sky whenever they appear.
In the entire history of radio astronomy, it has taken us until now to detect Fast Radio Bursts. What else are we missing? As Eliot Gillum pointed out in a recent presentation, we have no way of knowing what any extraterrestrial civilization may be doing, but we can say this: If their activity is bright but intermittent, all previous and current searches very likely won’t find it.
That’s why we need to look at the entire sky all the time.
by Paul Gilster | Jul 13, 2017 | Outer Solar System |
All of the Juno spacecraft’s instruments — including JunoCam — were operational during its July 10 flyby, giving us a close-up look at the Great Red Spot. Now 16,000 kilometers wide, the storm has been studied since 1830 and may be considerably older than that. Juno’s orbit took it to perijove (closest to Jupiter’s center) at 2155 EDT on the 10th (0155 UTC on the 11th), when it closed to about 3500 kilometers above the cloud tops. The passage across the Great Red Spot occurred eleven minutes later at some 9000 kilometers above the clouds.
While the data are being unpacked and analyzed, we can enjoy the efforts of citizen scientists who went to work on the raw images posted on the JunoCam site and processed them, a procedure done in coordination with the Juno team.
“These highly-anticipated images of Jupiter’s Great Red Spot are the ‘perfect storm’ of art and science. With data from Voyager, Galileo, New Horizons, Hubble and now Juno, we have a better understanding of the composition and evolution of this iconic feature,” said Jim Green, NASA’s director of planetary science. “We are pleased to share the beauty and excitement of space science with everyone.”
Image: This enhanced-color image of Jupiter’s Great Red Spot was created by citizen scientist Jason Major using data from the JunoCam imager on NASA’s Juno spacecraft. The image was taken on July 10, 2017 at 07:10 p.m. PDT (10:10 p.m. EDT), as the Juno spacecraft performed its 7th close flyby of Jupiter. At the time the image was taken, the spacecraft was about 13,917 kilometers from the tops of the clouds of the planet. Credit: NASA/JPL-Caltech/SwRI/MSSS/Jason Major.
And let’s go through the two others just posted by JPL.The one below was created by Kevin Gill, likewise working with raw data from the JunoCam imager.
And a final image from citizen scientist Gerald Eichstädt.
The English natural philosopher Robert Hooke (1635-1703) described what may have been the Great Red Spot in 1664, although he seems to have placed it on the wrong side of the Jovian equator, making the sighting problematic. The following year, however, Giovanni Cassini detected a ‘permanent storm’ that remained under observation into the 18th Century, although there are no observations between 1713 and 1830. The first certain record of the Great Red Spot occurs in an 1831 drawing by German amateur Samuel Heinrich Schwabe.
The red coloration of the Great Red Spot is problematic, with suggestions ranging from compounds of sulfur and phosphorus to organic material produced by high-altitude photochemical reactions or lightning discharges. The Voyager spacecraft returned spectacular imagery, including the Voyager 1 image in 1979 shown below, which revealed details as small as 160 kilometers and highlighted the spectacularly complex wave motion west of the Spot.
Image: Jupiter’s Great Red Spot (top right) and the surrounding region, as seen from Voyager 1 on March 1, 1979. Below the spot is one of the large white ovals associated with the feature. NASA/JPL
Since the Voyager encounters (and indeed, since the late 19th Century) the Spot has been shrinking. Voyager measured its length at 23,000 kilometers, and since 2012 it has been diminishing at a rate of about 900 kilometers per year. We’ll see what Juno can tell us about the giant storm’s energy source and its longevity as data are processed in the weeks ahead.
by Paul Gilster | Jul 12, 2017 | Exoplanetary Science |
‘Planetary mass binary’ is an unusual term, but one that seems to fit new observations of what was thought to be a brown dwarf or free-floating large Jupiter analog, and now turns out to be two objects, each of about 3.7 Jupiter masses. That puts them into planet-range when it comes to mass, as the International Astronomical Union normally considers objects below the minimum mass to fuse deuterium (13 Jupiter masses) to be planets. This is the lowest mass binary yet discovered.
A team led by William Best (Institute for Astronomy, University of Hawaii) went to work on the L7 dwarf 2MASS J11193254–1137466 with the idea of determining what they assumed to be the single object’s mass and age. It was through observations with the Keck II telescope in Hawaii that they discovered the binary nature of their target. The separation between the two objects is about 3.9 AU, based upon the assumption that the binary is around 160 light years away, the distance of the grouping of stars called the TW Hydrae Association.
Let’s pause on this for a moment. The TW Hydrae Association has come up in these pages in the past, as a so-called ‘moving group’ that contains stars that share a common origin, and thus are similar in age and travel through space together. Moving groups are obviously useful — if astronomers can determine that a star is in one, then its age and distance can be inferred from the other stars in the group. Best and colleagues determined from key factors like sky position, proper motion, and radial velocity that there was about an 80 percent chance that 2MASS J11193254–1137466AB is a member of the TW Hydrae Association.
Image: Keck images of 2MASS J11193254–1137466 reveal that this object is actually a binary system. A similar image of another dwarf, WISEA J1147-2040, is shown at bottom left for contrast: this one does not show signs of being a binary at this resolution. Credit: Best et al. 2017.
Determining a brown dwarf’s age is tricky business because these objects cool continuously as they age, which means that brown dwarfs of different masses and ages can wind up with the same luminosity. The authors point out that this mass-age-luminosity degeneracy makes it hard to figure out their characteristics without knowing at least two of the three parameters. Membership in a moving group like the TW Hydrae Association gives us an age of about 10 million years but also provides mass estimates from evolutionary models.
And a binary system hits the jackpot, for now we can study the orbits of the two objects to work out model-independent masses, which is how Best drilled down to the 3.7 Jupiter mass result for each binary member here. The authors consider the binary a benchmark for tests of evolutionary and atmospheric models of young planets, and go on to speculate about its possible origins:
The isolation of 2MASS J1119?1137AB strongly suggests that it is a product of normal star formation processes, which therefore must be capable of making binaries with ? 5 MJup components. 2MASS J1119?1137AB could be a fragment of a higher-order system that was ejected via dynamical interactions (Reipurth & Mikkola 2015), although the lack of any confirmed member of TWA within 10° (projected separation ? 5 pc) of 2MASS J1119?1137 makes this scenario unlikely. Formation of very low mass binaries in extended massive disks around Sun-like stars followed by ejection into the field has been proposed by, e.g., Stamatellos & Whitworth (2009), but disks of this type have not been observed.
Image: The positions of 2MASS J11193254–1137466A and B on a color-magnitude diagram for ultracool dwarfs. The binary components lie among the faintest and reddest planetary-mass L dwarfs. Credit: Best et al. 2017.
So there is much to learn here. An object’s composition, temperature and formation history all come into play when determining whether it is a brown dwarf or a planet, and some definitions of brown dwarf take us below the 13 Jupiter mass criteria. But at 3.7 Jupiter masses, these objects clearly warrant the authors’ careful tag of ‘planetary mass binary.’
The paper is Best et al., “The Young L Dwarf 2MASS J11193254?1137466 Is a Planetary-mass Binary,” Astrophysical Journal Letters Vol. 843, No. 1 (23 June 2017). Available online.
by Paul Gilster | Jul 11, 2017 | Outer Solar System |
The human expansion into the Solar System will demand our being able to identify sources of water, a skill we’re honing as explorations continue. On Mars, for example, the study of so-called ‘pitted craters’ has been used as evidence that the low latitude regions of the planet, considered its driest, nonetheless have a layer of underlying ice. The Dawn spacecraft discovered similar pitted terrain on Vesta, as you can see in the image below.
Image: These enhanced-color views from NASA’s Dawn mission show an unusual “pitted terrain” on the floors of the craters named Marcia (left) and Cornelia (right) on the giant asteroid Vesta. The views show that the physical properties or composition of the material in which these pits form is different from crater to crater. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/JHUAPL.
Vesta’s Marcia crater contains the largest number of pits on the asteroid. The 70-kilometer feature is also one of the youngest craters found there. So what accounts for this kind of terrain? Perhaps the water that formed the pits came from Vesta itself. Another possibility: Low-speed collisions with carbon-rich meteorites could have deposited hydrated materials on the surface, to be released in the heat of subsequent high-speed collisions within the asteroid belt. An explosive degassing into space could explain such pothole-like depressions.
But Dawn wasn’t through when it left Vesta, and what it has found at Ceres is proving invaluable at understanding what appears to be a common marker of volatile-rich material. In new work from Hanna Sizemore (Planetary Science Institute) and colleagues, we learn that Ceres is home to the same kind of pitted terrain. As Sizemore notes:
“Now, we’ve found this same type of morphological feature on Ceres, and the evidence suggests that ice in the Cerean subsurface dominated the formation of pits there. Finding this type of feature on three different bodies suggests that similar pits might be found on other asteroids we will explore in the future, and that pitted materials may mark the best places to look for ice on those asteroids.”
Image: Haulani Crater, Ceres, showing abundant pitted materials on the crater floor. Similar pitted materials have previously been identified on Mars and Vesta, and are associated with rapid volatile release following impact. Their discovery on Ceres indicates pitted materials may be a common morphological indicator of volatile-rich materials in the asteroid belt. Haulani Crater is 34 km in diameter. Color indicates topography. Credit: NASA/MPS/PSI/Thomas Platz.
Sizemore’s team studied the formation of pitted craters on Ceres through numerical models that explored the role of water ice and other volatiles. The morphological similarities between the Ceres features and what has been found on Mars and Vesta are striking. With water ice evidently significant in pit development on two asteroids and a planet, similar terrains will be of clear interest for future missions in terms of in situ resource utilization.
Image: Pitted terrain on Mars as seen by HiRISE aboard the Mars Reconnaissance Orbiter. Credit: NASA/JPL/University of Arizona.
The paper is Sizemore et al., “Pitted Terrains on (1) Ceres and Implications for Shallow Subsurface Volatile Distribution,” accepted at Geophysical Research Letters (preprint).
by Paul Gilster | Jul 10, 2017 | Exoplanetary Science |
Our theories of planet formation grow more mature as the exoplanet census continues, but I’ve always speculated about the first planets discovered and how they could have possibly been where we found them. The discovery of the planets around the pulsar PSR B1257+12 occurred in 1992, the work of the Polish astronomer Aleksander Wolszczan. Anomalies in its pulsation period — this is a millisecond pulsar with a period of 6.22 milliseconds — led Wolszczan and Dale Frail to produce a paper on the first extrasolar planets ever found.
We wouldn’t find such planets at all if it were not for the effect of their gravitational pull on the otherwise regular pulses from the pulsar. But how could the planets now know as Draugr, Poltergeist and Phobetor, the latter found in 1994, possibly have formed in such an environment? After all, a dense neutron star (a pulsar is a highly magnetized, rotating neutron star) is the result of a supernova that should have destroyed any planets nearby, making it necessary for planet formation to occur from raw materials around the resulting object.
Jane Greaves (University of Cardiff), working with Wayne Holland (UK Astronomy Technology Centre, Edinburgh) presented results at the recent National Astronomy Meeting that the duo have assembled into a new paper. The researchers focused on the Geminga pulsar, about 800 light years from the Sun in the constellation Gemini. A quick check of Wikipedia produced this delightful bit about the name Geminga: It’s a contraction of ‘Gemini gamma-ray source’ as well as a transcription of words meaning ‘it’s not there’ in the Lombard dialect of northern Italy.
Geminga really is there, and at one point the explosion that created it was considered the reason for the low density of the interstellar medium through which the Sun now passes, a theory that is now out of favor. Greaves and Holland observed Geminga at submillimeter wavelengths with the James Clerk Maxwell Telescope (JCMT) in Hawaii. The pulsar is surrounded by a pulsar wind nebula (PWN), a type of nebula found inside the shells of supernova remnants that is powered by pulsar ‘winds’ driven by the central pulsar.
Indeed, earlier data from the WISE mission had suggested a ‘shell-like parabolic structure comprising a number of clumps’ around Geminga, and the authors’ new work amplifies on that discovery. Multiple observing runs with different cameras showed Greaves and Holland faint material near the pulsar and an arc around it. Says Greaves:
“This seems to be like a bow-wave – Geminga is moving incredibly fast through our Galaxy, much faster than the speed of sound in interstellar gas. We think material gets caught up in the bow-wave, and then some solid particles drift in towards the pulsar.”
Image: Data at wavelength of 0.45 mm, combined from SCUBA and SCUBA-2 [the cameras used in this work] in a false-colour image. The Geminga pulsar (inside the black circle) is moving towards the upper left, and the orange dashed arc and cylinder show the ‘bow-wave’ and a ‘wake’. The region shown is 1.3 light-years across; the bow-wave probably stretches further behind Geminga, but SCUBA imaged only the 0.4 light-years in the centre. Credit: Jane Greaves / JCMT / EAO.
The hypothesis, then, is that dust from the interstellar medium interacting with the pulsar wind nebula around Geminga accumulates the raw materials for future planets, rather than interactions between the pulsar wind nebula and the pulsar itself. The pulsar’s movement through the interstellar medium is the key, at least for this pulsar. From the paper:
The origins of the rare pulsar planet systems are uncertain, with recent work (Margalit & Metzger 2017) favouring disruption of a companion over re-accretion of supernova fallback material. Here we find evidence that the middle-aged Geminga pulsar is surrounded by a shell of material which could have formed from compression of the local ISM. Preliminary calculations suggest that dust could penetrate the nebula, given the low space speed and local density, and this may provide an alternate source for dust near this pulsar. A candidate circum-pulsar disc would be the first to be found in the submillimetre, complementing the only infrared candidate (around the magnetar 4U 0142+61, Wang et al. 2006).
And the researchers seem to have found enough mass to do the job:
We are waiting for higher-resolution follow-up data, but can infer that any dust disc present around Geminga should exceed about 6 Earth-masses of dust. Thus it would have potential to form low-mass planets, such as the archetypes around PSR B1257+12 (Wolszczan & Frail 1992).
The authors have applied for time on the Atacama Large Millimeter Array (ALMA), hoping to tease out more detail, enough to demonstrate that the faint debris they have spotted so far around Geminga really is associated with the object. A confirmation there would lead to work on other pulsar systems to probe deeper into planet formation in unusual environments.
The paper is Greaves & Holland, “The Geminga pulsar wind nebula in the mid-infrared and submillimetre,” published online by Monthly Notices of the Royal Astronomical Society 15 June 2017 (abstract).
