Is High Definition Astrometry Ready to Fly?

In a white paper submitted to the Decadal Survey on Astronomy and Astrophysics (Astro2020), Philip Horzempa (LeMoyne College) suggests using technology originally developed for the NASA Space Interferometry Mission (SIM), along with subsequent advances, in a mission designed to exploit astrometry as an exoplanet detection mode. I’m homing in on astrometry itself in this post rather than the mission concept, for the technique may be coming into its own as an exoplanet detection method, and I’m interested in new ways to exploit it.

Astrometry is all about refining our measurement of a star’s position in the sky. When I talk to people about detecting exoplanets, I find that many confuse astrometry with radial velocity, for in loose explanatory terminology, both refer to measuring the ‘wobble’ a planet induces on a star. But radial velocity examines Doppler effects in a star’s spectrum as the star moves toward and then away from us, while astrometry looks for tiny changes in the position of the star in the sky, especially the kind of periodic shift that implies an otherwise unseen planet.

The problem has always been the level of precision needed to detect such minute variations. Long-time Centauri Dreams readers will recall Peter Van de Kamp’s work at Swarthmore on what he believed to be a planet detection around Barnard’s Star, but we can also mention Sarah Lippincott’s efforts at the same observatory on Lalande 21185. Kaj Strand, likewise working at Sproul Observatory at Swarthmore, thought he had detected a planet orbiting 61 Cygni all the way back in 1943. Instrumentation-induced errors unfortunately made these detections unlikely.

But astrometry has major advantages if we can reach the necessary accuracy, which is Horzempa’s point. The Gaia mission makes the case that astrometry is already in play in the exoplanet hunt. Thus the mission’s detections of velocity anomalies at Barnard’s Star, Tau Ceti and Ross 128. Gaia also found evidence for a possible gas giant around Epsilon Indi. For more on this, see Kervella et al., “Stellar and Substellar Companions of Nearby Stars from Gaia DR2,” Astronomy & Astrophysics Volume 623, A72 (March 2019). Abstract.

Image: Astrometry is the method that detects the motion of a star by making precise measurements of its position on the sky. This technique can also be used to identify planets around a star by measuring tiny changes in the star’s position as it wobbles around the center of mass of the planetary system. ESA’s Gaia mission, through its unprecedented all-sky survey of the position, brightness and motion of over one billion stars, is generating a large dataset from which exoplanets will be found, either through observed changes in a star’s position on the sky due to planets orbiting around it, or by a dip in its brightness as a planet transits its face. Credit: ESA.

What is significant here is that an era of astrometry 50 to 100 times more precise than Gaia may be at hand. Recall that minutes of arc are used to describe small angles. A circle of 360 degrees can be divided up into arcminutes and then arcseconds (1/60th of an arcminute). A milliarcsecond, then, is one-thousandth of an arcsecond, and a microarcsecond is one millionth of an arcsecond. You can see from this the precision needed for space-based astrometry.

Horzempa believes we are ready for 0.1 micro-arcsecond (?as), or 100 nano-arcseconds (nas) astrometry, a level anticipated for NASA’s canceled Space Interferometry Mission, and one for which hardware had already been constructed. Incorporating advances in laser metrology engineering since 2010, the Star Watch mission he is proposing would leverage the SIM work and make astrometry a valuable tool for complementing radial velocity and transit work. High-definition astrometry at 100 nas, he argues, is the best way to achieve the goal stated in a National Academy of Sciences’ 2018 report, which calls for developing ways to detect and characterize terrestrial-class planets in orbit around Sun-like stars.

Gaia is able to measure the position of brighter objects to a precision of 5 ?as, with proper motions established down to 3.5 ?as per year. For the faintest stars (magnitude 20.5), reports the European Space Agency, several hundred ?as is the working range. Bear in mind that, according to the NAS report, 20 ?as is a level of precision “sufficient to detect gas giant planets amenable to direct imaging follow-up with GSMTs” [giant segmented mirror telescopes].

If 10 microarcseconds is the size of a euro coin on the Moon as viewed from Earth, imagine what might be done with 100 nano-arcseconds, a high-definition astrometry that would offer many advantages over conventional radial velocity studies. Radial velocity can only offer a minimum mass determination because we cannot know the angle of inclination of unseen planets, and thus don’t know whether we are seeing a system edge-on or at any other angle.

Horzempa’s case is that astrometry can be used to tune up such radial velocity detections and tease out other worlds in the same systems that are undetected by RV. A case in point is Tau Ceti, which raises questions based on radial velocity work alone. From the white paper, where the author notes that this is a system that has been thought to hold 4 super-Earths::

This would be the case only if the RV m(sin i) mass values are the true values. However, if one assumes that the invariant plane for the system is the same as that of the observed disk (40 degrees), then the masses would range from 2.5 to 10 Me, putting them in the sub-Neptune to Neptune class. That would still be an interesting system but it would not be a system of super Earths. High-Definition astrometry will bring clarity, as these inner worlds of tau Ceti, if they exist, would generate signals that ranged from 1 to 15 uas, well within the reach of the next-generation 100 nano-arcsecond Probe. As a bonus, the Star Watch mission would be able to detect any true terrestrial worlds in the tau Ceti system.

Horzempa cites other intriguing possibilities, such as Zeta Reticuli, a star not known to have any planets through transit or radial velocity methods. Orbital inclination could rule out a transit detection in any case, while smaller planets might be too difficult a catch for RV around this binary system of two G-class stars in a wide orbit. 100-nas astrometry would be able, however, to detect Earth-class planets if they are there around either or both stars. If we are seeing ‘face-on’ systems, astrometry may be the only detection method applicable here.

There is no question about the advantages astrometry presents in being able to measure the true mass of planets it detects as opposed to a minimum mass, and as Horzempa writes, “…it will provide knowledge of the coplanarity of exoplanet systems, and it will discover extremely rare and valuable) planetary system ‘oddballs.’” All this shows the refinement of a method that has disappointed in the past. From the NAS report:

Astrometry has a spotty history involving exoplanet false positive detections and the cancellation of the Space Interferometry Mission. Only two “high-confidence” exoplanets have been discovered with this technique (Muterspaugh et al., 2010), neither of which has been independently confirmed. Despite its intrinsic merit, astrometry has not been viable as a search technique, given that a space mission is needed for large samples and low-mass planets.

It is a space mission that Horzempa now calls for, one building on technology developed for the Space Interferometry Mission and taking astrometry to unpreceded levels of precision. It is far too early to know how the idea will be received in the space community, but given the movement of astrometry toward the 100-nas level, a case can be made for supplementing existing methods with a strategy that at long last is beginning to deliver on its exoplanet promise. We’ll follow the progress of both astrometric measurement and this Star Watch mission concept with interest.

The paper is Horzempa, “High Definition Astrometry,” submitted to Astro2020 (Decadal Survey on Astronomy and Astrophysics) and available as a preprint.

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Insulating a Plutonian Ocean

An ocean inside Pluto would have implications for many frozen moons and dwarf planets, not to mention exoplanets where conditions at the surface are, like Pluto, inimical to life as we know it. But while a Plutonian ocean has received considerable study (see, for example, Francis Nimmo’s work as discussed in Pluto: Sputnik Planitia Gives Credence to Possible Ocean), working out the mechanisms for liquid ocean survival over these timeframes and conditions has proven challenging. A new paper now suggests a possible path.

Shunichi Kamata of Hokkaido University led the research, which includes contributions from the Tokyo Institute of Technology, Tokushima University, Osaka University, Kobe University, and the University of California, Santa Cruz. At play are computer simulations, reported in Nature Geosciences, that offer evidence for the potential role of gas hydrates (gas clathrates) in keeping a subsurface ocean from freezing. At the center of the work, as in so much recently written about Pluto, is the ellipsoidal basin that is now known as Sputnik Planitia.

Image: The bright “heart” on Pluto is located near the equator. Its left half is a big basin dubbed Sputnik Planitia. Credit: Figures created using images by NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.

Gas hydrates are crystalline solids, formed of gas trapped within molecular water ‘cages’. The ‘host’ molecule is water, the ‘guest’ molecule a gas or a liquid. The lattice-like frame would collapse into a conventional ice crystal structure without the support of the trapped molecules.

What is illuminated by the team’s computer simulations is that such hydrates are highly viscous and offer low thermal conductivity. They could serve, in other words, as efficient insulation for an ocean beneath the ice. The location and topography of Sputnik Planitia lead the researchers to believe that if a subsurface ocean exists, the ice shell in this area is thin. Uneven on its inner surface, the shell could thus cover liquid water kept viable by the insulating gas hydrates.

We wind up with a concept for Pluto’s interior that looks like the image below.

Image: The proposed interior structure of Pluto. A thin clathrate (gas) hydrate layer works as a thermal insulator between the subsurface ocean and the ice shell, keeping the ocean from freezing. Credit: Kamata S. et al., “Pluto’s ocean is capped and insulated by gas hydrates.” Nature Geosciences, May 20, 2019 (full citation below).

Methane is implicated as the most likely gas to serve within this insulating lattice, a provocative theory because of what we know about Pluto’s atmosphere, which is poor in methane but rich in nitrogen. What exactly is happening to support this composition? From the paper:

CO2 clathrate hydrates at the seafloor could have acted as a thermostat to prevent heat transfer from the core to the ocean. Primordial CO2, however, may have been converted into CH4 through hydrothermal reactions within early Pluto under the presence of Fe–Ni metals. As CH4 and CO predominantly occupy clathrate hydrates, the components that degassed into the surface–atmosphere system would be rich in other species, such as N2.

We arrive at an atmosphere laden with nitrogen but low in methane. This notion of a thin gas hydrate layer as a ‘cap’ on a subsurface ocean is one that could serve as a generic mechanism to preserve subsurface oceans in large, icy moons and KBOs. “This could mean there are more oceans in the universe than previously thought, making the existence of extraterrestrial life more plausible,” adds Kamata.

The authors point out that freezing of the ocean, causing the ice shell to thicken, would cause the radius and surface area of Pluto to increase, producing faults on the surface. New Horizons was able to observe these, and recent studies have shown that the fault pattern supports the global expansion of the dwarf planet. Thus we have a scenario consistent with a global ocean, perhaps one that is still partly liquid. Analyzing surface changes would offer constraints on the thickness of any potential layers of clathrates that could firm up the liquid water hypothesis.

The paper is Kamata et al., “Pluto’s ocean is capped and insulated by gas hydrates,” Nature Geosciences May 20, 2019 (abstract).

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New Horizons: Results and Interpretations

Another reminder that the days of the lone scientist making breakthroughs in his or her solitary lab are today counterbalanced by the vast team effort required for many experiments to continue. Thus the armies involved in gravitational wave astronomy, and the demands for big money and large populations of researchers at our particle accelerators. So, too, with space exploration, as the arrival of early results from New Horizons in the journals is making clear.

We now have a paper on our mission to Pluto/Charon and the Kuiper Belt that bears the stamp of more than 200 co-authors, representing 40 institutions. How could it be otherwise if we are to credit the many team members who played a role? As the New Horizons site notes: “[Mission principal investigator Alan] Stern’s paper includes authors from the science, spacecraft, operations, mission design, management and communications teams, as well as collaborators, such as contributing scientist and stereo imaging specialist (and legendary Queen guitarist) Brian May, NASA Planetary Division Director Lori Glaze, NASA Chief Scientist Jim Green, and NASA Associate Administrator for the Science Mission Directorate Thomas Zurbuchen.”

New Horizons still has the capacity to surprise us, or maybe ‘awe’ is the better word. That’s what I felt when I saw the now familiar shape of Ultima Thule highlighted on the May 17 Science cover in an image that shows us what the object would look like to the human eye. The paper presents the first peer-reviewed scientific results and interpretations of Ultima a scant four months after the flyby.

As to the beauty of that cover image, something like this was far from my mind during the 2015 Pluto/Charon flyby, when the occasional talk ran to an extended mission and a new Kuiper Belt target. I wouldn’t have expected anything with this degree of clarity or depth of scientific return from what was at that time only a possible future rendezvous, and one that would not be easy to realize. Another indication of how outstanding New Horizons has been and continues to be.

Image: This composite image of the primordial contact binary Kuiper Belt Object 2014 MU69 (nicknamed Ultima Thule) – featured on the cover of the May 17 issue of the journal Science – was compiled from data obtained by NASA’s New Horizons spacecraft as it flew by the object on Jan. 1, 2019. The image combines enhanced color data (close to what the human eye would see) with detailed high-resolution panchromatic pictures. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute//Roman Tkachenko.

I’d be hard-pressed to come up with a space-themed cover from Science that’s more memorable than that. For that matter, here’s an entertaining thought: What would the most memorable space-themed Science cover of, say, 2075 look like? Or 2250? Let’s hope we’re vigorously exploring regions from the Oort Cloud outward by the latter year.

Meanwhile, New Horizons is helping us look back toward our system’s earliest days.

“We’re looking into the well-preserved remnants of the ancient past,” says Stern. “There is no doubt that the discoveries made about Ultima Thule are going to advance theories of solar system formation.”

Indeed. The two lobes of this 36-kilometer long object include an oddly flat surface (nicknamed ‘Ultima’) and the much rounder ‘Thule,’ the two connected by a ‘neck’ that raises the question of how the KBO originally formed. We seem to be looking at a low velocity merger of two objects that had originally been in rotation. Their merger is important; remember, we’re dealing with an ancient planetesimal. From the paper:

For MU69 ’s two lobes to reach their current, merged spin state, they must have lost angular momentum if they initially formed as co-orbiting bodies. The lack of detected satellites of MU69 may imply ancient angular momentum sink(s) via (i) the ejection of formerly co-orbiting smaller bodies by Ultima and Thule, (ii) gas drag, or both. This suggests that contact binaries may be rare in CCKBO [Cold Classical Kuiper Belt Object] systems with orbiting satellites. Another possibility, however, is that the lobes Ultima and Thule impacted one another multiple times, shedding mass along with angular momentum before making final contact. But the alignment of the principal axes of MU69 ’s two lobes tends to disfavor this hypothesis.

Also interesting is the possibility of tidal locking of the two lobes before their merger:

In contrast, tidal locking could quite plausibly have produced the principal axis alignment we observe, once the co-orbiting bodies were close enough and spin-orbit coupling was most effective… Gas drag could also have played a role in fostering the observed coplanar alignment of the Ultima and Thule lobes… Post-merger impacts may have also somewhat affected the observed, final angular momentum state.

And let’s consider what the science team has to say about Ultima Thule’s color. Like many other objects found in the Kuiper Belt, it’s red, much redder than Pluto. This obviously has significant interest because of what it implies about the organic materials on the surface and how they are being modified by solar radiation. Here the mixture of surface organics differs from other icy objects previously examined, though not all:

There are key color slope similarities between the spectrum of MU69 and those of the KBO (55638) 2002 VE 95 (45) and the escaped KBO 5145 Pholus… Also, these objects all exhibit an absorption band near 2.3 mm, tentatively attributed to methanol (CH3OH) or perhaps more complex organic molecules intermediate in mass between simple molecular ices and tholins… Similar spectral features are also apparent on the large, dark red equatorial region of Pluto informally called Cthulhu… suggesting similarities in the chemical feedstock and processes that could operate there and on MU69.

The paper points out that we see evidence for H2O ice in the form of shallow spectral absorption features, indicating low abundance in the object’s upper surface, as compared with planetary satellites and even some Kuiper Belt objects with clearer H2O detections. As expected, we do not see the spectral signatures of volatile ices such as carbon monoxide, nitrogen, ammonia or methane, which were observed at Pluto. These were not expected at MU69 “…owing to the ready thermally driven escape of such ices from this object over time.”

We’re left with a lot of unanswered questions, including the nature and origin of surface features like ‘Maryland crater,’ the largest (8 kilometer wide) scar on the KBO, likely the result of an impact, and the various bright spots and patches. How much of the pitted surface can be related to outgassing via sublimation, or material falling into already existing underground spaces? Bear in mind that the data return from New Horizons is to continue until late summer of 2020. Meanwhile, the spacecraft continues to measure the brightness of other Kuiper Belt objects while also mapping the charged-particle radiation and dust environment of its surroundings.

The paper is Stern et al., “Initial results from the New Horizons exploration of 2014 MU69, a small Kuiper Belt object,” Science Vol. 364, Issue 6441 (17 May 2019). Abstract.

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A Neutrino Beam Beacon

If you want to look for possible artifacts of advanced civilizations, as do those practicing what is now being called Dysonian SETI, then it pays to listen to the father of the field. My friend Al Jackson has done so and offers a Dyson quote to lead off his new paper: “So the first rule of my game is: think of the biggest possible artificial activities with limits set only by the laws of physics and look for those.” Dyson wrote that in a 1966 paper that repays study today (citation below). Its title: The Search for Extraterrestrial Technology.” Dysonian SETI is a big, brawny zone where speculation is coin of the realm and the imagination is encouraged to be pushed to the limit.

Jackson is intrigued, as are so many of us, with the idea of using the Sun’s gravitational lens to make observations of other stars and their planets. Our recent email conversation brought up the name of Von Eshleman, the Stanford electrical engineer and pioneer in planetary and radio sciences who died two years ago in Palo Alto at 93. Eshleman was writing about gravitational lensing possibilities at a time when we had no technologies that could take us to 550 AU and beyond, the area where lensing effects begin to be felt, but he saw that an instrument there could make observations of objects directly behind the Sun as their light was focused by it.

Claudio Maccone has been working this terrain for a long time, and the complete concept is laid out in his seminal Deep Space Flight and Communications: Exploiting the Sun as a Gravitational Lens (Springer Praxis, 2009). There is much to be said about lensing and space missions, and it’s heartening to see interest in scientists within the Breakthrough Starshot project — a sail moving at 20 percent of lightspeed gets us to 550 AU and beyond relatively quickly. By my back of the envelope figuring, travel time is just a little short of 16 days.

There would be no need for Starshot to approach 550 AU at 20 percent of c, of course. The focal line runs to infinity, but as Jackson explains when running through gravitational lensing’s calculations, we can assume beam intensity gradually diminished by absorption in the interstellar medium, though all of this with little beam divergence. Just how to use the Sun’s gravity lens (a relay for returning data from a star mission, I assume) and how to configure mission parameters to get to the lensing region and use it are under debate.

Transmitting Through a Gravitational Lens

But back to Al Jackson’s paper, which offers us a take on gravitational lensing that I have never before encountered [see my addendum at the end of this post for a correction]. What he is proposing is that an advanced civilization of the kind Dyson is interested in would have the capability of using a gravitational lens to transmit data. He’s turned the process around, from observation to beacon or other form of communication. And he’s working with neutrinos, where attenuation from the interstellar medium is negligible.

A gravitational lens does not, of course, need to be a star, but could be a higher mass object like a neutron star. In Jackson’s thinking, a KII civilization could place a neutrino beam transmitting station around a neutron star. We make neutrino beams today via the decay of pi mesons, as the author reminds us, when large accelerators boost protons to relativistic energies that strike a target, producing pions and kaons that decay into neutrinos, electrons and muons. What counts for Jackson’s purposes is that pions and kaons can be focused to produce a beam of neutrinos.

For a stellar mass gravitational lens and 1 Gev neutrinos, the wavelength is about 10-14 cm, the gain is approximately 1020! The characteristic radius of main region of concertation is about one micron; however there is an effective flux out to about one centimeter.

And as mentioned above:

This beam intensity extends to infinity only diminished by absorption in the interstellar medium, encounters with a massive object like a planet or star and a very small beam divergence.

Image: This is Figure 6 from the paper. Caption: A schematic illustration of a possible neutrino accelerator-transmitter, the accelerator and lens (nothing to scale). Credit: A. A. Jackson.

A one-centimeter beam creates the problem of focusing on a specific target, one whose phenomenal pointing accuracy could only be left to our putative advanced civilization. Even so, increase the number of transmitters and detection becomes easier. Thus Jackson:

Suppose that a K2 type civilization capable of interstellar flight can reach a neutron star it should have the technological capability to build a beacon consisting of an array of transmitters in a constellation of orbits about the neutron star. Let this constellation consist of 1018 ‘neutrino’ transmitters 1 meter in characteristic size ‘covering’ the area of a sphere 1000 km in radius with 1018 particle accelerators in orbit… At the present time there is the development of plasma Wakefield particle accelerators that are meters in size [21, 22]. It is probable that a K2 civilization may construct Wakefield electron accelerators of very small size.

Jackson has heeded Dyson, that’s for sure. Remember the latter’s injunction: “…think of the biggest possible artificial activities with limits set only by the laws of physics and look for those.”

What emerges is a ‘constellation of neutrino beam transmitters,’ 1018 in orbit at 1000 neutron star radii, increasing the probability of detection, so that the detection rate at 10,000 light years becomes approximately 5 per minute. The transmitters must be configured to appear as point sources to the gravitational lens, again another leap demanding KII levels of performance. But if a civilization is trying to be noticed, a neutrino beam that takes neutrino detection well out of the range produced through stellar events in the galaxy should stand out.

Thus we have a method for producing directed neutrino signal transmission. Why come up with such? Harking back to Dyson, we can consider the need to examine the range of possible ETI technologies, hoping to create a catalog that could explain future anomalous observations. Jackson’s beam could be used as what he calls a ‘honey pot’ to attract attention to an electromagnetic transmitter broadcasting more sophisticated information. But we would not necessarily be able to understand what uses an advanced civilization would make of such capabilities. We might have to content ourselves merely with the possibility of observing them.

As Jackson points out, a KII civilization “…would likely have the resources to finesse the technology in a smarter way,” so what we have here is a demonstration that a thing may be possible, while we are left to wonder in what other ways a neutrino source can be used to produce detections at cosmic distances. Going deep to speculate on technologies far beyond our reach today should, as Dyson says, remind us to stay within the realm of known physics while simultaneously asking about phenomena that advanced engineering could produce. We hope through the labors of Dysonian SETI to recognize such signatures if we see them.

Appreciating Von Eshleman

And a digression: When Al brought up Von Eshleman in our conversations, I began thinking back to early gravitational lensing work and Eshleman’s paper, written back in 1979 but prescient, surely, of what was to come in the form of serious examination of lensing capabilities for space missions. Let me quote Eshleman’s conclusion from the paper, cited below. I have a lot more to say about Eshleman’s work, but let’s get into that at another time. For now:

It has been pointed out that radio, television, radar, microwave link, and other terrestrial transmissions are expanding into space at 1 light-year per year (2). Another technological society near a neighboring star could receive the strongest of these directly with substantial effort and could learn a great deal about the earth and the technology of its inhabitants. The concepts presented here suggest that on an imaginary screen sufficiently far behind that star, the short-wavelength end of this terrestrial activity is now being played out at substantial amplifications. Properly placed receivers with antennas of modest size could in principle scan the earth and discriminate between different sources, mapping such activity over the earth and learning not only about the technology of its inhabitants, but also about their thoughts. It is possible that several or many such focused stories about other worlds are now running their course on such a gigantic screen surrounding our sun, but no one in this theater is observing them…

I learned about Eshleman’s work originally through Claudio Maccone and often reflect on the tenacity of ideas as we go from a concept originally suggested by Einstein to a SETI opportunity realized by Eshleman to a mission concept detailed by Maccone, and now a potential actual mission seriously discussing using gravitational lensing as a relay opportunity for data return from Proxima Centauri (Breakthrough Starshot). As it always has, the interstellar field demands long-term thinking that crosses generations in support of a breathtaking goal.

[Addendum]: Although I hadn’t heard of discussions on using gravitational lensing for transmission, an email just now from Clément Vidal points out that both Claudio Maccone and Vidal himself have looked into this. In Clément’s case, the reference is Vidal, C. 2011, “Black Holes: Attractors for Intelligence?” at Towards a scientific and societal agenda on extra-terrestrial life, 4-5 Oct, Buckinghamshire, Kavli Royal Society International Centre. Abstract here. This quote is to the point:

“For a few decades, researchers have proposed to use the Sun as a gravitational lens. At 22.45AU and 29.59AU we have a focus for gravitational waves and neutrinos. Starting from 550AU, electromagnetic waves converge. Those focus regions offer one of the greatest opportunity for astronomy and astrophysics, offering gains from 2 to 9 orders of magnitude compared to Earth-based telescopes…It is also worth noting that such gravitational lensing could also be used for communication…Indeed, it is easy to extrapolate the maximal capacity of gravitational lensing using, instead of the Sun, a much more massive object, i.e. a neutron star or a black hole. This would probably constitute the most powerful possible telescope. This possibility was envisioned -yet not developed- by Von Eshleman in (1991). Since objects observed by gravitational lensing must be aligned, we can imagine an additional dilating and contracting focal sphere or artificial swarm around a black hole, thereby observing the universe in all directions and depths.”

The author of The Beginning and the End: The Meaning of Life in a Cosmological Perspective (2014), Vidal’s thinking is examined in The Zen of SETI and elsewhere in the archives.

I’m glad Clément wrote, especially as it jogged my memory about Claudio Maccone’s paper on using lenses as a communications tool, where the possibilities are striking. See The Gravitational Lens and Communications for more. I wrote about this back in 2009 and thus have no good excuse for letting it slip my mind!

The paper is Jackson, “A Neutrino Beacon” (preprint). The Dyson paper is “The Search for Extraterrestrial Technology,” in Marshak, ed. Perspectives in Modern Physics: Essays in Honor of Hans Bethe, New York: John Wiley & Sons 1966. The Eshleman paper is “Gravitational Lens of the Sun: Its Potential for Observations and Communications over Interstellar Distances,” Science Vol. 205 (14 September 1979), pp. 1133-1135 (abstract).

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Survivors: White Dwarf Planets

The term ‘destruction radius’ around a star sounds like something out of a generic science fiction movie, probably one with lots of laser battles and starship crews dressed in capes. It’s a descriptive phrase as used in this University of Warwick (UK) news release, but let’s go with ‘Roche radius’ instead. Dimitri Veras, a physicist at the university, probes the term in the context of white dwarfs in a new paper for Monthly Notices of the Royal Astronomical Society. Veras and collaborators are looking at what happens after the challenging transition between red giant and white dwarf, a time when planets will be in high turmoil.

The idea is to model the tidal forces that occur once a star collapses into a super-dense white dwarf, blowing away its outer layers in the process. We see the clear potential for dragging planets into new orbits, with some pushed out of their stellar systems entirely. The Roche radius, or limit, is the distance from the star where a self-gravitating object will disintegrate because of tidal forces, forming a disk of debris around the star. It’s not limited to white dwarfs, of course, and can be applied to the tidal forces acting between any two celestial bodies.

White dwarfs are generally comparable in size to the Earth, while their Roche limit extends outwards to about one stellar radius. They’re also known to have metallic debris in their photosphere, presumably from objects pulled within the Roche radius. Learning about the tidal interactions of such worlds will help us calculate a planet’s inward and/or outward drift, and determine whether an object in a particular orbit will reach the Roche radius and be destroyed.

Image: An asteroid torn apart by the strong gravity of a white dwarf has formed a ring of dust particles and debris orbiting the Earth-sized burnt out stellar core. Credit: University of Warwick/Mark Garlick.

I’m not aware of a lot of work on tidal effects in white dwarf scenarios, but Veras believes that now is the time to begin such investigations, for the number of planets around white dwarfs is sure to rise with the advent of new extremely large telescopes (ELTs). We learn here that according to the authors’ models, the more massive a planet, the more likely it will be destroyed by these tidal forces. We do have to factor in the fact that some simplifications needed to run these calculations will need to be further developed to help us characterize such systems.

“Our study, while sophisticated in several respects, only treats homogenous rocky planets that are consistent in their structure throughout,” adds Veras. “A multi-layer planet, like Earth, would be significantly more complicated to calculate but we are investigating the feasibility of doing so too.”

How likely are planets or asteroids to be pulled into the Roche limit? Tidal forces modeled here imply that an object of low viscosity — think of Enceladus as a local example — is ultimately absorbed by the star if it orbits at separations within five times the distance between the center of the white dwarf and the Roche limit. By contrast, a high viscosity world is likely to be eventually swallowed into the star only if it is found at distances closer than twice the separation between the center of the white dwarf and Roche radius. Viscosity counts. Observationally, we might be on the lookout for disruptions in white dwarf disks just outside the Roche limit.

By ‘high viscosity world,’ the paper refers to planets with a dense core of heavy elements, such as the iron- and nickel-rich SDSS J122859.93+104032.9, recently found in a star-grazing 2-hour orbit around a white dwarf by astronomers at the same university (for more, see White Dwarf Debris Suggests a Common Destiny — Veras was on the team that did this work).

The current work suggests that this object has avoided falling into the star because of its small size. Or as the paper puts it: “Because the magnitude of the stellar tides scales as the mass of the perturber, the orbital dynamics of the asteroids in the WD 1145+017 and SDSS J1228+1040 systems are unaffected by stellar tides.” The takeaway: The best chance for survival near the white dwarf is to be small and dense, packed with heavy elements. And a little distance doesn’t hurt. A distance of just .13 AU, a third of the distance between Mercury and the Sun, is enough to ensure survival for the kind of rocky, homogeneous planet this paper models.

We learn that massive super-Earths are more readily disrupted than lower-mass planets, while as seen above, planets of higher viscosity are more likely to survive than their low-viscosity counterparts. We’re dealing with a challenging environment indeed for surviving objects, but one that repays investigation considering its ubiquity. As the paper points out, almost every known exoplanet currently orbits a star that will become a white dwarf. The authors continue:

Exo-asteroids are already observed orbiting two white dwarfs in real time… Planets which then survive to the white dwarf phase play a crucial role in frequently shepherding asteroids and their observable detritus on to white dwarf atmospheres, even if the planets themselves lie just outside of the narrow range of detectability. Further, planets themselves may occasionally shower a white dwarf with metal constituents through post-impact crater ejecta and when the planetary orbit grazes the star’s Roche radius…

The paper’s goal is to create a computational framework for future observations that can grow out of this early study of a two-body system comprising an idealized solid planet and a white dwarf. Of note: “…the boundary between survival and destruction is likely to be fractal and chaotic,” reinforcing the challenge of characterizing these maximally stressed stellar systems.

An interesting question: Can second- or third-generation planets form from white dwarf debris disks? The paper briefly considers the possibility and notes that such worlds would be in near-circular orbits and possibly be out of the reach of detection by current technologies.

The paper is Veras et al., “Orbital relaxation and excitation of planets tidally interacting with white dwarfs,” Monthly Notices of the Royal Astronomical Society Vol. 486, Issue 3 (July, 2019), pp. 3831-3848 (abstract).

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