Centauri Dreams
Imagining and Planning Interstellar Exploration
Gas Giants on Eccentric Orbits: ‘Wrecking Balls’ for the Inner System?
We often think of Jupiter as a mitigating influence on asteroid or comet strikes in the inner system, its gravity changing the trajectories of potential impactors. That would make gas giants a powerful determinant of the survivability of Earth analogues, at least in terms of habitability. While we continue to investigate the question, it’s interesting to consider the damage a gas giant on an elliptical orbit might do to habitable zone planets. Stephen Kane (UC-Riverside), working with Caltech astronomer Sarah Blunt, decided to find out what would happen if, in their modeling, they introduced an elliptical gas giant into the system of an Earth twin.
You may remember Kane’s work earlier this year combining radial velocity with direct imaging methods to find three gas giants that had been previously unobserved (citation below). The monitoring of ten target stars continues even as this new work is published. We’re beginning to find more planets at ever larger distances from their stars as radial velocity and direct imaging methods improve, allowing us to better understand how the architecture of our own Solar System measures up to systems around other stars. Kane and Blunt’s paper implies that a gas giant on an elliptical orbit does not necessarily preclude a habitable planet’s survival.
The planetary system at HR 5183 is a little over 100 light years away in Virgo, home to an eccentric gas giant in a 75 year orbit that the researchers used in their modeling. The primary here is a G-class star. Its planet has one of the longest orbital periods currently known among exoplanets. The eccentricity of this world is e = 0.84, where e = 0 would be perfectly circular, and e = 1 would be a line segment. To find out whether such a world really would be a ‘wrecking ball’ for its neighbors, the researchers introduced a habitable zone terrestrial world as a test case to study extreme system architectures and their effects on habitability.
Image: Comparison of HR 5183b’s eccentric orbit to the more circular orbits of the planets in our own solar system. Credit: W. M. Keck Observatory/Adam Makarenko.
The dynamical simulations here involved the exploration of 200 evenly spaced semi-major axes between 1.0 and 3.0 AU, intended to encompass the range of the optimistic habitable zone around such a star. Kane and Blunt then placed an Earth-mass planet at randomized starting positions and propagated the effects of the eccentric gas giant over time. Says Kane:
“In these simulations, the giant planet often had a catastrophic effect on the Earth twin, in many cases throwing it out of the solar system entirely. But in certain parts of the planetary system, the gravitational effect of the giant planet is remarkably small enough to allow the Earth-like planet to remain in a stable orbit.”
This being the case, we’re called upon to imagine the view from the surface of a habitable zone planet in this system. The gas giant is on a 75 year orbit, something akin to Halley’s Comet in our own system. Kane says that when the gas giant makes its closest approach to the terrestrial planet during that orbit, it would appear 15 times brighter than Venus, a spectacular object that would dominate the night sky before receding once again into the outer reaches.
Here’s a clip from the paper talking about the significance of these findings. Note that the Milankovitch cycles discussed below are cyclical movements — eccentricity, axial tilt, and precession — related to a planet’s orbit around a star. From the paper:
The importance of such systems from a planetary habitability perspective arises from a thorough investigation of the dynamical stability of terrestrial planetary orbits, such as the one presented here. The careful analysis of the dynamical integrations demonstrates that planets can survive within a narrow range of locations in the HZ of such systems, even in the presence of a wrecking ball whose orbital origin is likely a chaotic event involving vast exchanges of angular momentum.
So we have planetary survival in certain locations, but habitability is severely challenged:
…the case of the HR 5183 system also shows that the presence of an eccentric planet will often have a profound effect on the Milankovitch cycles of the HZ terrestrial planetary orbits, causing significant orbital oscillatory behavior. The implications for the climate effects on such worlds may rule out temperate surface conditions, although the stabilizing effects of surface liquid water oceans can also potentially prevent a climate catastrophe.
In other words, our terrestrial world in its habitable zone orbit in a system with a highly eccentric gas giant is in a dangerous position indeed, though not one that completely rules out life. This seems to represent a slight widening of habitable zone possibilities as we examine exoplanetary systems, though the ‘wrecking ball’ hypothesis still seems the most likely outcome.
The paper is Kane & Blunt, “In the Presence of a Wrecking Ball: Orbital Stability in the HR 5183 System,” Astronomical Journal Vol. 158, No. 5 (31 October 2019). Abstract / Preprint. The paper on gas giant detection is Kane et al., “Detection of Planetary and Stellar Companions to Neighboring Stars via a Combination of Radial Velocity and Direct Imaging Techniques,” accepted at the Astronomical Journal. Preprint.
Benefits of a ‘Snow Line’ Neptune
The formation of planets like Neptune under the core accretion model involves a protoplanetary core that reaches around 10 Earth masses before beginning to pull in surrounding gas, the latter being a runaway process that quickly builds the atmosphere around the object. Core accretion is most efficient at doing this just outside the snow line, but if we want to understand and test the theory, we need to know a lot more about how planets are distributed in this region.
And that’s a problem, because recent microlensing surveys have found that planets like Neptune are most abundant much more distant from their host stars. Outward migration can account for such worlds, but we know little about exoplanets that form at the snow line, which is where the condensation of ices can factor into the emergence of a new world.
Is this just an artifact of our still evolving microlensing detection techniques? Perhaps, and exceptions to the rule can therefore be helpful. Recent work that began with a discovery by a Japanese amateur astronomer has now blossomed into a full-scale study of a snow line Neptune around a star that, unlike most viewed by microlensing, is actually fairly close. The amateur, Tadashi Kojima in Gunma Prefecture, Japan, found the object in Taurus, the beginning of observations from numerous observatories that uncovered the microlensing behind the discovery.
The planet Kojima-1Lb orbits a star 1600 light years away, while the star it passed in front of is some 2600 light years out. Remember that the curvature of spacetime in the presence of massive objects accounts for this phenomenon, as warped space around the nearby star acts as a lens that focuses the light from the background star. Within this brightening, a transient but useful phenomenon, changes in intensity can reveal a planet orbiting the foreground star, as happened here. This discovery is unusual because most microlensed planets have been observed toward galactic center, which makes sense given the sheer abundance of stars there. This one is found close and toward the galactic anticenter.
Image: Diagram illustrating the microlensing event studied in this research. Red dots indicate previous exoplanet systems discovered by microlensing. Inset: Artist’s conception of the exoplanet and its host star. Credit: The University of Tokyo.
76 days of observation by a team led by Akihiko Fukui at the University of Tokyo took advantage of 13 telescopes around the world, including two at the National Astronomical Observatory of Japan’s Okayama Astrophysical Observatory. The work, as revealed in a paper just published in the Astronomical Journal, show a Neptune-class planet orbiting a star on the border line between K and M-class dwarf status. The planet is about 20 Earth masses and orbits at 1.08 AU, snow line distance for this system.
So we’ve got a helpful Neptune at the snow line. The paper draws an interesting but highly tentative conclusion from this detection:
The orbit of Kojima-1Lb is a few times closer to the host star than the other microlensing planets around the same type of star and is likely comparable to the snow-line distance at its youth. We have estimated that the detection efficiency of this planet in this event is ?35%, which may imply that Neptunes are common around the snow line.
In other words, Fukui and colleagues calculate the a priori detection probability of this kind of planet at 35 percent, making this chance detection a possible indication of an abundance of such worlds around the snow line of other stars. The paper goes on to point out that the host star here is the brightest among all those studied in microlensed systems, offering the opportunity to do follow-up spectroscopic analysis to characterize the host star and to refine both mass and orbit of the planet through radial velocity studies.
The paper is Fukui et al. “Kojima-1Lb is a Mildly Cold Neptune around the Brightest Microlensing Host Star,” Astronomical Journal Vol. 158, No. 5 (November 1, 2019). Abstract / preprint.
Engineered Exogenesis: Nature’s Model for Interstellar Colonization
Is seeding life into the universe to be a part of the human future? Space probes conceivably could be doing this inadvertently, and the processes of panspermia also may be moving biological possibilities between planets and even stars. Robert Buckalew has his own take on what humans might do in this regard, as discussed below. Robert has written fiction and non-fiction since 2013 under the pen name Ry Yelcho for the blog Yelcho’s Muses. In 2015 he received the Canopus Award for Excellence in Interstellar Fiction from 100 Year Starship for the story “Everett’s Awakening.” His short story “The Interlopers” appears on Literally Stories. What follows draws on his speculative science article “Microbots—The Seeds of Interstellar Civilization,” which was awarded the Canopus Award for Original Non-Fiction. The essay that follows is based on his presentation at the Icarus Interstellar Starship Congress 2019.
by Robert Buckalew
The series of pivotal events that led to the development of intelligent life on Earth are so numerous and seemingly random that the occurrence of intelligent life at other places in the galaxy may be very rare. The chance extinction of the dinosaurs which led to the diversification and opportunistic evolution of mammals is but one of many such events. Assuming the extraordinary rarity of this occurrence elsewhere in the galaxy should be a compelling reason for humans to presume our gifts of intellect exceptional and assume the obligation to prevent this cosmic largess from vanishing through global natural disaster, nuclear war or self-made neglect. Even if intelligent life is found to be common in the universe, it is certain that our form of intelligent life is unique. If we wish, someday, to communicate and interact with these various sentient species and contribute our singular human culture to their diverse communities, we must project our species’ existence into cosmic time frames.
The creation of dispersed, self-sufficient human settlements both interplanetary and extra-solar is the best way to ensure our long term survival as a species. Because of the unimaginable distances to other star systems, most proposals for interstellar colonization involve large multi-generational starships, warp drives or wormholes. Although common plot devices to create science fiction stories, wormholes and warp drives appear unworkable travel methods given the constraints of known physics. Multi-generational starships come with their own technological, political, biological, social and psychological challenges that make their realization daunting to consider.
Nature, however, has developed efficient methods to spread life on Earth that could be employed for interstellar colonization. Engineered Exogenesis, modeled after successful natural processes, proposes a method to spread Earth-life throughout our local stellar neighborhood.
Exogenesis, in astrobiology, is the hypothesis that life originated elsewhere in the universe and was conveyed here to Earth. For example, there is evidence that life in our solar system originated on Mars and was brought to this planet aboard a meteorite. The plausibility of Earth-life having been transplanted is supported by our inability to create spontaneous life from primordial organic chemicals in the laboratory and by the fact that there is no known remnant of pre-genetic life on Earth.
Engineered Exogenesis will use an additional strategy derived from nature. The survival system used by plants, insects and many aquatic lifeforms is based on the overproduction of seeds, larvae or spores in order to overcome their natural failure rate. Mass produced microbots can be designed to intentionally deliver engineered genetics to prospective exoplanets in numbers sufficient to assure that some will likely reach their destination and survive. As with seeds, this may require the dissemination of thousands to millions of them, based on the projected failure rate of the delivery system and the expected germination rate.
We know that water and organic compounds, known as tholins, have been found on planets, moons, comets and asteroids throughout our solar system. These precursors for life are believed to exist on interstellar comets and asteroids as well. Planetary exo-systems are expected to offer a similar fertile environment ready for the introduction of earthly genetic material.
The major components of an Engineered Exogenesis system might include 1) a microbiotic vessel that travels to the extra-solar planet, 2) a space-based magnetic accelerator capable of providing the inertial energy to send the vessel to other solar systems, 3) a space-based laser providing communication and supplemental energy for solar sail navigation and maneuvering, and 4) the engineered genetic material capable of growing a bio-robotic agent on the exoplanet to prepare the planetary environment for humans.
The Microbot
The microbiotic vessel, hereafter referred to as the microbot, would transport hermetically encapsulated, genetic material to the destination exoplanet while providing radiation, magnetic and acceleration protection. This vessel would be designed to open in the presence of liquid water, deploy a biobot zygote and, if necessary, a photosynthesizing food source such as phytoplankton or other aqueous plant food. The engineering of the microbot would incorporate nano technology, bio-robotics, AI and neural networks. It could be very small, possibly the size of a grain of rice, and constructed of low mass materials to minimize the energy required for acceleration. Construction materials might include carbon fiber, graphene or Kevlar designed to withstand the high magnetic fields and high acceleration rates and to provide heat shielding for atmospheric entry. Microbot vessels would possess no on-board propulsion using only their initial inertial energy for space travel. A ferromagnetic mass at the leading end of the vessel would be required for magnetic acceleration and inertial stability. This mass might be separated for deceleration upon arrival at the planetary system or retained for atmospheric entry shielding.
Microbots would also use leading and trailing photo-sensors for navigational aids with the leading photo-sensor directed at the destination star and the trailing sensor pointed at Sol. Fore and aft modulated, bio-luminescent lasers would provide communication between traveling microbot ships reminiscent of fireflies on a summer night.
A series of pivoting, flat panels would be extended following launch. Each panel would use one side for solar energy collection. Solar electric storage might be achieved by capacitance of the microbot body. The obverse side, capable of adjustable reflectivity, would be used as a solar sail. The panel ends would also be capable of latching with other microbot panels for the creation of microbot arrays, connected clusters of individual microbots. A powdered iron substrate layer which would become magnetized during acceleration could aid in microbot arraying and later become a magsail for magnetic braking. A superconducting loop for magnetic braking could be incorporated into the perimeter of the solar panels or otherwise deployed as an independent loop. Finally, the panels could be positioned for autogyro aerobraking during atmospheric entry in the same way maple seeds can dissipate energy by helicoptering to Earth.
The Accelerator
Magnetic acceleration happens by activating each electromagnet ahead of the projectile to pull it forward. As the projectile accelerates, the rate of coil activation increases to stay ahead of the accelerating mass. Projectile acceleration would be limited by the inertial mass of the projectile and the force produced by the electromagnets. Magnetic accelerators can include a circular or linear motor configuration.
Aimed at the destination star, the microbots would be accelerated sequentially for a maximum exit velocity with a minimum energy expenditure. This must be a space based as the atmospheric heating from the very high exit velocity precludes microbots being launched from Earth. By timing the sequencing of the magnets the rate of acceleration can be optimized for the given projectile mass and the vulnerability of the vehicle and payload to the forces of acceleration. The length of a linear accelerator is inversely related to the acceleration needed for a given exit velocity – the longer the coil gun, the less acceleration needed. A circular, toroidal accelerator would not have this length constraint as it could use multiple cycles to obtain projectile terminal velocity. Once the system is deployed it could be used for numerous target stars. If microbots can be accelerated to 10% light speed, a trip to Alpha Centari (4.37 light years away) would take 43 years and a trip to Tau Ceti (10.4 light years away) would take 104 years. Reaching these speeds would be a function of the length and power of the accelerator and the mass of the microbot ship.
The Laser
The laser is not for propulsion, as suggested by Yuri Milner’s Breakthrough Starshot, but would be used for communication and programming updates by modulation of the laser beam. It could also provide energy for course correction and arraying maneuvers. It is also best located in space to reduce atmospheric light scattering.
Arrays
Microbots would be programmed to array although some microbots would necessarily remain as self sufficient individual ships. The primary purpose of arraying is to improve communication with Earth as a larger antenna area can enhance both transmission and reception performance. However, arraying may also be used to collectivize the solar power and energy storage and to organize the use of this power.
Implementing Arrays
Microbot arraying could be achieved through use of swarm intelligence, a naturally occurring function among social insects, migrating birds and fish schools. Known in robotics as distributed AI, microbots could communicate with each other while traveling in space through their fore and aft photo-sensors and modulated bio-luminescence. As the trip may take 100 years or more, there should be adequate time for arraying even considering the limited maneuvering power provided by the angular incident positioning and variable reflectivity of their panels. Although launched individually, the first vessels would be accelerated at a slower velocity than the later vessels causing their clumping in space as they travel and increasing their ability to form arrays.
Microbots Arrival and Descent to the ExoPlanet
Deceleration from near relativistic speeds to that of planetary orbit velocity is always problematic. Explosive ejection of the leading ferromagnetic mass could substantially decelerate the containment vessel while reducing the remaining microbot maneuvering mass. Employing the resistance of the reflective solar sails and the magsail, braking and maneuvering could be achieved by using the retrograde radiant photon pressure, plasma energy streams and charged magnetosphere of the destination star. Solar and magnetic braking could continue after entering an elliptical orbit of the star, until a matching planetary orbital velocity is obtained. With aerobraking the arrays could go from an elliptical orbit of the planet to a circular one. For the solitary microbots aerobraking would slow them for atmospheric entry.
Microbots entering the atmosphere would experience further braking through atmospheric drag. With reduced velocity, low gravitational attraction and high surface-to-mass ratios, atmospheric entry damage to the microbots might be kept to a minimum. Descent might be further dampened and controlled by positioning the panels for autogyro energy dissipation.
Engineered Genetic Materials
Genetics has become a game-changing technology allowing for man-made biological creativity. Genetic engineering has been revolutionized through CRISPR and the creation of artificial life by scientists like Craig Venter. Expanding the genetic alphabet beyond the 4 chemical bases found in DNA could add functions capable of assembling metal or silicon components into planet inhabiting biobots.
Although the microbot starship incorporated some bio-robotic functions such as neural networks and bio-luminescence for communication, it did not need the biobot ability to grow and reproduce. The biobots used for planetary exploration, terraforming and habitat construction would be grown from the genetic material in the microbot after acquiring a suitable watery environment for germination/gestation. These bio-life forms would be genetically designed to be suitable for the anticipated planetary environment but might also incorporate a greater degree of robotics into their biology for laser communication with Earth and reprogrammability.
Essential BioBot Characteristics
The first terrestrial biobots might best be designed to be amphibious for food access and terrain mobility and cold blooded for temperature tolerance. They might be capable of solar power or photosynthesis for their energy, but separate genetic material may be included to produce a photosynthesizing food source for the biobots.
Their instinctive behavior would include a work activity for communication and infrastructure building similar to instinctive nest, hive or web building found with Earth life. Their behaviors would be re-programmable from Earth allowing task changing capability. For this they would require a means to transmit and receive laser modulated signals with Earth as well as their interspecies communication using sound or light modulation.
Reproduction could be biological though it would likely be asexual. Reproduction would also be programmable through Earth communication in order to create a series of diverse offspring, specifically-tasked and specialized biobots. Eventually there would be terraforming for human habitation. After successful habitat construction and terraforming, the final genetic download would be human genomes for incubation. This would require specialized biobots for human gestation and nurturing. These humans could be genetically designed for the gravity, atmosphere and temperatures of the exoplanet by adjusting metabolism rate, body mass, lung capacity, skin color, fat, fur covering, etc. These modifications would prognosticate natural changes which would have evolved over time in humans in adapting to their new environment.
The simpler alternative to the more complex remote reproductive reprogramming would be to send sequential waves of microbots each containing subsequent genetics. However, the advantages of remote genetic reprogramming would not only result in faster colonization, but the genetic developments during the 100+ year microbot travel time could be incorporated in the transmitted genetic code.
Advantages of Engineered Exogenesis
There are some obvious advantages that come with with an Engineered Exogenesis approach to interstellar colonization. It would be scalable, specifically, the numbers of microbots manufactured and launch frequencies can be adjusted to suit political or financial circumstances. It would not be limited to one target planet, and any number of planets or newly discovered planets or moons could be added as targets over time. It would be tested in our solar system and modified with improvements before deployment. The seeding and growth of biobots could take place on Earth or in domed environments on the moon or Mars. Finally, it reduces the time scale for starship colonization, and eliminates human exposure to space travel. It, however, it lacks the drama and romance found in human adventure stories in space.
For Engineered Exogenesis to become a reality there are many technical problems to be resolved as well as numerous ethical issues to be considered such as the chauvinistic imposition of our genetics onto other evolving planetary systems, the creation and dissemination of synthetic, reproducing life forms and the alternation of our own human genome. Society, so far, has been accepting of test tube babies, GMO food crops and gene therapy, especially when it seems to improve our lives. Such controversial technological impositions may also be accepted as necessary in order to achieve a human interstellar presence.
Has This Happened Before?
If Engineered Exogenesis is a viable idea, would it not have been done by advanced alien civilizations? If so, why are there no alien biobots roving around on Earth? This is like the Fermi Paradox about space aliens. If they were sent here, maybe ocean Earth life feeds on these undeveloped alien genomes before they grow and reproduce. Possibly life on Earth is the result of an alien exogenesis. Then where are the microbot ships that carried them? They would be tiny, widely scattered and hard to find. Possibly the nascent search for micrometeorites on Earth may yet find one of these artificial nanobots.
References
1. Michael Noah Mautner, “Seeding the Universe with Life: Securing Our Cosmological Future,” The Interstellar Panspermia Society. http://www.panspermia-society.com/
2. N. Mathews, A. L. Christensen, R. O’Grady, F. Mondada, and M. Dorigo, “Mergeable nervous systems for robots,” Nature Communications 8(439), 2017 (full text).
3. Jennifer Doudna, “How CRISPR lets us edit our DNA,’ TED Talk September 2015. https://www.ted.com/talks/jennifer_doudna_we_can_now_edit_our_dna_but_let_s_do_it_wisely?language=en
4. J. Craig Venter, “Watch me unveil ‘synthetic life,'” TED talk. May 2010. https://www.ted.com/talks/craig_venter_unveils_synthetic_life?language=en
5. Daniela Rus, “Autonomous boats can target and latch onto each other,” MIT News June 5, 2019. http://news.mit.edu/2019/autonomous-robot-boats-latch-0605
6. Sarah Hörst, “What in the world(s) are tholins?” Planetary Society July 22, 2015. http://www.planetary.org/blogs/guest-blogs/2015/0722-what-in-the-worlds-are-tholins.html
7. Francesco Corea, “Distributed Artificial Intelligence: A Primer on Multi-Agent Systems Agent Based Modeling and Swarm Intelligence,” https://www.kdnuggets.com/2019/04/distributed-artificial-intelligence-multi-agent-systems-agent-based-modeling-swarm-intelligence.html
8. Paul Gilster, “Starship Surfing: Ride the Bow Shock,” Centauri Dreams March 21, 2012 https://www.centauri-dreams.org/2012/03/21/starship-surfing-ride-the-bow-shock/
Investigating a Pluto Orbiter
The spectacular success of New Horizons inevitably leads to questions about what an orbiter at Pluto/Charon might accomplish. It’s heartening that NASA has funded the Southwest Research Institute (SwRI) to look further into the matter, the Institute having already examined the question on its own. Now a Pluto orbiter becomes one of ten mission studies NASA is sponsoring as we look toward the next National Academy Planetary Science Decadal Survey. Beginning in 2020, the survey will outline science objectives and recommend missions over a ten year period.
The NASA decision leverages all the work SwRI has put into the Pluto orbiter concept, and brings the focus to what we might accomplish with such a mission that a flyby could not. Particularly significant will be the choice of science instruments, which a spacecraft achieving global coverage will demand. And because we have a system at Pluto with five moons, we have a range of targets that can be subjected to detailed study. There is even the possibility of taking the mission to other targets, as New Horizons principal investigator Alan Stern explained:
“In an SwRI-funded study that preceded this new NASA-funded study, we developed a Pluto system orbital tour, showing the mission was possible with planned capability launch vehicles and existing electric propulsion systems. We also showed it is possible to use gravity assists from Pluto’s largest moon, Charon, to escape Pluto orbit and to go back into the Kuiper Belt for the exploration of more KBOs like MU69 and at least one more dwarf planet for comparison to Pluto.”
Image: To follow up on NASA’s New Horizons mission that revealed Pluto’s “heart,” SwRI is studying a new Pluto orbiter mission for NASA. SwRI has shown it is possible to orbit Pluto and then escape orbit to tour additional dwarf planets and Kuiper Belt Objects. Credit: NASA/JHUAPL/SwRI.
New Horizons carries seven instruments, all of which are still functioning well, as we learned from Stern in his latest PI’s Perspective. Having flown past Ultima Thule (2014 MU69), New Horizons continues to explore the Kuiper Belt, and it will be instructive to see how long it continues to return data. The Voyagers have demonstrated longevity far beyond the expectations of those who built them, Right now the spacecraft is continuing to return data on the Ultima Thule flyby, a process that will last another year or so, according to Stern, but New Horizons is also continuing to observe KBOs as it moves ever further out..
The seven scientific instruments aboard the spacecraft have just been put through a thorough calibration, the first such campaign run since just before the Pluto/Charon flyby. As with the Ultima Thule data, the complete calibration results will be returned with the dataflow over the next year, though Stern says the instruments ‘performed flawlessly.’ The crucial Long Range Reconnaissance Imager (LORRI) has received a software upgrade designed to detect fainter targets than before as well as to enable longer exposures. The new capability will be in place by December for further Kuiper Belt exploration.
We still don’t have a dedicated mission to the interstellar medium, meaning one with an instrument package expressly designed for operations beyond the heliosphere, but we do have continuing dust and plasma observations of the outer heliosphere from New Horizons. This is useful stuff, because we are building a dataset that complements what the two Voyagers have given us, though the New Horizons instrument package is more capable. Planetary scientists will take advantage of observations like these in learning more about how the surfaces of KBOs and dwarf planets are affected by the environment through which they orbit.
A number of science papers on Pluto/Charon and Ultima Thule are about to be submitted, with dozens of new results reported at the recent Division for Planetary Sciences meeting in Geneva. You can see that New Horizons is very much an ongoing mission, even as we look toward the benefits of a Pluto orbiter that is at least under study not just at SwRI but NASA. The continued naming of Pluto surface features is a reminder of how much we’ve learned, but imagine how we can fill out these young maps with features yet to be observed in detail.
Image (click to enlarge): This map, compiled from images and data gathered by NASA’s New Horizons spacecraft during its flight through the Pluto system in 2015, contains Pluto feature names approved by the International Astronomical Union. Names from the newest round of nominations, approved in 2019, are in yellow. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Ross Beyer.
In Search of a Wormhole
A star called S2 is intriguingly placed, orbiting around the supermassive black hole thought to be at Sgr A*, the bright, compact radio source at the center of the Milky Way. S2 has an orbital period of a little over 16 years and a semi-major axis in the neighborhood of 970 AU. Its elliptical orbit takes it no closer than 120 AU, but the star is close enough to Sgr A* that continued observations may tell us whether or not a black hole is really there. A new paper in Physical Review D now takes us one step further: Is it possible that the center of our galaxy contains a wormhole?
By now the idea of a wormhole that connects different spacetimes has passed into common parlance, thanks to science fiction stories and films like Interstellar. We have no evidence that a wormhole exists at galactic center at all, much less one that might be traversable, though the idea that it might be possible to pass between spacetimes using one of these is too tempting to ignore, at least on a theoretical level. At the University at Buffalo, Dejan Stojkovic, working with De-Chang Dai (Yangzhou University, China and Case Western Reserve University), thinks the star S2’s behavior may offer a way to look for wormholes.
Image: An artist’s concept illustrates a supermassive black hole. A new theoretical study outlines a method that could be used to search for wormholes (a speculative phenomenon) in the background of supermassive black holes. Credit: NASA/JPL-Caltech.
Note that the authors are not saying they find such an object in the existing datasets on S2 (the object has only been monitored since 1995 at UCLA and at the Max Planck Institute for Extraterrestrial Physics). Rather, they’re arguing for using the behavior of objects near black holes, where extreme astrophysical conditions exist, to see whether they exhibit unusual behavior that could be the result of a wormhole associated with the black hole. So this is a methodological approach that advances a proposed course of observation.
You may remember that a 1995 paper from John Cramer, Robert Forward, Gregory Benford and other authors including Geoff Landis (see below) went to work on this question, though not using a star near the Milky Way’s center (see How to Find a Wormhole, a Centauri Dreams article from the same year). Cramer et al. argued for looking for an astrophysical signal of negative mass, which would be needed to keep a wormhole mouth open. Let me quote from something Geoff Landis told me about the paper:
“If the wormhole is exactly between you and another star, it would defocus the light, so it’s dim and splays out in all directions. But when the wormhole moves and it’s nearer but not in front of the star, then you would see a spike of light. So if the wormhole moves between you and another star and then moves away, you would see two spikes of light with a dip in the middle.”
That’s an astrophysical signature interesting enough to be noted. And from the paper itself:
“…the negative gravitational lensing presented here, if observed, would provide distinctive and unambiguous evidence for the existence of a foreground object of negative mass.”
Back to Stojkovic, whose new paper notes a property we would expect to exist in wormholes. Let me quote his paper on this:
The purpose of this work…is to establish a clear link between wormholes and astrophysical observations. By definition, a wormhole smoothly connects two different spacetimes. If the wormhole is traversable, then the flux (scalar, electromagnetic, or gravitational) can be conserved only in the totality of these two spaces, not individually in each separate space.
Interesting point. An example: A physical electric charge on one side of the wormhole would manifest itself on the other side. There, where there is no electric charge, an observer would notice the electric flux coming from the wormhole and assume that the wormhole is charged. There is, in fact, no real charge at the wormhole, but the flux is strictly conserved only if the entirety of both spaces connected by the wormhole is considered. And as the paper goes on to state, a gravitational source like a star orbiting the mouth of the wormhole should be observed as gravitational perturbations on the other side.
The message is clear. Again, from the Stojkovic paper:
As a direct consequence, trajectories of objects propagating in [the] vicinity of a wormhole must be affected by the distribution of masses/charges in the space on the other side of the wormhole. Since wormholes in nature are expected to exist only in extreme conditions, e.g. around black holes, the most promising systems to look for them are either large black holes in the centers of galaxies, or binary black hole systems.
By now it should be clear why S2 is an interesting star for this purpose. Its proper motion orbiting what is believed to be a supermassive black hole at Sgr A* could theoretically tell us whether the black hole harbors a wormhole. The extreme gravitational conditions make this the best place to look for a wormhole, and minute deviations in the expected orbit of S2 could indicate one’s presence. That means we need to assemble a lot more data about S2.
Stojkovic doesn’t expect to find a lot of traffic coming through any wormhole we do find:
“Even if a wormhole is traversable, people and spaceships most likely aren’t going to be passing through. Realistically, you would need a source of negative energy to keep the wormhole open, and we don’t know how to do that. To create a huge wormhole that’s stable, you need some magic.”
In the absence of magic, we can still put observational astronomy to work. We may be a decade or two away from being able to track S2 this closely, and in any case will need a lot more data to make the call, but the scientist cautions that even deviations in its expected orbit won’t be iron-clad proof of a wormhole. They’ll simply make it a possibility, leading us to ask what other causes on our own side of the presumed wormhole could be creating the perturbations. And any wormhole we do come to believe is there would not necessarily be traversable, but if the effects of gravity from a different spacetime are in play, that’s certainly something we’ll want to study as we untangle the complicated situation at galactic center.
The paper is Dai and Stojkovic, “Observing a Wormhole,” Phys. Rev. D 100, 083513 (10 October 2019). Abstract / preprint). The Cramer et al. paper is “Natural Wormholes as Gravitational Lenses,” Physical Review D (March 15, 1995): pp. 3124-27 (abstract).
Exoplanet Collision at BD +20 307?
NASA collaborates with the German Aerospace Center (DLR) on one of our more interesting observatories. SOFIA, the Stratospheric Observatory for Infrared Astronomy, is a Boeing 747 aircraft that flies an infrared telescope with a 2.7 m diameter mirror. Located on the port side of the fuselage near the tail, the telescope houses a number of instruments for infrared astronomy at wavelengths from 1-655 micrometers (μm). One of these is FORCAST (Faint Object Infrared Camera for the SOFIA Telescope), which has now spotted an intriguing phenomenon, one that may be flagging a collision of two exoplanets.
The stars in question form a double system called BD +20 307, some 300 light years from Earth. Note the age of this system, about one billion years, an important consideration in what follows. About ten years ago, observations from the Spitzer instrument as well as ground observatories produced evidence of warm debris here, whereas from age alone, we would have expected warm circumstellar dust to have disappeared, just as it has in our own system.
What SOFIA brings to the table is a new set of measurements that shows the infrared brightness from the debris at BD +20 307 has increased by more than 10 percent in a time period of 10 years. We don’t usually find this kind of rapid fluctuation when studying what ought to be the gradual evolution of a planetary system, especially not in one as mature as this. There should be little dust here to begin with, much less warm dust, and while there are other possible mechanisms in play (see below), the rapid pace implies a collision.
“This is a rare opportunity to study catastrophic collisions occurring late in a planetary system’s history,” said Alycia Weinberger, staff scientist at the Carnegie Institution for Science’s Department of Terrestrial Magnetism in Washington, and lead investigator on the project. “The SOFIA observations show changes in the dusty disk on a timescale of only a few years.”
Image: Artist’s concept illustrating a catastrophic collision between two rocky exoplanets in the planetary system BD +20 307, turning both into dusty debris. Ten years ago, scientists speculated that the warm dust in this system was a result of a planet-to-planet collision. Now, SOFIA found even more warm dust, further supporting that two rocky exoplanets collided. This helps build a more complete picture of our own Solar System’s history. Such a collision could be similar to the type of catastrophic event that ultimately created our Moon. Credit: NASA/SOFIA/Lynette Cook.
From the paper:
We investigated several mechanisms that could cause the observed changes in the disk flux, including making the dust grains hotter, either through an increase in stellar luminosity or moving the dust grains closer to the stars, or increasing the number of dust grains in the system. If the origin of the copious amount of warm dust orbiting BD +20 307 is an extreme collision between planetary-sized bodies, then this system may help unlock clues into planetary systems around binary stars, along with providing a glimpse into catastrophic collisions occurring late in a planetary system’s history.
It’s also true that gaining a stronger understanding of dusty debris disks should give us insights into how binary systems evolve, useful as we investigate such interesting places as the Alpha Centauri triple system. If we are indeed looking at the result of a major collision at BD +20 307, further work should illuminate the kind of catastrophes we find evidence for in the Solar System, from our Moon’s formation (likely through an impact with a Mars-sized object) to the huge axial tilt of Uranus, which is probably the result of multiple impacts. The authors argue that new SOFIA observations at a wider wavelength range out to 20 µm will allow us to draw more definitive conclusions.
The paper is Thompson et al., “Studying the Evolution of Warm Dust Encircling BD +20 307 Using SOFIA,” Astrophysical Journal Vol. 875, No. 1 (12 April 2019). Abstract / preprint.
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