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Investigating the ‘Halo Drive’

One of the interesting things about gravitational assists is their ability to accelerate massive objects up to high speeds, provided of course that the astrophysical object being used for the assist is moving at high speeds itself. Freeman Dyson realized, as we saw yesterday, that a pair of tightly rotating white dwarfs could offer such an opportunity, while a binary neutron star carried even more clout. When Dyson was writing his “Gravitational Machines” paper, neutron stars were still a theoretical concept, so he primarily focused the paper on white dwarfs.

Get two neutron stars in a tight enough orbit and the speeds they achieve would make it possible to accelerate a spacecraft making a gravity assist up to a substantial percentage of lightspeed. But what an adventure that close pass would be — the tidal forces would be extreme. I don’t recall seeing a neutron star propulsive flyby portrayed in science fiction (help me out here), though Gregory Benford offers a variant on the white dwarf idea in his early work Deeper than the Darkness (1970, later re-done as The Stars in Shroud, which ran in Galaxy in 1978), where a neutron star and an F-class star comprise a system he calls a ‘Flinger,’ with potential uses both for acceleration and deceleration.

David Kipping’s thoughts on extending Dyson to black holes were partially triggered by a fascinating paper from William Stuckey in 1993 entitled “The Schwarzschild Black Hole as a Gravitational Mirror” (citation below). The gravitational mirror in the title is what happens when photons skim near the event horizon and return to the source, a photon ‘boomerang’ that gains propulsive impact because the returning light rays receive a blueshift thanks to the black hole’s relative motion.

Thus the Kipping concept: Use laser beaming technologies to beam toward a precisely targeted member of a black hole binary. The light is returned in blue-shifted form to the spacecraft, the extra energy being used to push the craft. What’s especially intriguing here is that in this scenario, it is not the spacecraft that is making the messy plunge between two rapidly moving, tightly orbiting black holes, but rather the photon beam that is used for propulsion.

It is the light beam that gets the gravitational assist as photons are re-emitted and re-absorbed, for as the paper puts it, “…the halo drive transfers kinetic energy from the moving black hole to the spacecraft by way of a gravitational assist.” A less harrowing experience for any crew, to be sure, and one in which extreme time dilation can be avoided, not to mention the dangers of tidal disruption and radiation referred to above.

Image: David Kipping (Columbia University), creator of the ‘halo drive’ concept.

Kipping described the idea to me in an email recently:

The idea is to essentially to perform a Dyson slingshot remotely, by firing a collimated particle/energy beam just to the side of the event horizon of a Schwarzchild black hole. If you choose the angle carefully, the beam loops around (like a halo) and comes back to you. If the black hole is moving towards you (I envisaged a compact binary like Dyson), then the beam returns blue shifted. When you initially fire the beam, your ship receives a small momentum kick and when the beam returns and strikes your ship you get another. This is how the ship is propelled, much like a light sail. But the beam actually returns with more energy than it departed, since it siphoned some of the kinetic energy from the black hole. So not only did you accelerate, but your ship actually gained stored energy.

Do this right and the speed of your spacecraft eventually matches that of the black hole, but the cumulative blue shifts allow the craft to continue firing the laser past that point. No new ‘free’ energy is gained but energy from the stored cells aboard the craft can keep the acceleration going up to, the author calculates, 4/3 the speed of the black hole. It’s interesting to note that we are not limited to small masses in such a calculation. Unlike the Breakthrough Starshot energy issue we discussed yesterday, we can drive arbitrarily large masses up to potentially relativistic speeds. All of this by exploiting the fantastic energies available through astrophysical objects.

Image: This is Figure 1 from the paper. Caption: Figure 1. Outline of the halo drive. A spaceship traveling at a velocity βi emits a photon of frequency νi at a specific angle δ such that the photon completes a halo around the black hole, returning shifted to νf due to the forward motion of the black hole, βBH. Credit: David Kipping.

As you would imagine, for the gravitational mirror to function in this way, the beam must be precisely oriented. From the paper:

In order for the deflection to be strong enough to constitute a boomerang, this requires the light’s closest approach to the black hole to be within a couple of Schwarzschild radii, RS ≡ 2GM/c2. Light which makes a closest approach smaller than 3GM/c2 becomes trapped in orbit, known as the photon sphere, and thus typical boomerang geodesics skim just above this critical distance [italics mine].

Kipping calls this a ‘halo drive’ because the photons returning to the craft appear as a halo around the black hole. Single black holes as well as binaries can be used (and evidently Kerr black holes as well as Schwarschild black holes, though the author plans future work on this), but the paper notes that the potential for tight configurations at relativistic speeds makes binaries preferable. 10 million binary black holes are thought to exist in the galaxy [I’ve seen this figure reduced to 1 million recently — clearly, the issue is still open], raising the possibility of a network of starship acceleration points or, for that matter, deceleration stations.

A range of possible uses for binary black holes emerges, as the paper notes:

Although not the focus of this work, it is worth highlighting that halo drives could have other purposes besides just accelerating spacecraft. For example, the back reaction on the black hole taps energy from it, essentially mining the gravitational binding energy of the binary. Similarly, forward reactions could be used to not only decelerate incoming spacecraft but effectively store energy in the binary like a fly-wheel, turning the binary into a cosmic battery.

Another possibility is that the halos could be used to deliberately manipulate black holes into specific configurations, analogous to optical tweezers. This could be particularly effective if halo bridges are established between nearby pairs of binaries, causing one binary to excite the other. Such cases could lead to rapid transformation of binary orbits, including the deliberate liberation of a binary.

A natural question is why an interstellar civilization, one already capable of reaching a black hole binary and manipulating it in this manner would need to establish a galactic network of transit points. A possible answer is that the amount of energy liberated from such a black hole binary is, as with other kinds of gravitational assist, arriving essentially ‘free’ at the spacecraft. Thus a transportation network on the cheap could be established between specific locations, a mechanism for cost-savings that may well be too efficient to ignore.

Freeman Dyson’s interest in using astrophysical objects for propulsion included, of course, his abiding fascination with the technosignatures of advanced civilizations. Would a galactic infrastructure at work in the galaxy exhibit evidence of its use to distant astronomers? In my next post, I’ll look into the possibility.

The Dyson paper is “Gravitational Machines,” in A.G.W. Cameron, ed., Interstellar Communication, New York: Benjamin Press, 1963, Chapter 12. The Kipping paper is “The Halo Drive: Fuel-free Relativistic Propulsion of Large Masses via Recycled Boomerang Photons,” accepted at the Journal of the British Interplanetary Society (preprint). The Stuckey paper is “The Schwarzschild Black Hole as a Gravitational Mirror,” American Journal of Physics Vol. 61, Issue 5 (1993), pp. 448-456.



Pondering the ‘Dyson Slingshot’

Let’s start the week by talking about gravitational assists, where a spacecraft uses a massive body to gain velocity. Voyager at Jupiter is the classic example, because it so richly illustrates the ability to alter course and accelerate without propellant. Michael Minovitch was working on this kind of maneuver at UCLA as far back as the early 1960s, but it was considered even before this, as in a 1925 paper from Friedrich Zander. It took Voyager to put gravity assists into the public consciousness because the idea enabled the exploration of the outer planets.

Can we use this kind of maneuver to help us gain the velocity we need to make an interstellar crossing? Let’s consider how it works: We’re borrowing energy from a massive object when we do a gravity assist. From the perspective of the Voyager team, their spacecraft got something for ‘free’ at Jupiter, in the sense that no additional propellant was needed. What’s really happening is that the spacecraft gained energy at the expense of the planet. Jupiter being what it is, the change in its own status was invisible, but it lent enough energy to Voyager to prove enabling.

According to David Kipping (Columbia University), the maximum speed increase equals twice the velocity of the planet we’re using for the maneuver, and when you look at Jupiter’s orbital speed around the Sun (around 13.1 kilometers per second), you can see that we’re only talking about a fraction of what it would take to get us to interstellar speeds. But the principle is enticing, because traveling with little or no propellant is a longstanding goal, one that drives research into solar sails and their fast cousins, beamed lightsails. And it has been much on Kipping’s mind.

For gravitational assists from planets are only one aspect of the question, there being other kinds of astrophysical objects that can help us out. Depending on their orbital configuration, some of these are moving fast indeed. In the early 1960s, Freeman Dyson went to work on the physics of gravitational assists around binary white dwarf stars — he would ultimately go on to consider the case of neutron star binaries (back when neutron stars were still purely theoretical). Such concepts obviously imply an interstellar civilization capable of reaching the objects in the first place. But once there, the energies to be exploited would be spectacular.

While I want to begin with Dyson’s ideas, I’ll move tomorrow to Kipping’s latest paper, which addresses the question in a novel way. Kipping, well known for his work in the Hunt for Exomoons with Kepler project, has been pondering Dyson’s notions but also applying them to what would seem, on the surface of things, to be an entirely different proposition: Beamed propulsion. How he combines the two may surprise you as much as it did me, as we’ll see in coming days.

Image: An artist’s conception of two orbiting white dwarf stars. Credit: Tod Strohmayer (GSFC), CXC, NASA, Illustration: Dana Berry (CXC).

Nature of the Question

If we talk about manipulating astrophysical objects, a natural objection arises: Why should we study things that are impossible for our species today? After all, we can get to Jupiter, but getting to the nearest white dwarf, much less a white dwarf binary, is beyond us.

But big ideas can be productive. Consider Daedalus, conceived in the 1970s as the first serious design for a starship. The idea was to demonstrate that a spacecraft could be designed using known physics that could make a journey to another star. The massive two-stage Daedalus (54,000 tonnes) seems impossible today and doubtless will never be built. Was it worth studying?

The answer is yes, because once you’ve established that something is not impossible, you can go to work on ways to engineer a result that may differ hugely from the original. Breakthrough Starshot is built around the idea of using lasers to propel a different kind of spacecraft, not of 54,000 tonnes but of 1 gram, carried by a small lightsail, and designed to be sent not as a one-off mission but as a series of probes driven by the same laser installation.

Once again we’re stretching our thinking, but here the technologies to do such a thing may (or may not, depending on what Breakthrough Starshot’s analyses come up with) be no more than a few decades away. The current Breakthrough effort is all about finding out what is feasible.

Again we’re designing something before we’re sure we can do it. The challenges are obviously immense. Consider: To go interstellar with cruise times of several decades, we need to ramp up velocity, and that takes enormous amounts of energy. Kipping calculates that 2 trillion joules — the output of a nuclear power plant running continuously for 20 days — would be needed to send the Breakthrough Starshot ‘chip’ payload to Proxima Centauri. And that’s just for one ‘shot’, not for the multiple chips envisioned in what might be considered a ‘swarm’ of probes.

Working with Massive Objects

Are there other ways to generate such energies? Freeman Dyson’s extraordinary white dwarf binary gravitational assist appears in “Gravitational Machines,” a short paper that ran in a book A.G.W. Cameron edited called Interstellar Communication (New York, 1963). Conventional gravity assists aren’t sufficient because to be effective, a gravitational ‘machine’ would have to be built on an astronomical scale. Fortunately, the universe has done that for us. So we should be thinking about the principles involved, and what they imply:

…if our species continues to expand its population and its technology at an exponential rate, there may come a time in the remote future when engineering on an astronomical scale will be both feasible and necessary. Second, if we are searching for signs of technologically advanced life already existing elsewhere in the universe, it is useful to consider what kinds of observable phenomena a really advanced technology might be capable of producing.

Dyson’s considers the question in terms of binary stars, specifically white dwarfs, but goes on to address even denser concentrations of matter in neutron stars. Now we’re talking about a kind of gravitational assist that has serious interstellar potential. A spacecraft could be sent into a neutron star binary system for a close pass around one of the stars, to be ejected from the system at high velocity. If 3,000 kilometers per second appears possible with a white dwarf binary, fully 81,000 kilometers per second could occur — 0.27 c — with a neutron star binary.

Hence the ‘Dyson slingshot.’ (As an aside, I’ve always wondered what it must be like to have a name so famous in your field that everything from ‘Dyson spheres’ to ‘Dyson dots’ are named after you. The range of Dyson’s thinking on these matters certainly justifies the practice!).

The slingshot isn’t particularly effective with stars of solar class, where what you gain from a gravitational assist is still outweighed by the possibility of using stellar photons for propulsion. But as Dyson shows, once you get into white dwarf range and then extend the idea down to neutron stars, you’re ramping up the gravitational energy available to the spacecraft while at the same time reducing stellar luminosity. An advanced civilization, in ways Dyson explores, might tighten the orbital distance until the binary’s orbital period reached a scant 100 seconds.

Now a gravity assist has serious punch. In other words, there is the potential here for a civilization to manipulate astrophysical objects to achieve a kind of galactic network, where binary neutron stars offer transportation hubs for propelling spacecraft to relativistic speeds. As you would imagine, this plays to Dyson’s longstanding interest in searching for technological artifacts, and we’ll be talking about that possibility as we get into David Kipping’s new paper.

For Kipping will take Dyson several steps further, by looking not at neutron stars but black hole binaries and coming up with an entirely novel way of exploiting their energies, one in which a beam of light, rather than the spacecraft itself, gets the gravitational assist and passes those energies back to the vehicle. Kipping calls his idea the ‘Halo Drive,’ and we’ll begin our discussion of it, and a novel insight that inspired it, tomorrow.

The Dyson paper is “Gravitational Machines,” in A.G.W. Cameron, ed., Interstellar Communication, New York: Benjamin Press, 1963, Chapter 12. The Kipping paper is “The Halo Drive: Fuel-free Relativistic Propulsion of Large Masses via Recycled Boomerang Photons,” accepted at the Journal of the British Interplanetary Society (preprint). For those who want to get a head start, Dr. Kipping has also prepared a video on the Halo Drive that is available here.



Evidence of Passing Stars

The sheer range of possible outcomes in a planetary system is something we’re beginning to appreciate with each new exoplanet. Not long ago we looked at a possible collision between two large worlds in the young system Kepler 107, and the knowledge of how violent an evolving system can be informs our thinking about the formation of our own Moon and other Solar System phenomena. Now we’re learning to look for signs of another kind of early cataclysm, the migration of a planet caused by the close passage of one or more nearby stars.

None of this should be surprising when we think about the outer system today. We have a vast cloud made up of trillions of comets encircling a more disk-like belt of debris in the Kuiper Belt, and a host of small objects moving on orbits that challenge our theories of how they formed. Indeed, the orbits of ‘scattered disk’ objects influenced by Neptune and, even more intriguing, unusual trans-Neptunian objects like Sedna may implicate a yet undiscovered planet 9.

Some of what we are seeing may well be the result of a star passing near the Sun, and we know, for example, that the binary system known as Scholz’s star (WISE 0720−0846) passed through the Oort Cloud some 70,000 years ago. Close passes much earlier in the evolution of our protoplanetary disk could obviously have played a role in disrupting existing orbits.

An F-class star in the constellation Crux about 300 light years from Earth, HD 106906 may hold promising information about just such an event in another stellar system. The star is orbited by a directly imaged planet in a misaligned orbit that has been under investigation by UC-Berkeley’s Paul Kalas, working with Robert De Rosa (Kavli Institute for Particle Astrophysics and Cosmology). With a mass of about 11 Jupiters, the planet is tilted 21 degrees from the plane of the circumstellar disk. It’s also a whopping 738 AU out, 18 times farther from its star than Pluto from the Sun. That brings into doubt its in situ formation.

Image: Two binary stars, now far apart, skated by one another 2-3 million years ago, leaving a smoking gun: a disordered planetary system (left). Credit: UC-Berkeley. Credit: Paul Kalas.

A closer look using the Gemini Planet Imager and the Hubble Space Telescope produced the finding that this star is orbited by a belt of comets in an equally lopsided orbit. The signs of disruption were clear, and Kalas and De Rosa trace out a tortured history for this unusual world. Through gravitational instability induced by too close a passage to the central binary star (a finding discussed by Grenoble Observatory researchers led by Laetitia Rodet in 2017), the planet would have gone interstellar but for the close passage of a pair of passing stars. Their gravitational influence left it in the remote outer regions of its system on an eccentric orbit.

Image: Simulation of a binary star flyby of a young planetary system. UC Berkeley and Stanford astronomers suspect that such a flyby altered the orbit of a planet (in blue) around the star HD 106906 so that it remained bound to the system in an oblique orbit similar to that of a proposed Planet Nine attached to our own solar system. Animation credit: Paul Kalas.

Kalas and De Rosa used data from the European Space Agency’s Gaia mission to firm up this hypothesis. The scientists collected information on 461 stars from Gaia DR2 astrometry, all of them in the stellar grouping known as the Scorpius–Centaurus (Sco–Cen) association. Incorporating ground-based radial velocity work as well, the team calculated the positions of these stars backward in time, revealing the binary stars — HIP 59716 and HIP 59721 — as candidates for the stars that altered the young system some 3 million years ago.

“What we have done here is actually find the stars that could have given HD 106906 b the extra gravitational kick, a second kick so that it became long-lived, just like a hypothetical Planet Nine would be in our solar system,” Kalas said.

“Studying the HD 106906 planetary system is like going back in time to watch the Oort cloud of comets forming around our young sun. Our own giant planets gravitationally kicked countless comets outward to large distances. Many were ejected completely, becoming interstellar objects like ʻOumuamua, but others were influenced by passing stars. That second kick by a stellar flyby can detach a comet’s orbit from any further encounters with the planets, saving it from the prospect of ejection. This chain of events preserved the most primitive solar system material in a deep freeze far from the sun for billions of years.”

Image: Some 2 to 3 million years ago, in a young, newly formed planetary system, a planet was in danger of being kicked out of the system because of gravitational interactions with the central, binary star (left panel). A close pass by another binary star (not shown) within the same cluster gave the planet an extra kick that stabilized the orbit and rescued it from certain ejection (right panel). Credit: Paul Kalas.

The binary pair came into the system disk of HD 106906 on a trajectory that was within 5 degrees of the system disk, maximizing the extent of the encounter. From the paper:

HIP 59716 and HIP 59721 are the best candidates of the currently known members of Sco–Cen for a dynamically important close encounter with HD 106906 within the last 15 Myr. The flyby of these two stars fulfill many of the criteria for the stabilization scenario described in Rodet et al. (2017). Their trajectories are almost coplanar with the debris disk in its current orientation, their velocities relative to HD 106906 at closest approach are low (the change in velocity of the orbiting planet being inversely proportional to the relative velocity of the passing star at closest approach), and the distribution of closest approach distances for HIP 59716 is consistent with a dynamically significant encounter within 0.5 pc.

Continuing work on this system will investigate the relative radial velocities of the stars involved, which will mean future spectroscopic studies of the two candidate perturbers. The authors point out that the astrometry for each star will be improved with upcoming Gaia data releases. “We started with 461 suspects and discovered two that were at the scene of the crime,” says Kalas. “Their exact role will be revealed as we gather more evidence.”

The paper is De Rosa & Kalas, “A Near-coplanar Stellar Flyby of the Planet Host Star HD 106906,” accepted for publication at the Astronomical Journal (abstract).



Tuning Up HPF: The Habitable Zone Planet Finder

If you had a hot new instrument like the Habitable Zone Planet Finder (HPF) now mounted at the Hobby-Eberly Telescope (McDonald Observatory, University of Texas), how would you run it through its paces for fine-tuning and verification of its performance specs? The team behind HPF has chosen to deploy the instrument during its commissioning phase on a nearby target, Barnard’s Star, which for these purposes we can consider something of an M-dwarf standard.

Working at near-infrared wavelengths, HPF uses radial velocity methods to identify low-mass planets around nearby M-dwarf stars. The choice of wavelength is determined by the mission: M-dwarfs (also known as ‘red dwarfs’) are prey to substantial magnetic activity that shows up as spots and flares that disrupt instruments working in visible light, not to mention the fact that they are small to begin with and thus faint on the sky. In the near-infrared, close to but not in the visible spectrum, this category of star appears brighter and its surface activity more muted.

I mentioned Barnard’s Star as a kind of standard because it precisely suits astronomers’ needs for calibrating such an instrument. Here let me quote from a Penn State blog on HPF (Penn State built the instrument), which lays out the ideal for commissioning:

While the ultimate goal of any Doppler spectrograph is to find lots of exoplanets, boring is better during the commissioning phase. The only way to test the stability and precision of your end-to-end measurement system–from the telescope, through the fiber optics, and ultimately the optics and detector of the spectrograph–is to make repeated measurements of a star with little or no variability. That way, any variability seen in the measurements must be caused by the instrument, rather than the star itself. In other words, the less variability we measure in observations of our stable “standard star,” the better the instrument is performing.

Barnard’s Star fits the bill beautifully. For one thing, it’s close by, at about 6 light years, making it the second-closest system to the Sun. At 14 percent of the Sun’s mass, it’s also typical of the kind of stars HPF will survey. But the real value lies in its age, for Barnard’s Star is thought to be extremely old, possibly as old as the Milky Way itself. The star rotates slowly and shows little stellar activity of the kind that would mask the radial velocity signal in other M-dwarfs.

Image: The new Penn State-led Habitable Zone Planet Finder (HPF) provides the highest precision measurements to date of infrared signals from nearby stars. Pictured: The HPF instrument during installation in its clean-room enclosure in the Hobby Eberly Telescope at McDonald Observatory. Credit: Guðmundur Stefánssonn, Penn State.

To increase precision at the HPF, Penn State has added a laser frequency comb (LFC) to the mix. Custom-built by the National Institute of Standards and Technology (NIST), the comb is a kind of ‘ruler’ that is used to calibrate the near-infrared signal from other stars. Work like this demands a calibration source because a spectrum from the observed star will ‘drift’ slightly, a movement that must be corrected when astronomers are looking for signals in the area of 1 meter per second to identify a small planet in the habitable zone of an M-dwarf. This is a kind of false Doppler effect likely due to physical issues in the instrument itself. Measuring the spectra of two sources at once — one of them being the stable frequency comb — allows the correction to be made, letting the true Doppler effect induced by planets around the star be observed.

Atomic emission lamps have been used for such calibration in the past, but laser frequency combs produce spectra with finely calibrated emission lines that are stable and of uniform brightness. Adding a laser comb to HPF ensures maximum performance, says Suvrath Mahadevan (Penn State), who is principal Investigator of the HPF project:

“The laser comb…separates individual wavelengths of light into separate lines, like the teeth of a comb, and is used like a ruler to calibrate the near-infrared energy from the stars. This combination of technologies has allowed us to demonstrate unprecedented near-infrared radial velocity precision with observations of Barnard’s Star, one of the closest stars to the Sun.”

Image: An example comparison of calibration spectra for astronomical spectrographs. Credit: HPF / Penn State.

Mahadevan adds that the technical challenges of reaching this level of precision are substantial. The instrument is highly sensitive to any infrared light emitted at room temperature, which means operations must take place at extremely cold temperatures. Thus far, the results speak for themselves, as discussed in a paper in Optica that describes the Barnard’s Star work (citation below).

The current data series on Barnard’s Star shows a stability of about 1.5 meters per second, which tops anything achieved by an infrared instrument. This is actually close to the best earlier measurements of the star, which have come from the renowned HARPS spectrograph working at visible wavelengths (378 nm – 691 nm); these come in at 1.2 meters per second. The HPF goal is 1 meter per second, not yet attained, though the team continues to refine its numbers while searching for possible instrumental issues that may play a role. From the blog:

We would be remiss if we did not emphasize that working all of the kinks out of an ultra-precise Doppler spectrograph is a years-long process, and we are far from done making improvements to the instrument and our analysis techniques. With that said, our early observations of Barnard’s star are extremely promising!

Can HPF confirm the Pale Red Dot project’s super-Earth around Barnard’s Star? Not yet. Although the instrument has the precision to see Barnard’s Star b, a problem remains:

As it turns out, cosmic coincidence prevents us from having much information on Barnard b at this point. The orbit of the proposed planet is eccentric, which means the Doppler signal is more pronounced at some phases of its orbit than others. Through nothing but luck, our HPF-LFC observations completely missed the most dynamic section of the Barnard b phase curve. Thus, while our HPF measurements do not rule out the proposed planet, they cannot yet confirm it, either. This is just one of many examples of how exoplanet detection is a data-intensive process!

Image: The orbital model of Barnard b (blue), with HPF measurements (gold) folded to the orbital phase. Our measurements have not yet covered the maximum of the eccentric orbit. Credit: HPF team / Penn State.

The paper on applying laser frequency comb techniques to the HPF in studies of Barnard’s Star is Metcalf et al., “Stellar spectroscopy in the near-infrared with a laser frequency comb,” Optica Vol. 6, No. 2 (2019), pp. 233-239 (abstract).



Alternatives to DNA-Based Life

The question of whether or not we would recognize extraterrestrial life if we encountered it used to occupy mathematician and historian Jacob Bronowski (1908-1974), who commented on the matter in a memorable episode of his 1973 BBC documentary The Ascent of Man.

“Were the chemicals here on Earth at the time when life began unique to us? We used to think so. But the most recent evidence is different. Within the last few years there have been found in the interstellar spaces the spectral traces of molecules which we never thought could be formed out in those frigid regions: hydrogen cyanide, cyano acetylene, formaldehyde. These are molecules which we had not supposed to exist elsewhere than on Earth. It may turn out that life had more varied beginnings and has more varied forms. And it does not at all follow that the evolutionary path which life (if we discover it) took elsewhere must resemble ours. It does not even follow that we shall recognise it as life — or that it will recognise us.”

Bronowski wanted to show how human society had evolved as its conception of science changed — the title is a nod to Darwin’s The Descent of Man (1871), and the sheer elegance of the production reflected the fact that the series was the work of David Attenborough, whose efforts had likewise led to the production of Kenneth Clarke’s Civilisation (1969), among many other projects. If the interplay of art and science interests you, a look back at both these series will repay your time.

As to Bronowski, who died the year after The Ascent of Man was first aired, I can only imagine how fascinating he would have found new work out of the Foundation for Applied Molecular Evolution in Alachua, Florida. Led by Steven Benner, a team of scientists has addressed the question of alien life so unlike our own that we might not recognize it. Along the way, it has managed to craft a new informational system that, like DNA, can store and transmit genetic information. The difference is that Benner and team use eight, not four, key ingredients.

Image: This illustration shows the structure of a new synthetic DNA molecule, dubbed hachimoji DNA, which uses the four informational ingredients of regular DNA (green, red, blue, yellow) in addition to four new ones (cyan, pink, purple, orange). Credit: Indiana University School of Medicine.

DNA, a double-helix structure like the new “hachimoji DNA” (the Japanese term ‘hachi’ stands for ‘eight,’ while ‘moji’ means ‘letter’), is based upon four nucleotides that appear to be standard for life as we know it on Earth. ‘Hachimoji’ DNA likewise contains adenine, cytosine, guanine, and thymine, but puts four other nucleotides into play to store and transmit information.

We begin to see alternatives to the ways life can structure itself, pointing to environments where a different kind of structure could survive whereas DNA-based life might not. That could be useful as we’re beginning to put spacecraft into highly interesting environments like Europa and Enceladus, but to get the most out of our designs, we need to have a sense of what we’re looking for. What kinds of molecules could store information in the worlds we’ll be exploring?

Thus Mary Voytek, senior scientist for astrobiology at NASA headquarters:

“Incorporating a broader understanding of what is possible in our instrument design and mission concepts will result in a more inclusive and, therefore, more effective search for life beyond Earth.”

Creating something unusual right here on Earth is one way to approach the problem, but of course there are others, and I am reminded of Paul Davies work and his own notions of what he calls ‘weird life.’ The Arizona State scientist, a prolific author in his own right, has examined the concept of a ‘second genesis,’ a fundamentally different kind of life that might already be here, having evolved on our planet and remaining on it in what we might call a ‘shadow biosphere.’

Finding alternate life on our own planet would relieve us of the burden of creating new mechanisms to make life work in our labs, so perhaps the thorough investigation of deep sea hydrothermal vents, salt lakes and high radiation environments may cut straight to the chase, if such life is there. In any case, finding a second genesis would make it far more likely that we’re going to find life on other worlds, and such life, as Davies reminds us, might be right under our noses. Like ‘hachimoji DNA,’ such life would challenge and stimulate all our assumptions.

The paper is Hoskika et al., “Hachimoji DNA and RNA: A genetic system with eight building blocks,” Science Vol. 363, Issue 6429 (22 Feb 2019), pp. 884-887 (abstract). Thanks to Byron Rogers for an early tip on this work.



Ultima Thule at Highest Resolution

One of the most enjoyable interviews I’ve been involved with lately was with Ryan Ferris, who runs the podcast Cosmic Tortoise from Christchurch, New Zealand. Ryan’s questions were sharp and of a philosophic bent, plumbing issues like the purpose and direction of human exploration. From Thor Heyerdahl’s extraordinary experiments at shipbuilding and navigation to the impulses that took Polynesian sailors into unknown waters as they settled Pacific islands, is there an innate human impulse to explore? We kicked all this around, along with SETI, the ‘Oumuamua object, and the need for a re-orienting long-term approach to civilization.

Ultima Thule and the recent exploration of it by New Horizons fit comfortably within the narrative Ryan and I discussed, as an example of satisfying that drive to push into the unknown, and also as an early marker for the growth of infrastructure in the Solar System. The Kuiper Belt pushes us hard for now, but we learn with each mission. In the meantime, forays of growing complexity to the Moon and Mars, as well as nearby asteroids, will teach us many things about human and robotic operations in ways that can extend them more frequently to system’s edge.

I’m still jazzed about Ultima Thule. The New Horizons team is saying the recently released images have the highest resolution of any the spacecraft has taken during its mission. Note the surface detail including several bright areas, roughly circular, as well as dark pits near the terminator. We’ve got so much data yet to come as New Horizons continues to return its information, so there is much to figure out at this point. “Whether these features are craters produced by impactors, sublimation pits, collapse pits, or something entirely different, is being debated in our science team,” said John Spencer, deputy project scientist from SwRI.

Image: The most detailed images of Ultima Thule — obtained just minutes before the spacecraft’s closest approach at 12:33 a.m. EST on Jan. 1 — have a resolution of about 33 meters (110 feet) per pixel. Their combination of higher spatial resolution and a favorable viewing geometry offer an unprecedented opportunity to investigate the surface of Ultima Thule, believed to be the most primitive object ever encountered by a spacecraft. This processed, composite picture combines nine individual images taken with the Long Range Reconnaissance Imager (LORRI), each with an exposure time of 0.025 seconds, just 6 ½ minutes before the spacecraft’s closest approach to Ultima Thule (officially named 2014 MU69). The image was taken at 5:26 UT (12:26 a.m. EST) on Jan. 1, 2019, when the spacecraft was 6,628 kilometers (4,109 miles) from Ultima Thule and 6.6 billion kilometers (4.1 billion miles) from Earth. The angle between the spacecraft, Ultima Thule and the Sun – known as the “phase angle” – was 33 degrees. Credit: NASA/Johns Hopkins Applied Physics Laboratory/Southwest Research Institute, National Optical Astronomy Observatory

The success at Ultima Thule and the possibility of another KBO encounter in an extended mission will keep New Horizons in our thoughts for a long time to come — even after its KBO adventures have ended, we’ll still be tracking a live, outbound spacecraft just as we follow the Voyagers. The degree of precision exhibited in the Ultima Thule work is made possible by the stellar occultations we’ve discussed here in past months as well as data from the European Space Agency’s Gaia mission, so critical for star locations during the occultation campaigns.

Image: New Horizons scientists created this movie from 14 different images taken by the New Horizons Long Range Reconnaissance Imager (LORRI) shortly before the spacecraft flew past the Kuiper Belt object nicknamed Ultima Thule (officially named 2014 MU69) on Jan. 1, 2019. The central frame of this sequence was taken on Jan. 1 at 5:26:54 UT (12:26 a.m. EST), when New Horizons was 6,640 kilometers (4,117 miles) from Ultima Thule, some 6.6 billion kilometers (4.1 billion miles) from Earth. Ultima Thule nearly completely fills the LORRI image and is perfectly captured in the frames, an astounding technical feat given the uncertain location of Ultima Thule and the New Horizons spacecraft flying past it at over 14.3 kilometers per second. Credit: NASA/Johns Hopkins Applied Physics Laboratory/Southwest Research Institute.

Closing to within 3,500 kilometers of its target, the spacecraft moved three times closer to Ultima than when it flew past Pluto/Charon in July of 2015. No wonder Alan Stern is exultant:

“Getting these images required us to know precisely where both tiny Ultima and New Horizons were — moment by moment – as they passed one another at over 32,000 miles per hour in the dim light of the Kuiper Belt, a billion miles beyond Pluto. This was a much tougher observation than anything we had attempted in our 2015 Pluto flyby.

“These ‘stretch goal’ observations were risky, because there was a real chance we’d only get part or even none of Ultima in the camera’s narrow field of view,” he continued. “But the science, operations and navigation teams nailed it, and the result is a field day for our science team! Some of the details we now see on Ultima Thule’s surface are unlike any object ever explored before.”

The golden age of human discovery continues; indeed, it is just beginning. If you want to explore the raw imagery from the LORRI instrument, have a look at the New Horizons LORRI website. And give some thought to context. One thing we should recall as we ponder future exploration is a vast, island-dotted Pacific, and ancestors who navigated it by wind, currents and stars.



Hayabusa2: Asteroid Touchdown

For those of you who’ve been asking, I think the best way to keep up with the Hayabusa2 mission to asteroid Ryugu is via Twitter, @haya2e_jaxa. The news continues to percolate via websites and various publications, with a sustained ripple when the spacecraft successfully tested its sample mechanism and touched down on the asteroid. I’ll remind you too that the mission team now offers updated systems information in English on its Haya2NOW page for obsessives like me who want a really fine-grained look at what’s going on.

Hayabusa 2 is once again at what JAXA calls its ‘home position’ about 20 kilometers above the asteroid as the multi-part sample selection process continues. JAXA’s news release on the touchdown was to the point:

National Research and Development Agency Japan Aerospace Exploration Agency (JAXA) executed the asteroid explorer Hayabusa2 operation to touch down the surface of the target asteroid Ryugu for sample retrieval.

Data analysis from Hayabusa2 confirms that the sequence of operation proceeded, including shooting a projectile into the asteroid to collect its sample material. The Hayabusa2 spacecraft is in nominal state. This marks the Hayabusa2 successful touchdown on Ryugu.

But really, Twitter carries the excitement of the mission via tweets like these:

and photos like the one below, which was sent out to thank worldwide supporters for their thoughts and encouragement. Look at the size of the Hayabusa2 team! Congratulations to all of you.

Upon touchdown within the 6-meter circle selected on the asteroid, the spacecraft fired a tantalum ‘bullet’ into the surface to drive particles outward that the sampling instrument could collect. The craft then rose again, as vividly attested in the photo below, where its shadow is obvious. Two more samples are to be taken before Hayabusa 2 departs the asteroid, the final sampling involving a larger crater deliberately blasted into the asteroid to probe sub-surface materials.

Image: Image captured roughly 1 minute after touchdown at an estimated altitude of about 25m (error is a few meter). The color of the region beneath the spacecraft’s shadow differs from the surroundings and has been discolored by the touchdown. At the moment, the reason for the discoloration is unknown but it may be due to the grit that was blown upwards by the spacecraft thrusters or bullet (projectile). The photograph was taken with the Optical Navigation Camera – Wide angle (ONC-W1) on February 22, 2019 at an onboard time of around 07:30 JST. (Credit: JAXA, University of Tokyo, Kochi University, Rikkyo University, Nagoya University, Chiba Institute of Technology, Meiji University, University of Aizu, AIST.)

You may recall that the original first sample collection was scheduled for last October, but had to be delayed because the surface of the asteroid turned out to be rougher than expected. JAXA has been operating two small robotic rovers — MASCOT and MINERVA-II — on the surface, which produced the information that centimeter-sized gravel and larger were to be found there. As the agency reported online, a key question was whether the material was fine enough be released from the asteroid during the sampling events planned, which is why an artificial gravel experiment was performed in Tokyo at the end of December.

Image: Target simulating the surface of Ryugu (Credit: JAXA, University of Tokyo).

As JAXA went on to report:

In the ground test performed during the initial development, even large rocks with similar strengths to carbonaceous chondrite meteorites were crushed when a projectile made of metal (tantalum) with a mass of 5g was injected at about 300 m/s. It was confirmed that material formed from the resulting small pieces could be gathered by the sampler. So in this test, it was predictable that the bullet would crush material that it struck, but what would be the behavior of the gravel surrounding the focus of the shot?

From the results of the experiment, the fragments of gravel that were crushed were released into the surrounding gravel where they collided like billiards to break up the material. The resulting sample amount exceeded the initial assumption that would be released from the surface (Figure 4).

While the diameter of the collision site (crater) made by the impact of the projectile is smaller than when compared to that in a fine regolith layer, it was a sufficient size in comparison with the inner diameter of the open tip of the sampler horn.

The plan is for Hayabusa2 to depart Ryugu in December of this year, with return to Earth toward the end of 2020. Assuming a successful sample return, Hayabusa2 will mark the first time samples from a C-type (carbonaceous) asteroid — the most common, constituting 75% of those known — have been returned to Earth. Naturally we’ll also keep an eye on OSIRIS-REx and its operations at 101955 Bennu, another carbonaceous asteroid, for both sample returns should give us a window into early building blocks of our planet. The OSIRIS-REx sample return is scheduled for 2023.



A White Dwarf with Puzzling Rings

Backyard Worlds: Planet 9 is a project worth investigating. Using a database drawn from NASA’s Wide-field Infrared Survey Explorer (WISE), Backyard Worlds: Planet 9 is probing the cosmos at infrared wavelengths. Volunteers search the WISE data in a ‘citizen science’ effort that has already discovered more than 1,000 likely brown dwarfs. Now we have news of an intriguing white dwarf showing apparently multiple rings of gas and dust.

A ringed white dwarf isn’t unique. In fact, dust and rings have been observed around white dwarfs that were considerably younger than the one in question, J0207. As described in a paper in Astrophysical Journal Letters, the object is about 145 light years away in the constellation Triangulum and is thought to be about 3 billion years old. With a temperature of 5,800 degrees Celsius (10,500 degrees Fahrenheit), J0207 produced a strong infrared signal.

Bear in mind that a white dwarf is a remnant, a star left behind when its Sun-like predecessor, running out of nuclear fuel, has gone through red giant phase and ejected half of its mass, leaving the hot dwarf behind. As has been depicted in numerous science fiction tales, a swollen red giant can engulf its inner planets while pushing more distant planets and asteroids outward. Something of the future of our own Solar System is thus suggested.

Image: Backyard Worlds: Planet 9 volunteers scour infrared images from NASA, searching animated blinks for moving objects. Like other white dwarf stars, J0207 shows a bluish tinge in visible light (top), but also sports an orange hue in the infrared (bottom), indicating the unexpected presence of circumstellar dust rings. Credit: Digitized Sky Survey/WISE/NEOWISE, Aaron Meisner (NOAO).

What made J0207 stand out for discoverer (and paper co-author) Melina Thévenot, a citizen scientist based in Germany, was infrared emission indicating the star was surrounded by a dusty disk. We could be looking at a process of tidal disruption here, in which asteroids and comets in the stellar neighborhood are brought closer to the star by gravitational interactions with surviving planets. In any case, approaching the white dwarf, they would be torn apart by the star’s gravity. The ring that emerges will eventually dissipate, spiralling down onto the surface of the star.

Having discovered something odd about J0207, Thévenot assumed there were problems with the data. Consulting the European Space Agency Gaia archives for brown dwarfs, she identified J0207, and compared it to the source in the WISE data. Its brightness and distance confirmed it was not a brown dwarf, at which point she passed her results on to Backyard Worlds: Planet 9, where Adam Schneider (ASU), John Debes (Space Telescope Science Institute), and Marc Kuchner (NASA GSFC), who leads the Backyard Worlds: Planet 9 project, could examine them.

It would be Debes and Kuchner who, working with UC-San Diego astronomer Adam Burgasser, arranged follow-up observations of J0207 with the Keck II instrument in Hawaii. What makes J0207 stand out in relation to other white dwarfs with dust disks is its age. Given a process in which asteroids are ground apart by gravitational interactions with the star, we should get to the end of the line — with all such materials incorporated into the star — in a fairly short time. This is especially true if the white dwarf is not part of a binary, which J0207 is not.

Says Debes:

“This white dwarf is so old that whatever process is feeding material into its rings must operate on billion-year time scales. Most of the models scientists have created to explain rings around white dwarfs only work well up to around 100 million years, so this star is really challenging our assumptions of how planetary systems evolve.”

Image: The star, designated LSPM J0207+3331, is the oldest, coolest white dwarf known to be surrounded by a ring of dusty debris. This illustration depicts the ring with two distinct components, which scientists think best explains the system’s infrared signal, and an asteroid broken up by the white dwarf’s gravity. Credit: NASA Goddard Space Flight Center/Scott Wiessinger.

We may also be looking at a second point of interest, for the J0207 disk shows evidence of being composed of more than a single ring-like component. That would be a first in white dwarf observations, and for it I turn to the paper:

…the infrared excess seen for this disk requires a second, colder ring of dusty material that could potentially signal the presence of a gap in the system, or a component of dust that extends beyond the outer edge of the inner disk. If the second ring is confirmed, it would be the first example of a two-component ring system around a dusty white dwarf. If the dust disk has a gap near 0.94 R, this implies the possibility of a body that continuously clears dust from the system, since the PR drag timescale is so short.

So we have what the paper calls “an interesting test of dust disk accretion“ in J0207, given the age of the star. The researchers suggest further investigation in the form of optical spectra of this white dwarf to look for metal lines from accreted dust to examine the actual accretion rate.

The paper is Debes et al., “A 3 Gyr White Dwarf with Warm Dust Discovered via the Backyard Worlds: Planet 9 Citizen Science Project,” Astrophysical Journal Letters Vol. 872, No. 2 (19 February 2019). Abstract / Preprint.

I can’t say enough how much I support Backyard Worlds: Planet 9 and other citizen science projects that are not only producing high-quality results but also involving the wider public in our ongoing exploration of the cosmos.



Finding Neptune’s Smallest Moon

What a lively place Neptune used to be, at least back in the days when the planet captured Triton, doubtless a Kuiper Belt Object now in a retrograde orbit around the primary. Recent work led by Mark Showalter (SETI Institute) puts the Hubble Space Telescope to work in studying one result of the sudden acquisition of so massive an object. A first generation of small satellites was likely scattered and rearranged, its debris becoming the Neptunian moons we see today.

Among them is Hippocamp, once known as S/2004 N 1, which appears to be a fragment from Neptune’s second largest moon Proteus. What an interesting set of observations we have here. Discovered in 2013, Hippocamp is the outermost of the planet’s inner moons, and it orbits a scant 12,000 kilometers from Proteus. We can relate the 2013 discovery with what Voyager 2 found at Neptune in 1989: A large impact crater on Proteus.

“The first thing we realised was that you wouldn’t expect to find such a tiny moon right next to Neptune’s biggest inner moon,” saysd Mark Showalter. “In 1989, we thought the crater was the end of the story. With Hubble, now we know that a little piece of Proteus got left behind and we see it today as Hippocamp.”

Image: This artist’s impression shows the outermost planet of the Solar System, Neptune, and its small moon Hippocamp. Hippocamp was discovered in images taken with the NASA/ESA Hubble Space Telescope. Whilst the images taken with Hubble allowed astronomers to discover the moon and also to measure its diameter, about 34 kilometres, these images do not allow us to see surface structures. Credit: ESA/Hubble, NASA, L. Calçada.

The likely cause of the Proteus impact is a comet, striking long after the havoc created by Triton’s appearance. Jack Lissauer (NASA Ames) is a co-author of the new work:

“Based on estimates of comet populations, we know that other moons in the outer Solar System have been hit by comets, smashed apart, and re-accreted multiple times. This pair of satellites provides a dramatic illustration that moons are sometimes broken apart by comets.”

Image: This composite image shows the location of Neptune’s moon Hippocamp, formerly known just as S/2004 N 1, orbiting the giant planet Neptune, about 4.8 billion kilometres from Earth. The moon is only about 34 kilometres in diameter and dim, and was therefore missed by NASA’s Voyager 2 spacecraft cameras when the probe flew by Neptune in 1989. Several other moons that were discovered by Voyager appear in this 2009 image, along with a circumplanetary structure known as ring arcs. Mark Showalter of the SETI Institute discovered Hippocamp in July 2013 when analysing over 150 archival images of Neptune taken by Hubble from 2004 to 2009. The black-and-white image was taken in 2009 with Hubble’s Wide Field Camera 3 in visible light. Hubble took the colour inset of Neptune on August 19, 2009. Credit: NASA, ESA, and M. Showalter (SETI Institute).

Just 1/1000th the mass of Proteus, Hippocamp shouldn’t be where we see it, but that large impact crater Voyager found on Proteus tells the tale. It alone explains why Proteus didn’t assimilate or sweep aside Hippocamp long ago, thanks to an impact sufficient to have all but shattered Proteus while leaving Hippocamp behind. That cometary bombardment makes Hippocamp a third-generation satellite.

Image: This diagram shows the orbital positions of Neptune’s inner moons, which range in size from 17 to 420 kilometres in diameter. The outer moon Triton was captured from the Kuiper belt many billions of years ago. This tore apart Neptune’s original satellite system. After Triton settled into a circular orbit the debris from shattered moons re-coalesced into the second generation of inner satellites seen today. However, comet bombardment continued, leading to the birth of Hippocamp, which is a broken-off piece of Proteus. Therefore, Hippocamp is considered to be a third-generation satellite. Neither the size of the moons and Neptune, nor the orbits are to scale. Credit: NASA, ESA, and A. Feild (STScI).

Yesterday we looked at an exoplanet scenario for a massive planetary collision. The collision of Proteus with a comet is much smaller in scale, but powerful in its effects. From the paper:

We cannot rule out the possibility that Hippocamp formed in situ and has no connection to Proteus. However, its tiny size and peculiar location lead us to favour the proposed formation scenario, which illustrates the roles that collisions and orbital migration have played in shaping the Neptune system that we see today

The paper is Showalter et al., “The Seventh Inner Moon of Neptune,” Nature 566, pp. 350–353 (20 February 2019). Abstract.



Kepler 107: Collision of Worlds

It seems increasingly clear that the factors that govern what kind of a planet emerges where in a given stellar system are numerous and not always well understood. Beyond the snowline, planets draw themselves together from the ice and other volatiles available in these cold regions, so that we wind up with low-density gas or ice-giants in the outer parts of a stellar system. Sometimes. Rocky worlds are made of silicates and iron, elements that, unlike ice, can withstand the much warmer temperatures inside the snowline. But consider:

While we now have 2,000 confirmed exoplanets smaller than three Earth radii, the spread in their densities is all over the map. We’re finding that other processes must be in play, and at no insubstantial level. Low-density giant planets can turn up orbiting close to their stars. Planets not so dissimilar from Earth in terms of their radius may be found with strikingly different densities in the same system, and at no great distance from each other.

Which takes us to a new paper from Aldo S. Bonomo and Mario Damasso (Istituto Nazionale Di Astrofisica), working with an international team including astrophysicist Li Zeng (CfA). The collaboration has produced a new paper in Nature Astronomy that uses the planetary system around the star Kepler-107 to probe another possible formative influence: planetary collisions. Kepler-107 may be flagging a process that occurs in many young systems.

Image: The figure shows one frame from the middle of a hydrodynamical simulation of a high-speed head-on collision between two 10 Earth-mass planets. The temperature range of the material is represented by four colors grey, orange, yellow and red, where grey is the coolest and red is the hottest. Such collisions eject a large amount of the silicate mantle material leaving a high-iron content, high-density remnant planet similar to the observed characteristics of Kepler-107c. Credit: Zoe Leinhardt and Thomas Denman, University of Bristol.

Let’s take a deeper look at this curious system. The two innermost planets at Kepler-107 have radii that are nearly identical — 1.536 and 1.597 Earth-radii, respectively. They both orbit close to the host, a G2 star in Cygnus of about 1.25 solar masses, with orbital periods of 3.18 and 4.90 days. The scientists used the HARPS-N spectrograph at the Telescopio Nazionale Galileo in La Palma to determine the planets’ masses, and because they were working with known radii (thanks to Kepler’s observations of these transiting worlds), they were able to determine their densities. And now things get interesting.

For the innermost planet shows a density of 5.3 grams per cubic centimeter, while the second world comes in at 12.65 grams per cubic centimeter. The inner world, Kepler-107b, is thus about the same density as the Earth (5.5 grams per cubic centimeter), while Kepler-107c shows a much higher number (for comparison, water’s density is 1 gram per cubic centimeter).

The outer of the two is the denser world and by more than twice the inner world’s value, and given the proximity of their orbits, coming up with stellar radiation effects that could have caused mass loss is difficult to do, for such radiation should have affected both in the same way. The remaining strong possibility is that a collision between planets played a role in this system.

“This is one out of many interesting exoplanet systems that the Kepler space telescope has discovered and characterized,” says Li Zeng. “This discovery has confirmed earlier theoretical work suggesting that giant impact between planets has played a role during planet formation. The TESS mission is expected to find more of such examples.”

And this from the paper, which notes the possibility of extreme X-ray and ultraviolet flux in the young system, but dismisses it as operational here, at least to explain the discrepancy:

This imbalance cannot be explained by the stellar XUV irradiation, which would conversely make the more-irradiated and less-massive planet Kepler-107b denser than Kepler-107c. Instead, the dissimilar densities are consistent with a giant impact event on Kepler-107c that would have stripped off part of its silicate mantle.

Image: The video shows a hydrodynamical simulation of a high-speed head-on collision between two 10 Earth-mass planets. The temperature range of the material is represented by four colors grey, orange, yellow and red, where grey is the coolest and red is the hottest. Watch Video. Credit: Zoe Leinhardt and Thomas Denman, University of Bristol.

We can produce Kepler-107c, then, by a collision that results in a high-density remnant. We have apparent evidence for collisions even in our own Solar System. The composition of Mercury, with a dense metallic core and thin crust, may point to this; only Earth is more dense in our system. So too could the emergence of Earth’s moon through a planetary-sized impactor striking our planet. We can also see in the obliquity of Uranus — the planet’s axis of rotation is skewed by 98 degrees from what we would expect — the possibility that, as advanced in some theories, a large object struck the planet long ago.

The paper gives us some of the details about Kepler 107:

…the difference in density of the two inner planets can be explained by a giant impact on Kepler-107 c that removed part of its mantle, significantly reducing its fraction of silicates with respect to an Earth-like composition. The radius and mass of Kepler-107 c, indeed, lie on the empirically derived collisional mantle stripping curve for differentiated rocky/iron planets… Smoothed particle hydrodynamics simulations show that a head-on high-speed giant impact between two ~10M⊕ exoplanets in the disruption regime would result in a planet-like Kepler-107 c with approximately the same mass and interior composition… Such an impact may destabilize the current resonant configuration of Kepler-107 and thus it likely occurred before the system reached resonance. Multiple less-energetic collisions may also lead to a similar outcome.

Another possibility discussed by the authors: Planet c may have formed closer to the parent star and later crossed Kepler-107b in its orbit. But the authors note that to dampen the orbital eccentricities this would have produced, this scenario is unlikely to have had time to operate.

The paper is Bonomo & Zeng, “A giant impact as the likely origin of different twins in the Kepler-107 exoplanet system,” Nature Astronomy 04 February 2019 (abstract).