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New Planet Detected in Circumbinary System

The transit method has proven invaluable for exoplanet detection, as the runaway success of the Kepler/K2 mission demonstrates. But stars where planets have been detected with this method are still capable of revealing further secrets. Consider Kepler-47. Here we have a circumbinary system some 3340 light years away in the direction of the constellation Cygnus, and as we are now learning about circumbinaries — planets that orbit two stars — the alignment of the orbital plane of the planet is likely to change with time.

Let’s pause for a moment on the value of the detection method. Transits detected in the lightcurve have helped us identify 10 transiting circumbinary planets, with the benefit of allowing astronomers to measure the planets’ radius even as variations in the duration of transits and deviations from the expected timing of the transits establish the circumbinary orbit.

At Kepler-47, we’re looking at the only known multi-planet circumbinary system. Moreover, the orbital period of the binary stars is in the range of 7.5 days, making this the shortest known orbital period for any known circumbinary system. The first two Kepler planets were detected in 2012, but San Diego State University astronomers now find a middle planet between these, Kepler-47d, its strengthening transit signal the result of orbital plane adjustment. In fact, the transit depth for the hitherto undetectable world has become the deepest of the three planets.

Jerome Orosz (SDSU) is the paper’s lead author.

“We saw a hint of a third planet back in 2012, but with only one transit we needed more data to be sure. With an additional transit, the planet’s orbital period could be determined, and we were then able to uncover more transits that were hidden in the noise in the earlier data.”

And according to co-author and SDSU colleague William Welsh, this planet defied expectations by showing up not exterior to the previously known planets but between them. “We certainly didn’t expect it to be the largest planet in the system. This was almost shocking,” said Welsh.

Image: Artist’s impression of the third planet in the Kepler-47 circumbinary system. Credit: NASA/JPL Caltech/T. Pyle.

So what we see at Kepler-47, at least at this juncture, is an inner planet 3.1 times the size of Earth, in an orbit taking 49 days around this G-class star orbiting an M-dwarf. The other planets here are, respectively, 7 times Earth-size on an 187-day orbit (Kepler-47d), and 4.7 Earth-size with a 303 day orbit. Remember that we are talking about planets orbiting two stars, in a system compact enough to fit inside the Earth’s orbit of the Sun.

Kepler-47’s system may be telling us something interesting about planet formation. From the paper:

This is the first detection of a dynamically packed region in a circumbinary system, and it further confirms suspicions that planet formation and subsequent migration can proceed much like that around a single star, at least when far from the binary (Pierens & Nelson 2008, 2013; Kley & Haghighipour 2014, 2015). We also find that, although they are close to having integer commensurate periods, the middle and outer planets are not in a mean-motion resonance–and yet they are gravitationally interacting and exchanging angular momentum, as indicated by their anti-phased oscillations in inclination and eccentricity.

The authors find the planetary configuration dynamically stable for at least 100 million years, adding:

This nearly circular, co-planar, packed configuration is unlikely to have arisen as an outcome of strong gravitational scattering of the planets into their current orbits. Rather, the observations suggest that the planetary configuration is the result of relatively gentle migration in a circumbinary protoplanetary disk.

Image: This is Figure 28 from the paper. Caption: The conservative (dark green) and optimistic (light green) habitable zone regions are shown for the Kepler-47 system. The red circle shows the critical stability radius (Holman & Wiegert 1999), interior to which planetary orbits are most likely unstable. Credit: Jerome Orosz/William Welsh et al.

On the matter of habitability, there is little reason to expect life here. These are low density worlds, all three being less dense than Saturn, which implies substantial hydrogen and helium atmospheres. The outer planet receives an average insolation from its two stars that is 86.5 percent of what the Earth receives. But while that puts this world within the boundaries of the circumbinary habitable zone, the density implies a world somewhere in the range between Neptune and Saturn. The newly discovered middle planet skirts the inner edge of the habitable zone, but again its density makes life unlikely.

The paper is Orosz et al., “Discovery of a Third Transiting Planet in the Kepler-47 Circumbinary System,” Astronomical Journal Vol. 157, No. 5 (16 April 2019). Abstract / Preprint.



Huge White Light Flare on a Tiny Star

About 250 light years away there is a faint object that is on the borderline between brown dwarf and star. Only a tenth of the radius of our Sun, ULAS J224940.13-011236.9 was actually too faint for most telescopes to observe until a huge flare lit it up, turning this L dwarf, among the lowest mass objects that can still be considered a star, 10,000 times brighter than it was before. Very cool compared to the average red dwarf, L dwarfs emit radiation primarily in the infrared.

But this story also has to do with visible light, and the question of how such a small object can produce such a powerful explosion. This was a ‘white light’ flare, a type of flare that displays associated brightening in the visible light spectrum. Astronomers believe flares are driven by magnetic energy, the sudden release of which can cause charged particles to heat plasma. In this case the resulting optical, ultraviolet and X-ray radiation was copious.

James Jackman, a PhD student in physics at the University of Warwick (UK) and lead author of the paper on these observations, points to the rare nature of this flare:

“The activity of low mass stars decreases as you go to lower and lower masses and we expect the chromosphere (a region of the star which support flares) to get cooler or weaker. The fact that we’ve observed this incredibly low mass star, where the chromosphere should be almost at its weakest, but we have a white-light flare occurring, shows that strong magnetic activity can still persist down to this level.”

Image: A superflare on an L-dwarf. Credit & Copyright: University of Warwick/Mark Garlick.

And note this from the paper on this work, on the unusual strength of some L dwarf flares:

While seen regularly on GKM stars, observations of white-light flares on L dwarfs remain rare, with only a handful of stars showing them to date (e.g. Paudel et al. 2018). However, those observed have included some of the largest amplitude flares ever recorded, reaching up to V ≈ −11 (Schmidt et al. 2016). This shows that white-light flaring activity persists into the L spectral type, despite previous studies of L dwarfs showing their chromospheres and magnetic activity to be diminished compared to those of late M dwarfs.

So a borderline brown dwarf/star is giving us an interesting lesson, perhaps on the difference between the two, because we may be able to determine whether flares like these are limited to actual stars, learning at just what point the activity ceases. Are there other tiny stars, like ULAS J224940.13-011236.9 about the same size as Jupiter, that mark the limits of such flares, below which none occur? Whatever the case, few L dwarfs have produced a flare of this magnitude.

Embedded in future work will surely be the question of how tiny stars like this one store energy in magnetic fields, and their level of chromospheric activity. From the paper:

Ultracool dwarfs are also known to exhibit auroral activity…, which may account for observed Hα [hydrogen alpha] emission [when a hydrogen electron falls from its third to second lowest energy level], in these systems. It is expected that the transition from predominantly chromospheric to auroral Hα emission occurs during the L spectral type… Many ultracool dwarfs that show activity such as radio emission and flaring also tend to be fast rotators, with rotation periods on the order of hours. However, neither the L1 dwarf SDSSp J005406.55−003101.8 (Gizis et al. 2017a) nor the L0 dwarf J12321827−0951502 (Paudel et al. 2018) showed any sign of rapid rotation when observed by K2, despite showing large amplitude white-light flares. Consequently, we do not attempt to predict whether ULAS J2249−0112 is a fast rotator. Regardless of this, studies of white-light flares such as from ULAS J2249−0112 can aid in understanding exactly how far into the L spectral type chromospheric activity persists.

The J224940.13-011236.9 data come from the Next Generation Transit Survey (NGTS) facility at the European Southern Observatory’s Paranal Observatory, with further data from the Two Micron All Sky Survey (2MASS) and Wide-field Infrared Survey Explorer (WISE), a total observation period of 146 days. The flare occurred on 13 August 2017, with an energy equivalent of 80 billion megatonnes of TNT, surpassing the largest flare (the Carrington event of 1859) ever observed on the Sun.

The paper is Jackman et al., “Detection of a giant white-light flare on an L2.5 dwarf with the Next Generation Transit Survey,” Monthly Notices of the Royal Astronomical Society: Letters Vol. 485, Issue 1 (May 2019), L136-L140 (abstract).



Chinese Mission to an Earth Co-Orbital

This morning’s entry resonates with Jim Benford’s recent work on objects that are co-orbital with Earth (see A SETI Search of Earth’s Co-Orbitals). You’ll recall that Benford argues for close study of co-orbitals like Cruithne (3753), a 5-kilometer object with closest approach to Earth of 0.080 AU, and 2010 TK7, which oscillates around the Sun-Earth Lagrangian point L4. A number of other such objects are known in a 1:1 orbital resonance with Earth, but they are seldom studied or even mentioned in the literature.

Calling for SETI observations at radio and optical wavelengths, as well as lighting up the objects with planetary radar, Benford gives a nod to Ronald Bracewell, who speculated that one way for an extraterrestrial intelligence to study a stellar system would be to plant a probe within it that could inform the home civilization about events there. The Earth co-orbitals are made to order for such observation, so why not give them a look with all the tools in our SETI arsenal?

Now we learn that China plans to explore the near-Earth asteroid 2016 HO3, along with a main-belt comet designated 133P. An interesting move — 2016 HO3 is the closest, most stable quasi-satellite of Earth, with a minimum distance of 0.0348 AU. Also known as Kamoʻoalewa — a Hawaiian word for an oscillating object in the sky — 2016 HO3 has a minimum orbital intersection distance of 0.0348 AU (5,210,000) km, which is 13.6 times as far away as the Moon, although it seldom comes closer than about 38 lunar distances from us. The Center for Near Earth Object Studies (CNEOS) calculates this one has been in a stable orbit of our planet for about a century and will remain in its orbital pattern for centuries.

Image: Orbit of 2016 HO3. Credit: James Benford.

According to Liu Jizhong, director of the Lunar Exploration and Space Program Center of the China National Space Administration (CNSA), the current plan is to study 2016 HO3 from space before landing on it to collect samples for return to Earth. Following delivery of the sample return capsule, the probe is to proceed to comet 133P by means of gravity assists at Earth and Mars, in a mission lasting on the order of 10 years.

China is now soliciting proposals for eight types of scientific instruments for the mission among universities, research organizations and private companies both in China and abroad, according to a CNSA news release. Among the instruments needed, Liu says, are a color camera with an intermediate field of view, thermal emission spectrometer, visible and infrared imaging spectrometer, multispectral camera, detection radar, magnetometer, charged and neutral particle analyzer and dust analyzer. Quoting from the news release:

[Liu] said there might be two forms of onboard schemes. One possible scheme is to carry an independent detector on the rocket. After China’s main probe enters the orbit, the onboard detector will separate from the rocket and then perform independent tasks. Its mass should not exceed 200 kg. The other possible option is to let China’s main probe carry the onboard detector to the near-Earth asteroid or the main-belt comet and then release it. The detector could either perform independent scientific exploration or coordinate with the main probe.

If the onboard detector does not separate with the main probe, its mass should not exceed 20 kg. If the detector separates from the main probe near the asteroid, its mass should be no more than 80 kg. If it separates from the main probe near the comet, its mass should not exceed 20 kg.

The deadline for proposals is August 31, 2019, with those interested asked to contact CNSA.

Image: An animation of 2016 HO3’s orbit around Earth 2000-2300. Credit: Phoenix7777 – Own work. Data source: HORIZONS System, JPL, NASA. CC BY-SA 4.0.

This will not be China’s first experience with an asteroid mission. In December of 2012, its second lunar probe, Chang’e-2, made a close approach and flyby of asteroid 4179 Toutatis after completing its primary mission, approaching to within 3.2 kilometers and returning images. Now we have an ambitious mission to give us a close-up look at an Earth co-orbital, with comet operations to follow. We should learn a lot, for right now even the size of 2016 HO3 is not firmly established, though it is believed to be between 40 and 100 meters, depending on assumptions about its albedo, and we do know that it is a fast rotator.



TRAPPIST-1: Of Flux and Tides

Seven planets of roughly Earth-size make TRAPPIST-1 a continuing speculative delight, as witness the colorful art it generates below. And with three of the planets arguably in the star’s habitable zone, this diminutive star attracts the attention of astrobiologists anxious to examine the possible parameters under which they orbit. One thing that is only now receiving attention is the question of planet-to-planet tidal effects, as opposed to the star’s tidal effects on its planets.

Image: An artist’s impression of the perpetual sunrise that might greet visitors on the surface of planet TRAPPIST-1f. If the planet is tidally locked, the “terminator region” dividing the night side and day side of the planet could be a place where life might take hold, even if the day side is bombarded by energetic protons. In this image, TRAPPIST-1e can be seen as a crescent in the upper left of the image, d is the middle crescent, and c is a bright dot next to the star. Credit: NASA/JPL-Caltech.

In our Solar System, we’ve become familiar with the idea that tidal deformation can cause interior heating, a fact that could well support both Europa at Jupiter and Enceladus at Saturn with energy needed to retain temperatures suitable for life below their icy surfaces. The effects are extreme at Io (though hardly life-inducing!) and also noteworthy on Neptune’s large moon Triton. Here again TRAPPIST-1 stands out, because we know of no other system where planets, not moons, are so tightly wound that they can raise significant tides on each other.

Consider TRAPPIST-1g, the sixth planet in the system, which according to a study performed by Hamish Hay and Isamu Matsuyama (Lunar and Planetary Laboratory, University of Arizona) experiences the mixed effects of tidal heating from the central star and the other planets more strongly than any other planet in the system.

Tides from the other planets in a planetary system are rarely seen as a factor, say the scientists, but heating due to tidal deformation is definitely in play here. From the paper:

Such tides are typically negligible because the mass of the central tide raising body is usually far greater than other bodies in the system, and also because the distances between these bodies are vast and the strength of tidal forces decreases with the distance between them cubed. The seven planet extrasolar system TRAPPIST-1…is the first system to be discovered where this is not the case. The separation distance at conjunction is small enough that tides raised by neighbouring planets can become significant, and heating must occur as a result.

Similarly, TRAPPIST-1’s two inner planets come close enough to raise powerful tides on each other, possibly sustaining volcanic activity on worlds that would be too hot on the day side to support life. An atmosphere maintained by volcanic eruptions could move heat to the night side, assuming tidal lock.

Image: An artist’s concept for a view of the TRAPPIST-1 system from near TRAPPIST-1f. The system is located in the constellation Aquarius and is just under 40 light-years away from Earth. Credit: NASA/JPL-Caltech.

The Trouble with TRAPPIST-1e

We’ve also recently looked at Lisa Kaltenegger’s work on the effect of intense radiation on M-dwarf planets (see M-Dwarfs: Weighing UV Radiation and Habitability). Kaltenegger (Cornell University/Carl Sagan Institute) has been investigating possible ways for life to survive the intense flares and ultraviolet radiation that pummel such worlds. Various mechanisms suggest themselves, enough to keep open the possibility that planets like these could sustain life.

What Federico Fraschetti (Harvard Smithsonian Center for Astrophysics) and colleagues have been studying is the ability of a star so much cooler and less massive than the Sun to emit such quantities of radiation. The scientists have simulated the path of high-energy protons through the magnetic field of the star, finding that the first of the three TRAPPIST-1 planets thought to be in the habitable zone (TRAPPIST-1e) is receiving up to 1 million times more flux than Earth.

We’re fortunate, of course, in being protected by our planet’s magnetic field from our star’s energetic proton bath, but Fraschetti’s calculations show that to have the same effect at TRAPPIST-1e, the planet’s magnetic field would need to be hundreds of times more powerful than Earth’s. The conclusion is based on the star’s most likely field alignment, which brings its energetic protons directly to the surface of TRAPPIST-1e, where damaging biological effects could occur. But much depends upon how the star’s magnetic field is angled away from its axis of rotation, making this a key datapoint for future investigations. From the paper:

Based on the scaling relation between far-UV emission and energetic protons for solar flares by Youngblood et al. (2017), we estimate that the innermost putative habitable planet, TRAPPIST-1e, is bombarded by a proton flux up to 6 orders of magnitude larger than experienced by the present-day Earth. Such a bombardment of planets in this study is found to result largely from the misalignment of the B-field/rotation axis assumed for the star-proxy. Since the exact magnetic morphology and alignment of the magnetic field is currently unknown for TRAPPIST-1, and for M dwarfs in general, our results indicate that determination of these quantities for exoplanet hosts would be of considerable value for understanding their radiation environments.

TRAPPIST-1e, then, may need some of Lisa Kaltenegger’s proposed solutions to the radiation flux problem if it is to be considered habitable. Lithophilic life, or perhaps life beneath an ocean, is one solution among those that Kaltenegger has proposed, and of course there is the possibility of tidal lock, which could keep the ‘dark’ side of the planet free of the flux. Habitability, as we continue to learn, is by no means an easy call, no matter where a planet is located within or without the putative habitable zone of its host.

The papers are Fraschetti et al., “Stellar Energetic Particles in the Magnetically Turbulent Habitable Zones of TRAPPIST-1-like Planetary Systems,” Astrophysical Journal Vol. 874, No. 1 (18 March 2019) (abstract / preprint); and Hay & Matsuyama, “Tides Between the TRAPPIST-1 Planets,” Astrophysical Journal Vol. 875, No. 1 (9 April 2019) (abstract / preprint).



Detection of an Interstellar Meteor

Do we have a second interstellar visitor, following on the heels of the controversial ‘Oumuamua? If so, the new object is of a much different nature, as was its detection. In 2014, a meteor north of Manus Island, off the coast of Papua New Guinea produced a powerful blast that, upon analysis, implied a ∼ 0.45m meter object massing about 500 kg. Events like this, not uncommon in our skies, are cataloged by the Center for Near Earth Object Studies (CNEOS); this one shows up as being detected at 2014-01-08 17:05:34 UTC.

Image: This gorgeous wide-angle photo from the 1997 Perseid shower captures a 20-degree-long fireball meteor and another, fainter meteor trail in a rich area of the northern summer Milky Way. Showers like these are predictable, but could some solitary fireballs mark the end of a meteor with an interstellar origin? Credit & Copyright: Rick Scott & Joe Orman.

Now the CNEOS catalog, which covers the last three decades, is useful indeed, for it takes advantage of detectors maintained by the U.S. government to analyze the sound and light of the passage of objects through the atmosphere, producing information on velocity and position at the time of impact. Harvard’s Avi Loeb, a familiar face in the media thanks to the ‘Oumuamua discussion, worked with undergraduate student Amir Siraj, whom he set to calculating. What could we learn about the prior trajectory of meteors in the catalog, homing in on the fastest?

In a paper submitted to Astrophysical Journal Letters, Loeb and Siraj note the latter’s identification of the 2014 Manus Island meteor as interstellar in origin. The paper finds no substantial gravitational interactions between the meteor and any planet other than Earth. Indeed, based on the CNEOS-reported impact speed of 44.8 km s-1, Loeb and Siraj calculate a speed of 43.8 km s-1 outside the Solar System. For the object to be bound, the observed speed at impact would have to be off by more than 45%.

This meteor, then, was on an unbound hyperbolic orbit. We can go on from here to note the object’s relation to another useful metric. For measured relative to the Local Standard of Rest, this meteor entered the Solar System with a speed of 60 kilometers per second.

The Local Standard of Rest (LSR) is produced by averaging the motion of all stars in the Sun’s neighborhood. Siraj and Loeb speculate that this velocity could indicate ejection from a planetary system, specifically from the inner regions where orbital speeds are high. The object’s speed would imply a position inside the orbit of Mercury were it to come from a star like our own, but a red dwarf like Proxima Centauri would have an ejection speed from its habitable zone in this very regime. Recall that the habitable zone around Proxima Centauri is 20 times closer to the star than the HZ in our own system. So here’s an interesting thought: “Since dwarf stars are most common, the detection of this meteor offers new prospects for ‘interstellar panspermia,’ namely the transfer of life between planets that reside in the habitable zones of different stars.”

What I’m quoting from above is an as yet unpublished summation Loeb has recently written of the paper’s findings, one that goes on to speculate about its implications. Panspermia would require a larger object because it would have to survive the fiery passage through the atmosphere, but the notion that objects could be passed from star to star in this way is interesting (and note that Loeb is not identifying a Proxima Centauri origin for this meteor, but rather pointing to possible scenarios between stars). The point is that dwarf stars are the most common in the universe, and the detection of an interstellar meteor could point to what is perhaps a common form of transfer between stars.

Beyond that, consider the possibilities in studying interstellar materials when we may find them entering our own atmosphere. Says Loeb:

Using the Earth’s atmosphere as a detector for interstellar objects offers new prospects for inferring the composition of the gases they leave behind as they burn up in the atmosphere. In the future, Astronomers may establish an alert system that triggers follow-up spectroscopic observations to an impact by a meteor of possible interstellar origin. Alert systems already exist for gravitational wave sources, gamma-ray bursts, or fast radio bursts at the edge of the Universe. Even though interstellar meteors reflect the very local Universe, they constitute a “message in a bottle” with fascinating new information about nurseries which may be very different from the Solar System. Some of them might even represent defunct technological equipment from alien civilizations, which drifted towards Earth by chance, just like a plastic bottle swept ashore on the background of natural seashells.

Thus spectroscopy of gaseous debris burning up in the Earth’s atmosphere could offer us a way to make interstellar investigations of the kind we’ve been assuming would be decades (at least) off, assuming we can make a timely identification of likely targets.

The paper is Siraj & Loeb, “Discovery of a Meteor of Interstellar Origin,” submitted to Astrophysical Journal Letters (preprint).



Going Interstellar in Europe

Foundations of Interstellar Studies Workshop in UK

A workshop on interstellar flight titled Foundations of Interstellar Studies is to take place from 27 to 30 June of this year in the town of Charfield, Gloucestershire, United Kingdom, at the current headquarters of the Initiative for Interstellar Studies. This follows an initial ‘foundations’ conference in 2017 that was held at City College New York and the Harvard Club of New York; future conferences, “run jointly between several organisations depending on the host country,” are planned on a roughly two-year schedule. I immediately warmed to the theme that the Initiative for Interstellar Studies (i4IS) introduced by quoting Robert H. Goddard:

How many more years I shall be able to work on the problem I do not know; I hope, as long as I live. There can be no thought of finishing, for ‘aiming at the stars’ both literally and figuratively, is a problem to occupy generations, so that no matter how much progress one makes, there is always the thrill of just beginning.

Browsing through the conference materials I note that, with reference to famous physics conferences like Shelter Island, Pocono and Oldstone, the emphasis is on both academic rigor but also informal conversation, a format that i4IS president Kelvin Long hopes will energize the interstellar community. The aim is “to get researchers together and to maximize the social interaction time for idea swapping and information exchange and it is expected that the ideas and discussions (and maybe even calculations) should continue into the evening social sessions.”

The three days of discussions in this year’s conference will take place at the Bone Mill, which has been the i4IS headquarters since 2017. This is beautiful country, as those of you who have been to the Cotswolds will already know, in the village of Charfield, near Wootton-under-Edge, in the English county of Gloucestershire. The three themes under focus, each with a day devoted to it:

  • Living in Deep Space
  • Advanced Propulsion Technology & Missions
  • Building Architectural Megastructures

In addition to the formal scientific proceedings, there will be an opening social event on the evening of Thursday 27th June, starting at 18:00 hours at the Bone Mill. There will also be a formal dinner on Saturday 29th June starting at 19:00 hours at a venue to be announced.

An invitation will be made to submit papers from selected authors post-conference, to the Journal of the British Interplanetary Society (JBIS) and/or publication in the official conference proceedings. For more on the Foundations of Interstellar Studies Workshop 2019, including maps and information on accommodation, go to https://www.fisw.space/fisw-2019.

Horizon 2061 Synthesis Workshop in Toulouse

2061 will commemorate an interesting year in space exploration. It is the centennial not just of the first human flight into space by Yuri Gagarin but also of the speech by which John Kennedy propelled the US aerospace community into a determined drive for a lunar landing. But we might also add another memorable factor. In 2061, Comet Halley makes its return. The last time we saw Halley was in 1986, when five spacecraft ranging in origin from the European Space Agency to the Soviet Union and France as well as Japan studied the comet in the inner system.

Thus we had the first comet observed in detail by spacecraft, giving us information about the cometary nucleus, the coma and the tail, helping us understand cometary structure. The fact that the Halley expeditions were so determinedly multinational (although the studied US solar sail never materialized) gives impetus to an effort called Planetary Exploration Horizon 2061 which, according to its founders is creating a long-term analysis of four primary areas of space exploration, all of these addressed from a determinedly international perspective.

From the Horizon 2061 website:

By 2061, all the “frontiers” (or outer boundaries) of exploration should have moved dramatically outwards: human exploration might have reached Mars and perhaps the main asteroid belt; sample return missions should have reached, beyond the asteroid belt, the Trojan asteroids on the orbit of Jupiter and the icy moons of Jupiter and Saturn; robotic exploration should have reached the very local interstellar medium, well beyond the outer shock of the heliosphere, thus opening the way towards the closest stars and their planetary systems.

Thus the “the four pillars of planetary exploration” Horizon 2061 is examining:

  • Major scientific questions on planetary systems;
  • Representative space missions that answering these questions will demand;
  • Enabling technologies needed to make these missions happen;
  • Ground- and space-based infrastructure needed for mission support.

The overall goal:

[The year 2061] symbolically represents our intention to encompass both robotic and human exploration in the same perspective. Its distant horizon, located well beyond the usual horizons of the planning exercises of space agencies and of their standing committees, which generally address shorter time scales, avoids any possible confusion with them and is intended to trigger a joint foresight thinking of the scientific and technology communities of planetary exploration that will free the imagination of the planetary scientists, who are invited to formulate what they think are the most relevant and important scientific questions independently of the a priori technical feasibility of answering them; of the engineers and technology experts, who are invited to explore innovative technical solutions that will make it possible to fly by 2061 the challenging space missions that will allow us to address these questions.

Space missions are designed, the Horizon 2061 proponents note, around a Science Traceability Matrix (STM) in which mission science questions and objectives define the instruments needed, the mission profile and the kind of platform on which the mission will be flown. Unlike single missions, though, Horizon 2061 intends to write the STMs for a set of representative missions that will investigate everything from the origin of planetary systems to the detection of life. Observations to be made and destinations within the Solar System where such measurements can be performed will determine the type of space missions that emerge from this matrix.

Two meetings have already occurred, the first in Bern in September of 2016, the second in Lausanne in April of 2018. Coming now is the next step, devoted to the synthesis of the exercise. This will take place in an international colloquium hosted by the Université Paul Sabatier in Toulouse from June 5th to 7th, 2019.

The primary organizers will be the Institut de Recherche en Astrophysique et Planétologie (IRAP) and the Observatoire Midi-Pyrénées (OMP). This colloquium, placed under the sponsorship of COSPAR [Committee on Space Research, established by the International Council for Science in 1958], will complete the design of the four pillars and initiate the drafting of the final report, which will be edited and published under the auspices of COSPAR.

Tentative conclusions from the colloquium will be presented for discussion at the joint EPSC-DPS meeting (European Planetary Science Conference – AAS Division for Planetary Sciences) in Geneva (September 15th to 20th, 2019), and later for discussion and final approval at the COSPAR General Assembly (Sydney, August 15th to 23rd, 2020).

Meeting agenda, registration and other materials are available at https://h2061-tlse.sciencesconf.org/.



Proxima Centauri c?

A possible second planet around Proxima Centauri raises all kind of questions. I wasn’t able to make it to Breakthrough Discuss this year, but I’ve gone over the presentation made by Mario Damasso of Turin Observatory and Fabio Del Sordo of the University of Crete, recounting their excellent radial velocity analysis of the star. Proxima c is a fascinating world, if it’s there, because it would be a super-Earth in a distant (and cold) 1.5 AU orbit of a dim red star. Exactly how it formed and whether it migrated to its current position could occupy us for a long time.

But is it there? The first difficulty has to do with stellar activity, which Damasso and Del Sordo were careful to screen out; it’s one of the major problem areas for radial velocity work in this kind of environment, for red dwarf stars are often quite active. During the question and answer session, another key question emerged: We know from Kepler that many stars are orbited by multiple planets, and there is no reason to assume that Proxima Centauri has but one.

The question: If there are other, smaller worlds in play here, could the effect of their combined masses produce a ‘phantom’ Proxima c in the orbit Damasso and Del Sordo have discussed?

The two astronomers are completely open to this possibility, and point to the need for follow-up observations with ESPRESSO, not to mention the useful Gaia measurements that could give us even more detail. Flare activity is always an issue in any case — it may have affected the results of Anglada et al. in 2018 (citation below), when researchers found possibly two inner dust belts and one outer belt around the star (see Proxima Centauri Dust Indicates a Complicated System). The Damasso and Del Sordo work is comprehensive as far as it can go, but both were careful to note that we are dealing solely with a candidate, not a confirmed world. And it could well be the result of other, unseen planets affecting the star as well as stellar noise.

This work draws on the earlier Proxima Centauri radial velocity dataset compiled by Guillem Anglada-Escudé (University of London) and team, but folds in an additional 61 RV observations, with considerable attention to the question of filtering out the 85 day rotation period of the parent star and the associated noise of stellar surface perturbations. The instrument in play is the European Southern Observatory’s High Accuracy Radial Velocity Planet Searcher (HARPS) spectrograph at La Silla.

I suspect we’re going to find a number of small worlds around Proxima Centauri, so we’ll see how their gravitational interactions might affect the spectroscopic data and hence the confirmation of the current candidate. But if this detection is confirmed, this is what we’ve found: The planet would mass about six Earths — remember that because this is radial velocity, we can only measure a minimum mass, because we don’t know planetary inclination — and would orbit Proxima Centauri with a period of 1900 days at 1.5 AU. Not exactly a habitable place for the likes of our species. Del Sordo estimates temperatures there would be about 40 K.

We may know, via Gaia, whether Proxima Centauri c is an actual world by the end of this year. A key follow up question is, can we snag a direct image in visible light? If so, it would mark the first such detection of a planet outside our Solar System, the imaged worlds found thus far having been discovered via infrared. There is plenty, in other words, to like about the hypothetical Proxima Centauri c, provided it’s really there. Waiting a few more months could give us a firm answer.

On another matter, as a great admirer of Thoreau, I was pleased that Damasso and Del Sordo quoted him at the beginning of their presentation, and to good effect: “If you have built castles in the air, your work need not be lost; that is where they should be. Now put the foundations under them.” That’s a good metaphor for RV studies as exceedingly delicate as these. I’ll add a favorite bit from one of Thoreau’s poems:

For lore that’s deep must deeply studied be,
As from deep wells men read star-poetry…

There’s poetry indeed in the spectroscopic data of our nearest star, if we can just tease out its meaning. And here’s an image that might evoke a bit of poetry to close today’s entry.

Image: Rigil Kentaurus is the bright star near the top of this broad southern skyscape. Of course it’s probably better known as Alpha Centauri, nearest star system to the Sun. Below it sprawls a dark nebula complex. The obscuring interstellar dust clouds include Sandqvist catalog clouds 169 and 172 in silhouette against the rich starfields along the southern Milky Way. Rigil Kent is a mere 4.37 light-years away, but the dusty dark nebulae lie at the edge of the starforming Circinus-West molecular cloud about 2,500 light-years distant. The wide-field of view spans over 12 degrees (24 full moons) across southern skies. Credit & Copyright: Roberto Colombari.

The paper on dust belts around Proxima Centauri is from Guillem Anglada, “ALMA Discovery of Dust Belts Around Proxima Centauri,” Astrophysical Journal Letters Vol. 850 No. 1 (15 November 2017) (abstract). (Note: This is not Guillem Anglada-Escudé, despite the similarity in names!) The Damasso and Del Sordo paper is as yet unpublished, though undergoing peer review. Video of their presentation is available at https://www.youtube.com/watch?v=DLzzg9p0-AI&t=15648s (go to about 4:16:45 on the video).



Reflections on Messier 87’s Black Hole

Messier 87, a massive elliptical galaxy in the Virgo cluster, is some 55 million light years from Earth, and even though the black hole at its center has a mass 6.5 billion times that of the Sun, it’s a relatively small object, about the size of our Solar System. Resolving an image of that black hole is, says the University of Arizona’s Dimitrios Psaltis, like “taking a picture of a doughnut placed on the surface of the moon.” But the M87 black hole is one of the largest we could see from Earth, making it a natural target for observations, in this case using radio telescopes working at a frequency of 230 GHz, corresponding to a wavelength of 1.3mm.

A decade ago, working with Avery Broderick, Harvard’s Avi Loeb highlighted the advantages of M87 as an observational target, finding it in many ways preferable to the black hole at the heart of our own Milky Way:

M87 provides a promising second target for the emerging millimeter and submillimeter VLBI capability. Its presence in the Northern sky simplifies its observation and results in better baseline coverage than available for Sgr A*. In addition, its large black hole mass, and correspondingly long dynamical timescale, makes possible the use of Earth aperture synthesis, even during periods of substantial variability.

That paper, “Imaging the Black Hole Silhouette of M87: Implications for Jet Formation and Black Hole Spin,” is worth revisiting (abstract), for those intrigued with how these observations get made and the kinds of things we can learn from them.

I was reminded, when I first saw the now famous image, of the nature of M87 itself. Elliptical galaxies, unlike our barred spiral Milky Way, show slow rates of star formation, their primary population being older stars, and as you would imagine, they contain little gas and dust, while also housing a large number of globular clusters. Back in 2012, I ran across a paper by Falguni Suthar and Christopher McKay (NASA Ames) assessing habitability in such galaxies. What an environment to set a science fiction story! Consider the image below before we cut to the black hole image that is now center stage in the news, because here’s the context:

Image: A composite of visible (or optical), radio, and X-ray data of the giant elliptical galaxy, M87. M87 lies at a distance of 55 million light years and is the largest galaxy in the Virgo cluster of galaxies. Bright jets moving at close to the speed of light are seen at all wavelengths coming from the massive black hole at the center of the galaxy. It has also been identified with the strong radio source, Virgo A, and is a powerful source of X-rays as it resides near the center of a hot, X-ray emitting cloud that extends over much of the Virgo cluster. The extended radio emission consists of plumes of fast-moving gas from the jets rising into the X-ray emitting cluster medium. Credit: X-ray: NASA/CXC/CfA/W. Forman et al.; Radio: NRAO/AUI/NSF/W. Cotton; Optical: NASA/ESA/Hubble Heritage Team (STScI/AURA), and R. Gendler.

Could life survive in environments like this? I bring this up again as background, but also because yesterday we looked at the question of hardy microorganisms and their ability to withstand high levels of X-ray and UV radiation. Here’s what McKay and Suthar said in 2012:

Complex life forms are sensitive to ionizing radiation and changes in atmospheric chemistry that might result. However, microbial life forms, e.g. Deinococcus radiodurans, can withstand high doses of radiation and are more flexible in terms of atmospheric composition. Furthermore, microbial life in subsurface environments would be effectively shielded from space radiation. Thus, while a high level of radiation from nearby supernovae may be inimical to complex life, it would not extinguish microbial life.

It’s fascinating to me that we’ve begun studying such questions on a galactic scale. Fascinating too that we’re now peering into the heart of an active galaxy to reveal its powerhouse black hole. By now the image is familiar, but let’s see it again because it’s just extraordinary.

Image: Scientists have obtained the first image of a black hole, using Event Horizon Telescope observations of the center of the galaxy M87. The image shows a bright ring formed as light bends in the intense gravity around a black hole that is 6.5 billion times more massive than the Sun. Credit: Event Horizon Telescope Collaboration.

One thing I saw little attention given to in the coverage was that the Event Horizon Telescope, which produced the image, was supplemented by work from spacecraft. Remember that the EHT is comprised of telescopes located around the surface of our planet, to produce a planet-scale interferometer capable of making such an observation. But the Chandra X-ray spacecraft was also involved, as was the Nuclear Spectroscopic Telescope Array (NuSTAR), and the Neil Gehrels Swift Observatory. All of these, working at X-ray wavelengths, observed the M87 black hole at the same time it was under study by the EHT in April of 2017.

I point to this because while the space assets could not image the black hole, data from them were used to measure the brightness of the M87 jet, particles driven by an enormous energy boost from the black hole itself and surging away from it at nearly the speed of light. The hope here is that X-rays can help us measure particle events near the event horizon to coordinate with the black hole images. Also involved in space was the Neutron star Interior Composition Explorer (NICER), a NASA experiment on the International Space Station that looked at the center of the Milky Way and the black hole known as Sgr A*. Part of the EHT’s mandate is to study the origin of jets like this one, so these extraordinary interactions now become visible.

As to the ground-based observatories of the EHT themselves, what an accomplishment! The international team involved totalled over 200 astronomers, whose work is presented in a special issue of Astrophysical Journal Letters. In the black hole work, the EHT used an array of eight radio telescopes with worldwide coverage, from the Antarctic to Spain, Chile and Hawaii, all located in high-altitude settings where conditions are ideal for observation.

Jonathan Weintroub (CfA) coordinates the EHT’s Instrument Development Group:

“The resolution of the EHT depends on the separation between the telescopes, termed the baseline, as well as the short millimeter radio wavelengths observed. The finest resolution in the EHT comes from the longest baseline, which for M87 stretches from Hawai’i to Spain. To optimize the long baseline sensitivity, making detections possible, we developed a specialized system which adds together the signals from all available SMA dishes on Maunakea. In this mode, the SMA acts as a single EHT station.”

Spectacular. The very long baseline interferometry creates a virtual dish that is planet-sized, able to resolve an object to 20 micro-arcseconds. Working with a conjunction of four nights that would produce clear seeing for all eight observatories, the telescopes took in massive amounts of data — 5,000 trillion bytes of data in all — saved on 1,000 storage disks. Transmitting all that information for subsequent processing was ruled out, for air transport from FedEx could take the hard disks onto which the data had been recorded to a single location much faster. These are signals that needed to be aligned within trillionths of a second to achieve a valid result.

The resulting imagery is the payoff. The central dark region is surrounded by a ring of light, as Einstein’s equations led scientists to expect. We can’t, of course, see the black hole itself, but plasma emitted from its accretion disk, where matter piles up as material falls into the black hole, is heated to billions of degrees and accelerated almost to lightspeed. We get an image of the black hole’s shadow’ that is about 2.5 times larger than the event horizon. M87’s event horizon is thought to be some 25 billion miles across, making it 3 times the size of Pluto’s orbit.

“Once we were sure we had imaged the shadow, we could compare our observations to extensive computer models that include the physics of warped space, superheated matter and strong magnetic fields. Many of the features of the observed image match our theoretical understanding surprisingly well,“ said Luciano Rezzolla, professor for theoretical astrophysics at Goethe University and a researcher on the EHT. “This makes us confident about the interpretation of our observations, including our estimation of the black hole’s mass.“

Image: This artist’s impression depicts the paths of photons in the vicinity of a black hole. The gravitational bending and capture of light by the event horizon is the cause of the shadow captured by the Event Horizon Telescope. Credit: Nicolle R. Fuller/NSF.

This is a black hole massive enough that a planet orbiting it could move around it within a week while traveling, says MIT’s Geoffrey Crew, close to the speed of light. Crew’s colleague Vincent Fish, also at MIT’s Haystack Observatory, amplifies on the point:

“People tend to view the sky as something static, that things don’t change in the heavens, or if they do, it’s on timescales that are longer than a human lifetime. But what we find for M87 is, at the very fine detail we have, objects change on the timescale of days. In the future, we can perhaps produce movies of these sources. Today we’re seeing the starting frames.”

Now that’s something worth waiting for, movies of the accretion disk caught in the tortured spacetime of a galaxy’s central black hole. M87 anchors a jet stretching tens of thousands of light years, so we’re talking about seeing the dynamics of the jet’s interactions with the black hole. Fine-tuning EHT methods and expanding its sites points in the direction of further breakthrough imagery.

But what an accomplishment we’ve already achieved via instruments all over the world — ALMA and APEX in Chile, the IRAM 30 meter telescope in Spain, the James Clerk Maxwell telescope and the Submillimeter Array (both in Hawaii), the Large Millimeter Telescope (LMT) in Mexico, the Submillimeter Telescope (SMT) in Arizona and the South Pole Telescope (SPT) in Antarctica.

The papers are The Event Horizon Telescope Collaboration et al., “First M87 Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole,” Astrophysical Journal Letters Vol. 875, No. 1 (10 April 2019) (abstract); and from the same issue: “First M87 Event Horizon Telescope Results. II. Array and Instrumentation” (abstract); “First M87 Event Horizon Telescope Results. III. Data Processing and Calibration” (abstract); “First M87 Event Horizon Telescope Results. IV. Imaging the Central Supermassive Black Hole” (abstract); “First M87 Event Horizon Telescope Results. V. Physical Origin of the Asymmetric Ring” (abstract); and “First M87 Event Horizon Telescope Results. VI. The Shadow and Mass of the Central Black Hole” (abstract). The paper on M87 and galactic habitability is Suthar & McKay, “The Galactic Habitable Zone in Elliptical Galaxies,” International Journal of Astrobiology, published online 16 February 2012 (abstract).



M-Dwarfs: Weighing UV Radiation and Habitability

With 250 times more X-ray radiation than Earth receives and high levels of ultraviolet, would Proxima b, that tantalizing, Earth-sized world around the nearest star, have any chance for habitability? The answer, according to Jack O’Malley-James and Lisa Kaltenegger (Cornell University) is yes, and in fact, the duo argue that life under these conditions could deploy a number of possible strategies for dealing with the radiation influx. Their conclusions appear in a new paper in Monthly Notices of the Royal Astronomical Society.

Kaltenegger is director of Cornell’s Carl Sagan Institute, where O’Malley-James serves as a research associate. Modeling surface environments on four exoplanets that are prone to frequent flares — Proxima-b, TRAPPIST-1e, Ross-128b and LHS-1140b — Kaltenegger and O’Malley-James examined different atmospheric solutions that could suppress UV damage in living cells.

Thin atmospheres and a lack of ozone protection fail to block UV radiation well, no surprise there, and such atmospheres do not measure up favorably when compared to atmospheres like that of the Earth today. But go back four billion years and we find that the modeled planets receive radiation in the UV significantly lower than what the Earth experienced in that era of its development. Earth was at that time uninhabitable by human standards — had any humans been available — but life had indeed emerged and continued to thrive. Thus the authors write that UV radiation “…should not be a limiting factor for the habitability of planets orbiting M stars.”

Image: The intense radiation environments around nearby M stars could favor habitable worlds resembling younger versions of Earth. Credit: Jack O’Malley-James/Cornell University.

The extremophile Deinococcus radiodurans is key to this study, for it is one of the most radiation-resistant organisms known. By varying the UV wavelengths, the scientists assessed the mortality rates of the organism, in which it becomes clear that some wavelengths of UV are more damaging to biological molecules than others. From the paper:

…we use this as a benchmark against which to compare the habitability of the different radiation models. This action spectrum compares the effectiveness of different wavelengths of UV radiation at inducing a 90 per cent mortality rate. It highlights which wavelengths have the most damaging irradiation for biological molecules: for example, the action spectrum in Fig. 4 shows that a dosage of UV radiation at 360 nm would need to be three orders of magnitude higher than a dosage of radiation at 260 nm to produce similar mortality rates in a population of this organism.

Image: This is Figure 4 from the paper. Caption: Relative biological effectiveness of UV surface radiation on Proxima-b. (A) The biological effectiveness of UV on DNA and the radiation-resistant microorganism D. radiodurans (Voet et al. 1963; Diffey 1991) quantifies the relative effectiveness of different wavelengths of UV radiation to cause DNA destruction or, for D. radiodurans, mortality, which increases with decreasing wavelength. Biological effectiveness of UV damage for (B) oxygenic atmospheres and (C) anoxic atmosphere models shown as convolution of the surface UV flux and action spectrum over wavelength (solid line shows flaring, dashed line quiescent star), compared to present-day Earth (red solid) and early Earth (3.9 billion years ago) (red dashed). Credit: Lisa Kaltenegger/Jack O’Malley-James/Cornell University.

We can’t rule out organisms below ground or living in water or rock, not to mention such survival characteristics as biofluorescence or protective pigments. We know of microorganisms that can tolerate full solar UV in space exposure experiments, using protective cells or pigments as effective UV screens. Biofluorescence offers protection against radiation because UV can be upshifted to longer wavelengths that produce less harm. The authors think protective biofluorescence would be at its most useful during the intense UV flux of flares, although a constant level of high UV might produce continuous fluorescence.

Here we have a potential biosignature, cited by the authors in a previous paper:

Because biofluorescence is independent of the visible flux of the host star and only dependent on the UV flux of the star, emitted biofluorescence can increase the visible flux of a planet orbiting an active M-star by several orders of magnitude (O’Malley-James & Kaltenegger 2018) during a flare.

We may get our first look at such atmospheres by observing ozone, which is potentially detectable by the James Webb Space Telescope. On the other hand, a high-enough level of UV could also produce a biosphere below ground that would present, if any, only the weakest of biosignatures. Even so, the authors conclude that nearby planets around M-dwarfs like those studied here are serious candidates for biosignature examination by future observatories.

While a multitude of factors ultimately determine an individual planet’s habitability our results demonstrate that high UV radiation levels may not be a limiting factor. The compositions of the atmospheres of our nearest habitable exoplanets are currently unknown; however, if the atmospheres of these worlds resemble the composition of Earth’s atmosphere through geological time, UV surface radiation would not be a limiting factor to the ability of these planets to host life. Even for planets with eroded or anoxic atmospheres orbiting active, flaring M stars the surface UV radiation in our models remains below that of the early Earth for all cases modelled. Therefore, rather than ruling these worlds out in our search for life, they provide an intriguing environment for the search for life and even for searching for alternative biosignatures that could exist under high-UV surface conditions.

The paper is O’Malley-James & Kaltenegger, “Lessons from early Earth: UV surface radiation should not limit the habitability of active M star systems,” Monthly Notices of the Royal Astronomical Society Vol. 485 Issue 4 (June 2019), pp. 5598-5603 (full text).



A Major Hubble Survey of the Kuiper Belt

You’ll recall that well before New Horizons completed its primary mission at Pluto/Charon, the search was on for a Kuiper Belt Object that could serve as its next destination. Eventually we found Ultima Thule (2014 MU-69), from which priceless data were gathered at the beginning of January. Finding the target wasn’t easy given the distances involved and the small size of the relevant objects, which is why the Hubble Space Telescope was brought into the search.

The starfield in Sagittarius is crowded as we look toward galactic center, but despite the efforts of both the 8.2-meter Subaru telescope in Hawaii and the 6.5-meter Magellan telescopes in Chile, no KBOs among those found were within range of New Horizons. It was Hubble that made the difference, and Hubble which will presumably return a second target, if indeed the New Horizons team is granted an extended mission that can reach it. It’s worth noting, too, that it was Hubble that helped New Horizons in its discovery of Pluto’s smaller four moons, while also performing searches of the system for any dust rings that could harm the mission.

KBOs have never been heated by the Sun, so they provide the most pristine sample available of the earliest days of system formation. What we’ve learned about the Kuiper Belt so far is that there are a large number of binary objects within it, and as Southwest Research Institute scientist Alex Parker notes, many of these consist of two objects of similar mass. Parker will lead a new survey on the Kuiper Belt awarded to SwRI by the Space Telescope Science Institute (STScI), one that will put the emphasis on characterizing these binary populations.

“These binary systems are powerful tracers of the processes that built the planets,” says Parker. “We will use Hubble to test the theory that many planetesimals formed as binary systems from the get-go, and that today’s Kuiper Belt binaries did not come from mergers of initially solitary objects.”

Image: The SwRI-led Origins Legacy Survey will search for Kuiper Belt objects such as those shown in this artist’s illustration of a widely separated binary. Credit: Courtesy of Southwest Research Institute and Alex H. Parker.

Called the Solar System Origins Legacy Survey (SSOLS), the project represents the largest Hubble Solar System program ever, with 206 Hubble orbits around Earth allocated to it. SSOLS is conceived as a way to examine the primordial planetesimal disk with new and archival data. At stake are differing models of planetesimal formation, which predict different size and color distributions for solitary KBOs and their binary cousins.

The process of accretion would imply objects formed in isolation, later merging into binaries. In this case, the objects in binary systems would likely show dissimilar colors and a different size distribution than single KBOs. But if a process of rapid collapse was at work, producing some binary systems and some single KBOs quickly, then the expectation is for both objects in a binary system to have a similar surface color and a size distribution similar to what we find among solitary objects. At present, Hubble is the only instrument that can measure the binary occurrence rate in the Kuiper Belt, as well as the binary separation and color distribution.

SSOLS will characterize the binary and color properties of 221 KBOs, drawing on objects observed by the two largest Kuiper Belt surveys yet conducted, the Outer Solar System Origins Survey (OSSOS) and Canada-France Ecliptic Plane Survey (CFEPS). This earlier work becomes the framework within which the binary characterization of KBOs can proceed. For more, see the SSOLS website at https://www.ssols.space/, and ponder the need for the next outer system spacecraft that can take us into the realm New Horizons continues to explore.