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Problems with Red Dwarf Habitable Zones

Why all the fuss about red dwarf planets? We’re seeing so much ongoing work on these worlds because when it comes to terrestrial-class planets — in size, at least — those around red dwarfs are going to be our first targets for atmospheric characterization. A ‘habitable zone’ planet around a red dwarf throws a deep transit signature — small star, big planet — so that we can use transit spectroscopy to puzzle out atmospheric components. Getting an actual image would be even better, and modifications to the VISIR instrument at ESO’s Very Large Telescope, a project Breakthrough Initiatives is involved in along with the ESO, could eventually yield such.

We’ll know a great deal more about the possibilities as new missions come online, but for now, researchers are doing their best to apply models to what we know and deduce what surface conditions may be like around stars like TRAPPIST-1 and Proxima Centauri. Some of these results are not auspicious if it’s life we’re looking for. I’m looking at two papers from Chuanfei Dong (Princeton Plasma Physics Laboratory/Princeton University) that assess potential habitability, and in both cases there are significant reasons to question its likelihood.

Image: Princeton’s Chuanfei Dong. Credit: Princeton Plasma Physics Laboratory.

The assumption here is that an atmosphere must exist for long timescales to allow habitable conditions on the surface, and those long time-frames are precisely what is in question. Published earlier this year in the Astrophysical Journal Letters, the first of these papers develops models of the stellar wind, that outflow of charged particles that, in our own Solar System, defines the heliosphere around our Sun. This paper deals with the situation at Proxima Centauri b. The second paper, not yet published but available as a preprint, extends the study to the TRAPPIST-1 system, with results that are equally challenging for life.

While we have tended to focus habitability studies on factors like surface temperature (and this, in turn, is much dependent on atmosphere), Dong and colleagues are concerned about the effects of the stellar wind and atmosphere retention. Their simulations, performed using magneto-hydrodynamical (MHD) modeling that was originally developed for Venus and Mars, allow them to compute ion escape losses that would be expected at Proxima Centauri b.

Specifically, the Proxima paper examines the electromagnetic erosion of the atmosphere given the photo-chemical effects of the stellar wind, finding that the stellar inflow can ionize atoms in the planetary atmosphere, allowing these electromagnetic forces to sweep them into space. The result: Potentially severe atmospheric loss that would deplete the atmosphere of evaporated water, a cycle that could eventually leave the planetary surface dry.

A sufficiently high stellar wind pressure could cause extensive atmospheric loss, making any surface-based life that emerged a short-lived phenomenon. Says Dong:

“The evolution of life takes billions of years. Our results indicate that Proxima Centauri b and similar exoplanets are generally not capable of supporting an atmosphere over sufficiently long timescales when the stellar wind pressure is high. It is only if the pressure is sufficiently low, and if the exoplanet has a reasonably strong magnetic shield like that of the Earth’s magnetosphere, that the exoplanet can retain an atmosphere and has the potential for habitability.”

Image: Is the stellar wind capable of reducing a planetary atmosphere to the point where surface life is impossible? Credit: NASA/JPL-Caltech.

The paper finds that ion escape rates at Proxima b are two orders of magnitude higher than the terrestrial planets of our Solar System, assuming that the planet is unmagnetized. But even in the presence of a magnetosphere, ion escape rates are still higher than any we see in our system’s planets. The same issues apply at TRAPPIST-1, as noted in the new paper, which implicates stellar wind values as the primary driver in ion escape:

…as seen from our Solar system, the ion escape rates for Venus, Mars and Earth are similar despite their compositions, sizes and magnetic field strengths being wildly dissimilar (Lammer, 2013; Brain et al., 2016), thereby indicating that the ion escape rates may be more sensitive to stellar wind parameters; this is also partly borne out by the atmospheric ion escape rate calculations for Proxima b (Airapetian et al., 2017; Dong et al., 2017). Second, we observe that the inner planets of the TRAPPIST-1 system could have experienced significant losses of H2 and water over fast timescales (Bolmont et al., 2017; Bourrier et al., 2017), leaving behind other atmospheric components…

Dong also notes that planets close enough to be in a red dwarf’s habitable zone are likely tidally locked, producing constant bombardment on the star-facing side that would intensify the effects of atmospheric loss whether or not the planet has a protective magnetosphere. Earlier work has suggested that tidally locked planets are unlikely to have more than a weak magnetic field.

I’m focusing now on the TRAPPIST-1 paper because it’s the latest work on this matter, and it amplifies what was found in the earlier Proxima Centauri b work; I give citations for both papers below. Considering TRAPPIST-1 in light of stellar winds, the researchers argue that TRAPPIST-1h, viewed purely from the perspective of atmospheric loss, is the one most likely to have retained its atmosphere, but this is not a world where liquid water is possible on the surface. Dong and colleagues believe that TRAPPIST-1g thus represents “…the best chance for a habitable planet in this planetary system to support a stable atmosphere over long periods,” as the stellar wind effects diminish with distance.

It’s not a pleasant picture for those hoping for clement conditions on other planets around TRAPPIST-1 or Proxima Centauri. Oceans may once have existed there, but this work suggests that their surfaces today are probably dry. While the two papers focus on Proxima b and the TRAPPIST-1 worlds, Dong notes that the newly discovered planet around Ross 128 may have better prospects, as its star appears to be quieter than Proxima Centauri or TRAPPIST-1.

We should also note that an atmosphere can be replenished by outgassing, a reminder that analyzing an atmosphere over billion-year timeframes demands, as the paper notes, “…an in-depth understanding of the interplay between source and loss mechanisms.” Another issue: Stellar properties evolve, so that atmospheric escape rates change. This may not work to life’s advantage, however, for pre-main sequence M-dwarfs, according to Dong’s simulations, would produce even stronger stellar wind effects upon a young planet’s atmosphere.

The implications for other planetary systems seem clear:

For a given star, the mass-loss rate is fixed, implying that the escape rate is lower for smaller planets orbiting at greater distances. Hence, we suggest the following strategy for prioritizing studies of multi-planetary systems. If more than one exoplanet resides within the HZ of a given star, it may be more prudent to focus on the outward planet(s) since the atmospheric escape rates are likely to be lower. Similarly, when confronted with two planets with similar values of Rx and a [radius and semi-major axis], we propose that searches should focus on stars with lower mass-loss rates and magnetic activity.

The examination of the effects of stellar winds will proceed with soon-to-be-launched missions like the James Webb Space Telescope, allowing us to put theoretical work to the test. Given the surprises we’ve consistently found even with our interplanetary probes, Pluto being the most recent example, we can fairly confidently expect to modify our views with each new exoplanet atmospheric characterization. For now, though, these continuing studies raise serious questions about red dwarf planets as a clement venue for life.

The papers are Dong et al., “Is Proxima Centauri b habitable? — A study of atmospheric loss,” Astrophysical Journal Letters Vol. 837, No. 2 (10 March 2017) (abstract/ preprint); and Dong et al., “Atmospheric escape from the TRAPPIST-1 planets and implications for habitability,” accepted at Astrophysical Journal Letters (preprint).



Ozone Problematic for Biosignature Detection

TRAPPIST-1 and its seven interesting planets may be the most compelling stellar system we’re investigating, given the range of worlds here and the possibilities for analyzing an entire, nearby planetary system. But as we look toward examining systems like this with new space- and ground-based instruments, we may run into problems with searching for biosignatures. Both the TRAPPIST-1 planets and the promising Proxima Centauri b may be tough to characterize.

The problem: When searching for biosignatures, we’re looking for signs of metabolism, gases that are continually produced and remain out of balance in a planetary atmosphere. Ozone is one piece of the puzzle, one that signifies oxygen. Finding the latter in the same atmosphere with methane would be a compelling biosignature. But ozone could be hard to detect.

Ludmila Carone (Max Planck Institute for Astronomy) and colleagues now find that atmospheric circulation in planets close enough to red dwarfs to be in their habitable zone may mask the very signs we’re looking for. Ozone may become undetectable, trapped in equatorial regions.

The issue involves atmospheric flows, which will likely differ from what we see on Earth. The ozone in our stratosphere is formed when ultraviolet light from the Sun triggers chemical reactions in the oxygen molecules that make up approximately one-fifth of our atmosphere. The ozone protects us from harmful ultraviolet radiation from the Sun, and is distributed throughout the atmosphere owing to large-scale air flows moving from the equatorial regions to the poles.

Image: Earth’s atmosphere has a “transportation belt” of air flows which move ozone from the main production areas near the equator towards the poles. This mechanism is important for creating Earth’s global ozone layer. Credit: MPIA (L. Carone & Graphics Dept.)

The problem for exoplanetology is that red dwarf planets in orbits of less than 25 days may be tidally locked to their star, presenting the same face to it at all times. Tidal lock, which we’ve examined often in these pages, could conceivably still allow enough heat to circulate to avoid the kind of extremes between dark and light sides that could prevent life from emerging. But for planets like these, Carone’s models show major air flows moving in the opposite direction, from the poles to the equator. The ozone we seek, then, could be trapped in the equatorial region.

Image: As a new study by Ludmila Carone shows, certain exoplanets could have air flows that serve to trap ozone in the equatorial regions. This could present an unforeseen complication for the search for traces of life on these planets. For those who don’t speak German — my apologies, but I only have this image with German annotation. Credit: MPIA (L. Carone & Graphics Dept.)

This possibility is one we’ll need to consider as we begin the analysis of atmospheres on terrestrial-class planets around nearby red dwarfs. We may find no ozone on planets where ozone and oxygen might actually exist, though sequestered in places that might be difficult to detect. That’s a reverse take on the usual ‘false positive’ problem we’ve discussed here before, with abundant oxygen, for example, not necessarily being a marker for biological activity.

On the other hand, it’s worth asking whether a planet with ozone only in its equatorial regions would be capable of developing life in the first place. On that score, Carrone has an answer:

“In principle: yes. Proxima b and TRAPPIST-1d orbit red dwarfs, reddish stars that emit very little harmful UV-light to begin with. On the other hand, these stars can be very temperamental, and prone to violent outbursts of harmful radiation including UV. There is still a lot that we don’t know about these red dwarf stars. But I’m confident we will know much more in five years.”

True enough, assuming a successful launch of instruments like the James Webb Space Telescope, and the gradual emergence of the next generation of ground-based instruments. But as a marker for oxygen and potential life, exoplanet ozone may be a challenging detection.

The paper is Carone et al., “Stratosphere circulation on tidally locked ExoEarths,” Monthly Notes of the Royal Astronomical Society Vol. 473, Issue 4, pp. 4672-4685 (abstract).



Thinking About Saturn After Cassini

Several recent news items on Enceladus have me wanting to catch up with mission possibilities and the instruments that will drive them. NASA’s thinking in that direction takes in a remote sensing instrument called SELFI, an acronym standing for Submillimeter Enceladus Life Fundamentals Instrument. The plan here is to examine the chemical composition of the plumes of water vapor and icy particles that are regularly lofted into space from Enceladus’ south pole, in the region we’ve come to know as the ‘tiger stripes.’

Cassini data on the slight wobble in the orbital motion of Enceladus backs up the idea that the ocean beneath its ice is global, a body likely kept liquid by tidal energies as the moon is pulled and squeezed by Saturn in its orbit. The same process is likely the cause of the cracks that allow ocean water to escape into space, from perhaps as many as 100 sites on the surface.

Image: The Cassini spacecraft detected hydrogen in the plume of gas and icy material spraying from Enceladus during its deepest and last dive through the plume on Oct. 28, 2015. This graphic illustrates a theory on how water interacts with rock at the bottom of the moon’s ocean, producing hydrogen gas. A Goddard team wants to develop an instrument that would reveal even more details about the hydrothermal vents and perhaps help answer if life exists on this ocean world. Credit: NASA/JPL-Caltech/Southwest Research Institute.

Gordon Chin (NASA GSFC), principal investigator for SELFI, describes it as a significant improvement over current submillimeter-wavelength technologies. Says Chin:

“Submillimeter wavelengths, which are in the range of very high-frequency radio, give us a way to measure the quantity of many different kinds of molecules in a cold gas. We can scan through all the plumes to see what’s coming out from Enceladus. Water vapor and other molecules can reveal some of the ocean’s chemistry and guide a spacecraft onto the best path to fly through the plumes to make other measurements directly.”

The GSFC team is using NASA R&D funding to increase the spectrometer’s sensitivity in the 557 GHz range, where the strongest signal from water is to be found. The goal is to explore the entire system of surface vents on Enceladus, measuring water and traces of other gases. The work also includes creating a radio frequency data-processing system, and a digital spectrometer for the RF signal that will convert it into digital signals to allow the measurement of the gases emerging from the plumes in terms of their quantity, temperature and velocity.

Calling it “one of the most ambitious submillimeter instruments ever built,” Chin says in this NASA overview that SELFI should be able to detect and analyze 13 molecular species, ranging from water in various isotopic forms to methanol, ammonia, ozone, hydrogen peroxide, sulfur dioxide, and sodium chloride. Work on the instrument is sufficiently encouraging that the GSFC team believes SELFI will eventually be part of a proposal for a future mission to Saturn.

Image: Dramatic jets of ice, water vapor and organic compounds spray from the south pole of Saturn’s moon Enceladus in this image captured by NASA’s Cassini spacecraft in November 2009. Credit: NASA/JPL-Caltech/Space Science Institute.

Assuming it flies, the instrument should allow us to deduce the composition of the global ocean, and its potential for hosting extraterrestrial life. We have no idea whether Enceladus has warm hydrothermal vents of the sort that sustain life at the bottom of Earth’s ocean, but the prospect is enticing not only for this moon but for many of the other icy moons in the outer Solar System.

Into Saturn’s Depths

On the subject of Saturn, be aware as well of an interesting mission possibility called Hera, proposed as an M-class mission led by the European Space Agency, with collaboration from NASA (thanks Mike Fidler for the tip). The plan here is to detach an atmospheric probe from a ‘carrier-relay’ spacecraft as it approaches Saturn, letting the Hera probe enter Saturn’s atmosphere to study its tropopause, descending to pressure levels of at least 10 bars.

From the proposal page:

The primary science objectives will be addressed by an atmospheric entry probe that would descend under parachute and carry out in situ measurements beginning in the stratosphere to help characterize the location and properties of the tropopause, and continue into the troposphere to pressures of at least 10 bars. All of the science objectives, except for the abundance of oxygen, which may be only addressed indirectly via observations of species whose abundances are tied to the abundance of water, can be achieved by reaching 10 bars.

The Hera proposal recalls two previous missions, the first being the Galileo probe that was carried aboard the main Galileo spacecraft to Jupiter, entering the Jovian atmosphere on December 7, 1995 and continuing to function for close to an hour as it descended. The other analog is, of course, Cassini carrying the Huygens probe for the spectacular 2005 landing on Titan. That was a collaboration between ESA and NASA that paid off handsomely, and it provides a model for the carrier/data relay spacecraft model that Hera would use.



New Work on Planetary Inflation

Once in space in 2018, the Transiting Exoplanet Survey Satellite (TESS) will be observing, among many other things, hundreds of thousands of red giant stars across the entire sky. Planets around red giants are an interesting topic, because such stars point to an evolutionary outcome our own Sun will share, and we’d like to know more about what happens to existing planets in such systems as the host star swells and reddens, engulfing inner worlds.

New work out of the University of Hawaii Institute for Astronomy now examines two ‘hot Jupiters’ around red giants, stellar systems where we see the gas giants swelling up as the result of processes that remain controversial. The inflated size of planets like these can be explained in at least two ways, one of which involves a slowing of the cooling in the planet’s atmosphere, which causes the planet to inflate soon after formation. But the data presented here, drawn from NASA’s K2 mission, tend to corroborate the thinking of co-author Eric Lopez (NASA GSFC) that direct energy input from the host star is the dominant cause of this planetary inflation.

Image: Upper left: Schematic of the K2-132 system on the main sequence.
Lower left: Schematic of the K2-132 system now. The host star has become redder and larger, irradiating the planet more and thus causing it to expand. Sizes not to scale. Main panel: Gas giant planet K2-132b expands as its host star evolves into a red giant. The energy from the host star is transferred from the planet’s surface to its deep interior, causing turbulence and deep mixing in the planetary atmosphere. The planet orbits its star every 9 days and is located about 2000 light years away from us in the constellation Virgo. Credit: Karen Teramura, UH IfA.

The work is now available in The Astronomical Journal, where lead author Samuel Grunblatt and team show that each of the two planets is about 30 percent larger than Jupiter, though in each case only about half as massive. The two planets — K2-132b and K2-97b — are similar in orbital periods, radii and masses. Each orbits its red giant star in about nine days, with planetary radii being calculated at 1.31 ± 0.11 RJ and 1.30 ± 0.07 RJ respectively.

The researchers used models to analyze the evolution of planets like these over time, determining that their radii are typical for planets receiving their current level of radiation, but calculating back to main sequence values of radiation, they find the gas giants would have been considerably smaller. Stellar flux flowing to the planets’ deep convective interiors could therefore explain their current size, an indication that planet ‘inflation’ is directly tied to stellar irradiation rather than delayed atmospheric cooling after the planets’ formation.

But other factors remain to be tested, metallicity in particular. From the paper:

Further studies of planets around evolved stars are essential to confirm the planet re-inflation hypothesis. Planets may be inflated by methods that are more strongly dependent on other factors such as atmospheric metallicity than incident flux. An inflated planet on a 20 day orbit around a giant star would have been definitively outside the inflated planet regime when its host star was on the main sequence, and thus finding such a planet could more definitively test the re-inflation hypothesis. Similarly, a similar planet at a similar orbital period around a more evolved star will be inflated to a higher degree (assuming a constant heating efficiency for all planets). Thus, discovering such a planet would provide more conclusive evidence regarding these phenomena.

Also in play is the issue of heating efficiency, which may well vary between planets depending on their composition. Back to TESS, whose investigations should complement these results. Grunblatt and team point out that TESS should be able to observe additional planets in roughly 10 day orbits around more evolved stars, including oscillating red giants. The data should allow us to distinguish between the delayed cooling possibility and stellar irradiation scenarios.

The paper is Grunblatt et al., “Seeing Double with K2: Testing Re-inflation with Two Remarkably Similar Planets around Red Giant Branch Stars,” Astronomical Journal Vol. 154, No. 6 (27 November 2017). Abstract / preprint.



Cassini’s Exquisite Last View

The release of Cassini’s last images of Saturn and its rings is a welcome event, a capstone to the mission that has taught us so much. What we see below is a series of images that have been grafted together, 42 red, green and blue images that allow us to see a wide-angle mosaic of Cassini’s view. The images were taken by the spacecraft’s wide-angle camera on September 13, and include the moons Prometheus, Pandora, Janus, Epimetheus, Mimas and Enceladus.

Image: After more than 13 years at Saturn, and with its fate sealed, NASA’s Cassini spacecraft bid farewell to the Saturnian system by firing the shutters of its wide-angle camera and capturing this last, full mosaic of Saturn and its rings two days before the spacecraft’s dramatic plunge into the planet’s atmosphere. During the observation, a total of 80 wide-angle images were acquired in just over two hours. This view is constructed from 42 of those wide-angle shots, taken using the red, green and blue spectral filters, combined and mosaicked together to create a natural-color view. Credit: NASA/JPL-Caltech/SSI.

I was glad to see some Cassini team members reminiscing about Voyager, whose journeys opened up the outer Solar System to close view, and continue to inform us about the interstellar medium. Thus Carolyn Porco, former Voyager imaging team member and Cassini’s imaging team leader at the Space Science Institute in Boulder, Colorado:

“For 37 years, Voyager 1’s last view of Saturn has been, for me, one of the most evocative images ever taken in the exploration of the solar system. In a similar vein, this ‘Farewell to Saturn’ will forevermore serve as a reminder of the dramatic conclusion to that wondrous time humankind spent in intimate study of our Sun’s most iconic planetary system.”

Here is a brightened view, processed to bring out detail in the fainter areas of the image. The six moons mentioned above show up faintly, but the annotations should help.

Image: The ice-covered moon Enceladus — home to a global subsurface ocean that erupts into space — can be seen at the 1 o’clock position. Directly below Enceladus, just outside the F ring (the thin, farthest ring from the planet seen in this image) lies the small moon Epimetheus. Following the F ring clock-wise from Epimetheus, the next moon seen is Janus. At about the 4:30 position and outward from the F ring is Mimas. Inward of Mimas and still at about the 4:30 position is the F-ring-disrupting moon, Pandora. Moving around to the 10 o’clock position, just inside of the F ring, is the moon Prometheus. Credit: NASA/JPL-Caltech/SSI.

We’re looking toward the sunlit side of the rings from about 15 degrees above the ring plane, with Cassini at approximately 1.1 million kilometers from the planet and on its final approach.

Titan’s Polar Vortex

Although Cassini is gone, we have vast amounts of data that will continue to generate new discoveries for quite some time, as witness the latest, a paper out of the University Of Bristol that discusses the atmospheric chemistry on Saturn’s largest moon, Titan. Lead author Nick Teanby has been studying Titan’s upper atmosphere, where in the polar regions we are seeing unexpected cooling, a process that differs from what we see on Earth, Venus and Mars.

Indeed, Titan’s polar vortex seems to be extremely cold. What we see on the other worlds is that the high altitude polar atmosphere on a planet’s winter hemisphere is warmed as a result of sinking air heating as it is compressed. It was Cassini’s Composite Infrared Spectrometer (CIRS) instrument that produced the observations Teanby has used to study this anomaly on Titan.

CIRS data showed the expected polar hot-spot beginning to develop in 2009, but temperatures dropped significantly by 2012 and remained as low as 120 K until late 2015, after which the expected hot-spot returned. Teanby explains what’s happening this way:

“For the Earth, Venus, and Mars, the main atmospheric cooling mechanism is infrared radiation emitted by the trace gas CO2 and because CO2 has a long atmospheric lifetime it is well mixed at all atmospheric levels and is hardly affected by atmospheric circulation. However, on Titan, exotic photochemical reactions in the atmosphere produce hydrocarbons such as ethane and acetylene, and nitriles including hydrogen cyanide and cyanoacetylene, which provide the bulk of the cooling.”

Image: Titan’s winter polar vortex imaged by the Cassini Spacecraft’s ISS camera. The vortex is now in deep winter and can only be seen because the polar clouds within the vortex extend high above Titan’s surface into the sunlight. The vortex was extremely cold from 2012-2015, giving rise to unusual nitrile ice clouds. Credit: NASA/JPL-Caltech/Space Science Institute/Jason Major.

Hydrocarbons and nitriles appear high in the atmosphere and are strongly susceptible to vertical atmospheric circulation, meaning that over the southern winter they accumulate in great amounts over the pole, creating the cooling Teanby and team are studying. The work also draws on Cassini data from 2014, when hydrogen cyanide ice clouds were observed over the pole, a reminder not only of Titan’s intriguing chemistry but the continuing vitality of Cassini data.

The paper is Teanby et al., “The formation and evolution of Titan’s winter polar vortex,” published online by Nature Communications 21 November 2017 (full text).



A Thought for the Weekend

From Arthur C. Clarke’s Interplanetary Flight: An Introduction to Astronautics (London: Temple Press Limited, 1960):

There is no way back into the past; the choice, as Wells once said, is the universe–or nothing. Though men and civilizations may yearn for rest, for the dream of the lotus-eaters, that is a desire that merges imperceptibly into death. The challenge of the great spaces between the worlds is a stupendous one; but if we fail to meet it, the story of our race will be drawing to its close.



Email Delivery Problems

Several readers have told me that their email deliveries of Centauri Dreams have not been coming through. This has been an on-again, off-again problem for some time and I’m now trying to switch providers to take care of it. Bear with me, as I hope to have the problem resolved within a few days.



Shards, Axis Ratios and Interstellar Objects

It being the day after the Thanksgiving holiday here in the States, I hadn’t planned a post, but a few more things have cropped up about ‘Oumuamua that I can quickly tuck in here. Now that I’ve learned how to pronounce it (oh MOO-uh MOO-uh), it doesn’t seem nearly as intimidating — it’s the lineup of vowels that trips up the unwary. On the other hand, Jim Benford suggested on Wednesday that we avoid the vowels altogether and call this thing ‘the Shard.’

Here’s the photo Jim sent of the Shard, a 95-story London skyscraper sometimes called The Shard of Glass. It’s 309.7 metres high, the tallest building in the United Kingdom, featuring 11,000 panes of glass with a total surface area of 56,000 square metres. What draws Jim’s attention is the 6 to 1 aspect ratio, with ‘Oumuamua’s thought to be 10 to 1. Jim might also have referenced London’s BT Tower (8 to 1), but what the Guinness Book of World Records calls the “most slender tower” turns out to be the i360 observation tower in Brighton, at a whopping 41 to 1.

With ‘Oumuamua, though, we now have to ask whether the 10 to 1 ratio is actually correct, as Jason Wright noted in a recent post. The problem here is that, unlike the situation with Boyajian’s Star, we have a small dataset to work with, and according to Wright (Penn State), researchers are getting different aspect ratios, ranging all the way from the aforementioned 10 to 1 down to a relatively ordinary 3 to 1.

If the latter is the case, the interstellar object may look something more like Haumea than the Shard. “I’ll need to see a lot more data and hard, critical analysis of the anomalies in ‘Oumuamua before I get interested in the SETI angle at the level I am for Tabby’s Star,” adds Wright.

Greg Laughlin (UC-Santa Cruz) also weighed in on ‘Oumuamua with a new paper (citation below) and accompanying article in Scientific American. Laughlin dubs our visitor “exhilaratingly bizarre” and goes on to describe its unusual arrival, in which, despite accruing 20 kilowatts of energy per square meter at perihelion, it showed no evidence of cometary activity. It was fun to see that Greg also refers to ‘Oumuamua at one point as “a crazily elongated shard.”

But what drew my attention in Greg’s post was how difficult the ejection of debris from a newly forming planetary system seems to be. Getting such a shard free from the host star demands a gravitational assist from a massive planet located at a large distance from the star — terrestrial worlds in our Solar System would fall far short, though the gas giants could manage the feat.

If objects like ‘Oumuamua are common and if they are made predominantly of ice, as we would expect in an outer stellar system, then it implies, says Laughlin, that almost every star in the Milky Way hosts a Neptune-class planet at roughly the distance of our own Neptune from the Sun.

And if it really is rock or metal? Then we deal with another scenario entirely:

…in the highly unlikely event ‘Oumuamua is indeed a refractory slab of rock or metal, as suggested by its complete lack of coma, then its appearance is extremely hard to understand. Only a few percent of stars host planets that are capable of ejecting volatile-free debris from warm regions deep within a gravitational well. They flat-out can’t generate the vast overall swarm implied by ‘Oumuamua’s recent passage, suggesting that another visit by a similar object won’t happen for a very long time.

A long time indeed. According to Laughlin’s calculations of that scenario, ‘Oumuamua could travel for 10 quadrillion years before coming into similar proximity to another star.

At that far distant time, the galaxy will be a very different place, in which all the stars that now shine warmly down on planets will be expired white dwarfs, warmed a few degrees above absolute zero by the flicker of proton decay.

The paper is Laughlin, G. & Batygin, K. (2017) “On the Consequences of the Detection of an Interstellar Asteroid,” submitted to Research Notes of the AAS (abstract).



`Oumuamua: Listening To An Interstellar Interloper

The buzz about `Oumuamua, our first known visitor from another stellar system, seems likely to continue given yesterday’s news that the object’s axis ratio is a startling 10 to 1. Given all that, Jim Benford wondered whether there were SETI implications here. Was anyone on the case from our major SETI organizations? The answer is below, as we learn that the effort is ongoing. A frequent contributor to these pages, Jim is President of Microwave Sciences in Lafayette, California, which deals with high power microwave systems from conceptual designs to hardware. He also heads up the critical sail subcommittee for Breakthrough Starshot, the effort to send small beamed sails with miniaturized payloads to a nearby star.

By James Benford

I contacted Jill Tarter and Andrew Siemion about whether SETI researchers are conducting observations of the interstellar interloper, Oumuamua. Both say yes.

Jill said that the Allen Telescope Array has been looking at it for a while. Andrew said that Breakthrough Listen was using the Green Bank Telescope for a few hours last weekend. This was actually looking for water via hydroxyl lines using broadband 1.1-1.9 GHz data. No water was immediately evident in the coarse spectra from the standard data reduction. Breakthrough Listen is working on incorporating the appropriate windowing capabilities necessary to analyze this data, so as to use their data analysis pipeline.

Therefore there are some observations in parts of the microwave spectrum.

Image: This diagram shows the orbit of the interstellar asteroid ‘Oumuamua as it passes through the Solar System. Unlike all other asteroids and comets observed before, this body is not bound by gravity to the Sun. It has come from interstellar space and will return there after its brief encounter with our star system. Its hyperbolic orbit is highly inclined and it does not appear to have come close to any other Solar System body on its way in. Credit: ESO/K. Meech et al.

Besides astronomical observations of this unique object, there is also this remote possibility: That this interloper is an interstellar survey probe, having perhaps dropped down to interplanetary-scale velocities in order to take data during its transit of our solar system, before going on to another star.

If this is the case, then perhaps we ought to be looking rather broadly in the electromagnetic spectrum for any signal it might send to us, having easily detected leakage from Earth. That assumes it would try to respond to us using frequencies it knows we use. That would certainly include the microwave bands 1-10 GHz, where most of our radiation leakage radiation is.

I think at present the frequencies most observable coming from Earth are leakage of uplink transmissions to our satellites, of which there are now about 1200 active in orbit. Those frequencies tend to be in the upper end of the microwave where the wavelength is smaller, so we can use smaller apertures on both Earth and satellite. Downlinks, of course, would be absorbed in the Earth and not observable from afar.

Or, because they know enough about our atmosphere’s transmission windows and the Sun’s radiation spectrum, they might be signaling in the visible. Therefore our SETI optical observatories ought to be watching as well.

I would look for a pulsed beacon signal, which is more noticeable. That would be like a pulsar, but of course with no interstellar dispersion.

This matter has a very low probability of success, of course. However, it’s our first opportunity to observe at close range a truly interstellar object.

Image: This plot shows how the interstellar asteroid `Oumuamua varied in brightness during three days in October 2017. The large range of brightness — about a factor of ten (2.5 magnitudes) — is due to the very elongated shape of this unique object, which rotates every 7.3 hours. The different coloured dots represent measurements through different filters, covering the visible and near-infrared part of the spectrum. The dotted line shows the light curve expected if `Oumuamua were an ellipsoid with a 1:10 aspect ratio, the deviations from this line are probably due to irregularities in the object’s shape or surface albedo. Credit: ESO/K. Meech et al.



Unusual Visitor: A Deeper Look at ‘Oumuamua

When I first wrote about the interstellar interloper now called ‘Oumuamua, I made reference to Arthur C. Clarke’s Rendezvous with Rama because of the delightful symmetry between the novel and the object, though noting that ‘we’re unlikely to find that A/2017 U1 is as intriguing as Clarke’s mysterious starship bound for the Magellanics’ (see An Interstellar Visitor?). Still, an interstellar object entering the Solar System only to go careening back out of it could not help but recall Clarke, whose ‘asteroid’ 31/439 wound up being artificial.

Then came the paper from Karen Meech (University of Hawaii Institute for Astronomy, where the object was first detected with the Pan-STARRS1 telescope). Drawing on data from telescopes around the world, Meech’s team has been able to characterize our first nearby object from another stellar system, with equally delightful results. For it turns out that ‘Oumuamua (pronounced oh MOO-uh MOO-uh) has an unusual axis ratio, being about ten times longer than it is wide. Jim Benford couldn’t resist suggesting I show a cover from Rendezvous with Rama depicting just such an axis ratio, and I agreed wholeheartedly.

Any science fiction fan familiar with Clarke (and are there any who aren’t?) will have fun with the similarities, but how much do we actually know about ‘Oumuamua? Meech’s team based its conclusions on the object’s shape on the fact that its brightness changed so dramatically as it rotated (spinning on its axis every 7.3 hours). Lance Benner, who specializes in radar imaging of near-Earth and main-belt asteroids at JPL, calls the axis ratio here ‘truly extraordinary.’ We know of no Solar System objects elongated more than 3 times longer than they are wide.

Nothing in our Solar System, in other words, quite matches an object shaped like this. Of course, it might also look like the image below, courtesy of the European Southern Observatory.

Image: This artist’s impression shows the first detected interstellar asteroid: `Oumuamua. This unique object was discovered on October 19, 2017 by the Pan-STARRS 1 telescope in Hawai`i. Subsequent observations from Gemini, ESO’s Very Large Telescope in Chile, CFHT, UKIRT, and other observatories around the world show that it was on a path which must have been travelling through interstellar space for millions of years before its chance encounter with our star system. `Oumuamua seems to be a dark red highly-elongated metallic or rocky object, about 400 meters long, and is unlike anything normally found in the Solar System. Credit: ESO/M. Kornmesser.

On the other hand, the object’s light curve, as examined through data from the Canada-France-Hawaii Telescope (CFHT), can show us rotation and likely axis ratio, but the overall observations — from the United Kingdom Infrared Telescope (UKIRT), the Keck Telescope on Mauna Kea, the Gemini South telescope, and the European Southern Observatory (ESO) Very Large Telescope (VLT) in Chile as well as CHFT — show similarities with local objects as well as differences. Here’s Karen Meech on those findings:

“While study of `Oumuamua’s colors shows that this body shares characteristics with both Kuiper Belt objects and organic-rich comets and trojan asteroids, its hyperbolic orbit says it comes from far beyond.”

The object’s dark red color is similar to Kuiper Belt objects, and the researchers believe it is dense and rocky or with high metal content, an asteroid that, according to this ESO news release, lacks significant amounts of water or ice, and one whose surface darkening and reddening is the result of millions of years of irradiation. Current length estimates are in the range of 400 meters.

So while its composition is similar to objects in our Solar System, its shape is unique and so, of course, is its origin. Observations of the object had to be quickly compiled, for while it was only 85 times the Earth-Moon distance away when discovery was announced earlier this month, it is moving away rapidly. It was clear to the discoverers that they had little time to gather data on this enigmatic visitor. Thus Gemini director Laura Ferrarese, who applied Gemini South’s resources to the study for Meech’s group: “Needless to say, we dropped everything so we could quickly point the Gemini telescopes at this object immediately after its discovery.”

Both Hubble and Spitzer are also tracking the object the week of November 20, according to JPL. And here’s another science fictional bit: Preliminary orbital calculations show `Oumuamua came from the approximate direction of Vega, in the constellation Lyra. That just might recall Carl Sagan’s novel Contact, where a SETI signal containing data is found to be coming from Vega, 26 light years away, leading to the construction of a most unusual device.

But there the similarities end: Our object is simply traveling too slow to have been sent from Vega, which would not have been nearby when `Oumuamua was there 300,000 years ago.

Image: This composite was produced by combining 192 images obtained through three visible and two near-infrared filters totaling 1.6 hours of integration on October 27 at the Gemini South telescope. Processing removes the background stars. The field of view represents a patch of sky 5,000 km on a side at the distance of `Oumuamua. Visible colors for the image were assigned to each filter as follows:
g (398-552 nm) = blue
r (562-692 nm) = green
i (706-850 nm) = yellow
z (830-925 nm) = orange
Y (970-1070 nm) = red
While assigning visible colors to filtered images is somewhat subjective, the resulting color of `Oumuamua in optical light is similar to the hue of some of the moons of outer planets in our Solar System, and possibly indicates a similar composition (a combination of minerals, carbon, iron, and organic compounds). Color composite produced by Travis Rector, University of Alaska Anchorage, using Gemini South GMOS data obtained and processed by Meech et. al. Credit: Gemini Observatory/AURA/NSF

Are we going to find more such visitors? Ejected comets and asteroids, forced into the deep through interactions with larger planets, may be relatively common. In fact, Robert Jedicke, who is part of the team working with Meech, estimates that a similar object, of similar size, is out there between the Earth and the Sun at any given time, up to perhaps 10 every year. If so, it will be surveys like Pan-STARRS and the future Large Synoptic Survey Telescope (LSST) that begin to compile statistics on the frequency of visitors like `Oumuamua.

`Oumuamua is now traveling at 38.3 kilometers per second relative to the Sun, about 200 million kilometers from Earth, and will pass the orbit of Jupiter in May of 2018, headed in the general direction of the constellation Pegasus. We should be able to continue to refine its trajectory as it leaves the Solar System through mid-December, when it becomes too faint to detect.

I am not saying this is going to happen, but wouldn’t it be fascinating if we learned it was headed toward another nearby star…

The paper is Meech et al., “A brief visit from a red and extremely elongated interstellar asteroid,” published online by Nature 20 November 2017 (abstract).