Centauri Dreams
Imagining and Planning Interstellar Exploration
A Relatively Nearby Earth-Sized Planet
Given my abiding interest in red dwarf stars and the planets that circle them, I always keep an eye on what’s happening with the MEarth project. Two arrays of robotically controlled telescopes are involved in MEarth (pronounced ‘mirth’), one at the Fred Lawrence Whipple Observatory on Mt. Hopkins (AZ), the other a cluster of eight at the Cerro Tololo Inter-American Observatory in Chile. Both these arrays are controlled from MEarth’s offices in Cambridge (MA). MEarth is all about observing nearby M-dwarfs in the hunt for Earth-class planets.
My fascination in these stars is simply a result of the numbers. We’ve learned that M-dwarfs comprise as much as 80 percent of the stars in the Milky Way. Earth is not, in other words, orbiting the most common type of star out there. We also know that M-dwarfs host planets. If we learn that conditions on such worlds can support life, then we’ve dramatically expanded the search space for astrobiology. The prospect of a living world, probably tidally locked to its star, conjures images strange and wonderful, a world where shadows are permanent and half of the planet is an ice-covered waste, as Stephen Baxter recently portrayed in a planet called ‘Per Ardua’ that circles Proxima Centauri in his novel Ultima (Roc, 2015).
The latest news from MEarth comes out of MEarth-South, whose 40-centimeter instruments have detected an interesting light curve around the star GJ 1132, finding a dip of approximately 0.3 percent in the starlight. The signal, confirmed by other instruments in Chile, flags a planet that is roughly 1.2 times the size of the Earth, in a tight 1.6-day orbit. Given the star’s radius and the amount of light it blocks, researchers led by Zachory Berta-Thompson (MIT) calculate this is a planet with about 1.6 times Earth’s mass, a world that may well be rocky.
Image: In this artist’s rendering of GJ 1132b, a rocky exoplanet very similar to Earth in size and mass, circles a red dwarf star. GJ 1132b is relatively cool (about 226 degrees C) and could potentially host an atmosphere. At a distance of only 39 light-years, it will be a prime target for additional study with Hubble and future observatories like the Giant Magellan Telescope. Credit: Dana Berry.
The world is probably tidally locked. And life on GJ 1132b looks to be unlikely, given an estimated average temperature of 500 K (226 degrees Celsius). Says Berta-Thompson:
“The temperature of the planet is about as hot as your oven will go, so it’s like burnt-cookie hot. It’s too hot to be habitable — there’s no way there’s liquid water on the surface. But it is a lot cooler than the other rocky planets that we know of.”
That’s a useful fact because while surface conditions appear inimical to life, the planet is cool enough to retain a substantial atmosphere. GJ 1132 is a mere 39 light years from Earth, making it the closest Earth-sized exoplanet yet discovered. Recently we’ve been discussing the next generation of space telescopes, and now we find a world that will surely be a target for scrutiny, especially by the James Webb Space Telescope, which should be able to analyze the chemical constituents of the planet’s atmosphere and even detect the patterns of its winds.
“If we find this pretty hot planet has managed to hang onto its atmosphere over the billions of years it’s been around, that bodes well for the long-term goal of studying cooler planets that could have life,” adds Berta-Thompson. “We finally have a target to point our telescopes at, and [can] dig much deeper into the workings of a rocky exoplanet, and what makes it tick.”
Berta-Thompson points out that some 500 star systems are known to be closer to us than GJ 1132, and instruments like TESS (Transiting Exoplanet Survey Satellite) and CHEOPS (Characterizing Exoplanets Satellite), both scheduled for launch in 2017, will help us study many more targets. Learning how to analyze the atmospheres of nearby worlds is critical for our investigations into exoplanetary life. Planets like GJ 1132b will be useful in refining our tools as we begin to turn them to worlds with better prospects for living things.
The paper is Berta-Thompson et al., “A rocky planet transiting a nearby low-mass star,” Nature 527 (12 November 2015), 204-207 (abstract).
Quantifying KIC 8462852 Power Beaming
Plasma physicist James Benford, CEO of Microwave Sciences, is well known here on Centauri Dreams. Today he is joined by his son Dominic, whose work focuses on the development of ultrasensitive technologies for far-infrared through millimeter-wave astronomy. The younger Dr. Benford is Program Scientist for NASA’s WFIRST mission, which is designed to conduct major surveys in the near-infrared to answer fundamental questions on the nature of dark energy, the distribution of dark matter, the occurrence of planets around other stars, and even to enable the direct imaging of planetary systems. Previously, Dominic was Chief Scientist for the Cosmic Origins Program Office, as well as Deputy Mission Scientist for WISE, the Wide-field Infrared Survey Explorer. In today’s entry, the Benfords look at the SETI Institute’s recent observations of KIC 8462852 and analyze the detectability of power beaming at these distances.
by James and Dominic Benford
The recent report from the SETI Institute of radio observations of the anomalous star KIC 8462852 has immediate implications. That report concluded that, using the Allen Array, no narrowband radio signals were found above a few hundred Janskys in 1 Hz channels and no “wideband” signals above 100 Janskys are seen in 100 kHz channels. This is for observations taking place for 2 weeks, observing half the time. This implies about 180 hours of observations, although only about 1% of the time is spent at any individual frequency.
The purpose of the observations is to see whether the anomalous star is the site of a super-civilization that might be incidentally radiating sufficient power that we can observe, i.e., leakage radiation. They might even be intentionally producing signals for us to detect. The easiest way to do that is to ‘piggyback’, to put a message onto the power beams.
The thresholds they have reported, above which no signals are present, have implications for the presence of power beams in the anomalous star system. Beaming power on astronomical scales has been a frequent topic on this site and it has long been pointed out that the beaming of power for various purposes could be observable at astronomical distances.
The missions suggested for power beaming involve Earth-to-space applications such as launching spacecraft to orbit or raising satellites from a lower orbit to a higher one. Several workers have studied interplanetary missions, meaning space-to-space transfers of cargo. Finally, launch into the outer solar system and for interstellar precursors and ultimately for starships has also been quantified.
We have examined the thresholds in light of concepts proposed for beaming power in and around our solar system. By comparing the reported thresholds set by the SETI Institute, the non-observation of leakage signals at their stated thresholds implies the following:
- Orbit raising missions, which require lower power, are not detectable at the thresholds of the Allen Array.
- Launch from a planetary surface into orbits would be bright enough to be seen by the 100 kHz observations. However, the narrow bandwidth 1 Hz survey would not see them.
- Interplanetary transfers by beam-driven sails should be detectable in their observations, but are not seen. This is for both the narrow 1 Hz and for the “wideband” 100 kHz observations.
- Starships launched by power beams with beamwidths that we happen to fall within would be detectable, but are not seen.
These results must be qualified by noting:
- Power beaming is not an isotropic endeavor, and so the geometry of the transmitter and the intended recipient will produce a conjunction from our point of view only episodically. The observations were conducted for only a limited time and further observations would provide a more stringent constraint.
- Even the “wideband” observation is actually quite narrow compared with the kinds of sources that would be used in power beams, based on our current understanding of microwave physics. For the applications discussed here, the 100 kHz bandwidth observed would be about 10 to 100 millionths of the center frequency of the Beamer. But high-power devices are inherently not designed for such narrow bandwidths.
- The frequencies we would use for power beaming are in the millimeter band, so are outside the microwave range the Allen Array observed.
Therefore the observations by the Allen Array are not sufficiently broad to produce firm conclusions about realistic Beamers.
Readers are encouraged to consult the original paper: Harp et al., “Radio SETI Observations of the Anomalous Star KIC 8462852” (preprint). Previous discussions on this matter can be found in the following reports:
“A Path Forward for Beamed Sails”: https://centauri-dreams.org/?p=20962
“Seeing Alien Power Beaming”: https://centauri-dreams.org/?p=34133
“Microwave Beaming: A Fast Sail to Mars”: https://centauri-dreams.org/?p=1176
“The Case for Beamed Sails”: https://centauri-dreams.org/?p=20924
Alpha Centauri Planet Reconsidered
Finding a habitable world around any one of the three Alpha Centauri stars would be huge. If the closest of all stellar systems offered a blue and green target with an atmosphere showing biosignatures, interest in finding a way to get there would be intense. Draw in the general public and there is a good chance that funding levels for exoplanet research as well as the myriad issues involving deep space technologies would increase. Alpha Centauri planets are a big deal.
The problem is, we have yet to confirm one. Proxima Centauri continues to be under scrutiny, but the best we can do at this point is rule out certain configurations. It appears unlikely, as per the work of Michael Endl (UT-Austin) and Martin Kürster (Max-Planck-Institut für Astronomie), that any planet of Neptune mass or above exists within 1 AU of the star. Moreover, no ‘super-Earths’ have been detected in orbits with a period of less than 100 days. This doesn’t rule out planets around Proxima, but if they are there, so far we don’t see them.
Image: The Alpha Centauri stellar system, consisting of the red dwarf Proxima Centauri and the two bright stars forming a close binary, Centauri A and B. Credit: NASA.
Centauri B, the K-class star in close proximity to G-class Centauri A, was much in the news a few years back with the announcement of Centauri Bb, a candidate world announced by Swiss planet hunters. This is radial velocity work based on data gathered by the HARPS (High Accuracy Radial Velocity Planetary Searcher) spectrometer on the 3.6-meter telescope at the European Southern Observatory in La Silla, Chile. The signal that Xavier Dumusque and team drew out of the data was 0.5 meters per second, a fine catch if confirmed.
What we thought we had in Centauri Bb was a mass just a little over the Earth’s and an orbit of a scant 3.24 days. As the blistering first planet detected around one of the Centauri stars, it would be a significant find even if it’s a long way from the temperate, life-sustaining world we’d like to find further out. The putative Centauri Bb supported the idea that there might be other planets there, and we’ve known since the work of Paul Wiegert and Matt Holman back in the 1990s that sustainable habitable zone orbits are possible around both the primary Alpha Centauri stars.
But Centauri Bb has remained controversial since Artie Hatzes (Thuringian State Observatory, Germany), using different data processing strategies, looked at the same data and found a signal he considered too noisy, indicating that what might be a planet might also be stellar activity on Centauri B itself. Debra Fischer’s team at Cerro Tololo Inter-American Observatory has also been studying Centauri Bb using the CHIRON spectrometer but has not been able to confirm it. And while a transit search using the Hubble Space Telescope did find a promising lightcurve (about which more in a moment), it couldn’t confirm Centauri Bb.
Image: Of the three stars of Alpha Centauri, the dimmest, Proxima Centauri, is actually the nearest star to the Earth. The two bright stars, Alpha Centauri A and B form a close binary system; they are separated by only 23 times the Earth – Sun distance. This is slightly greater than the distance between Uranus and the Sun. The Alpha Centauri system is not visible from much of the northern hemisphere. The image above shows this star system and other objects near it in the sky. Credit/copyright: Akira Fujii / David Malin Images.
Now we have a new paper from Vinesh Rajpaul (University of Oxford) and colleagues that makes Centauri Bb look more unlikely than ever. Rajpaul praises the thorough work of Xavier Dumusque and the team at the Geneva Observatory, but notes that their attempts to filter stellar activity out of their data evidently boosted other periodic signals that had nothing to do with a planet. The signal grows out of the time sampling, or ‘window function,’ of the data.
What is left behind is what the paper calls ‘the ‘ghost’ of a signal’ that was present all along. The paper argues that when a signal is sampled at discrete times (and the Dumusque team had to use the La Silla instrument only when it was not otherwise booked), periodicities can be imposed on the signal. Rajpaul was able to simulate a star with no planets, generating synthetic data out of which the exact same 3.24-day planetary signal emerged. The problem is particularly acute when working with planetary ‘signals’ as weak as these. From the paper:
D12’s data set [i.e., the data gathered by Dumusque and team] was particularly pathological because the window function happened to contain periodicities that coincided with the stellar rotation period of α Cen B, and its first harmonic; when these signals were filtered out, the significance of the 3.24 d signal was preferentially boosted.
All this is going to be quite useful if it helps us refine our techniques for identifying small planets. Rajpaul proposes that his team will carry out a new study of the spurious but coherent signals that can emerge from noisy datasets that should help us learn how to mitigate the problem:
We alluded to a number of other tests we believe worth carrying out when considering the reliability of planet detections from noisy, discretely-sampled signals. These include using the same model used to detect the planet instead to fit synthetic, planet-free data (with realistic covariance properties, and time sampling identical to the real data), and checking whether the ‘planet’ is still detected; comparing the strength of the planetary signal with similar Keplerian signals injected into the original observations; performing Bayesian model comparisons between planet and no-planet models; and checking how robust the planetary signal is to datapoints being removed from the observations.
Xavier Dumusque praises the Rajpaul team in this story in National Geographic, saying “This is really good work… We are not 100 percent sure, but probably the planet is not there.” We’re going to get a lot out of this investigation even though we lose Centauri Bb.
But back to that HST transit study run by Brice-Olivier Demory (University of Cambridge). I mentioned that it could detect no transit of Centauri Bb, which certainly fits with what we’ve just seen, but there was an interesting lightcurve suggesting a different possible planet, this one in an orbit that might range from 12 to 20 days. If this planet exists, radial velocity confirmation would be even more challenging than for Centauri Bb. Its signal, as Andrew LePage notes in The Discovery of Alpha Centauri Bb: Three Years Later, would be only half that of Centauri Bb.
LePage’s work at Drew ex Machina is definitive, and he has devoted a good deal of attention to Alpha Centauri. Here he explains why that second ‘planet’ is going to be so hard to spot:
Unfortunately with such a poorly constrained orbit, three weeks of nearly continuous photometric monitoring of α Centauri B will be required to confirm this hypothesis. HST is too busy to accommodate a dedicated search of this length and no other space telescope currently available is capable of making the needed observations. In addition, since the radial velocity signature for this planet would be expected to be maybe half that of α Centauri Bb, this method has little likelihood of providing independent confirmation of this sighting any time soon. Once again, we will have to wait for a few more years for new telescopes to become available such as NASA’s TESS (Transiting Exoplanet Survey Satellite) mission or ESA’s CHEOPS (Characterizing Exoplanets Satellite) which are both scheduled for launches in 2017 and may be capable of making the required observations of such a bright target.
Alpha Centauri is frustrating in many ways because you would expect the closest stellar system to have revealed more of its secrets by now. One of the problems, though, and a huge one, is that the angular separation (as viewed from Earth) of the primary Centauri stars has been decreasing as they move through their orbits. It won’t be until December of this year that they’ll reach minimum separation as seen from Earth. We’ll need to give Alpha Centauri a little time, in other words, before we can hope to get data on other possible worlds around Centauri B.
Image (click to enlarge): Apparent and true orbits of Alpha Centauri. The A component is held stationary and the relative orbital motion of the B component is shown. The apparent orbit (thin ellipse) is the shape of the orbit as seen by an observer on Earth. The true orbit is the shape of the orbit viewed perpendicular to the plane of the orbital motion. According to the radial velocity vs. time [10] the radial separation of A and B along the line of sight had reached a maximum in 2007 with B being behind A. The orbit is divided here into 80 points, each step refers to a timestep of approx. 0.99888 years or 364.84 days. Credit: Wikimedia Commons.
The Rajpaul paper is Rajpaul, Aigrain & Roberts, “Ghost in the time series: no planet for Alpha Cen B,” accepted for publication at Monthly Notices of the Royal Astronomical Society (preprint). The Hatzes paper is “Radial Velocity Detection of Earth-Mass Planets in the Presence of Activity Noise: The Case of α Centauri Bb”, The Astrophysical Journal, Vol. 770, No. 2, (2013) (preprint).
SETI: No Signal Detected from KIC 8462852
I’ve mentioned before that I think the name ‘Tabby’s Star’ is a wonderful addition to the catalog. It trips off the tongue so much more easily than the tongue-twisting KIC 8462852, and of course it honors the person who brought this unusual object to our attention, Yale University postdoc Tabetha Boyajian. 1480 light years away, Tabby’s Star is an F3 with a difference. It produces light curves showing objects transiting across its face, some of them quite large, and the search is on to find an explanation that fits within the realm of natural causes.
Five articles about Tabby’s Star have already appeared in these pages, with the most likely explanation being some kind of cometary activity, an answer that seems to satisfy no one. We’ve also consulted both Boyajian’s paper on the subject and a paper by Jason Wright and colleagues out of the Glimpsing Heat from Alien Technologies project at Penn State. The light curves we’re looking at do fit the scenario of a ‘Dyson swarm,’ a cluster of power-collecting surfaces that an advanced civilization might create to extract maximum energy from its star.
Thus we can’t rule out the possibility of an extraterrestrial civilization, but no one is claiming that we’ve found one. The point is that in terms of Dysonian SETI, which looks for signs of another civilization’s activity in our astronomical data, Tabby’s Star is the most interesting target we’ve found, so it only makes sense to investigate it. Assuming we do deduce a natural cause for its signature, we will have learned something about an unusual astrophysical process, and that is all to the good. The sole driver here is to investigate and find out what is happening.
The SETI Institute has a natural interest in all this and has been deploying the Allen Telescope Array on Tabby’s Star for more than two weeks. Now we have an update on what the Institute has found. The effort used the Array’s 42 antennas north of San Francisco to look for narrow-band signals (approximately 1 Hz in width) that could be part of an interstellar beacon. In general, SETI at radio and optical frequencies (SETI, that is, of the non-Dysonian kind) looks for this kind of signal, a deliberate attempt by a civilization to declare its presence.
Image: Allen Telescope Array. Credit: Seth Shostak, SETI Institute.
But the SETI Institute also looked for broadband signals, an interesting choice. Here we are asking whether, if there really is an enormous astro-engineering effort going on around this star, there would be spacecraft sent out to service it. Our own investigations into quick travel around the Solar System point to microwave beaming as a feasible solution, the basis for an interplanetary infrastructure. Such intense microwave beams might well be visible, a kind of ‘leakage’ from the civilization’s activities that implies nothing about communication.
Here’s the result, from the SETI Institute’s paper on the work (Jy stands for jansky, a unit of density used in radio astronomy):
The observations presented here indicate no evidence for persistent technology-related signals in the microwave frequency range 1 – 10 GHz with threshold sensitivities of 180 – 300 Jy in a 1 Hz channel for signals with 0.01 – 100 Hz bandwidth, and 100 Jy in a 100 kHz channel from 0.1 – 100 MHz.
So no clear evidence for either kind of signal between 1 and 10 GHz. The paper goes on:
These limits correspond to isotropic radio transmitter powers of 4 – 7 1015 W and 1020 W for the narrowband and moderate band observations. These can be compared with Earth’s strongest transmitters, including the Arecibo Observatory’s planetary radar (2 1013 W EIRP [effective isotropically radiated power]). Clearly, the energy demands for a detectable signal from KIC 8462852 are far higher than this terrestrial example (largely as a consequence of the distance of this star).
What this initial search does is to place upper limits on anomalous emissions from Tabby’s Star. It tells us that we can rule out omnidirectional transmitters broadcasting narrow-band signals at approximately 100 times today’s total terrestrial energy usage, as well as broadband emissions of ten million times terrestrial energy usage. These numbers are high, as the Institute notes, but the paper goes on to say that required transmitter power for narrow-band signals could be reduced considerably if a signal were being beamed in our direction intentionally. It’s worth remembering, too, that any civilization of K2 status (capable of building a Dyson swarm) should have approximately 1027 watts to work with, the energy output of its star.
In any case, says Institute astronomer Seth Shostak, we keep looking:
“The history of astronomy tells us that every time we thought we had found a phenomenon due to the activities of extraterrestrials, we were wrong. But although it’s quite likely that this star’s strange behavior is due to nature, not aliens, it’s only prudent to check such things out.”
Exactly so. The authors add that the star will be the subject of observations for years to come.
Addendum: The Boquete Optical SETI Observatory in Panama is also going to be brought into the search, as per this story.
The paper is Harp et al., “Radio SETI Observations of the Anomalous Star KIC 8462852” (preprint). A SETI Institute news release is also available.
A 3D Look at GJ 1214b
An old friend used to chide me about the space program, asking good-naturedly enough why it mattered to travel nine years to get to a place like Pluto (this was not long after the New Horizons launch). ‘Just another rock,’ he would say. ‘Why go all that way to look at just another rock?’ Although we had many disagreements, Abe was one of the shrewdest people I’ve ever known. I had met him when he was in his sunset years, but in his prime he had run a large financial operation, been the subject of a story on the front page of the Wall Street Journal and had made a serious fortune in real estate speculation.
So what about this ‘just another rock’ meme? Abe died a few years back but I think about him in relation to things like yesterday’s story on Charon. The point is, it’s not just another rock. It’s this particular rock. And maybe it’s not a rock at all; maybe it’s a ball of icy slush. And maybe, as we’ve learned, it’s a seriously interesting thing that surpasses expectation. Each time we get to one of these places, or study a planet with transits and radial velocity methods, we’re seeing something never seen before. It may be able to teach us about conditions far different from those we experience and explain deep questions about the history of our own solar system.
Beyond that, each new ‘rock’ is part of a process of building understanding. We need to know what is around us because while we cannot solve all mysteries, we are compelled to solve the ones in front of us, the ones we can get to or develop the tools to investigate. The thing is, each time we go looking we seem to find something surprising, and then we need an answer for the conundrum. It’s a process that presumably began early in the development of our species.
I used to kid Abe by responding, ‘What does another dollar matter? It’s just another dollar.’ He got the point. Abe had a natural touch with money and knew how to make it multiply. Each new business venture was for him a kind of exploration. We just differed in the direction of our passions. I’ll take Charon or GJ 1214b over stock exchanges in New York or Tokyo, and that’s probably while you’ll never be reading about me on the front page of the Wall Street Journal.
An Exoplanet’s Clouds
So let’s talk about exoplanets. GJ 1214b is likewise ‘just another planet’ when viewed with the wrong filter. Viewed as we view things on Centauri Dreams, it’s one particular planet, and like so many we’ve found, it has its own set of things to teach us. Its star, an M-dwarf, is 42 light years away in the constellation Ophiuchus. Consider this planet a ‘mini-Neptune,’ one of the first discovered. What makes GJ 1214b so interesting is that it’s so close to our own system and gives us such a rich transit signature every 1.6 days.
Image: Comparing the sizes of exoplanets GJ 436b and GJ 1214b with Earth and Neptune. Credit: NASA / ESA / A. Feild and G. Bacon, STScI.
We can use what is called transmission spectroscopy to study the atmospheres of places like this even though we can’t actually see the planet other than through its light curve. A planet passing in front of its star as seen from Earth offers us the opportunity to parse starlight that has been filtered through its atmosphere. The spectrum thus produced tells us about the composition of that atmosphere, its molecules and dust grains. It’s a method that has been used with great effect on worlds like HAT-P-11b and the ‘hot Jupiter’ HD 189733b.
The problem is, when we apply the techniques of transmission spectroscopy to GJ 1214b, nothing much happens. Studies using the Hubble instrument see little variation, a ‘flat spectrum’ that rules out an atmosphere of hydrogen, water, carbon dioxide or methane. Something in the planet’s atmosphere is evidently blocking out light. In a new paper, Benjamin Charnay (University of Washington), working with the university’s Victoria Meadows and several other researchers, has been attacking the problem, setting up models of varying atmospheric temperatures and composition that simulate a three-dimensional cloud structure.
Using a climate model developed by Charnay’s former research group in Paris, the scientist applied models previously used to study Titan to this intriguing exoplanet. GJ 1214b is close enough to its star that atmospheric temperatures are high, exceeding the boiling point of water. Clouds on a world like this would, Charnay believes, most likely be made of potassium chloride (KCl) or zinc sulfide (ZnS), lifted high into the atmosphere to produce such a flat spectrum. The model relies on a robust atmospheric circulation to boost these clouds to altitude.
But there is another potential source of GJ 1214b’s flat spectrum: A photochemical haze. Charnay is now looking at modeling hazes that could produce the same kind of spectrum. Data from the James Webb Space Telescope, scheduled to be launched in 2018, will be needed to rule out alternatives. Assuming 0.5 µm particles (necessary to produce the flat spectra observed), the paper finds that potassium chloride clouds produce a constant reflectivity at visible wavelengths, while zinc sulfide clouds do not absorb at 0.5 µm, producing a peak of reflectivity. Organic haze, on the other hand, strongly absorbs at short visible wavelengths.
A stratospheric thermal inversion should show up at infrared wavelengths. From the paper:
The observation of a few primary/secondary eclipses or full orbits by JWST could provide very precise spectra and phase curves revealing GJ1214b’s atmospheric composition and providing clues on the size and optical properties (i.e. absorbing or not) of clouds. Non-absorbing clouds would suggest KCl particles. Absorbing clouds would favor ZnS particles or organic haze. In that case, the best way for determining the composition of cloud particles would be direct imaging or secondary eclipses/phase curves of reflected light in the visible. The different clouds/haze have characteristic features in visible reflectivity spectra. Future large telescopes such as ELT may have the capabilities for measuring this.
Charnay’s model shows how potassium chloride or zinc sulfide clouds would be created and lifted into the upper atmosphere, while also predicting the effect the clouds would have on planetary weather. The atmospheric circulation in this model is strong enough to carry KCl particles to high altitude regions while producing a minimum of cloud cover at the equator.
Just another rock? Look what we’re doing here: We’re studying layers of atmosphere on a planet we cannot see by using the spectroscopic signatures produced by a star 42 light years from the Sun. In astronomical terms, of course, that’s close, and GJ 1214b’s proximity makes it ideal for the study of mini-Neptunes. Moreover, it may be useful for understanding how our own planet’s atmosphere has changed over time, as Charnay notes in this UW news release:
“Worlds like Titan and this exoplanet have complex atmospheric chemistry that might be closer to what early Earth’s atmosphere was like. We can learn a lot about how planetary atmospheres like ours form by looking at them.”
The trick is in knowing what to look for and developing the tools to investigate — later resources, both space- and ground-based, will then be brought to bear to further the analysis. Given the wild multiplicity of exoplanets, we’re sure to be applying lessons learned at GJ 1214b to other mini-Neptunes, and generalizing from there to broader models of atmospheric evolution. Uncovering things, learning, pushing deeper is a compulsive process. We all have our obsessions, but I can’t think of a better one than the drive to explain other worlds.
The paper is Charnay et al., “3D modeling of GJ1214b’s atmosphere: formation of inhomogeneous high clouds and observational implications,” Astrophysical Journal Letters, Volume 813, Number 1 (abstract / preprint).
Unusual Crater on Charon
Another surprise from New Horizons, in a year which will surely see a few more before it ends. After all, we have a long flow of data ahead as the spacecraft continues to return the information it gathered during the July flyby of Pluto/Charon. Now we focus on Charon and the crater being called Organa, which produced an anomaly when scientists studied the highest resolution infrared compositional scan of the moon available. This crater and some of the surrounding materials show infrared absorption at about 2.2 microns, indicating frozen ammonia.
Not far away on Charon’s Pluto-facing hemisphere is Skywalker crater, which under infrared scrutiny shows the same composition as the rest of Charon’s surface. Here water ice — not ammonia — dominates. As this JHU/APL news release notes, ammonia absorption was first detected on Charon as far back as 2000, but what we’re seeing here is unusually concentrated. In any case, why is Organa so different from Skywalker and the rest of Charon’s craters?
Image: This composite image is based on observations from the New Horizons Ralph/LEISA instrument made at 10:25 UT (6:25 a.m. EDT) on July 14, 2015, when New Horizons was 81,000 kilometers from Charon. The spatial resolution is 5 kilometers per pixel. The LEISA data were downlinked Oct. 1-4, 2015, and processed into a map of Charon’s 2.2 micron ammonia-ice absorption band. Long Range Reconnaissance Imager (LORRI) panchromatic images used as the background in this composite were taken about 8:33 UT (4:33 a.m. EDT) July 14 at a resolution of 0.9 kilometers per pixel and downlinked Oct. 5-6. The ammonia absorption map from LEISA is shown in green on the LORRI image. The region covered by the yellow box is 280 kilometers across. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.
Organa and Skywalker are roughly the same size, about 5 kilometers in diameter, and both show the same ‘rays’ of ejected material, although Organa’s central areas are darker. But there seems to be no correlation: The ammonia-rich material extends beyond the dark area. One possibility is that the impactor that created Organa was rich in ammonia. An alternative posited by Will Grundy (New Horizons Composition team lead, Lowell Observatory) is that the crater could have been the result of an impact into a pocket of ammonia-rich subsurface ice.
“This is a fantastic discovery,” says Bill McKinnon (Washington University, St. Louis), deputy lead for the mission’s Geology, Geophysics and Imaging team. “Concentrated ammonia is a powerful antifreeze on icy worlds, and if the ammonia really is from Charon’s interior, it could help explain the formation of Charon’s surface by cryovolcanism, via the eruption of cold, ammonia-water magmas.”
Meanwhile, New Horizons has now completed the third of its four trajectory-adjusting maneuvers needed for intercept of Kuiper Belt object 2014 MU69. This was a 30-minute burn with the craft’s hydrazine thrusters and all indications are that it was successful. The fourth and final targeting maneuver is scheduled for today (November 4), although adjustments will be made later in the mission as more information about the KBO’s orbit is obtained. Remember that we only found 2014 MU69 in 2014, after a long search for a Kuiper Belt candidate.
Image: Projected route of NASA’s New Horizons spacecraft toward 2014 MU69, which orbits in the Kuiper Belt about 1.6 billion kilometers beyond Pluto. Planets are shown in their positions on Jan. 1, 2019, when New Horizons is projected to reach the small Kuiper Belt object. NASA must approve an extended mission for New Horizons to study the ancient KBO. Credit: JHU/APL.
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