by Paul Gilster | Nov 24, 2023 | Exoplanetary Science |
Although I think most astronomers have assumed Proxima Centauri was bound to the central binary at Alpha Centauri, the case wasn’t definitively made until fairly recently. Here we turn to Pierre Kervella (Observatoire de Paris), Frédéric Thévenin (Côte d’Azur Observatory) and Christophe Lovis (Observatoire Astronomique de l’Université de Genève). We last saw Dr. Kervella with reference to a paper on aerographite as a sail material, but his work has appeared frequently in these pages, analyzing mission trajectories and studying the Alpha Centauri system. Here he and his colleagues use HARPS spectrographic data to demonstrate that we have at Centauri a single gravitationally bound triple system. This is important stuff; let me quote the paper on this work to explain why (italics mine):
Although statistical considerations are usually invoked to justify that Proxima is probably in a bound state, solid proof from dynamical arguments using astrometric and radial velocity (RV) measurements have never been obtained at a sufficient statistical significance level. As discussed by Worth & Sigurdsson (2016), if Proxima is indeed bound, its presence may have impacted planet formation around the main binary system.
This is a six-year old paper, but I want to return to it now because a new paper from the same team will tighten up its conclusions and slightly alter some of them. We’ve gone from resolving whether Proxima is bound to the A/B binary to pondering the issues involved in the dynamical history of this complex system. That in turn can inform the ongoing search for planets around Centauri A and B at least in terms of explaining what we might find there and how these two systems evolved. The original paper on this work lays out the challenges involved in tracing the orbit of the red dwarf. For HARPS is exquisitely sensitive to the Doppler shifts of starlight, and these data, obtained between 2004 and 2016, contain potential booby traps for analysis.
Image: Pierre Kervella, of the Observatoire de Paris/PSL.
Convective blueshift is one of these. We’re looking at the star’s spectral lines as we calculate its motion, and some of these are displaced toward the blue end of the spectrum because of the structure of its surface convection patterns. The lifting and sinking of hot internal gases has to be factored into the analysis and its effect nulled out. The spectral lines are displaced toward the blue, in effect a negative radial velocity shift, although the effect is stronger for hotter stars. In the case of Proxima, Kervella’s team finds a relatively small convective blueshift, though still one to be accounted for.
A similar though more significant issue is gravitational redshift, which occurs as photons climb out of the star’s gravity well. Here the effect is “an important source of uncertainty on the RV of Proxima” whose value can be established and corrected. How the astronomers went about making these corrections is laid out in a discussion of radial velocities that aspiring exoplanet hunters will want to read.
Image: Orbital plot of Proxima showing its position with respect to Alpha Centauri over the coming millenia (graduations in thousands of years). The large number of background stars is due to the fact that Proxima is located very close to the plane of the Milky Way. Credit: P. Kervella/ESO/Digitized Sky Survey 2/Davide De Martin/Mahdi Zamani.
Out of all this we learn that Proxima’s elliptical orbit around Centauri A and B’s barycenter extends from 800 billion kilometers when closest (periastron) to 1.9 trillion kilometers at apastron, its farthest distance, with an orbital period of approximately 550,000 years. The orbital phase is currently closest to apastron.
The Astronomy & Astrophysics site (this is the journal in which the paper above appeared) is currently down, so I’m quoting from the version of the paper on arXiv, which after noting that the escape velocity of Alpha Centauri at Proxima’s distance (545 +/- 11 m/s) is about twice as large as Proxima’s measured velocity, goes on to speculate in an intriguing way:
Proxima could have played a role in the formation and evolution of its planet (Anglada-Escudé et al. 2016). Conversely, it may also have influenced circumbinary planet formation around αCen (Worth & Sigurdsson 2016). A speculative scenario is that Proxima b formed as a distant circumbinary planet of the αCen pair, and was subsequently captured by Proxima. Proxima b could then be an ocean planet resulting from the meltdown of an icy body (Brugger et al. 2016). This would also mean that Proxima b may not have been located in the habitable zone (Ribas et al. 2016) for as long as the age of the αCen system (5 to 7 Ga; Miglio & Montalbán 2005; Eggenberger et al. 2004; Kervella et al. 2003; Thévenin et al. 2002).
The idea of Proxima b as a captured planet has not to my knowledge appeared anywhere else in the literature. I was fascinated, enough so that I dashed off a quick email to Dr. Kervella asking about this as well as the current status of the orbital calculations. And indeed, his response indicates new work in progress:
… we identified a mistake in our 2017 determination of the orbital parameters of Proxima. In the papier, they are expressed in the Galactic coordinate system, and the orbital inclination is thus not directly comparable to that of the Alpha Cen AB orbit. We are preparing a new publication with revised orbits and parameters for all three stars. The main difference is that now the orbital plane of Proxima is better aligned with that of AB. The gravitationally bound nature of Proxima with Alpha Cen AB is also strengthened, as we include new astrometry and radial velocities.
I’ll cover the new paper as soon as it appears. Dr. Kervella also observes that confirming a scenario of Proxima b as a captured planet would be difficult (Proxima b has ‘forgotten’ the history of its orbital evolution, as he puts it), meaning that working with astrometric data alone will not be sufficient. But the arrival of telescopes like the Extremely Large Telescope, now under construction in Chile’s Atacama Desert with first light planned for 2028, should signal a treasure trove of new information. A spectrum obtained by ELT could show us whether Proxima b is indeed an ocean planet.
The paper on Proxima Centauri’s orbit is Kervella, Thévenin & Lovis, “Proxima’s orbit around α Centauri,” Astronomy & Astrophysics Vol. 598 (February 2017), L7 (abstract/preprint).
by Paul Gilster | Oct 11, 2023 | Missions |
Larry Niven played around with an interesting form of suspended animation in his 1966 Ballantine title World of Ptavvs. While the usual science fictional imagining is of a crew in some sort of cryogenic deep freeze, Niven went all out and envisioned a means of suspending time itself. It’s an ingenious concept based on an earlier short story in Worlds of Tomorrow, one that so aggressively pushes the physics that the more subtle delights of characterization and perspective come almost as afterthoughts. Niven fans like myself will recognize it as taking part in his ‘Known Space’ universe.
In the absence of time manipulation, let’s plumb more modest depths, though these can be tantalizing in their implications. In the last post, Don Wilkins described new work out of Washington University on inducing states of torpor – life processes slowed along with temperature – in laboratory experiments involving rodents. The spectrum from torpor to suspended animation has intervals that may suit our purpose. At the Interstellar Research Group’s Montreal symposium, John Bradford examined technologies with interplanetary as well as interstellar implications, though as he pointed out, they differ greatly from our cryogenic imaginings.
No deep freeze, in other words, and an experience for the crew that is far more like sleep – a very deep sleep – than the profound stoppage of metabolism that might keep a crew on ice for centuries. Bradford, who is CEO and principal engineer at Atlanta-based SpaceWorks, has explored the subject in two NIAC studies. His review for the Montreal audience took note of where we are today, when placing humans into a reduced metabolic state is relatively well understood, and therapeutic hypothermia is a medical treatment that can involve inhaled gases, cold saline injection and ice packs.
What particularly intrigued me in Bradford’s presentation (available here) is that there are numerous documented stories of human survival in extreme conditions, including one Anna Bagenholm, who survived submersion in an ice-covered lake for three hours. There are even some indications that some form of hibernation may have been present in early hominins, as evidenced by remains found in Spain involving changes in bone structure and density that imply cycles of use and disuse. Bradford’s NIAC work involved a mission design for Mars and Ceres in which the crew cycled through torpor states alternating with active periods. This notion of cyclic hibernation seems promising if we can discover ways of using it that maintain crew health both physical and mental.
Image: Suspended animation the ‘old-school’ way. This is an image from the 2016 film Passengers, involving a crew that is put into stasis inside hibernation pods.
Can we implement torpor in deep space? It’s a fascinating issue, one that may offer a path to manned interstellar travel if we can reduce trip times down to perhaps 50 years, a number Bradford chooses because, in conjunction with increases in human longevity, it offers missions where the balance of a human life is lived before and after the mission. The challenges are huge and already familiar from our space activities aboard the ISS. Muscle atrophy can be profound, as can bone loss and demineralization. An interstellar crew would confront exposure to galactic cosmic rays, while shielding exacts a mass penalty. There is also the matter of consumables.
Let’s face it, the human body is simply not designed for being off-planet, nor do we have even a fraction of the information we need to really assess such things as interplanetary missions with human crews from the physiological point of view. Until we have a dedicated research lab in orbit, we’re reduced to theorizing based on data from the ISS and manned missions that have never gone beyond the Moon.
If we can induce torpor states, we could drastically reduce the bulk of crew consumables needed for missions to another star, but let’s be clear about the kind of mission we’re talking about. A breakthrough in some sort of Alcubierre-like drive would eliminate the need for hibernation. On the other hand, absent the ability to take humans to 10 to 20 percent of lightspeed, we need to look to generation ships as the only viable way to move adult humans to an exoplanet, perhaps considering the possibility of using AI and human embryos raised at destination.
That’s why Bradford is really looking at ships moving fast enough that an interstellar crossing could be made within decades, and here we can look at such possibilities as induced ‘profound hypothermia’ that drops the body temperature below 20 ℃, a procedure used today in extreme cases and generally as a last resort. Perhaps more useful and certainly less drastic is gene-editing to enhance what may be in-built hibernation capabilities. Combine increased human longevity with some form of induced torpor and you come up with mission scenarios involving cycles of torpor and full wakefulness. Indeed, a 4-week cycle of torpor between periods of wakeful activity can reduce the perception of a 50-year transit to another star to a ten year period.
Plenty of work is going on in terms of longevity extension, ranging from research groups like the Methuselah Foundation and Altos Labs to drug trials involving replacement of molecules that tend to diminish with age and supplements like resveratrol and taurine that have promise in increasing lifespan. I’m not familiar with the details of this research, but Bradford said that there are voices in the scientific community arguing that 150 years is a reasonable goal for the average human, with the current record-holder (Jeanne Calment) having managed a startling 122.
Our scenario, then, is one in which we use induced torpor in a cyclical manner to reduce the time perception of an interstellar crossing. Therapeutic hypothermia (TH) currently involves periods of no more than three days, but as the chart above shows (drawn from Bradford’s slides), we can think in terms of two weeks in stasis with four or five days active as an achievable goal based on current research. Bradford’s NIAC work involved missions to Mars and Ceres using this cycle. Going beyond it raises all kinds of interesting questions about how the body responds to lengthy torpor states and, just as significantly, what happens to human cognition.
by Paul Gilster | Oct 6, 2023 | Culture and Society |
Suspended animation shows up early in science fiction after a long history in prior literature. In Shakespeare, it’s the result of taking a “distilling liquor” (thus Juliet’s ‘sleep,’ which drives Romeo to suicide). In the SF realm, an early classic is John Campbell’s 1938 story “Who Goes There?”, which became the basis for the wonderful “The Thing from Another World” (1951). Here an alien whose spacecraft has crashed remains in frozen suspension for millennia, only to re-emerge as the barely recognizable James Arness. In the essay below, Don Wilkins points us toward a new study that could have implications for achieving the kind of suspended animation that one day might get a crew through a voyage lasting centuries. A frequent contributor to Centauri Dreams, Don is an adjunct instructor of electronics at Washington University, where the work took place. Echoes of van Vogt’s “Far Centaurus”? Read on. I’ll have another take on this topic in the next post.
by Don Wilkins
Humans have often observed with envy the ability of certain animals to extend sleeping periods from mere hours to months. If bears can do it, why cannot a suitably prepared person do it? Artificial hibernation is often used in science fiction to transport an individual into a distant future without the bother of aging or achieving relativistic speeds or conserving scarce resources. A practical hibernation system, in a more terrestrial function, provides medical support, improving survival by decreasing metabolic activity of a critically ill patient. Some writers hypothesize that a certain number of sleepers, particularly hibernations of decades or more, will suffer disabilities or death.
Research has focused on inducing torpor, a condition of significantly decreased metabolic rates and body activity, producing hibernation without adverse side-effects or horrifying experiments with cryogenics. A practical system remains within the realm of science fiction.
Torpor, like hibernation, is a physiological state in which various animals, including certain fish, reptiles, insects and mammals, actively suppress metabolism, lower body temperature and slow other life processes to conserve energy and survive fatal conditions and cold environmental temperatures.
A research team led by Yaoheng Yang (Washington University, St. Louis) has demonstrated a novel method for inducing torpor in rodents: Deep ultrasonic stimulation of a mammal’s brain [1]. Animals in torpor states experience reduced metabolism and body temperatures. Ultrasound was selected as the stimuli as it noninvasively and safely penetrates bone, and can be tightly focused with millimeter precision. The team hypothesizes that the central nervous system organizes the multitude of reactions needed to induce torpor.
In the experiments, as shown in Figure 1, a mouse wore a tiny “hat”, a lead zirconate titanate ceramic piezoelectric ultrasonic device with a center frequency of 3.2 MHz. The output was focused on the animal’s brain in the hypothalamus preoptic area (POA). Activating the POA neurons induced a torpor state for periods greater than 24 hours.
Fig. 1: Ultrasound device for inducing a torpor-like hypothermic and hypometabolic state. a, Illustration of ultrasound (US)-induced torpor-like state. b, Illustration of the wearable US probe (top). The probe was plugged into a baseplate that was glued on the mouse’s head. MRI of the mouse head with the wearable US probe shows that ultrasound was noninvasively targeted at the POA (insert). Photograph of a freely moving mouse with the wearable US probe attached is shown at the bottom. c, Illustration of the US stimulation waveform used in this study. ISI, inter-stimulus interval; PD, pulse duration; PRF, pulse repetition frequency. d, Calibration of the temperature (T) rise on the surface (top) and inside (bottom) the US probe. The temperature inside the probe was measured between the piezoelectric material and the mouse head when US probes were targeted at the POA or the cortex.
When the POA was stimulated, the body temperatures of the test animals dropped approximately three degrees C, although the environment was held to room temperature. Metabolism switched from carbohydrates and fat to solely fat. Heart rates declined about 47%.
The system used an automatic closed-loop feedback controller with the animal’s body temperature as the feedback variable. Tests which kept the subject’s body 32.95 ℃ for 24 hours were successfully concluded when the ultrasonic influence was removed and the animal returned to normal body temperature. According to previous studies, the body temperature must be below 34 ℃ to induce torpor.
Increasing the acoustic pressure and duration of the ultrasound stimulus further lowered body temperature and slowed metabolism. Each ultrasonic pulse produced consistent neuronal activity increases together with body temperature reductions in the test subjects.
The team, through genetic sequencing, discovered ultrasound could restrain the TRPM2 ultrasound-sensitive ion channel in the POA neurons. The precise mechanism providing the torpid state is unknown.
In a rat, which does not naturally enter torpor or hibernation, ultrasound simulation of POA neurons reduced skin temperature and core body temperature.
Ultrasonics can, with great spatial accuracy, reach deeply within the brain to stimulate the POA neurons. This approach could serve as the foundation of a system providing long term, noninvasive, and safe induction of torpor.
Reference
1. Yang, Y., Yuan, J., Field, R.L. et al., “Induction of a torpor-like hypothermic and hypometabolic state in rodents by ultrasound,” Nature Metabolism 5, 789–803 (2023). Full text.
by Paul Gilster | Jun 22, 2023 | Uncategorized |
It’s no surprise that the James Webb Space Telescope’s General Observers program should target TRAPPIST-1 with eight different efforts slated for Webb’s first year of scientific observations. Where else do we find a planetary system that is not only laden with seven planets, but also with orbits so aligned with the system’s ecliptic? Indeed, TRAPPIST-1’s worlds comprise the flattest planetary arrangement we know about, with orbital inclinations throughout less than 0.1 degrees. This is a system made for transits. Four of these worlds may allow temperatures that could support liquid water, should it exist in so exotic a locale.
Image: This diagram compares the orbits of the planets around the faint red star TRAPPIST-1 with the Galilean moons of Jupiter and the inner Solar System. All the planets found around TRAPPIST-1 orbit much closer to their star than Mercury is to the Sun, but as their star is far fainter, they are exposed to similar levels of irradiation as Venus, Earth and Mars in the Solar System. Credit: ESO/O. Furtak.
The parent star is an M8V red dwarf about 40 light years from the Sun. It would be intriguing indeed if we detected life here, especially given the star’s estimated age of well over 7 billion years. Any complex life would have had plenty of time to evolve into a technological phase, if this can be done in these conditions. But our first order of business is to find out whether these worlds have atmospheres. TRAPPIST-1 is a flare star, implying the possibility that any gaseous envelopes have long since been disrupted by such activity.
Thus the importance of the early work on TRAPPIST-1 b and c, the former examined by Webb’s Mid-Infrared Instrument (MIRI), with results presented in a paper in Nature. We learn here that the planet’s dayside temperature is in the range of 500 Kelvin, a remarkable find in itself given that this is the first time any form of light from a rocky exoplanet as small and cool as this has been detected. The planet’s infrared glow as it moved behind the star produced a striking result, explained by co-author Elsa Ducrot (French Alternative Energies and Atomic Energy Commission):
“We compared the results to computer models showing what the temperature should be in different scenarios. The results are almost perfectly consistent with a blackbody made of bare rock and no atmosphere to circulate the heat. We also didn’t see any signs of light being absorbed by carbon dioxide, which would be apparent in these measurements.”
The TRAPPIST-1 work is moving relatively swiftly, for already we have the results of a second JWST program, this one executed by the Max Planck Institute for Astronomy and explained in another Nature paper, this one by lead author Sebastian Zieba. Here the target is TRAPPIST-1 c, which is roughly the size of Venus and which, moreover, receives about the same amount of stellar radiation. That might imply the kind of thick atmosphere we see at Venus, rich in carbon dioxide, but no such result is found. Let me quote Zieba:
“Our results are consistent with the planet being a bare rock with no atmosphere, or the planet having a really thin CO2 atmosphere (thinner than on Earth or even Mars) with no clouds. If the planet had a thick CO2 atmosphere, we would have observed a really shallow secondary eclipse, or none at all. This is because the CO2 would be absorbing all of the 15-micron light, so we wouldn’t detect any coming from the planet.”
Image: This light curve shows the change in brightness of the TRAPPIST-1 system as the second planet, TRAPPIST-1 c, moves behind the star. This phenomenon is known as a secondary eclipse. Astronomers used Webb’s Mid-Infrared Instrument (MIRI) to measure the brightness of mid-infrared light. When the planet is beside the star, the light emitted by both the star and the dayside of the planet reach the telescope, and the system appears brighter. When the planet is behind the star, the light emitted by the planet is blocked and only the starlight reaches the telescope, causing the apparent brightness to decrease. Credits: NASA, ESA, CSA, Joseph Olmsted (STScI)
What JWST is measuring is the 15-micron mid-infrared light emitted by the planet, using the world’s secondary eclipse, the same technique used in the TRAPPIST-1 b work. The MIRI instrument observed four secondary eclipses as the planet moved behind the star. The comparison of brightness between starlight only and the combined light of star and planet allowed the calculation of the amount of mid-infrared given off by the dayside of the planet. This is remarkable work: The decrease in brightness during the secondary eclipse amounts to 0.04 percent, and all of this working with a target 40 light years out.
Image: This graph compares the measured brightness of TRAPPIST-1 c to simulated brightness data for three different scenarios. The measurement (red diamond) is consistent with a bare rocky surface with no atmosphere (green line) or a very thin carbon dioxide atmosphere with no clouds (blue line). A thick carbon dioxide-rich atmosphere with sulfuric acid clouds, similar to that of Venus (yellow line), is unlikely. Credit: NASA, ESA, CSA, Joseph Olmsted (STScI).
I should also mention that the paper on TRAPPIST-1 b points out the similarity of its results to earlier observations of two other M-dwarf stars and their inner planets, LHS 3844 b and GJ 1252 b, where the recorded dayside temperatures showed that heat was not being redistributed through an atmosphere and that there was no absorption of carbon dioxide, as one would expect from an atmosphere like that of Venus.
Thus the need to move further away from the star, as in the TRAPPIST-1 c work, and now, it appears, further still, to cooler worlds more likely to retain their atmospheres. As I said, things are moving swiftly. In the coming year for Webb is a follow-up investigation on both TRAPPIST-1 b and c, in the hands of the system’s discoverer, Michaël Gillon (Université de Liège) and team. With a thick atmosphere ruled out at planet c, we need to learn whether the still cooler planets further out in this system have atmospheres of their own. If not, that would imply formation with little water in the early circumstellar disk.
The paper is Zieba et al., “No thick carbon dioxide atmosphere on the rocky exoplanet TRAPPIST-1 c,” Nature 19 June 2023 (full text). The paper on TRAPPIST-1 b is Greene et al., “Thermal emission from the Earth-sized exoplanet TRAPPIST-1 b using JWST,” Nature 618 (2023), 39-42 (abstract).
by Paul Gilster | Jun 13, 2023 | Exotic Physics |
A friend and I were sitting in a diner some time back talking mostly about old movies (my passion is for black-and-white films from 1927 to the death of Bogart in 1957). Somehow the topic of gravity came up, I suspect because we had homed in on early 50’s science...
