Centauri Dreams
Imagining and Planning Interstellar Exploration
Whispering At The Stars
Recent activity in sending signals to the stars has caught the attention of plasma physicist Jim Benford. The CEO of Microwave Sciences and chairman of the Sail Subcommittee for Breakthrough Starshot, Jim has more than a few doubts about the efficacy of these signals, and questions the rush to send them. Is the recent EISCAT signal detectable at interstellar distances? A look at the science of such signals follows, and thoughts on the caution with which we ought to proceed.
By James Benford
Yet another ‘Message’
Recently, advocates of METI (Messaging to Extraterrestrials) sent a ‘message’ consisting of prime numbers followed by 36 music pieces to Luyten’s Star. It was a collaboration of METI International, led by Doug Vakoch, with the Catalonia Institute of Space Studies. This star is 12.4 light years from Earth and has a potentially habitable exoplanet (GJ 273b).
This was sent from the EISCAT facilities near Tromsø, Norway, using a microwave antenna. The music pieces are 10 seconds long, therefore contain only 1500 bits, so are quite simple.
Can this deliberate transmission from Earth be detected at the distances of nearby stars? What is the reality of claims that the low power messages sent to date are ‘practically detectable’? Such qualitative statements are not useful in a quantitative science. We will see that the message is faint and very unlikely to be detected, even if aimed at nearer stars.
How detectable will the message actually be?
Image: EISCAT Tromsø site with the EISCAT3D test facility in the foreground. Credit: Craig Heinselman.
Can it be heard?
No. This group is in fact whispering at the stars.
I base this on what we know about Vakoch’s transmission: power 2MW, 32 meter dish, frequency 929-930.2MHz, bit rate 125 bits/s, encoded to 8 bits PCM and to a frequency of 6.4 kHz, repeating the 33 minute signal three times over three days.
Analysis of this EISCAT (European Incoherent Scatter Scientific Association) METI, accounting for differences in power, aperture and frequency, is that the power density at long range (EIRP, effective isotropic radiated power) is only 1/100th of Arecibo, and a 10th of that used by Alexander Zaitsev in his messages. Therefore, the several observations and conclusions that John Billingham and I made 3 years ago about the unobservable Zaitsev ‘messages’ are true in spades for this failed transmission as well (“Costs and Difficulties of large-scale METI, and the Need for International Debate on Potential Risks”, John Billingham and James Benford, JBIS 67, pg. 17, 2014).
The conclusion is: This will not be detectable as a message by radio telescopes such as we have on Earth. The energy might be detected with radio telescopes larger than any we have, integrating the signal, but there isn’t much integration time, and integration would destroy the content of Vakoch’s transmission. So it will not be recoverable as a message by ETI if their radio telescopes are comparable to or substantially greater than ours. (One can of course assume a Supercivilization only 12 light years away with vastly larger radio telescopes. But if our leakage were detectable by them, as the METI-ists claim, then their greater leakage radiation would surely be detectable by us. But we do not see it.)
Dave Messerschmitt, who is in METI International’s Advisory Council, but wasn’t consulted about this message, observes:
“This METI signal is a simple on-off keying scheme, which dates to the 1837 invention of the telegraph. It has the virtue of extreme simplicity and transparency. However, there are modulation and coding techniques known today that operate near the fundamental limits of data-rate vs energy, such as is described in my paper “Design for Minimum Energy for Interstellar Communications.
“For the same average power (and energy consumption) such signal designs can considerably increase the distance over which information can be reliably extracted. They have other benefits, such as easier discovery and less susceptibility to local sources of radio-frequency interference. However energy-efficient signals will be essentially unobservable by long-term spectrum analysis. Rather, transient (short-term energy) analysis is more effective for such signals. They also require a transmitter implementation capable of high peak-to-average power ratios.”
Note also that the following must all occur for ETI to detect this weak signal:
1) Their system must stare in the very small part of the sky where our sun is, i.e., they must be interested in our system. (To get high sensitivity, the antenna area must be large, so the targeting angle is very small.) This could be because they’ve detected our out-of-equilibrium atmosphere, thus possible life here. This has been true for billions of years.
Or they could have detected our leakage radiation. But the bandwidth of incoherent leakage sources, such as TV and radar, is too wide and the power too unfocused for such signals to escape the Solar System, let alone reach other stars, before it’s indistinguishable from noise.
2) They would have to guess the bit rate of the message. Processing the stored signal with successive assumed rates and seeing which gives the best signal could do this.
3) They would have to deduce that we’re using simple on-off keying instead of another of our many modulation methods, so must analyze the received signal against a list of such stratagems.
While all the above could occur, this is by no means certain.
A decent respect for the opinion of mankind…
They are Star Whisperers. They show no decent respect for the opinion of mankind, to quote a certain historical document, in speaking for Earth.
What we see here is yet another attempt by the METI-ists to announce ourselves to the stars with weak signals that have no serious possibility of being received and interpreted. After several such transmissions in the last decade, they continue to make false claims and send silly signals, paying no attention to the scientific fact that their messages cannot be heard. And they continue to advertise such matters as the following:
“I would say, on behalf of the Klingons, that I prefer to listen to some good music than to the empty whistle of SFO’s radar.” – Seth Shostak
Seth Shostak surely knows that the San Francisco Airport short-range radar, being weak in power with a very low gain antenna, cannot be heard beyond the moon. He certainly knows this if he does any quantitative calculation. They claim, quite falsely, that we have announced ourselves by leakage radiation or intentional transmissions in recent years. This is not true. Therefore these are simply claims to excite the public. This is not an intellectually defensible position.
I advise the METI-ists to restrain themselves from trying to signal ETI. They are not being given access to seriously high power facilities such as Arecibo because they have no rationale for sending messages. They have no claim to speak for Earth.
In 2014 John Billingham and I made several suggestions in our paper referenced above. The time has come to address the METI issue on an international scale by establishing international symposia on transmitting from Earth to ETI. I advocate a moratorium on METI until an international consensus has been reached about announcing ourselves to the stars.
A Second Super-Earth for K2-18
The transiting red dwarf K2-18 is about 111 light years out in the general direction of the constellation Leo, with a mass of 40 percent of Sol’s. A super-Earth, K2-18b, was detected here in 2015 through light curve analysis of data from the reconfigured Kepler K2 mission, and we now have the first measurement of the planet’s mass, drawing on radial velocity data from HARPS. The two planet detection methods in conjunction thus firm up our knowledge of a possible habitable zone planet.
But they also reveal, in the analysis of Ryan Cloutier (University of Toronto) and colleagues, a second super-Earth, K2-18c, which turns out to be non-transiting, and therefore non-coplanar with K2-18b. As we saw yesterday, HARPS (High Accuracy Radial Velocity Planet Searcher), is capable of drilling down to about one meter per second in the analysis of the stellar wobbles that radial velocity methods examine. The current data set gives us another interesting world while reminding us of the capabilities of the ESPRESSO spectrograph I talked about yesterday.
Image: An artist’s conception of planet K2-18b, its star K2-18 and the second planet K2-18c. Credit: Alex Boersma (www.alexboersma.com). Two things I like about this image: Its evocative feel, an imaginative take on a world whose actual conditions are speculative, and the fact that the artist is given credit by the Université de Montréal Institute for research on exoplanets (iREx). Artists should always get credit but all too often do not in press materials.
When it came to K2-18b, the challenge was to figure out whether the planet was a gaseous mini-Neptune or a predominately rocky world. Hence the importance of the radial velocity measurements, giving us in combination with earlier transit data a tentative mass as well as a radius, allowing the planet’s density to be inferred. The researchers believe K2-18b is either a large, rocky planet with a small gaseous atmosphere or a water planet with an ice crust. The range is explained by the uncertainty in the measured mass, which the paper cites at 24%.
It’s interesting to note that this planet’s mass is close to that of another possible habitable zone planet, LHS 1140b, though the dissimilarities are also striking:
K2-18b is of a similar mass to the habitable zone planet LHS 1140b (Dittmann et al. 2017) and receives a comparable level of insolation despite being ? 1.6 times larger than LHS 1140b. Analyzing the mass-radius relationship of these small planets over a range of equilibrium temperatures is a critical step towards understanding which of these systems have retained significant atmospheric content thus making them more suitable to extraterrestrial life.
Given the inability to distinguish between the two outcomes, we have to look to future instruments, like the upcoming James Webb Space Telescope. Cloutier points out that JWST should be able to probe the atmosphere of K2-18b to make the call. We keep bumping up against this ceiling as we wait for new instruments to come online. A nearby transiting world, this planet should climb toward the top of the target list for atmospheric analysis. K2-18 is now the second brightest M dwarf with a transiting habitable zone planet behind LHS 1140.
On JWST, by the way, be sure to read the always reliable Lee Billings’ article What Will NASA’s Biggest-Ever Space Telescope Study First? for a look at the inevitable prioritization of observing time and also the potential of the space telescope. A quick bit from the essay:
Unlike Kepler, which simply surveyed a single star-packed field of view for transits, Webb can zoom in on individual transiting worlds for deeper study. Astronomers should be able to use it to detect water vapor, methane, carbon dioxide and other gases in some silhouetted planets’ upper atmospheres by monitoring the starlight streaming through. They could also record a planet’s passage in front of and then behind its star, using the difference between the two observations to crudely measure a world’s temperature, weather patterns and clouds. “Webb is going to be great for exoplanets,” says Kepler project scientist Natalie Batalha, an astronomer at NASA Ames Research Center who leads the most time-intensive ERS program, which will use nearly 80 hours of Webb’s time to study transiting worlds. “It’s just that this is a difficult game to play, because the signals we’re looking for are really tiny. Seen [in transit] from another star, Venus blocks one part per ten-thousandth of the sun’s light, and its atmosphere intercepts one two-hundredth of that. It’s tough to see—you need a big mirror and great instrumentation to do it.”
As to newly detected K2-18c, the researchers looked for but did not find significant transit timing variations (TTV) in the K2 data. The Cloutier paper continues:
The orbit of the newly discovered K2-18c lies interior to that of K2-18b and yet the planet is non-transiting. This implies that the orbital planes of the planets are mutually inclined. In order for K2-18c to not be seen in-transit the planetary system requires a mutual inclination of just ? 1.4° which is consistent with the observed distribution of mutually inclined multi-planet systems (Figueira et al. 2012; Fabrycky et al. 2014).
The team’s dynamical simulations show that the oscillation timescale of the planets’ orbital inclinations is in the range of 106 years, meaning that it may be a long time before K2-18c can be seen in a transit from Earth. And the authors believe that finding the second planet through radial velocity data emphasizes the prevalence of Earth- to super-Earth size planets in M-dwarf systems, with multi-planet systems offering potential insights into planet formation processes around M-dwarfs. Comparative planetology here we come.
The paper is Cloutier et al., “Characterization of the K2-18 multi-planetary system with HARPS: A habitable zone super-Earth and discovery of a second, warm super-Earth on a non-coplanar orbit,” submitted to Astronomy & Astrophysics (preprint).
First Light for ESPRESSO
What great news that ESPRESSO, the Echelle SPectrograph for Rocky Exoplanet and Stable Spectroscopic Observations, has just achieved ‘first light.’ The spectrograph is installed on the European Southern Observatory’s Very Large Telescope at the Paranal Observatory in northern Chile and its powers are prodigious. For ESPRESSO makes it possible, for the first time, to combine the light of all four telescopes at the VLT. This creates an instrument with the light collecting power of a 16-meter telescope, a major enhancement to the exoplanet hunt.
Image: The room where the light beams coming from the four VLT Unit Telescopes are brought together and fed into fibres, which in turn deliver the light to the spectrograph itself in another room. One of the points where the light enters the room appears at the back of this picture. Credit: ESO/P. Horálek.
Thus the enthusiasm of lead scientist Francesco Pepe (University of Geneva):
ESPRESSO isn’t just the evolution of our previous instruments like HARPS, but it will be transformational, with its higher resolution and higher precision. And unlike earlier instruments it can exploit the VLT’s full collecting power — it can be used with all four of the VLT Unit Telescopes at the same time to simulate a 16-metre telescope. ESPRESSO will be unsurpassed for at least a decade — now I am just impatient to find our first rocky planet!”
Image: This colorful image shows spectral data from the First Light of the ESPRESSO instrument on ESO’s Very Large Telescope in Chile. The light from a star has been dispersed into its component colours. This view has been colourised to indicate how the wavelengths change across the image, but these are not exactly the colours that would be seen visually. Close inspection shows many dark spectral lines in the stellar spectra and also the regular double spots from a calibration light source. The dark gaps are features of how the data is taken, and are not real. Credit: ESO/ESPRESSO team.
We’re going to be hearing a lot from ESPRESSO because it will greatly improve our powers of radial velocity observation. Remember what we are doing when we use these techniques. Radial velocity involves extracting the tiny Doppler signature of star motion as the star is pulled first one way, then another, by the planets around it. Compared to the size of the star, the movements are small but we can trace them in the star’s light spectrum.
Repeating changes to the spectrum as it shifts toward red, toward blue, back toward red, give us the data we need to identify a planet, and until the Kepler mission came along, radial velocity was the primary means we used to find such worlds. 51 Pegasi’s planet, the first found to orbit a main-sequence star, was found using radial velocity methods in 1995. Now we use a mixture of methods including transit studies, direct imaging and gravitational microlensing.
Image: The Echelle SPectrograph for Rocky Exoplanet and Stable Spectroscopic Observations (ESPRESSO) successfully made its first observations in November 2017. Installed on ESO’s Very Large Telescope (VLT) in Chile, ESPRESSO will search for exoplanets with unprecedented precision by looking at the minuscule changes in the properties of light coming from their host stars. This view shows the inside of one of the ESPRESSO front-ends where all the active components of the spectrograph are located. Credit: Giorgio Calderone, INAF Trieste.
Combining methods can be hugely useful, for while radial velocity lets us measure the mass and orbit of the planet (I won’t, for the purposes of this post, get into the problem that RV mass measurements can produce only minimum mass estimates), transits can help us deduce its density. But for those planets that do not transit, we’d like to move further and further down the scale, making detections of ever smaller worlds, terrestrial-class planets perhaps like our own.
Image: the moment of first light, with the team jubilant in the VLT control room. Credit: Giorgio Calderone, INAF Trieste.
From an ESO fact sheet on ESPRESSO:
The radial velocity technique has been so far the most productive in terms of extra-solar planet detections. Low mass planets (one to few Earth masses) are especially interesting because according to formation models they could represent the bulk of the planet population. However they are more elusive and require extremely stable instruments. The HARPS instrument, with a precision better than 1m/s, has discovered up to now the vast majority of planets with masses smaller than Neptune, giving an invaluable experience in view of the realization of more precise instruments. With a radial velocity precision better than 10cm/s, an Earth mass planet in the habitable zone of a low mass star can be detected.
Exactly so. ESPRESSO picks up where HARPS left off. While HARPS could achieve a precision of one meter per second, ESPRESSO gets us down below 10 centimeters per second. The upscaling is the result of ESPRESSO’s placement to tap the four VLT telescopes as well as advances in spectroscopic technology. The benefit will be in characterizing much less massive planets unavailable for our scrutiny through transits or direct imaging, further bulking up the exoplanet catalog and deepening our statistical analysis of planets near us in the galaxy.
Europa: Two Takes on Plate Tectonics
Could plate tectonics occur on Europa? It’s an intriguing notion because the surface crust of the Saturnian moon offers oxidants and other chemicals useful for life. The process called subduction, in which a tectonic plate slides underneath another plate and sinks, would offer a way for these materials to come into contact with the subsurface ocean. We would have, in other words, a first-class mechanism for feeding the ocean with nutrients.
Image: Previous studies have hinted that something like subduction may have been happening on Jupiter’s moon, Europa. A new study provides geophysical evidence that it could indeed be happening on the moon’s icy shell. Credit: NASA/JPL-Caltech/SETI Institute.
There has been earlier evidence of possible plate tectonic activity on Europa, based on studies of surface geology. In places, the icy crust seems to be expanding, and here we can draw an analogy with mid-ocean ridges on Earth, which spread as a result of tectonic activity. In our oceans, material from the crust can sink into the mantle because it is cooler than the mantle material, and therefore denser. Negative buoyancy results, empowering subduction.
A temperature gradient could be in play on Europa. One line of thinking is that the ice shell on the moon is bi-layered, with the thin and cold outer layer atop a somewhat warmer layer of convective ice. Push colder ice from the surface into the warmer ice and it would sink until it warmed to the temperature of the ice around it. The image below illustrates how this might work, and is based on work that was done in 2014.
Image: A subduction model for Europa based on temperature, drawing on a 2014 paper by Simon Kattenhorn (University of Idaho) and Louise Prockter (JHU/APL), This conceptual illustration of the subduction process shows how a cold, brittle, outer portion of Europa’s 20-30 kilometer-thick ice shell moved into the warmer shell interior and was ultimately subsumed. A low-relief subsumption band was created at the surface in the overriding plate, alongside which cryolavas may have erupted. Credit: Noah Kroese, I.NK.
But a new study out of Brown University hints at another, perhaps more robust option. The work of Brandon Johnson, who developed it with the help of graduate students in a class on ocean worlds he was teaching, the paper looks at differences in salt content on Europa. The team’s computer modeling tracked the temperature and compacting of ice as it sinks into warmer ice below, analyzing density changes as warm ice becomes easier to compact. The temperature, salt content and level of ice compaction determine the buoyancy of the ice.
Johnson’s work shows that variations in salt content in Europa’s ice shell of no more than a few percent can cause subduction to occur. Moreover, we have several mechanisms for producing these variations in salt, with upwellings of water from the ocean — suggested by surface geology — offering similarities to magma rising from Earth’s mantle. This process, along with possible cryovolcanism, could deliver salty ocean water directly onto the surface.
Thus we have a mechanism for making salt variations global on Europa, and a process that can feed the ocean below with nutrients. If subduction is occuring on Europa, it would not necessarily be dependent on temperature differences. Or perhaps both processes are in play. In any case, Europa would be only the second place in the Solar System where plate tectonics occurs, and it would carry astrobiological implications. “If indeed there’s life in that ocean, subduction offers a way to supply the nutrients it would need,” Johnson says, and adds:
“It’s fascinating to think that we might have plate tectonics somewhere other than Earth. Thinking from the standpoint of comparative planetology, if we can now study plate tectonics in this very different place, it might be able to help us understand how plate tectonics got started on the Earth.”
The paper is Johnson et al., “Porosity and salt content determine if subduction can occur in Europa’s ice shell,” accepted at the Journal of Geophysical Research: Planets 4 December 2017 (abstract). The earlier Kattenhorn and Prockter paper is “Evidence for subduction in the ice shell of Europa,” Nature Geoscience 7 (2014), 762-767 (abstract).
Voyager and Mission Longevity
Sometimes it’s helpful to look back at the original intent of a space mission. Extending missions is all about continuing to do good science, and it’s often a major benefit of missions as successful as Voyager. But consider the Voyager parameters when the two craft launched in 1977. The plan: Study Jupiter and Saturn, as well as their larger moons and Saturn’s rings, with spacecraft that were built to last five years.
That primary mission, of course, was completed and led on to Voyager 2’s flybys of Uranus and Neptune, and Voyager 1’s crossing into the interstellar medium, a 40-year mission still returning data. Voyager 2 will make a similar crossing within the next few years.
I’ve said a lot about Voyager in this space and have even advocated a final thruster burn for each when the two craft reach the end of their energy supplies, in a purely symbolic trajectory change that would bring them closer to nearby stars than they otherwise would travel (see Voyager to a Star).
This goes back to a Carl Sagan notion that Jim Bell also discussed in his book The Interstellar Age (Dutton, 2015). The two stars in question are Gliese 445 (Voyager 1) and Ross 248 (Voyager 2). Here’s a snip from my essay on the matter:
Carl Sagan and the team working on the Voyager Golden Record wondered whether something could be done about the fact that neither Voyager was headed for another Solar System. Is it possible that toward the end of the Voyagers’ active lifetimes (somewhere in the 2020s), we could set up a trajectory change that would eventually lead Voyager as close as possible to one of these stars? Enough hydrazine is available on each craft that, just before we lose radio contact with them forever, we could give them a final, tank-emptying burn. Tens of thousands of years later, the ancient craft, blind, mute but still more or less intact, would drift in the general vicinity of a star whose inhabitants, if any, might find them and wonder.
As I said, purely symbolic, but I think the symbol is powerful. But someday we’ll be sending craft on long-duration missions with the hopes of delivering more than an ancient, silent relic. We’ll want deep journeys coupled with robust data return. Thus a key question for any deep space travel is the lifetime of a spacecraft, and what a heartening example the Voyagers have set by outlasting their original parameters. Can we really build craft that could return data for centuries on missions as far out as the Oort Cloud, or beyond?
Firing Up Voyager
We learn with each new mission, but in terms of Voyager, we’ve just been given another example of how robust even these early spacecraft are. In a move that scientists believe will extend the lifetime of Voyager 1 by two to three years, engineers have fired backup thrusters that have not been used for 37 years. Made by Aerojet Rocketdyne, Voyager’s MR-103 thrusters were highly useful during the Jupiter and Saturn flybys, orienting the craft as they made observations of the planets and their huge number of moons.
The trajectory correction maneuver (TCM) thrusters are located on the back side of the spacecraft, identical to the craft’s attitude control thrusters, but because Voyager 1’s last planetary flyby was at Saturn, there had been no need to use them since November 8, 1980. During the Saturn encounter they were in continuous firing mode rather than the short bursts that the attitude control thrusters normally employ.
On November 28, 2017 engineers fired the four TCM thrusters on Voyager 1 and tested spacecraft orientation changes using 10-millisecond pulses. A wait of 19 hours and 35 minutes followed, reminding us just how far from home (141.3 AU, or 21.1 billion kilometers from Earth as of this morning) Voyager 1 now travels. But the TCM thrusters worked without flaw.
Waking up these thrusters is fascinating in its own right, but it was also a necessary move. The attitude control thrusters on Voyager 1 had been degrading, requiring more firing to achieve the same effect. In any case, Voyager 1 was already using its set of backup attitude thrusters.
But switching to the TCM thrusters to perform spacecraft orientation wasn’t cut and dried. It required unearthing data that hadn’t been examined in decades. Given the amount of time since the original software was written, the Voyager team had to wade into assembler language that is now long outdated. Todd Barber, a propulsion engineer on the mission at JPL, catches a bit of the mood as the news of successful operation finally came in:
“The Voyager team got more excited each time with each milestone in the thruster test. The mood was one of relief, joy and incredulity after witnessing these well-rested thrusters pick up the baton as if no time had passed at all.”
Image: An artist concept depicting one of NASA’s twin Voyager spacecraft. Humanity’s farthest and longest-lived spacecraft are celebrating 40 years in August and September 2017. Credit: NASA/JPL-Caltech.
Now that the TCM thrusters have been tested, the plan is to switch to them in January, although there is a price: Voyager 1 will need to turn on one heater per thruster, itself a use of precious power. When the heaters are no longer an option, the plan will be to switch back to the attitude control thrusters. A similar test will probably take place for Voyager 2.
Can we build spacecraft capable of enduring generations as they tackle increasingly distant missions beyond the Solar System? The evidence from the Voyagers is that the idea is realistic. What a tribute to the original engineering of these craft that they are still in the news.
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).
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