Centauri Dreams
Imagining and Planning Interstellar Exploration
New Insights into Beta Pictoris b
Beta Pictoris b continues to instruct us in the ways of exoplanet finding. Consider: The young world was identified in 2008 through direct imaging via the Very Large Telescope at the European Southern Observatory site at Cerro Paranal (Chile). Actually seeing an exoplanet is no small feat. We are in this case talking about a bright A-class star some 63 light years away in the wash of whose light we can pick out a comparatively small planet. But it was also a young planet, putting out plenty of heat amidst the large debris disk, the first such disk ever imaged.
The earliest detections of planets around main sequence stars have involved radial velocity, using Doppler methods that can tell us the rate at which the star moves toward and then away from the Earth as it is affected by the planet orbiting it. But radial velocity is a tough call at Beta Pictoris because these changes are tiny, and we are dealing with a star those fast rotation and stellar pulsations obscure the needed signal. Radial velocity, in other words, is far more suited to planets in systems that are well beyond the early period of planet formation.
Image: The planet Beta Pictoris b is visible orbiting its host star in this composite image from the European Southern Observatory’s (ESO) 3.6-m telescope and the NACO instrument on ESO’s 8.2-m Very Large Telescope. The Beta Pictoris system is only about 20 million years old, roughly 225 times younger than the Solar System. Observing this dynamic and rapidly evolving system can help astronomers shed light on the processes of planet formation and early evolution. Credit: ESA.
Now we have what is being called the first successful estimate of a young planet’s mass taken by means of astrometry. Rather than measuring the star’s ‘wobble’ along the line of sight, as we do with radial velocity methods, the new astrometric data allow scientists to measure its deviations on the plane of the sky. Crucial to the work is a lengthy period of observation, which means taking advantage not only of data from the Gaia mission but also the older Hipparcos mission. The latter observed Beta Pictoris 111 times between 1990 and 1993.
Gaia’s second data release includes 22 months of observations, including 30 observations of Beta Pictoris. The combined measurements, examined by Ignas Snellen and Anthony Brown (Leiden University, The Netherlands) show the star’s long-term proper motion. Essentially, the scientists have measured the deviation from what would have been expected of a star without a planet, interpolating the mass of the planet from the size of this deviation.
“By combining data from Hipparcos and Gaia, which have a time difference of about 25 years, you get a very long term proper motion,” adds Brown. “This proper motion also contains the component caused by the orbiting planet. Hipparcos on its own would not have been able to find this planet because it would look like a perfectly normal single star unless we had measured it for a much longer time. Now, by combining Gaia and Hipparcos and looking at the difference in the long term and the short term proper motion, we can see the effect of the planet on the star.”
Image: Astronomers can measure the mass of exoplanets by looking at tiny deviations in the trajectories of their host stars caused by the gravitational pull of the orbiting planets. These can be observed either along the line of sight, looking for small changes in a star’s radial velocity, or on the plane of the sky, using astrometric measurements. To be able to make accurate assessments, the astrometric observations need to cover a period of many years. In this picture, the white dashed spiral shows the evolution of a star’s trajectory observable from the Earth, caused by the combination of parallax and proper motion. The brown band shows the range of deviations of the star’s trajectory caused by a possible planet orbiting it. Credit: ESA.
We learn from all this that Beta Pictoris b is a gas giant of between 9 and 13 times the mass of Jupiter. Useful in itself, the finding also highlights what we can expect from Gaia in the years ahead. Thousands of exoplanets are expected to be discovered in the course of the mission, but the tiny astrometric wobbles being sought can only be detected over sufficiently long periods of time, which is why the windfall of Gaia planets isn’t expected until late in the mission.
As to where this work fits in on the spectrum of young exoplanet studies, we see in Beta Pictoris b and other examples of young gas giants the chance to study the formation and evolution of such worlds. Thus far we have had no reliable mass measurements to distinguish between different formation models. The new Beta Pictoris b work implies a planet that has formed from gravitational instability, with disk gas collapsing to form the planet. Such a mechanism, with the infant giant planet retaining most of it initial entropy, is sometimes called a ‘hot start.’
The authors go on to point out that the collection of data leading to mass measurements for other young gas giants will help to constrain other models of early planetary evolution.
The paper is Snellen & Brown, “The mass of the young planet Beta Pictoris b through the astrometric motion of its host star,” Nature Astronomy 20 August 2018 (abstract).
The Prevalence of ‘Water Worlds’
The first time I ran into the term ‘water world,’ it had a seductive quality. After all, we think of habitable zones in terms of water on the surface, and a world with an overabundance of water suggested a kind of celestial Polynesia, archipelagos surrounded by a planet-circling, azure sea.
But we immediately run into problems when we think about planets with substantially more water than Earth. For one thing, we may have no land at all. Let’s leave aside the icy moons of our Solar System that may well contain oceans beneath their surface and concentrate on exoplanets in the interesting size range of two to four times the size of Earth. We have to ask what would happen if a planet were completely covered with water, with no run-off of nutrients from exposed rock. Such an ocean could be starved of key elements like phosphorus.
Or how about a planet with a high-pressure zone of ice effectively cutting off the global ocean from the rocky mantle? A world with enough water — 50 times that of Earth has been considered — could create enough pressure on the seafloor to prevent geological activity, blocking the kind of carbon-silicate cycle that adjusts the atmospheric composition we find here on Earth. Cayman Unterborn (Arizona State) thinks liquid water on the 5th planet in the TRAPPIST-1 system could be as much as 200 kilometers deep, 20 times deeper than the Marianas Trench.
Image: A water world as envisioned in a photo-illustration by Christine Daniloff/MIT/ESO.
We don’t want to be too doctrinaire about this. For instance, Ramses Ramirez (Tokyo Institute of Technology) and Amit Levi (Harvard-Smithsonian Center for Astrophysics), have argued that there are ways to exchange greenhouse gases between deep sea ice and the atmosphere (citation below). In other words, we can have a carbon cycle without rock weathering, subject to a number of constraints like stellar type (hotter stars work best here) and rotation rate (thanks to Alex Tolley for his overview of this paper in private correspondence).
This is a work I hope to write about soon, but for now, let me quote from an essay by Shannon Hall on the topic that cites both Ramirez and Edwin Kite (University of Chicago), with a useful reminder from both on not being too quick to limit our thinking to Earth-centric models:
“What I’ve taken away from this project is the inadequacy of working from Earth’s analogy,” says Kite, admitting this conclusion is ironic given that he is a geologist by training. “I love rocks and Earth-history, but you really need to build up from basic physics and chemistry, rather than relying on Earth’s analogy in order to tackle exoplanet problems.” This consideration will be important when astronomers have to determine which individual worlds to further assess with large telescopes like the James Webb Space Telescope or when they have to choose between future missions that would survey hundreds of worlds and those that would study a handful of Earth clones in detail. But there is no consensus yet. “I think it could be dangerous just thinking about everything in an Earth-mindset,” Ramirez says. “You might be missing out on other possibilities.”
Exactly so, and when telescope time is precious, as it will continue to be for all our space-based resources in particular, target selection is critical. Meanwhile, I’ve run across the presentation that Li Zeng (Harvard University) and colleagues made at the recent Goldschmidt conference in Boston. The researchers point to data from both Kepler and the Gaia mission indicating that many known exoplanets may contain as much as 50 percent water. Here we can definitely toss the Earth-centric model out the window. Consider that Earth’s water content is 0.02% by weight. If these data are correct, huge numbers of exoplanets are entirely awash.
Zeng and team have been developing a model of the internal structure of two kinds of exoplanets: Those with a radius averaging about 1.5 times that of Earth, and those averaging 2.5 times Earth’s radius. According to their developing model, those with a radius 1.5 times the Earth’s are generally rocky planets, perhaps five times as massive as the Earth. Those with a radius 2.5 times that of Earth, massing about 10 Earth masses, are likely water worlds.
The model the researchers have developed tracks the changes in mass and radius when planets grow from a rocky core and later accrete either ices or hydrogen/helium gas, with the observed radius and mass-radius distribution reproduced in the model’s simulations. Many of the interesting planets in this range turn out to be water worlds.
“Our data indicate that about 35% of all known exoplanets which are bigger than Earth should be water-rich,” says Zeng. “These water worlds likely formed in similar ways to the giant planet cores (Jupiter, Saturn, Uranus, Neptune) which we find in our own solar system. The newly-launched TESS mission will find many more of them, with the help of ground-based spectroscopic follow-up. The next generation space telescope, the James Webb Space Telescope, will hopefully characterize the atmosphere of some of them. This is an exciting time for those interested in these remote worlds.”
On the larger planets, we can throw out my fanciful ‘Polynesia’ model. The researchers believe the surface temperature here would be in the range of 200 to 500 degrees Celsius. We would see an atmosphere dominated by water vapor, with a liquid water layer beneath, and high-pressure ices below. Our next generation of telescopes can put these ideas to the test.
“It’s amazing to think that the enigmatic intermediate-size exoplanets could be water worlds with vast amounts of water,” says MIT planet-hunter Sara Seager. “Hopefully atmosphere observations in the future–of thick steam atmospheres—can support or refute the new findings.”
The abstract for the Zeng et al. presentation at the Goldschmidt Conference, “Growth Model Interpretation of Planet Size Distribution,” is here. The Unterborn paper is “Inward migration of the TRAPPIST-1 planets as inferred from their water-rich compositions,” Nature Astronomy 19 March 2018 (abstract). The Ramirez and Levi paper is “The Ice Cap Zone: A Unique Habitable Zone for Ocean Worlds,” Monthly Notices of the Royal Astronomical Society 477, 4 (2018), 4627-4640 (abstract / preprint).
The Breakthrough Starshot Opportunity
When we think about what is usually called ‘planetary protection,’ we’re talking about the probes we send to possibly life-bearing places like Mars or Europa. It would confound our investigations if we couldn’t be sure we hadn’t contaminated such a place with microorganisms from Earth, unwittingly carried aboard a lander that was not properly stripped of such passengers. Even our Cassini Saturn orbiter was guided into the planet as a way of ensuring that it would not, at some future date, crash into a place as biologically interesting as Enceladus.
Yesterday, having looked at an essay by Ethan Siegel, I asked rhetorically whether we should think up some kind of exoplanetary protection policy as well. After all, we’re fleshing out an actual mission design through Breakthrough Starshot, aiming to reach nearby stars in coming decades. Siegel (Lewis & Clark College) had expressed his concern that Breakthrough Starshot might inadvertently start an interstellar war. The idea is extreme, but I use the word ‘war’ because it was right there in the title of Siegel’s piece.
What Siegel worries about is that among the large number of payloads Breakthrough Starshot would like to send to nearby stars there might be one that accidentally impacted the planet under study. He’s concerned about the ‘cone of uncertainty’ that would be involved in any trajectory aimed at making a close pass of a planet like Proxima b and argues that at 60,000 km/sec, which is the Starshot goal of 20 percent of c, a tiny 1-gram payload will strike with the force of a 1 tonne asteroid moving at 60 km/sec, a hit like Chelyabinsk.
Harvard’s Avi Loeb, who chairs the Breakthrough Starshot advisory board, finds little merit in the concern. I had been wondering as I wrote yesterday’s post just what Dr. Loeb would have to say about Siegel’s views, only to find an email with a link to his new essay in Forbes in my mailbox shortly after I published — his piece had already appeared that morning. Loeb points out that at a tenth of the speed of light, a payload of Starshot size would “…merely carry the energy of a common asteroid, only a few meters in size — of order the height of a person. Such asteroids hit the Earth a few times per year and burn up in the atmosphere.”
Here is a map of asteroid impacts that Loeb linked to in his essay.
Image: This diagram maps the data gathered from 1994-2013 on small asteroids impacting Earth’s atmosphere to create very bright meteors, technically called “bolides” and commonly referred to as “fireballs”. Sizes of red dots (daytime impacts) and blue dots (nighttime impacts) are proportional to the optical radiated energy of impacts measured in billions of Joules (GJ) of energy, and show the location of impacts from objects about 1 meter (3 feet) to almost 20 meters (60 feet) in size. Credit: Planetary Science.
The map, the work of NASA’s Near Earth Object Program, is instructive. All told, 556 small asteroids impacted our atmosphere in a 20-year period, almost all of them disintegrating in the atmosphere, with the obvious exception of the Chelyabinsk event, which was caused by the largest asteroid to strike in this timeframe. My understanding about Chelyabinsk is that the meteor was approximately 20 meters in size, moving at roughly 19 kilometers per second.
But wouldn’t a one-gram payload moving at Starshot speeds still wreak havoc if it fell on a planetary surface? Evidently the event wouldn’t happen. Let me quote Loeb on this:
…a gram-scale starchip would burn up in the atmosphere of a planet much more easily than a tonne-mass asteroid. Its impact on a planet would be no more irritating than the impact of a dust grain on the skin of a grazing cow. The interplanetary medium is full of debris that poses far greater risks to a planet, as the dinosaurs realized when wiped out by the impact of a 10-15 kilometer asteroid (a trillion times more massive than we just considered) some sixty five million years ago.
In any case, Loeb sees the chances of collision at something less than one part in a million, given the fact that knowing the relative positions of planet and spacecraft over a journey lasting decades is impossible with the payload as envisioned. What Breakthrough Starshot does expect is a closest approach thousands of times larger than the surface of any planet.
We should pause on the deceleration question, which comes up repeatedly whenever I talk about Starshot probes with people new to the project. While it is certainly true that deceleration and orbit in, say, the Proxima Centauri system would be a huge benefit, there is no way to make it happen given the current constraints on the payload, which relies on miniaturization and nonetheless requires vast energies to be brought up to cruising speed. Our initial interstellar efforts will surely be flybys for this reason, and there is a wealth of science that can be gathered.
The other thing to be remembered is that Breakthrough Starshot is an ongoing effort to demonstrate the feasibility of beamed laser sail technologies. Submissions from the first Requests for Proposals are being evaluated as sail stability and materials are considered by way of moving toward experimental work in what we can hope will be a dedicated sail facility. The laser array that would drive these craft presents huge challenges of its own, as do the many other issues, like communications, that will have to be resolved before any interstellar mission can be flown.
We’re looking at an interstellar capability that, if everything were to go well, would still take 30 years to produce a working infrastructure. To those new to Breakthrough Starshot, I recommend looking at the list of technical problems that face the project as it determines whether the concept is sound enough to move to prototype stage enroute to a full deployment. This is going to take time, and we won’t know for quite a while whether the Starshot concept, as is, is workable.
What we do already know is that the commitment of resources underlying the project is already paying off, and will continue to pay off, in the form of newly funded research.
Anyone familiar with the history of interstellar flight studies will know how rare it is to have this level of cooperation and participation among the scientists best equipped to tackle the problem. For most of the past century, researchers interested in the interstellar conundrum did their work in their spare time and met only at the occasional conference, with infrequent journal articles maintaining the thread. The research Breakthrough Starshot will produce will thus be a solid advance for the entire field, no matter if, when or how often the actual missions are flown.
On the Enigma of Arrival
The death of V.S. Naipaul (1932-2018), that cross-grained and all too combative man who saw so unflinchingly into the post-colonial lands from which he drew his heritage, invariably brings to mind his strangest novel, The Enigma of Arrival (Vintage Books, 1987). Temporarily settled into a cottage in Wiltshire in rural England, the author looks back on his career in search of a renewal as cyclic as the seasons. Landscape inspires creativity in this deeply visualized microcosm, even as Naipaul broods over the painting that gives the book its title.
The novel is an odd, self-indulgent work, one I completed more out of a sense of duty (I was reviewing it for a newspaper) than enthusiasm. Yet its introspective imagery keeps resonating. Naipaul was obsessed with the sub-story of the painting, showing the arrival of a visitor at a strange port city and implying a subsequent journey that would in some way parallel his own career.
The work of Giorgio de Chirico (1888-1978), ‘The Enigma of Arrival and the Afternoon’ is a canvas showing this surreal cityscape, a world fitting into Naipaul’s autobiographical meditation with its characters disembarked in a place suggestive of antiquity under a brooding Levantine sky.
A classical scene, Mediterranean, ancient-Roman — or so I saw it [writes Naipaul]. A wharf; in the background, beyond walls and gateways (like cutouts), there is the top of the mast of an antique vessel; on an otherwise deserted street in the foreground there are two figures, both muffled, one perhaps the person who has arrived, the other perhaps a native of the port. The scene is of desolation and mystery…
Which gets us to interstellar flight and other, more exotic arrivals. I’ve always believed that if we ever do discover hard evidence of an extraterrestrial civilization, that experience will not translate into trade opportunities or galactic encyclopedias but mysteries that leave us in some ways more baffled about the nature of intelligence than ever before. I draw a distinction here between ‘contact’ and ‘encounter,’ which are entirely different things, and wonder what this kind of arrival would look like to humans finding evidence for extraterrestrial intelligence.
Here I’ll invoke the splendid novel Roadside Picnic, by Arkady and Boris Strugatsky (1971). Alien artifacts appear at various places on Earth, so-called ‘zones of visitation’ filled with bizarre items, many of them dangerous, and inexplicable happenings. Unseen themselves, the aliens are glimpsed only through what they leave behind in our region of spacetime before moving on. Protagonist “Red” Schuhart is a ‘stalker,’ one who defies the danger to enter the alien zones in search of artifacts. What happens in a Zone can never be predicted.
He had never experienced anything like this before outside the Zone. And it had happened in the Zone only two or three times. It was as though he were in a different world. A million odors cascaded in on him at once—sharp, sweet, metallic, gentle, dangerous ones, as crude as cobblestones, as delicate and complex as watch mechanisms, as huge as a house and as tiny as a dust particle. The air became hard, it developed edges, surfaces, and corners, like space was filled with huge, stiff balloons, slippery pyramids, gigantic prickly crystals, and he had to push his way through it all, making his way in a dream through a junk store stuffed with ancient ugly furniture … It lasted a second. He opened his eyes, and everything was gone. It hadn’t been a different world—it was this world turning a new, unknown side to him. This side was revealed to him for a second and then disappeared, before he had time to figure it out.
Ursula Le Guin was quick to note the connection between the Strugatskys and the work of Stanislaw Lem, the great Polish novelist whose Solaris (1961) is widely admired and twice filmed. Here too, coping with a planet-wide ocean with its own kind of sentience, the human characters come up hard against their own preconceptions and the failure of their paradigms to understand an alien presence. Lem chided what he called ‘the myth of our cognitive universalism,’ a myth the Strugatsky’s equally exploit in the ravishingly strange Roadside Picnic. Like Naipaul, Lem has his own obsession with arrivals.
“We don’t want to conquer the cosmos, we simply want to extend the boundaries of Earth to the frontiers of the cosmos… We are searching for an ideal image of our own world: we go in quest of a planet, a civilization superior to our own but developed on the basis of a prototype of our primeval past. At the same time, there is something inside us which we don’t like to face up to, from which we try to protect ourselves, but which nevertheless remains, since we don’t leave Earth in a state of primal innocence. We arrive here as we are in reality, and when the page is turned and that reality is revealed to us – that part of our reality which we would prefer to pass over in silence – then we don’t like it anymore.”
Is an arrival always a wakening of self-knowledge? Here I might also mention Arthur C. Clarke’s Rendezvous with Rama (1973), in which an alien starship passes through the Solar System, an approach that reminded many Centauri Dreams readers of the recent appearance of ‘Oumuamua. A survey vessel despatched to study the object called Rama discovers geometric structures and a ‘cylindrical sea,’ along with an atmosphere that turns out to be breathable. But Rama will not tarry. After a gravitational slingshot maneuver, it departs the Solar System for the Magellanics.
It is an arrival wrapped in mystery but, as Clarke goes on, “at least we have answered one ancient question. We are not alone. The stars will never again be the same to us.”
Could anything ever be the same? In tales like these, we confront the unknowable in disturbing ways, conflicted by our inability to establish the kind of contact that would explain and enlarge our own existence. No one can know what an actual alien contact might involve, but I suspect that dealing with an entirely separate lifeform deriving from an evolution under wholly different skies will prove to be as enigmatic as anything written by the Strugatskys, Lem or Clarke.
Arrival as Threat?
In that vein, I recently ran across an essay by astrophysicist Ethan Siegel, sent on by several readers who knew how often we’ve discussed Breakthrough Starshot in these pages. What Siegel does is to reverse the story. He wonders how the recipients of an arrival from Earth would react, and speculates that if we’re not careful, we may send entirely the wrong signal.
Breakthrough Starshot, after all, envisions pushing a fleet of small sails carrying payloads of a gram or so to a nearby stellar system. Right now the obvious target is Proxima Centauri, where we know we have a planet in the habitable zone, but we may find equally promising possibilities around Centauri A or B. The goal here is not contact but simply the opportunity to perform flybys of an interesting planet and return data and imagery, and Siegel is quick to note that sails boosted to 20 percent of lightspeed have no deceleration mechanism available to them.
The problem: A ‘cone of uncertainty’ exists for any trajectory that will take us close enough to the planet to retrieve good data, an aiming problem requiring unprecedented levels of precision. Thus the possibility that we could be entering an inhabited stellar system and colliding with a living world cannot be ruled out. Siegel notes the invariable relationship between kinetic energy and speed: Double the speed and you get four times the energy. Even our tiny 1-gram spacecraft moving at 60,000 km/sec, says Siegel, will hit with the force of a 1 tonne asteroid moving at 60 km/sec, which in effect means we could cause a Chelyabinsk-like event, or more than one.
If you were an alien on this world that got struck by these relativistic masses, what would you conclude? You’d know that these were too massive and too fast-moving to be created naturally; they were made by an intelligent civilization. You’d know that you were being intentionally targeted; space is too vast for these to strike you by random chance. And?—?worst of all?—?you’d assume this civilization had a malicious intent. No benevolent aliens would launch something so recklessly and carelessly given the damage it would cause. If we’re smart enough to send a spacecraft across the galaxy to another star, surely we can be wise enough to reckon the disastrous consequences of doing so.
What an arrival this would be. And yet, in order to study nearby worlds, we have yet to come up with a plan remotely as feasible as the admittedly longshot Breakthrough Starshot. Are there ways we can minimize this risk or eliminate it altogether while still finding a way to begin interstellar explorations? Because we do need to consider how we are perceived when we probe into the utterly unknown, and if the odds seem long that there is a civilization on Proxima b or that we might inadvertently hit the planet we are studying, they are still not zero.
The enigma of arrival is magnified and transformed when we are arriving at a place we are seeing for the first time, just as our arrival at Pluto/Charon opened up two worlds while posing new mysteries about the surfaces we flew past. Naipaul referred to de Chirico’s scene as the depiction of ‘a dangerous classical city,’ one in which the newcomer sought orientation and meaning. How much more enigmatic might our own arrival be if perceived by other intelligences? Should the need for ‘exoplanetary protection’ join our other mission parameters?
[Addendum]: Be sure to look at Avi Loeb’s spirited response to Siegel in Why Humanity Probably Won’t Accidentally Start An Interstellar War With An Alien Civilization. More on this tomorrow.
Omega Centauri: Improbable Venue for Life?
Although it appears in the same constellation as seen from Earth (Centaurus), Omega Centauri has nothing to do with the Alpha Centauri stars that so interest interstellar flight theorists. The brightest globular cluster visible in our skies, Omega Centauri is anything but close (16,000 light years out) and, containing several million stars, is the largest globular cluster in our galaxy. We may in fact be looking at the core of a dwarf galaxy once absorbed by the Milky Way.
But although it’s quite distant, Omega Centauri may be the source of the relatively nearby Kapteyn’s Star, just 13 light years from the Sun. Where this gets intriguing is that Kapteyn’s Star, (named after Dutch astronomer Jacobus Kapteyn) is known to have at least two planets, one of them considered the oldest known potentially habitable planet — let’s call it a ‘temperate Super-Earth’, as Guillem Anglada-Escudé and team have done — at 11 billion years old.
Steven Kane (UC-Riverside), working with graduate student Sarah Deveny (San Francisco State) has now produced a paper taking a deep dive into Omega Centauri and its own prospects for habitability. Beginning with 470,000 stars in the cluster’s core, the duo focused on a subset of 350,000 of these, using color as a gauge of temperature and age in making their choice. The idea was to calculate habitable zones around each star, in which a rocky world might have liquid water on the surface.
Image: There are colorful stars galore, but likely no habitable planets, inside the globular star cluster Omega Centauri. Credit: NASA, ESA, AND THE HUBBLE SM4 ERO TEAM.
The paper Kane and Deveny produced makes the point that most exoplanet searches have occurred around field stars, but globular clusters are attractive hunting grounds given the age of their stars and the possibility they offer to study planet formation and evolution as affected by the cluster. But so far planet hunting in such clusters has not proven profitable. A 2000 study using the Hubble instrument to study 34,000 stars in the core of the cluster 47 Tucanae revealed not a single transit, nor did follow-ups in the less crowded outer regions of the cluster.
Nor have transiting planets been found in the cluster NGC 6397. Undaunted, Kane and Deveny here analyze Hubble data on the core of Omega Centauri and calculate habitable zones for the observed stars. The idea is to determine how conditions in the Omega Centauri core affect the potential habitability of planets around main sequence stars, the latter being determined by a color-magnitude diagram with data drawn from filters described in the paper. Luminosity and effective temperatures were calculated using Hubble instrument filters from the Dartmouth Stellar Evolution Database, assuming 11.5 billion years as the age in the applied model.
Kane and Deveny use habitable zone relationships and methodologies found in the literature, going on to calculate the conservative habitable zone (CHZ) and optimistic habitable zone (OHZ), boundaries that Kane previously used in creating a catalog of Kepler habitable zone planets. Given the star properties determined by the above analysis, which showed that the sample under consideration was dominated by low-mass stars, the researchers found that about 50 percent of the sample had an outermost habitable zone boundary within 0.5 AU of the star.
No surprise there, as most of the stars in the Omega Centauri core are red dwarfs, implying habitable zones much closer than those around G- and K-class stars. Compact planetary systems of the kind we see, for example, around small M-dwarfs like TRAPPIST-1 would seem to stand a better chance of survival from disruption by nearby stars, but conditions here are truly tight. Our Sun has a separation of over 4 light years from its nearest stellar neighbors, but within Omega Centauri’s core, the average distance is a scant 0.16 light years. That would set up frequent encounters between stars, on the order of one every 1 million years.
The result does not bode well for these planets, as the paper notes:
…the compact nature of the HZ regions is more than offset by the potential disruption of planetary systems, where close encounters of only 0.5 AU are expected to occur on average every 1.65 × 106 years. Though the large resulting population of free-floating terrestrial planets are intrinsically interesting from formation and dynamical points of view, the potential for habitability in the ω Cen core environment is significantly reduced by such scattering events. The primary lesson that can be extracted from this analysis is the underlining of the importance of quantifying the long-term dynamical stability of orbits inside HZ regions taking into account both internal (planetary) dynamics and external (stellar) interactions.
Image: The globular cluster Omega Centauri — with as many as ten million stars — is seen in all its splendor in this image captured with the WFI camera from ESO’s La Silla Observatory. The image shows only the central part of the cluster — about the size of the full moon on the sky (half a degree). North is up, East is to the left. This colour image is a composite of B, V and I filtered images. Note that because WFI is equipped with a mosaic detector, there are two small gaps in the image which were filled with lower quality data from the Digitized Sky Survey. Credit: ESO.
Because we have no firm knowledge of how common planets are in this environment, we’re left to speculate, but the paper’s calculations show that numerous compact planetary systems could exist in Omega Centauri. But even assuming a large population of rocky worlds, cluster dynamics would keep encounters between these planetary systems happening on a regular basis. The scenario of planets stripped from their host stars seems inimical to life.
But a possibility remains. Terrestrial-class worlds disrupted from their home systems could constitute a large population of free-floating worlds that might be analyzed through gravitational microlensing. There are some studies in the literature arguing that free-floating worlds with a thick hydrogen atmosphere could retain habitable conditions at the surface, so as the authors note, habitable planets cannot be entirely ruled out despite the problem of stability.
And, of course, encounter rates between stars will vary depending on the size and density of the cluster:
“The rate at which stars gravitationally interact with each other would be too high to harbor stable habitable planets,” Deveny said. “Looking at clusters with similar or higher encounter rates to Omega Centauri’s could lead to the same conclusion. So, studying globular clusters with lower encounter rates might lead to a higher probability of finding stable habitable planets.”
Such studies will doubtless be pursued. After all, the close separation among ancient stars at a globular cluster’s core would seem to give any civilizations that formed near them the opportunity to explore nearby systems and eventually the entire cluster. We’ve talked before about the work of Rosanne Di Stefano (Harvard-Smithsonian Center for Astrophysics), who makes this case (see Globular Clusters: Home to Intelligent Life?). Says Di Stefano:
“Interstellar travel would take less time… The Voyager probes are 100 billion miles from Earth, or one-tenth as far as it would take to reach the closest star if we lived in a globular cluster. That means sending an interstellar probe is something a civilization at our technological level could do in a globular cluster.”
In fact, Di Stefano argues that portions of many globular clusters other than the core can be considered ‘sweet spots’ where habitable orbits are stable for long periods. So the question of habitability and planet formation itself inside globular clusters is one of continuing interest.
One thing is clear. When we talk about habitable zones, we have to take into account the dynamical stability of the orbits involved, examining not just the planets themselves but the possibility of encounters with nearby stars. In dense environments, such interactions and the scattering they cause would be game-changers. Cluster outskirts may offer better opportunities.
The paper is Kane and Deveny, “Habitability in the Omega Centauri Cluster,” accepted at The Astrophysical Journal (preprint). The Di Stefano paper mentioned above is “Globular Clusters as Cradles of Life and Advanced Civilizations,” Astrophysical Journal Vol. 827, No. 1 (5 August 2016). Abstract.
Probing Ultrahot Jupiters
Speaking of getting really, really close to a star, as we were yesterday in our discussion of the Parker Solar Probe, I couldn’t help but turn to new computer models of the ‘ultrahot Jupiter’ WASP-121b. I still find it delightful that the earliest exoplanet detections involved a category of planet that few scientists had imagined existed. These days we routinely discuss gas giants blisteringly close to their hosts, and even manage to extract information about their atmospheres through transmission spectroscopy, but few people expected such planets when we began to discover them.
In fact, Apollo 11’s Buzz Aldrin had a role to play in what may be considered to be the first prediction of the worlds we would start calling ‘hot Jupiters.’ Working with John Barnes on his novel Encounter with Tiber (Grand Central, 1996), Aldrin asked physicist Greg Matloff whether a hydrogen-helium atmosphere as found in a Jupiter-class world could survive in an inner stellar system. Here’s how Matloff recalls the discussion:
Although I was initially very skeptical since then-standard models of solar system formation seemed to rule out such a possibility, I searched through the literature and located the appropriate equation (Jastrow and Rasool, 1965)….To my amazement, Buzz was correct. The planet’s atmosphere is stable for billions of years. Since I was at the time working as a consultant and adjunct professor, I did not challenge the existing physical paradigm by submitting my results to a mainstream journal. Since “Hot Jupiters” were discovered shortly before the novel was published, I am now credited with predicting the existence of such worlds.
Indeed, for this was just at the time when 51 Pegasi b swam into our consciousness in 1995 (the book was finished but not yet published when the discovery was made). Now we knew that hot Jupiters were out there, and the radial velocity method of exoplanet discovery ensured that large planets close to their star would be the most likely to be detected in our earliest efforts.
The Realm of the ‘Ultrahot’
But hot Jupiters are but one variety of inner system gas giants. Today we can catalog a planet like WASP-121b as a member of a still more unique class of worlds, distinguishing between ‘hot Jupiters’ and those gas giants that come astoundingly close to their stars.
Dubbed ‘ultrahot Jupiters,’ these worlds reflect about as much light as charcoal. What distinguishes them is a temperature that on the dayside causes them to glow like an ember. Thus the image below, in which WASP-121b is simulated based on computer models that draw on observations of the planet conducted by the Spitzer and Hubble space instruments.
Image: These simulated views of the ultrahot Jupiter WASP-121b show what the planet might look like to the human eye from five different vantage points, illuminated to different degrees by its parent star. The images were created using a computer simulation being used to help scientists understand the atmospheres of these ultra-hot planets. Credit: NASA/JPL-Caltech/Aix-Marseille University (AMU).
The word ‘hellish,’ so often used to describe the surface of Venus, can with even more justice be applied to ultrahot worlds like this. They orbit closer to their host stars than Mercury does to the Sun, tidally locked and with temperatures that can range between 2,000 and 3,000 degrees Celsius. Even the nightside of such a world can reach 1,000 degrees Celsius, though as we’ll see, this makes enough of a difference to explain some anomalous observations.
For new work from Vivien Parmentier (Aix Marseille University, France) and colleagues goes into the question of why we find no water vapor in the atmospheres of these worlds. Hot Jupiters — gas giants in an inner system that experience dayside temperatures below 2,000 degrees Celsius — have been found with abundant water vapor in their atmospheres. But ultrahot Jupiters seem to lack it. One theory on why is that these planets formed with high levels of carbon instead of oxygen, but the new study points to the occasional traces of water that have been detected at the dayside-nightside boundary as a refutation of the idea.
Parmentier’s team, applying a model of brown dwarf atmospheres developed by co-author Mark Marley (NASA Ames), went to work on ultrahot Jupiter atmospheres as if they were the atmospheres of stars. After all, says Parmentier, “The daysides of these worlds are furnaces that look more like a stellar atmosphere than a planetary atmosphere. In this way, ultrahot Jupiters stretch out what we think planets should look like.”
The team used Spitzer observations of WASP-121b at infrared wavelengths to probe carbon monoxide levels in its atmosphere. CO molecules have a bond strong enough to withstand the dayside heat. The result: The planet’s atmosphere reveals a strong temperature gradient, burning hotter higher up than further down. A uniform atmosphere could have masked the signature of water molecules, providing one explanation for the apparent lack of water.
But the carbon monoxide work showed that the answer lies elsewhere. According to the study, hydrogen and oxygen atoms are indeed found on ultrahot Jupiters, but the strong irradiation on the dayside simply tears the water molecules apart. The researchers have placed WASP-121b in the context of recently published studies authored by Parmentier, co-author Michael Line (ASU) and others on fellow ultrahot Jupiters WASP-103b, WASP-18b and HAT-P-7b.
They have concluded that the fierce stellar winds of the dayside blow the broken water molecules onto the nightside, where they can recombine and condense into clouds before, inevitably, drifting back onto the dayside to undergo the destructive process again.
The paper sees the transmission spectrum of WASP-121b as being consistent with a solar composition atmosphere having partial cloud coverage. Within its dayside atmosphere, molecules are being continually sundered. And here is why we see no water:
Ultra hot Jupiters with dayside temperatures larger than 2200K are good targets for thermal emission measurements. However, the majority of the observed planets have weaker-than-expected spectral features in the 1?2µm range. Using the example of WASP-121b, we interpret this lack of strong features as being due to a combination of a vertical gradient in molecular abundances due to thermal dissociation, and to the presence of H? [the hydrogen anion, or negative ion of hydrogen] absorption at wavelengths shorter than 1.4µm.
Thermal dissociation affects all spectrally important molecules in the atmospheres of ultra hot Jupiters except CO. It creates a large vertical gradient in the molecular abundances. We show analytically that the presence of such a molecular gradient weakens the features in emission spectra. This is a qualitatively different effect than a global depletion of the abundances.
Image: Jupiter-like exoplanets are 99 percent molecular hydrogen and helium with smaller amounts of water and other molecules. But what their spectra show depends strongly on temperature. Warm-to-hot planets form clouds of minerals, while hotter planets make starlight-absorbing molecules of titanium oxide. Yet to understand ultrahot Jupiter spectra, the research team had to turn to processes more commonly found in stars. Credit: Michael Line/ASU.
What a place an ultrahot Jupiter must be. The idea of this kind of circulation in the atmosphere is given force by the previous detection by Hubble of clouds at the boundary between night and day. The same process may affect titanium oxide as well as aluminum oxide. Because the latter is the basis for the gemstone ruby, this JPL news release speculates that there could be clouds producing rains of liquid metals and fluidic rubies in the atmospheres of ultrahot Jupiters. That would make the ultrahot Jupiter about as exotic an environment as we can conceive.
The paper is Parmentier et al., “From thermal dissociation to condensation in the atmospheres of ultra hot Jupiters: WASP-121b in context.” submitted to Astronomy & Astrophysics (preprint).
Follow with RSS or E-Mail
