Centauri Dreams

How close would a passing star have to come to produce drastic results on the outer Solar System? According to researchers at the Max Planck Institute for Radio Astronomy in Bonn, roughly 3 times Neptune’s distance would be disruptive enough to explain what we see beyond that planet’s orbit today. Led by Susanne Pfalzner, the scientists have been modeling close stellar flybys of other planetary systems for years, but have only recently turned their attention to the eccentricities of our own system, where conditions beyond Neptune pose questions.

Image: Artist’s concept of a stellar system in the making with a protoplanetary disk surrounding a young star. Credit: NASA JPL-Caltech.

A look at the Solar System’s formation highlights the problematic nature of the process. Out to the orbit of Neptune, planets, dwarf planets and asteroids orbit with only small differences in orbital inclination, indicative of the flatness of the original disk from which all these objects drew their birth materials. But we find that many trans-Neptunian objects move along eccentric, inclined orbits, with objects like Sedna occupying orbits well outside the range of the planets.

What Pfalzner and team added to the mix was another observation: In this distant region, the cumulative mass of the objects we know about drops by almost three orders of magnitude. The assumption is that the outer Solar System must have been modified by processes that took place after its formation. The fact that the drop-off occurs just outside Neptune’s orbit provoked enough interest that the scientists began to perform computer simulations, starting with a young circumstellar disk around the Sun and introducing the close passage of another star.

The result is striking. A star of 0.5 to 1 solar masses moving past our outer system would produce a good fit for the current Solar System, and as we’ll see, in more ways than one:

Starting point of our simulations was the Sun surrounded by a disc, which could either be a protoplanetary or a debris disc, possibly containing already formed planets. We performed an extensive parameter study for the effect of close flybys on discs concentrating on the ones that lead to a steep drop in the mass distribution at 30-35 AU as that observed for the solar system. We found that fly-bys of stars with masses in the range 03.-1.0 M at perihelion distances of between 50 and 150 AU inclined between 50 to 70 degree and at an angle of periastron between 60 to 120 degree are the most promising candidates for a fly-by shaping the outer solar system. Such fly-bys lead to Sednoids, a hot and cold Kuiper belt population and various other properties characteristic for the outer solar system. What distinguishes this model from others, is that only a single event is necessary to create all this features.

Image: Simulation of the stellar intruder scenario for a mass of 0.5 solar masses and a perihelion distance of 100 astronomical units or 15 billion kilometers for the perturbing star (three times the distance between Sun and Neptune). a) average positions of the particles after the fly-by, colors showing eccentricity of their orbits increasing from blue to green. b) particle positions before the fly-by with different eccentricity populations (colors) from the top row Grey regions: particles that became unbound due to the fly-by event. Credit: S. Pfalzner et al.: The Astrophysical Journal (2018).

As the paper notes, a virtue of this model is its simplicity, allowing a single event (the close flyby) to explain a multitude of features about the current system. For there is another puzzle to be explained: The separation between the giant planets around the Sun increases as their mass decreases with distance from the Sun, which is usually considered to reflect conditions in the protoplanetary disk. But Neptune, with a mass of 17.2 Earths, is more massive than Uranus, at 14.6 M_Earth.

The Pfalzner paper explains this, for in its flyby simulations, some portion of the matter beyond Neptune moves inward and concentrates between 30 and 35 AU. In fact, the matter involved corresponds to approximately 1-2 Earth masses, pointing to the possibility that Neptune grew from the incoming material to its present disproportionate size. Some other models have suggested that Neptune and Uranus were originally positioned in opposite order than today and moved to their present positions by gravitational instabilities, an idea the flyby model discounts.

A final note is that the so-called ‘cold Kuiper Belt,’ made up of objects that move on fairly circular, low-inclination orbits, does not fit well with any current planet-scattering models, while it does fit within the parameters of the stellar flyby simulations. Moving to the realm of prediction, these simulations make it clear that if the flyby scenario is correct, many additional outer system bodies at high inclinations may yet be found, perhaps including an object of planetary mass.

Recently we looked at Omega Centauri a cluster whose dense composition caused researchers to question habitability given that planetary orbits were so likely to be disrupted by encounters with other stars. It’s worth remembering that stars like the Sun were born in large groups of stars that were originally densely packed, making stellar encounters more likely.

The numbers from subsequent simulations performed by these researchers are interesting, showing that the odds on such an encounter were fairly high:

Performing simulations of the dynamics of star clusters – gas expulsion, expansion and finding a new quasi-equilibrium – we find that such type of fly-bys are probable not only during the early phases but also on Gyr time scales, with a 5-7% chance of all solar-type stars experiencing such a fly-by during the first 10 Myr and a 20-30% chance in the next Gyr. If planet formation is as fast as anticipated from the ring structures in some discs, even a fly-by during the first 10 Myr would be an option. We showed that the probability of such an event even in the consecutive 4.56 Gyr is competitive to that of other models.

Thus we have an alternative to the ‘Nice Model’ and its offshoots for early Solar System formation, one that manages to explain multiple features of the outer system through a single event rather than a series of gravitational encounters. Calling it a ‘realistic alternative’ to present models, Pfalzner points out that it remains a hypothesis, but one intriguing in its simplicity.

The paper is Pfalzner et al., “Outer Solar System Possibly Shaped by a Stellar Fly-by,” The Astrophysical Journal, Vol. 863, No. 1 (09 August 2018). Abstract / preprint.

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 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).

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.