Reconsidering Gliese 581

Gliese 581 continues to occupy the attention, and understandably so. At least three planets orbit this M-dwarf, one of which sprang into the public consciousness with the announcement that it might be in its star’s habitable zone. But both Gl 581c and d are interesting from the habitability standpoint, even if subsequent discussions have pointed out just how problematic it is to make such judgments on insufficient data.

Ponder how tricky the call can be. For being in a circumstellar habitable zone only means that a terrestrial-size planet can have liquid water on its surface. A new paper by Franck Selsis (Centre de Recherche Astrophysique de Lyon), James Kasting (Pennsylvania State) and colleagues wades right into this morass, pointing out how many other factors could make such a planet remain uninhabitable:

  • Water may not be available
  • A high impact rate may prevent the emergence of life
  • The thus far unknown minimum ingredients for life’s formation may not be present
  • Gravity may be too low to retain a dense atmosphere
  • The planet may have formed an atmosphere that keeps the surface pressure too high for water to remain liquid

And so on. The point being that the habitable zone is no guarantee of life. For that matter, we have a long way to go in understanding how to put all these factors together. The Selsis/Kasting paper, already under discussion in various messages here, sees Gl 581c as definitely problematic but not completely out of the life-forming picture. While it’s unlikely to have liquid water, the uncertainties involved in its cloud properties and cloud cover leave the question at least slightly open. From the paper:

…Gl 581c would be habitable only if clouds with the highest re?ectivity covered most of the daytime hemisphere. A 50% cloud cover is not sufficient to prevent a runaway greenhouse effect on Gl 581c, which receives 30% more energy than Venus today. This problem is exacerbated by the fact that Venus has a much higher albedo than the expected value for a habitable planet at the orbital distance of Gl 581c. The composition of the atmosphere of Gl 581c depends on the mass of the initial water reservoir on the planet, and on the efficiency of the gravitational escape of H.

The paper goes on to describe possible scenarios, including one involving water forming “…a mantle of hot and high-pressure ice underneath a ?uid envelope of supercritical H2O,” as well as a Venus-like scenario leaving a CO2 rich atmosphere with surface temperatures as high as 1000 K. But we’re not through — change the models around and things get much iffier. The authors note, for example, how much depends on the carbonate-silicate cycle, which stabilizes surface temperature and the amount of CO2 in the atmosphere. Even at 1 AU, Earth itself would not be close enough to the Sun to maintain water above the freezing point without enough atmospheric CO2.

What happens if we do away with the greenhouse effect on Gl 581c, or take albedo to the extreme? How about geochemical processes and their effect on whatever atmosphere exists there? Dig into this paper for the speculative details. With so many questions, it’s clear how much we need by way of further data before we can make any calls. The authors put it this way:

Because of the uncertainties in the precise location of the HZ boundaries, planets at the edge of what is thought to be the HZ are crucial targets for future observatories able to characterize their atmosphere. At the moment, our theory of habitability is only con?rmed by the divergent fates of Venus and the Earth. We will have to confront our models with actual observations to better understand what makes a planet habitable. The current diversity of exoplanets (planets around pulsars, hot Jupiters, hot Neptunes, super-Earths…) has already taught us that Nature has a lot more imagination when building a variety of worlds than we expected from our former models inspired by the Solar System.

Amen to that. And as for Gl 581d, it’s conceivable that we’re looking at an early Mars scenario, a situation thought to have involved plentiful water at the surface. Add a greenhouse effect from CO2 ice clouds and planet d might just be the best bet for habitability in this odd and fascinating system. But here again, we don’t know enough about the geochemical processes involved that could stabilize such an atmosphere. A mission like Darwin or whatever emerges from the US equivalent could tell us much about the atmospheres of both worlds, searching in particular for water vapor bands in the atmosphere of Gl 581d, and on Gl 581c distinguishing between a CO2 atmosphere and an H2O rich alternative.

Prime targets for follow-up work with next generation space technology? You bet. The paper is Selsis et al., “Habitable planets around the star Gl 581?” accepted for publication in Astronomy and Astrophysics, available online in preprint form and, as one of our readers has already noted, as absorbing and thorough a take on these distant worlds as we’ve yet seen.

A Closer Look at Vesta

It seems extraordinary to speak of picking up pieces of an asteroid on the surface of the Earth, but the meteorites known as eucrites are confidently identified with Vesta, the brightest asteroid in the sky (and the only one visible with the naked eye). With the Dawn mission on its way to both Ceres and Vesta, we’ll learn much more about the composition of both, but Vesta is coming into its own as a most unusual object that has contributed much to the surrounding system.

Hubble model of Vesta

For the 330-mile wide asteroid sports a huge gouge taken out of its south pole, apparently the result of a collision between protoplanetary objects. The hole, some eight miles deep, once contained a half million cubic miles of asteroid material that was subsequently blasted into interplanetary space, where interaction with Jupiter came into play. Gravitational tugging changes orbits, and some of these objects were put onto trajectories that brought them to Earth.

Image: A 3-D computer model of the asteroid Vesta synthesized from Hubble topographic data. The crater’s 8-mile high central peak can clearly be seen near the pole. The surface texture on the model is artificial, and is not representative of the true brightness variations on the asteroid. Elevation features have not been exaggerated. Credit: Ben Zellner (Georgia Southern University), Peter Thomas (Cornell University) and NASA.

Thus we can study at least part of Vesta — materials from its primordial surface — right here. Says Christopher Russell (UCLA):

“Meteorites are hardy objects indeed. Eucrites are a specific type of meteorite that the science community is confident came from Vesta’s surface. We believe that when Vesta was forming, there was molten rock that flowed onto its surface that cooled rapidly. That rapid cooling created small crystals.”

Indeed, the spectral signature of these meteorites is identical with that of Vesta. The isotopes in eucrites are unlike isotopes found in any other rocks on Earth, the Moon or other meteorites. “Simply put,” adds Russell, “we cannot find another place in the Solar System they could be from.”

This JPL feature offers background. We follow Dawn with interest, but note that finding fragments is the best way to encounter an asteroid on Earth, a reminder that a darker alternative is the kind of impact that leads to planetary catastrophe. Our deepening understanding of asteroids should have us sorting out our options for missions to near-Earth objects in hopes of learning as much about their composition as we’re learning about Vesta, and deducing from that how we go about deflecting them.

Hot Jupiters Co-existing with Earth-like Worlds?

One of the surprises of the early planet-hunting era has been the discovery of ‘hot Jupiters,’ giant planets orbiting extremely close to their parent star. That these planets should be prolific in our catalog at present makes sense given the nature (and limitations) of radial velocity detection methods, but before we started finding them, there seemed little reason to believe gas giants would exist at orbits within 0.1 AU. Now we see them as evidence that protoplanets can migrate during the formation period, probably causing havoc as they pass through the inner system.

Are hot Jupiters the bane of terrestrial planets? You would think so, given the above scenario, with a gas giant clearing planet-forming materials out of the inner disk during its passage. But Martyn Fogg and Richard Nelson (University of London) think otherwise. Their new paper looks at models of terrestrial planet formation and finds that inner disks survive the passage of the inbound giant, resuming their planetary formation once the hot Jupiter has closed to its new, searingly close orbit.

A sample scenario goes like this, using a simulation based on a system that has evolved for a million years before the giant planet appears and begins its migration:

The results show that the passage of the giant does not sweep the inner system clear of planet-forming material. Instead, the giant planet shepherds the solids disk inward, compacting it and exciting the orbits of objects captured at mean-motion resonances. Much of this excited material eventually experiences a close encounter with the giant planet and is expelled into an exterior orbit, augmenting a new disk of solid material that progressively builds up in orbits external to the final position of the hot-Jupiter. In this particular case, 86% of the solids disk survives, with 82% of it residing in the external scattered disk.

If this work is correct, the passage of a wandering gas giant has a different aspect than we have assumed. The inner disk is only slightly diluted by the event. Moreover, the materials left behind are prompted to new planetary formation with volatile-rich material moving inward. And note this comment on orbital eccentricity (internal references deleted for brevity):

The results of further simulation of accretion in this scattered disk show that the initially eccentric orbits of protoplanets are rapidly damped and circularized via dynamical friction exerted by smaller bodies and possibly via tidal drag exerted by the remaining gas. Planetary growth resumes and over the following ~ 10-100 Myr gives rise to a set of water-rich terrestrial planets in stable orbits external to the hot-Jupiter.

Thus another paradigm shift: This paper suggests that all those hot Jupiter systems we’ve been more or less writing off for Earth-like planets are back in the game. These systems account for roughly one-quarter of all exoplanets thus far found, so the finding isn’t insignificant. Can Kepler, Darwin or perhaps a mission like ESA’s proposed PLATO track down a terrestrial world in such a system? The technology should be up to the task, but the first step is the realization, more than a little surprising, that we may need to add hot Jupiter systems to the target list.

The paper is Fogg and Nelson, “Can Terrestrial Planets Form in Hot-Jupiter Systems?” to appear in Extreme Solar Systems, ASP Conference Series, eds. Debra Fischer, Fred Rasio, Steve Thorsett and Alex Wolszczan (abstract). Personally, I’m finding the interest in extreme systems fascinating, because part of the study is figuring out which systems are actually extreme, and which scenarios, however unlike our own, may be common.

Notes & Queries 10/27/07

More notes on the ‘wandering planet’ scenario advanced by John Debes (Carnegie Institution of Washington) and Steinn Sigurðsson (Penn State), which suggests that planets ejected from their stars as their solar systems formed could conceivably keep enough internal heat to maintain an atmosphere and sustain a liquid ocean under ice. Debes’ simulations show that a planet with a large moon could survive the ejection process.

Noting that between four and five percent of the simulations the duo ran on an Earth-mass planet with Luna-like companion resulted in the ‘Earth-Moon system surviving, Debes had this to say in an article in Sky & Telescope:

“Anytime something happens in astronomy a few percent of the time, it is interesting to us because on the grand scale of things, it means it’s happening a lot and people should probably know about it.”

Interesting indeed, because a large moon means tidal energies between moon and planet that could cause the interior of the planet to warm. Debes and Sigurðsson think the heating would be localized in hot spots of volcanism or other geothermal processes, making the case for extremophiles like those along Earth’s mid-ocean ridges as a perhaps common form of life in the cosmos. The paper is Debes and Sigurðsson, “The Survival Rate of Ejected Terrestrial Planets with Moons,” Astrophysical Journal 668 (October 20, 2007), L167-L170 (abstract).
The unexpected outburst on Comet 17P/Holmes has made it brighter than any comet in the past decade. Now shining at 2nd or 3rd magnitude, it’s visible all night long at mid-northern latitudes, spanning an apparent diameter of 90 arc-seconds. You can find sky charts for viewing here. Nice to see cometary enthusiasm so intense that, when the Harvard-Smithsonian Center for Astrophysics asked their staff for viewing reports and images, responses poured in despite the competing attraction of the Red Sox in World Series game two. The comet, says one observer: “…looks like a big yellow globular cluster through binoculars. Truly a one of a kind object.” Even more one of a kind would be a Rockies comeback after their hard times at Fenway Park, but hope persists.
Vinton Cerf, now a Google vice president but best known for the invention of the TCP/IP protocols that drive the Internet, told a gathering in Seoul that the InterPlanetary Internet project is on course. According to this AFP story, Cerf expects a key part of the IPN, which would establish broad standards for space communications, to be completed in three years: “This effort is now bearing fruit and is on track to be space qualified and standardized in the 2010 time frame.” Adapting Net protocols to the huge latency problems of deep space missions will result in a robust interplanetary communications infrastructure, one that will maximize the potential of the Deep Space Network as spacecraft collect and pool their data before phoning home.
North Carolina State’s PULSTAR reactor is in the news with the production of the most intense low-energy positron beam operating anywhere. The idea, says NCSU’s Ayman Hawari, is simple: “…if we create this intense beam of antimatter electrons – the complete opposite of the electron, basically – we can then use them in investigating and understanding the new types of materials being used in many applications.” The focus now turns to new intstrumentation such as antimatter spectrometers and microscopes, but propulsion theorists will want to keep a long-term eye on the implications of such work for increasing our ability to produce and deploy antimatter.

A Gravitational Explanation for Dark Matter

Because dark matter has never been directly observed, we’re left trying to figure it out using deductions based on its presumed effects on visible matter. Seven dwarf satellite galaxies of the Milky Way — Carina, Draco, Fornax, Leo I, Leo II, Sculptor and Sextans — offer a case in point. Stars in these galaxies do not move more slowly the farther they are from their galaxy’s core. Is dark matter the explanation?

Mario Mateo (University of Michigan) has been studying the velocity of almost 7,000 stars in the seven dwarfs. His observations lead to the same kind of deduction already been made for larger spiral galaxies, that the matter we see does not account for the apparent distribution of mass throughout the galaxy. All that, of course, depends upon subjecting these observations to established theory. Mateo colleague Matthew Walker (now at the University of Cambridge) puts it this way:

“We have more than doubled the amount of data having to do with these galaxies, and that allows us to study them in an unprecedented manner. Our research shows that dwarf galaxies are utterly dominated by dark matter, so long as Newtonian gravity adequately describes these systems.”

But there are those who argue that it does not. You can explain dark matter under other theories as well, but when you do this, you have to start tinkering with some basics of modern physics. While the Michigan team travels to Cambridge MA to present its findings on the 30th, Canadian researchers are pushing a different model, one based on the idea that there is no dark matter. Modified Gravity theory (MOG) takes Newtonian/Einsteinian gravity in a new direction, and if correct, could explain not only the motion of distant galaxies but objects closer to home.

Bullet Cluster

For John Moffat (University of Waterloo, Ontario) and graduate student Joel Brownstein are arguing that they can explain stellar and galactic motion as well as the anomalous deceleration of the Pioneer 10 and 11 space probes by using Modified Gravity theory, which Moffat has been developing for the last thirty years. Their work involves the Bullet Cluster, two merging clusters of galaxies some three billion light years from Earth in the direction of the constellation Carina.

Image: This composite image shows the galaxy cluster 1E 0657-56, also known as the “bullet cluster.” This cluster was formed after the collision of two large clusters of galaxies. Hot gas detected by Chandra in X-rays is seen as two pink clumps in the image and contains most of the “normal,” or baryonic, matter in the two clusters. The bullet-shaped clump on the right is the hot gas from one cluster, which passed through the hot gas from the other larger cluster during the collision. An optical image from Magellan and the Hubble Space Telescope shows the galaxies in orange and white. The blue areas in this image show where astronomers find most of the mass in the clusters.
Credit: X-ray: NASA/CXC/CfA/M.Markevitch et al.; Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al.; Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/D.Clowe et al.

Although previous studies have concluded that gravitational lensing associated with the Bullet Cluster demonstrates the presence of dark matter, Moffat and Brownstein think that normal matter in the cluster can account for the observed lensing. Assuming, of course, that the MOG theory is right. And we should be getting more answers as the search for dark matter continues. Says Moffat:

“If the multi-billion dollar laboratory experiments now underway succeed in directly detecting dark matter, then I will be happy to see Einsteinian and Newtonian gravity retained. However, if dark matter is not detected and we have to conclude that it does not exist, then Einstein and Newtonian gravity must be modified to fit the extensive amount of astronomical and cosmological data, such as the bullet cluster, that cannot otherwise be explained.”

Fair enough, and the beauty of that is that either way we win, with a deeper understanding of dark matter through direct observation, or a growing knowledge of how current gravitational theory can be enhanced. The Michigan study is Walker et al., “Velocity Dispersion Profiles of Seven Dwarf Spheroidal Galaxies,” Astrophysical Journal 667 (September 20, 2007), L53-L56 (abstract). The Modified Gravity paper is Brownstein and Moffat, “The Bullet Cluster 1E0657-558 evidence shows Modified Gravity in the absence of Dark Matter,” accepted for November publication in Monthly Notices of the Royal Astronomical Society (abstract).