by Paul Gilster | Jul 14, 2008 | Exoplanetary Science |
The news about planetary prospects around the Centauri stars has been positive enough lately that a paper suggesting otherwise introduces a rather jarring note (to me, at least). After all, we’ve detected more than forty extrasolar planets in multiple systems, a significant percentage of all detected exoplanets, and while most of these are in systems where the stars are widely spaced, there are planets around stars like Gliese 86 or Gamma Cephei where the separations are in the range of a Centauri-like 20 AU. Moreover, key studies have shown that planetary orbits in the habitable zone of the Centauri stars are viable.
But what Philippen Thébault (Stockholm Observatory), Francesco Marzari (University of Padova) and Hans Scholl (Observatoire de la Côte d’Azur) bring to the table is a different question. Never mind that planetary orbits may be stable — how likely are planets to form in these settings in the first place? It turns out that the last stage of planet formation has been studied, with no deal-breaking restraints on at least some planets in close binary systems if the stars are separated by more than 5 AU. The new work, however, focuses on an earlier stage, the accretion of small planetesimals that leads to so-called planetary ’embryos.’
And here the news is much less positive for those of us hoping to find terrestrial worlds around such stars. The companion star can cause such perturbations that the kilometer-sized planetesimals involved collide at high speeds, moving too rapidly to allow them to gradually merge into a larger object. Earlier work by these authors has shown that the efficiency of runaway growth is quite sensitive to encounter velocities in budding planetary systems. The current paper now applies these results to the situation around Centauri A.
The results: Beyond a close 0.5 AU, Centauri A “…is hostile to kilometre-sized planetesimal accretion.” In fact, say the authors, using any realistic distribution of planetesimals, these high-impact events outnumber low-velocity encounters by far. What’s worse, “…our approach can be regarded as conservative regarding the extent of the impact velocity increase which is reached, the ‘real’ system being probably even more accretion-hostile than in the presented results.” That makes Centauri A an unlikely place for habitable worlds like our own.
Image: Artist’s conception of the debris disk surrounding the Sun-like star, HD 12039. Planetesimals like these are thought to be the building blocks of planets. But will they form planets in the binary environment of Centauri A? Credit: NASA/JPL-Caltech; T. Pyl, SSC.
Contrasting this outcome with earlier studies that found planet formaton possible in the region inside 2.5 AU of Centauri A, the authors explain why their work comes to such a radically different conclusion:
This is because these studies focused on the later embryo-to-planet phase, thus implicitly assuming that the preceding planetesimal-to-embryo phase was successful. The present study shows that this is probably not the case. This con?rms that the planetesimals-to-embryos phase is more affected by the binary environment than the last stages of planet formation.
How to extrapolate these results to other stars? The authors are reluctant to do so, not even to nearby Centauri B, the perturbing star in the Centauri A scenario. The reason is that the mathematical modeling here is based both upon the perturbations caused by the nearby star as well as the relationship between those perturbations and the effects of gas drag in the early system, which shape the movements of planetesimals. The byplay between these forces is complex and obviously varies from star to star, which is one reason we have so far to go in understanding how planets form in binary systems.
Can we imagine planetary migration from inner orbits into the habitable zone? Perhaps, but this would assume interactions between multiple planets and a sizeable disk of remaining planetesimals. The authors run through other scenarios that would produce Centauri A planets at 1 AU or greater, including the possibility that the Centauri stars were originally in a different configuration than today, thus setting up a new set of initial conditions. The latter can’t be ruled out, but the authors’ conclusion remains:
…we think that our results on planet formation, with the present binary con?guration, are relatively robust. Our main result is that it is very di?cult for s < 30 km planetesimals to have accreting encounters beyond 0.5-0.75 AU from the primary, which makes planet formation very unlikely in these regions.
Observationally, of course, we lack the data to know just what we might find around 1 AU at Centauri A. Radial velocity studies have shown that we can say only one definitive thing about planets around this interesting star: If they exist, they are smaller than 2.5 Jupiter masses. The new work, which takes what the authors believe is a more realistic set of assumptions for differences in sizes among planetesimals, suggests that if we find smaller worlds around this star, they will be relatively close to the star, too close for habitable conditions to exist there. A look at Centauri B using these parameters would be welcome.
The paper is Thébault et al., “Planet formation in Alpha Centauri A revisited: not so accretion-friendly after all,” accepted for publication in Monthly Notices of the Royal Astronomical Society (abstract). Centauri Dreams has covered a good deal of recent work by Elisa Quintana, Jack Lissauer, Greg Laughlin and others — you can use the search function on the top page to find these entries, or to get a representative sample, look here and here and here, where citations are available.
by Paul Gilster | Jul 12, 2008 | Outer Solar System |
The latest Carnival of Space is now available at the Space Disco site, where Dave Mosher has put together a helpful slideshow of entries handsomely illustrated and linked to the originals. With seven new blogs coming online at Discovery Space, we’ll doubtless be seeing contributions from many of these fine writers, people such as Ray Villard, Chris Lintott and Mosher himself. I’m particularly looking forward to Jennifer Ouellette’s Twisted Physics blog, which this week offers a backgrounder on tachyons.
In terms of our usual beat, deep space from the outer planets into interstellar space, I’ll send you to David S.F. Portree’s Altair VI site, where the author has gone to considerable trouble to present the results of a study by Science Applications, Inc. (SAI) on the possibilities of futuristic missions to Titan. This material was originally presented in 1983 at a NASA workshop and offers a view of what should still be a viable game plan: To seed the clouds of Titan with floating laboratories to study the chemical evolution occurring in its atmosphere, with an eye toward examining life’s building blocks.
Image: A blimp hovers explores Titan’s lower atmosphere. Credit: American Blimp Corporation/NASA.
The SAI concept involved an orbiter that would provide the necessary radio relay between systems in the atmosphere and on the surface of Titan and scientists on Earth. It’s fascinating to contrast what we did with the Huygens probe with what SAI came up with 25 years ago, including a ‘penetrator’ probe that would collect surface data for return to the orbiter. And how about these exotic systems:
SAI’s most novel and picturesque Titan exploration systems were its large and small buoyant stations. Delivered into Titan’s atmosphere by the flyby bus, these probes would be based on the Galileo Jupiter atmosphere probe design. The small stations, packed into aeroshells the same size as the Galileo probe (1.25 meters), would be balloons. The large station, packed into an aeroshell twice as large, would be either a large balloon or a powered blimp. Small buoyant stations would operate between 100 and 10 kilometers above Titan, while large buoyant stations would operate between 10 kilometers of altitude and Titan’s surface.
Getting such missions to Saturn using Space Shuttle boosts to Earth orbit imposed severe limitations upon the planners, but assuming a future capability of on-orbit assembly and refueling allowed the full spectrum of concepts to be explored, including a complex mission design with 28 experiments and no need for planetary gravity assists. It’s fascinating to read through these ideas, and frustrating to realize how much the cost estimates have changed in the years since. But we can certainly envision blimps like these being put to use in a variety of exploratory settings — Venus also comes to mind, as do all the outer planets — as we refine robotic systems for environments beyond the reach of manned missions.
by Paul Gilster | Jul 11, 2008 | Asteroid and Comet Deflection |
Photons streaming outward from the Sun can impart momentum, which is how a solar sail works. But even more subtle effects produced by the warming of irregular objects may have visible results. A new study of asteroid moons and how they form invokes the tongue-twister known as the Yarkovsky, O’Keefe, Radzievskii, Paddack effect, mercifully shortened to ‘YORP effect’ by those who study it. A body warmed by the Sun gives off infrared radiation, which carries momentum as well as heat. An asteroid’s spin can thus be speeded or slowed by sunlight.
Add plenty of time and things get interesting. Start with the kind of asteroid that is little more than a pile of rocky rubble held together by gravity, then spin that rubble pile up slowly over a period of millions of years and eventually material will be slung off from the asteroid’s equator. Colliding materials of this nature can eventually coalesce into the satellite we see orbiting its parent, says Patrick Michel (Cote d’Azur Observatory, France), who goes on to note the implications for defending Earth against an incomng asteroid:
“Based on our findings, the YORP effect appears to be the key to the origin of a large fraction of observed binaries. The implications are that binary asteroids are preferentially formed from aggregate objects [rubble piles], which agrees with the idea that such asteroids are quite porous. The porous nature of these asteroids has strong implications for defensive strategies if faced with an impact risk to Earth from such objects, because the energy required to deflect an asteroid depends sensitively on its internal structure.”
Image: Three views of the binary asteroid KW4, a ‘rubble pile’ that may have spawned its own moon. Credit: NASA/JPL.
A binary impact is quite a thought, but the authors of the study point out that doublet craters formed by the nearly simultaneous impact of similar objects can be found on Earth as well as other planets. Learning how to counter such asteroids is going to push our technology to the limit, involving as it will missions to different types of asteroids to assess their makeup and figure out which methods for trajectory change are most likely to succeed.
If the YORP theory proves out, it will have solved an earlier conundrum. Small binary asteroids are not thought to have formed early in the history of the Solar System, so what brought them about later? You could create a model of collisions and planetary encounters to account for the binaries, but their sheer numbers make that model dubious. Fully fifteen percent of near-Earth and main belt asteroids (two dynamically different environments indeed) with diameters of less than ten kilometers are now believed to have satellites. The YORP model, which fits the test case of binary asteroid KW4, may well hold the key.
The paper is Walsh et al., “Rotational breakup as the origin of small binary asteroids,” Nature 454, (10 July 2008), pp. 188-191 (abstract)
by Paul Gilster | Jul 10, 2008 | Exotic Physics |
The continuing activity on the Practical Positronic Rocket threads has made it clear that we need a place for speculations that do not flow out of particular posts. What we’re aiming at down the road is to implement discussion software that will make such threads easy to follow and contribute to, but for now we’re dealing with weblog software that is not optimized for the task. Hence this thread, which is open to rational theorizing about interstellar issues in comments that do not reflect content found in the posts elsewhere on the site. If your idea is ‘blue sky’ and not related to a particular post, this is the place to put it.
by Paul Gilster | Jul 10, 2008 | Exoplanetary Science |
We’d all like to think our Solar System is a run-of-the-mill place, filled with the kind of planets, including our own, likely to be found around other stars. But maybe it’s not so ordinary after all. For recent work suggests that stars like the Sun aren’t all that likely to form planets the size of Jupiter or larger. So while small, rocky worlds may or may not be common — we’re still finding the answer to that one — the combination of rocky worlds and gas giants we take for granted may actually be distinctive.
Once again I’m reminded how many conjectures go into our projections of habitable worlds. Here’s one possibility: Without a large gas giant in the outer solar system to act as a gravitational shield for the inner system, planets in the habitable zone of a star might be so pelted by space debris that life would be unlikely to form on them. So it’s conceivable that any findings about the scarcity of gas giants are a blow to our astrobiological hopes around other stars. At least, around stars like our own.
The work in question looks at the Orion Nebula, that fertile breeding ground for new stars. Only a million years old, an infant in cosmic terms, this is quite a dense place, from which the view must be striking: Pack a thousand stars into a cube several light years to the side and you’ve got the idea. The Sun’s origin some four billion years ago is commonly thought to have occurred in a dense, open cluster like Orion. These clusters come apart with age, their stars gradually separating until no trace of the original cluster remains.
Using the Combined Array for Research in Millimeter Astronomy (CARMA), researchers from the University of California at Berkeley, Caltech, and the Harvard-Smithsonian Center for Astrophysics have found that only eight percent of the stars in Orion’s central region have the surrounding dust needed for a gas giant. That would be a disk with a mass greater than one-hundredth the mass of the Sun. Indeed, the average mass of a protoplanetary disk in this region was only one-thousandth of a solar mass.
Only one in ten of the stars showed radiation characteristic of any dust disk whatsoever. The paper on this work explains the ramifications: “Evidently, giant planet formation is either advanced (having thus depleted the small dust grains in the disk) or impossible around most stars in the ONC [Orion Nebula Cluster].”
Image: While a Hubble Space Telescope image of visible light emitted by a protoplanetary disk in the Orion Nebula called proplyd 170-337 shows hot, ionized gas (red) surrounding and streaming off of a disk (yellow), 1.3 mm radio observations by CARMA and SMA reveal the dust disk hiding within the hot gas (contours). This protoplanetary disk has a mass more than one hundredth that of the sun, the minimum needed to form a Jupiter-sized planet. Credit: Bally et al 2000/Hubble Space Telescope & Eisner et al 2008/CARMA, SMA.
The findings may well relate to how tightly packed the Orion stars are. John M. Carpenter (Caltech), relating the work to previous studies of the Taurus cluster (where twenty percent of the stars showed enough dust to form planets), had this to say about Orion:
“Somehow, the Orion cluster environment is not conducive to forming high mass disks or having them survive long, presumably due to the ionization field from the hot, massive OB stars , which you might expect would photoevaporate dust and lead to small disk masses.”
This (from the paper) is also interesting, while highlighting the obvious need for further work:
… our observations show no clear correlation between stellar mass and disk mass, but suggest that massive disks may be more likely to be found around lower mass stars. The percentage of detected disks is lower for stars more massive than 1 M? , and the most massive disks detected are associated with the relatively low stellar mass stars in the sample. However, larger numbers of stellar and disk mass measurements in the ONC are needed to build up better statistics and further constrain the relationship between stellar and disk properties.
Let’s relate all this this to broader exoplanet findings. Six percent of the stars thus far surveyed have planets the size of Jupiter or larger. This does not mean that smaller disks of the sort that could give rise to rocky planets are not out there, and perhaps in abundance. Improved instrumentation, like the unfinished Atacama Large Millimeter Array (ALMA) now being built in Chile, may tell us how numerous they are. But it does imply that what we assume to be a common way of producing stars like the Sun is less likely to deal up Jupiter-class worlds, with ramifications not yet fully understood. (See the comments re a slight change to the text above).
An abridged version of the paper is available online. It’s Eisner et al., “Proplyds and Massive Disks in the Orion Nebula Cluster Imaged with CARMA and SMA,” accepted for publication this August in the Astrophysical Journal.
