by Paul Gilster | Sep 7, 2006 | Exoplanetary Science |
Planetary migration — as when a ‘hot Jupiter’ moves inward toward its parent star during system formation — may not be as disruptive as we once thought. In fact, according to a new study led by the University of Colorado at Boulder and Pennsylvania State, solar systems with hot Jupiters may well harbor Earth-like planets covered with deep oceans.
The research team’s paper, just published in Science, paints a positively benign scenario, one in which the gas giant’s migration actually becomes the trigger for the formation of water worlds that could well turn out to be habitable. Let’s look at this more closely, because it’s quite a shift from earlier studies, which assumed that a hot Jupiter’s migrations would eject protoplanetary materials from the system or else absorb them.
Working with computer simulations, the Colorado/Penn State researchers now think the hot Jupiters force rocky debris outward in the system, helping the formation of rocky planets. At the same time, and this is crucial, the dense surrounding gas slows the orbit of small, icy objects in the outer reaches of the protoplanetary disk, causing them to move inward and deliver water to the newly formed rocky worlds. To achieve these results, the study used simulated protoplanetary disks containing more than 1000 rocky, icy bodies and ran them through 200 million years of planetary evolution based on the best information we have on how planets form.
“Upcoming space missions such as NASA’s Kepler and Terrestrial Planet finder and ESA’s COROT and Darwin will discover and eventually characterize Earth-like planets around other stars,” write the authors in Science. “We predict that a significant fraction of systems with close-in giant planets will be found to have a Hot Earth or potentially habitable, water-rich planets on stable orbits in the Habitable Zone.”
The remarkable conclusion is that one out of every three known exoplanetary systems may shelter Earth-like planets in its habitable zone. Habitability means temperatures that involve orbiting well outside the hot Jupiter’s orbit, but the team notes that rocky planets inside that orbit — ‘hot Earths’ — occur in the simulations as well.
Centauri Dreams‘ take: These results show again how early in the planet-hunting process we are, and how many assumptions we will have to revise as we proceed. Hot Jupiters themselves were almost inconceivable until first detected; now they comprise about 40 percent of all known exoplanets. It made sense to see their movements as inimical to life-bearing planets and some models assume this, but if they are not, our models of planetary formation begin to point to Earth-like planets in a wider variety of settings than we had ever imagined.
If they do exist, these would be water worlds with an exclamation point. The team’s findings show that such planets could have up to 100 times the water present on Earth today, oceans miles and miles deep, with all that implies about creating sustainable biospheres. One difference, however, is that these planets would probably have a lower percentage of iron than Earth, which could have significant effects on how their atmospheres evolved. We need space-based telescopes and spectroscopic studies to learn more.
Colorado researcher Sean Raymond has this to say about life on one of these water worlds:
“I think there are definitely habitable planets out there. But any life on these planets could be very different from ours. There are a lot of evolutionary steps in between the formation of such planets in other systems and the presence of life forms looking back at us.”
Just how different is something we can hope to learn when our technology produces viable propulsion methods for interstellar probes.
But read the whole study, one of the most heartening to come across my desk since Greg Laughlin and Jeremy Wertheimer’s work on Proxima Centauri. It’s Raymond et al., “Exotic Earths: Forming Habitable Worlds with Giant Planet Migration,” in the September 8 issue of Science. One can only imagine Webster Cash’s interest in this paper. The Boulder-based Cash’s New Worlds Imager design, now being considered by NASA, is just what we need in space to actually get an image of some of these water worlds.
by Paul Gilster | Sep 6, 2006 | Exoplanetary Science
Putting a brown dwarf into the same stellar system with one or more known planets seems like a dicey proposition, but we know it sometimes occurs. Radial-velocity studies have already detected systems like HD 38529 and HD 168443 that include a brown dwarf and known planet. In both these cases, the brown dwarf involved is known to revolve around the exoplanet host star in an orbit at least ten times wider than that of the planet found there. But there are more unusual possibilities: A brown dwarf around the star HD 202206 actually moves inside the orbit of the known exoplanet there. Look no further if you need a new science fiction setting.
Until now, we’ve never had a direct image of a brown dwarf around an exoplanet host star, but a new paper changes all that. A German team led by Markus Mugrauer (University of Jena) provides just such a detection, as shown in the image below. The star is HD 3651, a K-class dwarf near the boundary between Pegasus and Pisces some 11 parsecs from Earth; its mass is about 80 percent that of the Sun. What we know about its planet is that it is a small Saturn-class world in a highly eccentric orbit with a semi-major axis of 0.284 AU (think of the ‘major’ axis of an ellipse as its longest diameter, while the ‘semi-major’ axis is half of that).
Image: Note the arrow isolating HD 3651 B, a brown dwarf in an exoplanetary system. Other candidate objects, which did not turn out to be associated with the primary star, are shown in circles. Credit: Markus Mugrauer and team, ESO/NTT and UKIRT (United Kingdom Infrared Telescope).
The team detected five faint objects around HD 3651, as shown in the image, but found that only one shared the host star’s proper motion. Based on these observations, the brown dwarf now called HD 3651 B is thought to be about 480 AU from the primary, and the possibility that it is a white dwarf can be ruled out because of its faintness in the optical spectral range. We are evidently looking at a T7 brown dwarf with a temperature between 800 and 900 K.
Interesting? You bet. The more we can learn about brown dwarfs in exoplanetary systems, the more we can refine our models of how such objects are formed, and how both brown dwarfs and planets might emerge around the same primary star. The paper is Mugrauer et al., “HD 3651 B: the first directly imaged brown dwarf companion of an exoplanet host star,” accepted for Monthly Notices of the Royal Astronomical Society Letters and available online here.
by Paul Gilster | Sep 5, 2006 | Exoplanetary Science |
Among the 200-plus exoplanets discovered thus far, the system around the red dwarf Gl 876 stands out. For one thing, it contains the closest thing to a terrestrial-sized planet yet found, with a mass of about six times that of Earth and a tight, two-day orbit around its primary. For another, it houses two gas giants, the only planets of this type known to orbit an M-dwarf, and they exhibit a 2:1 mean-motion resonance — one of them orbits the star twice in the same amount of time the other makes a single orbit. Such resonances provide clues in the study of how systems like these formed and changed over time.
These gas giants, one in a 30-day orbit, the other in a 60, are fascinating in their own right, but the detection of the small inner planet by Doppler techniques shows just how far planetary detection methods have come in the past decade. Now a paper slated for publication in The Astrophysical Journal looks at Gl 876 in terms of planetary transits, for to arrive at the true mass of a planet, independent measures of its orbital inclination are needed. The team’s goal was to discover whether the two outer planets around Gl 876 make such transits, and the answer seems to be no.
This is still useful information, for it provides a tune-up to the existing radial velocity data. And it points to the singularly productive nature of further transit studies. If small, terrestrial-style worlds do form commonly around stars like Gl 876, we have reason to believe they are detectable. As the paper points out, an Earth-sized planet transiting such a star will block about 1 percent of the stellar flux from that star. If in a habitable 3.85-day orbit, the planet will make a transit that lasts about forty minutes. These are workable numbers, as the paper says:
A 1% photometric dip is readily detectable. Amateur astronomers who participate in the Transitsearch collaboration routinely achieve detection thresholds of considerably better than 1%, as evidenced by confirming detections of HD 149026b which has a transit depth of just 0.3%. Indeed, capable amateur observers have demonstrated the photometric capability to detect the passage of a Mars-sized body in front of an 11th magnitude 0.1 solar mass red dwarf.
For the properly equipped amateur astronomer, Transitsearch is an intoxicating challenge. What could be more significant than the detection of an Earth-mass planet around another star? Nearby red dwarfs of great interest include Proxima Centauri, Barnard’s Star, Wolf 359, and Ross 154, among others listed in this paper. “We estimate,” the paper continues, “that each one has a ~ 1% chance of harboring a detectible transiting, potentially habitable planet – readily detectible by distributed photometric observation.” Given these numbers, we may be no more than a year or so away from the first detection of a habitable world.
The paper is Shankland, Rivera, Laughlin et al., “On the Search for Transits of the Planets Orbiting Gl 876,” published in the December 10 issue of The Astrophysical Journal (Vol. 653, p. 701) and available online.
by Paul Gilster | Sep 4, 2006 | Culture and Society |
A recent acquisition has me looking backward rather than forward to begin the week. It’s the January 1940 issue of Astounding Science Fiction, a magazine then in its glory years under the editorship of John Campbell. Some of the authors here ring few bells: Sam Weston, for example, weighs in with “In the Day of the Cold,” as does D.L. James with “Moon of Delirium.” But this is also the issue of Robert Heinlein’s “Requiem,” and it contains good work by Lester del Rey and Edward E. Smith as well.
Astounding‘s science articles mixed with its ever reliable stories gave it a special place in the history of the pulp magazines, and many a scientist has told me that it was through Astounding or its later incarnation as Analog Science Fiction & Fact that a career path in physics or astronomy emerged. Which brought to mind science fiction writer Frederick Pohl, a distinguished editor in his own right.
Pohl’s 1978 memoir The Way the Future Was catches the magazine’s exciting heyday. He evokes the old building on Seventh Avenue where Astounding was produced, and recalls “…going up and down staircases, squeezing past rolls of paper stored to feed the ground-floor presses, reveling in the fascinating smells of printer’s ink and rotting wood.” In the passage that follows, Pohl recalls conversations with Campbell and their link to Astounding‘s editorials:
After a few such conversations, and after reading the editorials in Astounding a month or two after each of them, I figured out what was happening. That was how John Campbell wrote editorials. On the first of every month he would choose a polemical notion. For three weeks he would spring it on everyone who walked in. Arguments were dealt with, objections overcome, weak points shored up — and by the end of each month he had a mighty blast proof-tested against a dozen critics.
Every word he said I memorized:
On atmosphere: “I hate a story that begins with atmosphere. Get right into the story, never mind the atmosphere.”
On motivating writers: “The trouble with Bob Heinlein is that he doesn’t need to write. When I want a story from him, the first thing I have to do is think up something he would like to have, like a swimming pool. The second thing is to sell him on the idea of having it. The third thing is to convince him he should write a story to get the money to pay for it, instead of building it himself.”
On rejection letters: “When there’s something wrong with a story, I can tell you how to fix it. When it just doesn’t come across, there’s nothing I can say.”
On plot ideas: “When I think of a story idea, I give it to six different writers. It doesn’t matter if all six of them write it. They’ll all be different stories, anyway, and I’ll publish all six of them.”
On the archetypal sf story: “I want the kind of story that could be printed in a magazine of the year two thousand A.D. as a contemporary adventure story. No gee-whiz, just take the technology for granted.”
That latter item, of all Campbell’s dicta, seems to be the one most followed by modern science ficton, not always to its benefit since it can encourage a needless obscurity. Little of which you will find, it must be said, in today’s Analog, which continues to publish under the editorship of Stanley Schmidt, whose own tenure there is longer than any other editor save Campbell himself. Its science component is still robust, as witness the current issue’s discussion of current technology and interstellar possibilities by Gregory Matloff and Les Johnson.
And as to those old pulp issues like the one in the illustration above, they’re harder and harder to come by (most were destroyed in paper drives during World War II), but a small collector’s market still flourishes online in places like eBay, or at annual gatherings like PulpCon. Faced with a malfunctioning scanner, I poached the image above from the wonderful Pulps & Magazines Americains index of science fiction magazine covers and content. A few moments there, whether you read French or not, will trigger limitless nostalgia.
by Paul Gilster | Sep 2, 2006 | Outer Solar System
The seventh and final instrument aboard New Horizons has now been tested in space and found to return good data. The Pluto-bound spacecraft used its Long Range Reconnaissance Imager (LORRI) to take a picture of the Messier 7 cluster on August 29. Stars down to 12th magnitude are visible in the image, which means the instrument checks out with pre-launch calculations and is operating nominally.
“Our hope was that LORRI’s first image would prove not only that the cover had opened completely, but that LORRI was capable of providing the required high-resolution imaging of Pluto and Charon,” says Andy Cheng, LORRI principal investigator, from Johns Hopkins University Applied Physics Laboratory, which built and operates the spacecraft. “Our hopes were not only met, but exceeded.”
The mirror remained in focus even after its temperature dropped by more than 50 degrees C (120 degrees F) when its cover door opened. Next for LORRI are observations of Jupiter as New Horizons begins to focus on the gas giant on September 4. The spacecraft will pass within 1.4 million miles of the planet next February as it gains a gravity assist toward Pluto and Charon. Expect solid results from the Jupiter observations, but of course the best news is that all of New Horizons’ instruments are sound. Planet or not, Pluto is not many years from revealing at least some of its secrets.
