by Paul Gilster | May 17, 2005 | Exoplanetary Science |
The first of the giant, close-in exoplanets (widely known as ‘hot Jupiters’) was discovered a decade ago around 51 Pegasi. Since then, we’ve found numerous other examples of such objects, challenging our theories of planetary formation and revising estimates of how common habitable worlds may be. Now a team of Canadian astronomers using the MOST (Microvariability & Oscillations of STars) space telescope has found yet another interesting anomaly: a planet that seems to force its star to rotate in synchrony with the planet’s orbit.
The planet is in orbit around the star tau Bootis. MOST has revealed that the star varies in light output in relation to the orbit of the planet around it. The explanation offered by the team studying tau Bootis b is that the planet has forced the outer envelope of the star to rotate so it always keeps the same face toward the planet. The effect is probably limited to the outer layers of gas in the star, in much the same way the Moon can cause a tidal bulge in the water layer on Earth’s surface that results in ocean tides.
However limited, the effect is still remarkable, and probably made possible by the size of tau Bootis b, about four times the mass of Jupiter, and the fact that it orbits only 1/20th of an AU from the star. Paul Butler and Geoff Marcy first found the planet back in 1997, studying the wobble it induced in its parent. We now know that its orbit around tau Bootis a takes 3.3 days, causing all kinds of other interactions including starspots, tidal distortion and magnetic activity on the star.
Image: A computer rendering of the MOST space telescope in its polar orbit above Earth. Credit: MOST.
The variation between exoplanets is considerable. MOST monitored nine systems in 2004 and 2005, finding each displayed a unique set of properties. From a press release from the University of British Columbia:
“The nature of the light variations is different for each of the nine exoplanet orbits monitored by MOST in 2004 and 2005,” said Gordon Walker, an exoplanet pioneer and MOST Science Team member at UBC. “The explanation for all the variability will have to include intrinsic stellar effects, like rotation, and planet-induced effects, like heating caused by tides and magnetic fields – a complex model, to be sure.”
The announcement of the tau Bootis findings was made at the annual meeting of the Canadian Astronomical Society in Montreal on May 16. A second significant report from the conference describes how the MOST telescope was used to study the exoplanet HD209458b (orbiting the star HD209458a), examining the dip in light that occurs when the planet disappears behind the star. The way a planet reflects light offers information about its atmospheric composition and temperature. In this case:
“We can now say that this puzzling planet is less reflective than the gas giant Jupiter in our own Solar System,” MOST Mission Scientist Dr. Jaymie Matthews announced… “This is telling us about the nature of this exoplanet’s atmosphere, and even whether it has clouds.”
Like tau Bootis b, HD209458b orbits at only 1/20th of the Earth-Sun distance. A scientific paper on these results is expected to be submitted soon. More on this one at a Canadian Astronomical Society press release. You can read more about the MOST mission here.
by Paul Gilster | May 16, 2005 | Exoplanetary Science
Many stars close to the Sun have familiar names, like the Centauri triple-star system, Barnard’s Star, Epsilon Eridani and Tau Ceti. But the catalog of nearby stars is by no means complete, as we are reminded periodically by the discovery of stars showing large proper motion as observed from Earth. That motion flags the object as close, but the fact that so many of the galaxy’s stars are M-class red dwarfs (up to 70 percent in some estimates) makes detecting them tricky. These are small, dim stars; some may have been in our catalogs for years before astronomers realized how close they were.
Some scientists, in fact, measure the number of missing stellar systems at 25 percent or more, even so close as 10 parsecs from the Sun. Now an international team of researchers has found three of the missing stars. Using data from the Two-Micron All Sky Survey (2MASS), the Deep Near Infrared Survey (DENIS) and the SuperCOSMOS Sky Survey (SSS), the team uncovered the three close neighbors by noting their high proper motion in SSS data from four different sets of observations, and cross-checking against data in the other databases. Further observations were made at the European Southern Observatory facility at La Silla (Chile).
The three stars, known as L 449-1, L 43-72, and LP 949-15, are all thought to be within 10 parsecs. The findings are summarized in Scholz, R.-D., Lo Curto, Mendez et al., “Three active M dwarfs within 8 pc: L 449-1, L 43-72, & LP 949-15,” now available in preprint form at the arXiv site and accepted for publication by Astronomy & Astrophysics. From the paper:
Although all three stars are known X-ray sources and were earlier classified as proper motion stars, we are not aware of any publication treating them as nearby stars. Our cross identification with the 2MASS providing accurate photometry and the follow-up low-resolution spectroscopy allowed us to uncover them as close neighbours to the Sun. As such, they are certainly worth investigating further, with more accurate distances to be obtained in a trigonometric parallax program.
Centauri Dreams note: We need to complete the catalog of nearby star systems for several reasons. Not only should they be excellent candidates for identifying extrasolar planets, but their proximity gives us the chance to study stars of this stellar type in detail. They will likely be added to the target list for upcoming planet-identification missions like the Space Interferometry Mission and Terrestrial Planet Finder.
Be aware, too, of the Wide-field Infrared Survey Explorer, a mission that will scan the sky in infrared looking for such objects. An earlier post here quoted WISE Principal Investigator Dr. Edward Wright of the University of California, Los Angeles: “Approximately two-thirds of nearby stars are too cool to be detected with visible light. The Wide-field Infrared Survey Explorer will see most of them.”
by Paul Gilster | May 14, 2005 | Outer Solar System
Huygens’ Descent Imager Spectral Radiometer (DISR) team has now produced mosaic images of the probe’s descent to Titan’s surface. These were created by combining images taken by Huygens as it rotated on its axis, the first image showing the view from approximately 20 kilometers altitude. The photos were taken in groups of three as the probe descended through the atmosphere last January.
Image: (click to see enlargement): This stereographic projection of DISR images from ESA’s Huygens probe combines 60 images in 31 triplets, projected from a height of 3000 metres above the black ‘lakebed’ surface. The bright area to the north (top of the image) and west is higher than the rest of the terrain, and covered in dark lines that appear to be drainage channels. Credits: ESA/NASA/JPL/University of Arizona.
The stereoscopic image shown above is intriguing because of what appears to be going on in the north and west (top and top left of the image). Be sure to click on the image to see it in higher resolution. According to an ESA news release, this area is covered in dark lines that give the appearance of drainage channels:
These lead down to what appears to be a shoreline with river deltas and sand bars…The current interpretation of these lines is that they are cut by flowing liquid methane. Some of them may have been produced by precipitation run-off, producing a dense network of narrow channels and features with sharp branching angles. Some others may have been produced by sapping or sub-surface flows, giving shape to short stubby channels that join at 90 degree angles.
And later:
The bright shapes to the north-east and east look to be ridges of ice gravel that are slightly higher than the flats around them, and the probe landing is believed to be just south-west of the semi-circular shape. The light and dark areas to the south are still of unknown nature.
The other image is a ‘gnomonic’ projection (the method makes the surface appear to be flat — again, click to enlarge). It was assembled from images taken at roughly 800 meters, as the landing site is approaching, and shows dark channels some 30 to 40 meters wide.
Image: This gnomonic projection of DISR images from ESA’s Huygens probe combines 17 image triplets, projected from an altitude of 800 metres. The area covered is approximately 1300 metres across (north at the top of the image). The smallest visible objects visible are less than five metres across, and the dark channels are 30-40 metres wide. Credits: ESA/NASA/JPL/University of Arizona.
by Paul Gilster | May 13, 2005 | Outer Solar System
Titan continues to live up to its billing as a model of the early Earth. Recent observations by Cassini tell us much about the moon’s atmosphere, about 98 percent nitrogen (with most of the remainder being methane), and laden with organic molecules. Sunlight appears to break these molecules apart as they rise in the atmosphere, where their fragments form heavier organic molecules — propane, ethane, acetylene, hydrogen cyanide. Cold air over the winter pole then causes this material to sink, with the result that heavy organics build up in the stratosphere over the course of the winter.
Supplying the data is Cassini’s Composite Infrared Spectrometer instrument (CIRS); a paper on its findings is scheduled to appear in the May 13 issue of Science. The polar vortex phenomenon is similar to what occurs on Earth; strong winds circulating around Titan’s north pole isolate the atmosphere over the pole during the polar night, inhibiting mixing with the lower regions of the atmosphere. Something similar happens over Antarctica, leading to the infamous ‘ozone hole’ over the region. There, spring sunlight decomposes molecular chlorine that formed during the winter. Titan has no ozone, but the isolation of the polar atmosphere during the winter night may lead to unusual and complex chemistry.
Image: This natural color composite shows approximately what Titan would look like to the human eye: a hazy orange globe surrounded by a tenuous, bluish haze. The orange color is due to the hydrocarbon particles which make up Titan’s atmospheric haze. The image comes from the April 16, 2005 Cassini flyby. Credit: NASA/JPL/Space Science Institute.
“We don’t know if there are even more similarities to Earth’s ozone hole process, like polar clouds that react with molecules in the atmosphere, simply because we haven’t seen them yet,” said Dr. Michael Flasar, principal investigator for the CIRS instrument at Goddard Space Flight Center. “But we wouldn’t be surprised to discover them, nor would we be surprised to find that Titan has some unique twists of its own. This is what makes science so exciting. Nature is too rich for us to predict exactly what we will find when we go exploring.”
Note that Titan’s axis of rotation, like Earth’s, is tilted, setting up the long winter night. But on Titan, polar winter is years long. The moon’s northern hemisphere is currently in early winter. By studying the temperature differences between the north pole and the equator, the team was able to derive the speed of winds around the pole, while CIRS data show that the concentration of several heavy organic compounds peaks in the region. These molecules are the cause of Titan’s now famous orange haze.
A NASA feature story on Titan’s atmosphere can be found here. Also see this news release from Lawrence Livermore National Laboratory.
by Paul Gilster | May 12, 2005 | Autonomy and Robotics |
Even in best-case scenarios, a probe to Alpha Centauri or other nearby stars will take decades to reach its target, perhaps centuries. That puts the premium on spacecraft autonomy, an interesting take on which is the ability of machinery to repair itself. Cornell researchers have just announced a milestone in this regard, the creation of a robot that makes copies of itself. Previous self-replicating designs have existed only as computer simulations or were much simpler than Cornell’s new devices.
As reported in the May 12 issue of Nature, the university’s Hod Lipson and colleagues have created machines made up of modular cubes called ‘molecubes.’ Each is identical and contains the computer program that allows it to reproduce. Using electromagnets on their faces, the cubes can adhere to one another selectively; a complete robot is made up of an assembly of such molecubes. And because the cubes are divided, robots composed of them can bend to various angles or manipulate other cubes.
Thus rebuilding a robot can occur when the machine simply replaces defective modules, tapping a store of such parts. To replicate, a robot would remove its own top cube and add new cubes to it, making a copy of itself, with the new robot assisting in the completion of its own construction. Such flexibility could have immediate benefits for interplanetary missions, such as robotic exploration of the Martian surface, as well as for hazardous environments here on Earth where machines are likely to experience failures.
Image: Frames from a video show the replication process. A robot consisting of a stack of four cubes begins by bending over and depositing one of its cubes on the table. The remaining three cubes pick up additional cubes from “feeding stations” and transfer them to the new robot, which assists in the process by standing itself up. Credit: Hod Lipson, Cornell University.
Replication is very much dependent on the environment, tapping power sources in the laboratory and relying on a steady supply of additional modules. But this is true in nature as well, where replication is favored in some contexts more than others. From a Cornell University press release:
…the researchers point out that human beings reproduce but don’t literally self-replicate, since the offspring are not exact copies. And in many cases, the ability to replicate depends on the environment. Rabbits are good replicators in the forest, poor replicators in a desert and abysmal replicators in deep space, they note. “It is not enough to simply say they replicate or even that they replicate well, because these statements only hold in certain contexts,” the researchers conclude.
Lipson’s page on self-replication is here; it contains video of robotic replication and useful links to further information. Lipson sees self-replication as the ultimate form of self-repair, but notes that robots built along these lines will not have designs as targeted as machines designed for a specific task. From his site:
We see that robotic systems are becoming more complex, and in some cases like space exploration, they need to sustain operation for long periods of time without human assistance. If you send a robot to Mars, for example, and it breaks, there is little you can do. But if instead of sending a fixed robot you send a robot with a supply of modules, then that robot may be able to self-repair and even make more and possibly different robots if the mission needs change unexpectedly. Of course, there are many tradeoffs: These robots would not be as optimally designed as a fixed robot for any specific task, and they would be more complex, but maybe overall they would be more robust. We still need to do the exact calculations to see when and where it would be a viable solution.
Centauri Dreams note: As self-replication techniques improve, watch the size of robotic modules begin to shrink. Lipson already talks of using microscale techniques to create systems with thousands of modules, an early stage in the robotic self-evolution that may emerge as an enabler for interstellar probes.
The paper is Victor Zykov, Efstathios Mytilinaios, et al., “Robotics: Self-Reproducing Machines,” Nature 435, 163-164 (12 May 2005). For background, see Lipson and Jordan Pollack’s work on the Golem Project, which explored artificial evolution and 3-D printing for robots that were unable to self-reproduce. Wedding Golem’s methods with the self-reproduction of the current work is high on Lipson’s list of priorities. Video is available here.
