by Paul Gilster | Nov 12, 2004 | Exoplanetary Science |
An extrasolar planet the size of Neptune is news, and when that planet is in an orbit roughly analagous to Neptune’s in our own solar system, researchers take special note. After all, almost all the planets we’ve discovered around other stars are huge gas giants orbiting extremely close to their parent stars.
And this extrasolar planet is more anomalous still. It was back in May that a team from the University of Rochester led by Drs. Dan Watson and William Forrest, using NASA’s Spitzer Space Telescope, discovered a gap in the dust around the star CoKu Tau 4, one of five young stars they surveyed in the constellation Taurus.
The central part of the dust disk around the star was missing, a ‘hole’ that can only be explained by the presence of a planet, and a young one, at that. In fact, this planet is assumed to be between 100,000 to half a million years old, a toddler by any astronomer’s definition. CoKu Tau 4 is itself about one million years old; by contrast, Earth is approximately 4.5-billion years old.
Image: The relative locations of the RCW 49 and Taurus star-forming regions. Also surveyed by the Spitzer telescope, RCW 49 is approximately 13,700 light-years from Earth in the constellation Centaurus. The top panel shows an artist’s concept of our own Milky Way Galaxy as seen from above, with the relative locations of the Sun, the nearby Taurus region, and the more distant RCW 49 nebula. In the bottom panel, these regions are marked in the sky as seen from Earth in nearly opposite directions. The top half shows the stars of the Milky Way, and the bottom half shows the far infrared view of the dust clouds in the Milky Way. Credit: ESO (top), 2MASS (middle), IRAS/DIRBE (bottom).
Now a new team of Rochester planetary theorists has verified the CoKu Tau 4 findings. And that means that a problem persists: our current models show that planets form by accretion of dust into granules and thence into the rocks that form into asteroids and planets. But this method should require at least ten million years for the process to complete.
The other theory of planetary formation is gravitational instability, where a huge cloud of gas is pulled together quickly by its own gravity. This method could create a young planet, but would require one much larger than was detected. As the team reported at the AAS Division for Planetary Sciences meeting today, “…we find that only if the planet mass is larger than about 10 Jupiter masses, allowing for a high enough surface density without inducing migration, would formation by direct gravitational instability be possible.”
A press release from the University of Rochester quotes Adam Frank, professor of physics and astronomy on the recent work:
“Even though it doesn’t fit either model, we’ve crunched the numbers and shown that yes, in fact, that hole in that dust disk could have been formed by a planet,” says Frank. “Now we have to look at our models and figure out how that planet got there. At the end of it all, we hope we have a new model, and a new understanding of how planets come to be.”
More information on the original Rochester findings can be found on this Spitzer Web page. The DPS presentation was “On the Planet and the Disk of CoKuTau/4,” (A. C. Quillen, E. B. Blackman, A. Frank, P. Varniere), with precis available here.
by Paul Gilster | Nov 11, 2004 | Outer Solar System
Roughly 1,000 Kuiper Belt objects have been discovered orbiting beyond Neptune since the first was found in 1992. Now researchers are suggesting that these icy objects — considered to be leftover building blocks of the solar system — are much smaller than was originally thought. The key is albedo, a measure of how much light an object reflects. Using a presumed albedo of four percent, which is the figure for comets, astronomers had calculated the size of the Kuiper Belt objects, and believed there were more than 10,000 KBOs with diameters greater than 100 kilometers (62 miles), compared to 200 asteroids known to be that large in the main asteroid belt between Mars and Jupiter.
But all these measurements depend on an accurate read on albedo. A higher albedo means a more reflective object, forcing a reassessment of how large the KBO objects really are.
Image: Kuiper Belt Object 2002 AW197 (Image: NASA/JPL/John Stansberry, University of Arizona)
And as reported at the ongoing meeting of the American Astronomical Society’s Division of Planetary Science in Louisville, results from a recent survey of 30 Kuiper Belt objects do indeed reveal a much higher albedo. One object, a KBO designated 2002 AW197, reflects 18 percent of its incident light and is thus calculated to be about 700 kilometers (435 miles) in diameter. The data came from the Spitzer Space Telescope’s Multiband Imaging Photometer (MIPS).
From a University of Arizona press release:
…MIPS detected heat from a Kuiper Belt Object with a surface temperature of around minus 370 degrees Fahrenheit at an astonishing distance of 4.4 billion miles (7 billion kilometers), or one-and-a-half times farther away from the sun than Pluto.
Without MIPS, astronomers operating under the assumption that 2002 AW197 reflects four percent of its incident light would calculate that it is 1500 kilometers (932 miles) in diameter, or two-thirds as large as Pluto…
The reality: the object is about half the size of Pluto’s moon Charon, and only about thirty percent as large as Pluto itself. And the notion that Pluto is just another Kuiper Belt object rather than a planet in its own right comes into serious question; it’s just too big.
More tomorrow on a possible extrasolar planetary find using the Spitzer instrument.
by Paul Gilster | Nov 10, 2004 | Deep Sky Astronomy & Telescopes, Exoplanetary Science
We’ve talked about adaptive optics here recently, particularly in regard to the W.M. Keck observatory complex at Mauna Kea (Hawaii). Keck’s new adaptive system essentially removes atmospheric distortion and improves data processing of the raw image. What you wind up with is a stunningly clear view, as has become apparent in new images of Uranus released by the observatory.
The images show Uranus and its ring system, first with the adaptive optics system shut off, then with it on. You can see how much more visible the rings are in the second image, but notice too the deep atmospheric cloud structure in the images on the right. More images are available at the Keck site’s article on these findings.
Image Credit: Heidi Hammel, Space Science Institute, Boulder, CO/Imke de Pater, University of California, Berkeley/ W. M. Keck Observatory.
From the Keck information, quoting a scientist who conducted a second set of observations of the planet:
Dr. Lawrence Sromovsky, principal investigator for the Wisconsin observations said, “Twenty years ago we simply couldn’t see the types of details in the outer solar system the way we can today with large, ground-based telescopes like Keck. These images actually reveal many more cloud features than the Voyager spacecraft found after traveling all the way to Uranus.”
Incidentally, the storms that show up in these Keck images would engulf the United States, but with Uranus more than 1.6 billion miles away, they’re barely detectable without such advanced imaging techniques. The clarity of the images is a reminder of how far we’ve come. Twenty years ago when we talked about viewing the outer planets, we could only imagine sharp images being taken by space-based telescopes like Hubble. Adaptive optics now offers the chance to see details that are absent almost all atmospheric distortion.
What stunning vistas lie ahead, particularly as we mount efforts to image extrasolar planets, can only be imagined, but keep the fortunes of the Terrestrial Planet Finder mission firmly in your sights. Twenty years from now, we may have equally stunning images of planets orbiting distant stars.
by Paul Gilster | Nov 9, 2004 | Advanced Rocketry, Sail Concepts
The conference of the Division for Planetary Sciences of the American Astronomical Society continues in Louisville. Among the papers presented at today’s Advanced Propulsion session were three of particular interest for interstellar advocates.
Les Johnson, who heads up NASA’s In-Space Propulsion Technology Program, gave an overview on the technology portfolio now being examined. “Some of the most promising technologies for achieving these goals use the environment of space itself for energy and propulsion and are generically called, ‘propellantless’ because they do not require on-board fuel to achieve thrust,” Johnson wrote in a precis of the talk. “Propellantless propulsion technologies include scientific innovations such as solar sails, electrodynamic and momentum transfer tethers, aeroassist, and aerocapture.” Both solar sails and aerocapture are candidates for flight validation as early as 2008.
Two other presentations of particular note: “Solar Sail Propulsion: A Simple, Propellantless, Rapidly Maturing Technology, ” E. E. Montgomery, L. Johnson, R. M. Young, J. B. Presson (NASA MSFC), C. Adams (Gray Research, Inc.), and…
“Emerging Propulsion Technologies,” J. A. Bonometti (NASA MSFC). This one examines so-called ‘tether-based propulsion,’ another propellantless concept that taps the Earth’s magnetic field and solar power to propel spacecraft. Think of miles of cable rotating like an enormous sling, scooping up spacecraft and flinging them into higher orbits.
Here’s more on Momentum-Exchange Tether Propulsion (MXER) from the project home page:
Momentum-exchange tether propulsion transfers momentum from one object to another by briefly linking a slow-moving object with a faster one. Much the same way ice skaters play “crack the whip,” the slower object’s speed could be dramatically increased as momentum and energy is transferred to it from the faster object. Similarly, a spinning tether facility in an elliptical Earth orbit might snare slower-moving spacecraft in low-Earth orbit and throw them into much higher-energy orbits.
NASA researchers currently are developing the technologies needed to realize this advanced form of propulsion. The “Momentum-Exchange Electrodynamic Reboost” tether propulsion system, or MXER tether, could use momentum-exchange to transfer satellites from low-Earth orbit to geosynchronous transfer orbit — an elliptical orbit stretching from 200 miles out to 22,300 miles above the equator — and beyond. After throwing the payload, the MXER tether would then use energy collected from solar panels to drive electrical current through the tether. The Earth’s magnetic field would push against the current and reboost the tether’s orbit, restoring the energy that was transferred to the payload.
MXER would use a tether some 100 to 150 kilometers (62 to 93 miles) in length, made from a lightweight, insulated conductive material that could carry electrical current as needed to reboost the tether. This stuff is tricky, but the Web page cited above provides some helpful animations. And check this fact sheet from Marshall Space Flight Center on tether concepts (PDF warning).
When I talked to Les Johnson at Marshall Space Flight Center in Huntsville about MXER, he pointed out how early in the game we still are: “Rendezvous and capture are big issues for MXER,” Johnson said. And so is the accuracy with which you release the payload on the right trajectory. How do you do that on a big non-rigid structure that’s rotating? There are lot of problems still to be resolved… I would put MXER at the same level of understanding right now that we have of the plasma sail. But is this promising technology? Absolutely.”
Tethers Unlimited (Bothell, WA), from whose Web page the above image is drawn, is deeply involved in this work; TUI was founded in 1994 by Robert Forward and Robert P. Hoyt, and was the place where Forward spent the last of his seemingly inexhaustible energy before his untimely death in 2002. Tethers don’t help us with interstellar propulsion, but they do point to where the action is: finding ways to circumvent the mass ratio problem by leaving the fuel behind. Hence the beauty of the solar sail (and laser-pushed lightsail) — the spacecraft has no need for bulky fuel supplies and can carry more payload.
A complete schedule of upcoming talks at the Louisville conference is here.
by Paul Gilster | Nov 8, 2004 | Missions, Outer Solar System
When it comes to interstellar work, don’t forget the Kuiper Belt. Although amateur astronomer Kenneth Edgeworth was the first to predict its existence, the Belt was named for Gerard Kuiper, who analyzed it in 1951. It is a region of thousands (and perhaps millions) of small, icy moons and cometary debris that exists from the orbit of Neptune well into deep space. Our first interstellar missions will be explorations of this area and the vast Oort cloud of comets that may extend as much as a light year out from the Sun.
And yes, in a true sense, the Voyager probes could be considered interstellar missions, still reporting data as they move on toward the heliopause. But we may learn a good deal about Kuiper Belt objects by studying the findings of a spacecraft considerably closer, the Cassini Saturn orbiter.
Cassini’s Ultraviolet Imaging Spectrometer tells us that Phoebe, a tiny world about one-fifteenth the diameter of Earth’s moon, is probably itself a Kuiper Belt object that was captured long ago by Saturn’s gravity. A University of Colorado at Boulder professor named Larry Esposito has been discussing Phoebe at the 36th annual Division of Planetary Sciences Meeting held in Louisville, KY from today until November 12. From a UC Boulder press release:
“UVIS sees the absorption signature of water ice on its surface, showing Phoebe was born in the outer solar system,” Esposito said. Exhibiting an unusual retrograde, or backward, orbit, Phoebe likely was lassoed by Saturn’s powerful gravitational field during the planet’s formative years, he said.
One powerful clue about Phoebe: its unusual retrograde (backward) orbit. “UVIS sees the absorption signature of water ice on its surface, showing Phoebe was born in the outer solar system,” Esposito added.
We may well find that other outer-system moons, like Neptune’s Triton, have their origin in the Kuiper Belt. Other UVIS data include high-detail images of Saturn’s rings that show the influence of the planet’s moons in creating density waves — ripples — in the so-called Cassini Division, the gap between the bright A and B rings of Saturn. UVIS also found, according to Esposito, a bright glow in Titan’s upper atmosphere that appears to be “…the glow of nitrogen atoms, molecules and ions energized by electrons striking the upper atmosphere.”
Centauri Dreams will have more in the next few days on the meeting’s other sessions with interstellar implications. They include an advanced propulsion meeting tomorrow and discussions on extrasolar planets to be held on Friday. Particularly fascinating from the latter should be “Specular Reflection of Starlight off Distant Planetary Oceans,” to be presented by D.M. Williams (Penn State Erie, The Behrend College) and E. Gaidos (U. Hawaii). More as information comes in.
