четвъртък, 28 октомври 2010 г.
сряда, 20 октомври 2010 г.
неделя, 17 октомври 2010 г.
Watching a Planet's Death Spiral?
Fifteen years ago, Geneva Observatory researchers Michel Mayor and Didier Queloz discovered a planet more massive than Jupiter circling a mere 5 million miles (8 million km) from the Sunlike star 51 Pegasi.
Some "hot Jupiters," like the exoplanet designated OGLE-TR-56b depicted here, orbit their stars so closely that tidal forces are gradually dragging them to their doom.
David A. Aguilar / Center for Astrophysics
Dynamicists have struggled to understand how this and the many other "hot Jupiters" found since then managed to orbit so tightly without being gobbled up. Some were perhaps flung into their star-skirting orbits after losing tugs-of-war with other massive planets in their systems, and some were probably dragged inward along with the natal disks of matter around their young Suns. After the disks dissipated, these massive worlds were deposited in precarious but seemingly stable orbits
There they might remain, roasting for eons, were it not for the tides these close-in planets incite in their stars. For some, long-term survival is not in the cards.
Why so? Consider how the Moon raises tides in Earth's oceans. Because our satellite takes a month to complete one orbit, whereas Earth spins in just 24 hours, the exchange of tidal energy is a net gain for the Moon, which is edging ever farther from Earth.
But a hot Jupiter completes its orbit in a few days — sometimes in less than 24 hours — creating stellar tides that have the opposite effect. Slowly, inexorably, the planet is dragged to its doom.
Why Earth's Rotation Slows Down
Earth tugs on the Moon and the Moon tugs back, causing tides in our oceans. Because of tides, an energy transfer takes place: Earth's rotation slows (days get longer), while the Moon gains orbital energy, causing it to slowly migrate farther away. Some hot Jupiters, however, orbit faster than their suns spin, so the tidal exchange gradually draws them inward instead.
Night Sky: Gregg Dinderman
That's the fate of OGLE-TR-113b, a hottie with 1.3 times Jupiter's mass crowding a Sunlike star 1,800 light-yeas away in Carina. Because it transits, or passes directly in front of its star, astronomers can time the planet's orbital period exactly. (TR in its designation is for transit; OGLE stands for Optical Gravitational Lensing Experiment.)
As Elisabeth Adams told a meeting of planetary scientists on Friday, precise timing of six such transits over the past few years reveals that OGLE-TR-113b's 34⅓-hour orbital period does seem to be very gradually shrinking. Using the timings she and her observing team made with the 6.5-m Magellan Telescopes in 2007-09, Adams finds that OGLE-TR-113b shaves about 60 milliseconds off its period each year.
It's possible, she notes, that the system contains a massive but as-yet unseen companion that's yanking the planet around. Some kind of librating resonance with a second planet isn't out of the question either, though the perturber couldn't have more than twice Earth's mass. Instead, the simplest explanation is that astronomers have, for the first time, captured a hot Jupiter in its death spiral.
So when might we kiss OGLE-TR-113b good-bye? Adams emphasizes that it'll happen long before the planet actually reaches the star's surface. Rather, the point of no return occurs when the orbital period shrinks to 10.8 hours. By then the planet will be close enough for tidal forces to rip it apart. Depending on assumptions about how strongly the star and planet are interacting, the end could come only 1½ to 2 million years from now.
Magellan Telescopes
Twin 6.5-m Baade and Clay telescopes were built for the Magellan project in 2000 and 2002 by the Carnegie Institution of Washington. They are sited at Las Campanas Observatory in Chile.
Carnegie Observatories
"This detection is still quite tentative," writes the team in a paper submitted to the Astrophysical Journal, in part because there's a less-than-perfect fit to other timings made in 2002 and 2006. Clarity should come early next year, when OGLE-TR-113b again becomes observable and more transits can be clocked.
"Other people have been on this trail," Adams adds, with the hot Jupiter OGLE-TR-56b also getting special attention. Discovered in 2002 and orbiting just 2.1 million miles (3.4 million km) from its stellar host, it might meet its doom in as little as a million years.
Posted by Kelly Beatty, October 12, 2010
събота, 9 октомври 2010 г.
Comet Hartley 2 At Its Best
In the second week of October Comet Hartley 2 sails past some of the most spectacular scenery in the sky, including the famous Double Cluster in Perseus. The comet symbols in this chart are placed at 0:00 UT, which falls on the evening of the previous date in North America. Click the image for a larger, printable, black-on-white chart.
Sky & Telescope illustration
http://www.skyandtelescope.com/observing/home/102632669.html
петък, 8 октомври 2010 г.
"Potentially Habitable" Planet Found
As soon as you can read these words when the news embargo lifts, the media worldwide are going to light up with the discovery of the first "potentially habitable" planet beyond Earth. Readers in the know, however, will realize that this milestone is less revolutionary than it sounds.
Gliese 581 system
Artist's concept of the dim red dwarf star Gliese 581 and company. In reality, even a "red" star would be dazzling yellow-orange if seen close up.
European Southern Observatory
"This is both an incremental and a monumental detection," sums up Sara Seager, an exoplanet specialist at MIT and frequent Sky & Telescope author. Many planets nearly like this one have already been found, so everyone in the exoplanet community was expecting a discovery like this any day now. What's special about this planet is that it orbits in its star's "Goldilocks" temperature zone where liquid water could exist on the surface. But even if such a discovery was expected, this is the one that will forever go into the record books.
Tha planet is Gliese 581g, one of at least six low-mass worlds orbiting the dim red-dwarf star Gliese 581 just 20 light-years away in Libra. The orbits were teased apart from 240 high-precision radial-velocity measurements, spanning 11 years, tracking the tiny gravitational wobbles that the planets induce in the star.
The star is Gliese 581, a dim M3 dwarf with about a third the Sun's diameter, a third the Sun's mass, and 1.3% of the Sun's energy output. Although it's one of our near stellar neighbors, it shines at only magnitude 10.6; it's visible in most amateur telescopes but not binoculars (at right ascension 15h 19.4m, declination –7° 43′).
Its six known planets all circle it more closely than Earth orbits the Sun. They form a miniature solar system, as befits such a miniature star. Two of the previously known planets, c and d, orbit just inside and outside the liquid-water temperature zone. The new one, g, lands between them. The exact definition of the "habitable zone," however, assumes that the planets have atmospheres providing greenhouse effects not too different from Earth's. And for a long time, that will really be just a guess.
The new planet has between 3.1 and 4.3 Earth masses, based on its gravitational effect and an upper limit set by orbital stability of the whole system. It is probably mostly rock and iron like Earth. That would give it a diameter 1.2 to 1.4 times Earth's diameter (allowing for its stronger gravity to compress its interior more), and a surface gravity of about 1.5 to 2 gs. In other words, after your starship landing craft sets down you could stand up and walk around with effort. Not that we know Gliese 581g even has a solid surface to land on. It could be swathed in a super-deep, super-dense atmosphere, or it could be a global-ocean waterworld.
The planet orbits closely enough to its dim star (with a period of 36.6 days) that its rotation has surely become tidally locked to its period of revolution, so that one side faces the star in permanent day and the other faces away in permanent night. This situation is not as problematic as exoplanet buffs once feared. A tidally locked world, it turns out, would need an atmosphere only about 1% as thick as Earth's to distribute enough heat from the dayside to the nightside so that the atmosphere would not freeze out as a solid crust on the nightside.
Nor would the winds be supersonic screamers tearing everything flat, as planetologists once speculated. Based on detailed atmospheric modeling "the surface winds would be quite benign," said team member Steven Vogt (Lick Observatory) at a press conference this afternoon. High-altitude jet streams would do most of the heat transport; the surface would experience Earthly sorts of breezes of about 0 to 40 knots. Parts of such a world, said Vogt, might be "a benign, comfortable place to live."
If the atmosphere isn't too thick, you could even pick your ideal temperature zone between the hot permanent noon point and the cold permanent midnight point. "Any emerging life forms would have a wide range of stable climates to choose from and to evolve around, depending on their longitude," Vogt said.
Of course, there's a big difference between "potentially habitable" and habitable for real. Seager says she already has a student working on the range of all possible atmospheres that a 3-Earth-mass body could realistically be expected to have (as best as we know), to get an idea of what fraction of such worlds might indeed have liquid water.
Really, however, the most important thing about Gliese 581g is its closeness. Only 116 stars of any kind are nearer to us, and only 9 of these have been tracked thoroughly enough to find any such planets if they exist. The fact that one was found so close so soon, says team member Paul Butler, suggests that at least 15% or 20% of of all stars have habitable worlds. And that's even before we can detect planets with 1 Earth mass or less. So it's safe to day that there are tens of billions of potentially life-friendly worlds in our Milky Way galaxy.
But we kinda knew that.
Says Seager, "There should be many more of these soon."
For lots more information see the team's full paper. It's more readable than research papers often are, especially the introduction.
Earlier paper on other planets in this system.
Повече подробности на: http://www.skyandtelescope.com/community/skyblog/newsblog/104031014.html
четвъртък, 7 октомври 2010 г.
EAAE-European Association for Astronomy Education
EAAE-European Association for Astronomy Education: "- изпратено чрез Google Toolbar"
неделя, 3 октомври 2010 г.
World Space Week October 4 – 10 annually
How You Can Use Space in Your Classroom
During World Space Week
October 4 – 10 annually
But what is World Space Week?
World Space Week is an international celebration of space and in particular the contributions that
space makes improving the human condition on Earth. It was officially declared by the United Nations
General Assembly in 1999. During World Space Week, events and educational programs are held
around the world and online, educating the public about space and inspiring youth.
When is World Space Week?
World Space Week is October 4-10 annually. These two dates commemorate key milestones in
space: October 4, 1957 was when Sputnik I, the first artificial satellite was launched. October 10, 1967
was when the Outer Space Treaty took effect, the first legal framework for space activities.
Who Celebrates World Space Week?
World Space Week is the world’s largest annual space education and outreach event. Last year it was
celebrated in 550 events in 63 countries; and in space! In the 2008 Annual Report [1 MB], you can
exactly see who/what/where... Many individuals, schools, museums, government agencies and
companies organize classroom activities, public events, Web meetings and generate publicity.
How Can I as a Teacher Participate?
World Space Week is the ideal time for you as a teacher to use space to excite students about math,
science, any subject in fact! Easy-to-use materials and free Teacher Activity Guides are available from
our website and other organizations directly linked from our website. Schools are, of course,
encouraged to organize events involving many parents and/or the local community.
What Should I Do?
Now – Put World Space Week, October 4 - 10, on your teaching calendar
Before October – Go to worldspaceweek.org to get ideas and free support materials.
Before October – Add your plans to our Calendar to get recognition for you and your school.
October 4-10 – Excite your students! Discuss / teach / have fun with space in your classroom!
Afterwards – Tell us what you did! We’ll include it in our widely distributed Annual Report.
Afterwards – In fact send us materials that you created and be eligible to our Educational Awards.
Afterwards – Invite colleagues to join you in participating to World Space Week next year.
Please go to www.worldspaceweek.org for more...
Thank you for your interest in World Space Week!
The astronomical clock of Lyon
http://artsandstars.ens-lyon.fr/ArtsAndStars/history/2-description-of-the-astronomical-clock
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