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Eight Planets and New Solar
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| Title |
Eight Planets and New Solar System Designations |
| Explanation |
How many planets are in the Solar System? This popular question now has a new formal answer according the International Astronomical Union [ http://en.wikipedia.org/wiki/International_Astronomical_Union ] (IAU): eight. Last week, the IAU voted [ http://www.iau2006.org/mirror/www.iau.org/iau0603/index.html ] on a new definition for planet [ http://www.iau2006.org/mirror/www.iau.org/iau0603/index.html ] and Pluto [ http://antwrp.gsfc.nasa.gov/apod/ap010319.html ] did not make the cut. Rather, Pluto was re-classified as a dwarf planet [ http://en.wikipedia.org/wiki/Dwarf_planet ] and is considered as a prototype for a new category of trans-Neptunian objects [ http://en.wikipedia.org/wiki/Trans-Neptunian_object ]. The eight planets now recognized by the IAU are: Mercury [ http://antwrp.gsfc.nasa.gov/apod/ap040912.html ], Venus [ http://antwrp.gsfc.nasa.gov/apod/ap040516.html ], Earth [ http://antwrp.gsfc.nasa.gov/apod/ap050102.html ], Mars [ http://antwrp.gsfc.nasa.gov/apod/ap060730.html ], Jupiter [ http://antwrp.gsfc.nasa.gov/apod/ap050911.html ], Saturn [ http://antwrp.gsfc.nasa.gov/apod/ap041225.html ], Uranus [ http://antwrp.gsfc.nasa.gov/apod/ap010826.html ], and Neptune [ http://antwrp.gsfc.nasa.gov/apod/ap010821.html ]. Solar System objects now classified as dwarf planets are: Ceres [ http://antwrp.gsfc.nasa.gov/apod/ap060821.html ], Pluto [ http://antwrp.gsfc.nasa.gov/apod/ap060624.html ], and the currently unnamed 2003 UB313 [ http://antwrp.gsfc.nasa.gov/apod/ap060207.html ]. Planets, by the new IAU definition, must be in orbit around the sun, be nearly spherical, and must have cleared the neighborhood around their orbits. The demotion of Pluto [ http://www.nineplanets.org/pluto.html ] to dwarf planet status is a source of continuing dissent [ http://www.badastronomy.com/bablog/2006/08/18/colbert-takes-neil-tyson-down/ ] and controversy [ http://newswire.ascribe.org/cgi-bin/behold.pl?ascribeid=20060818.063045&time=07%2006%20PDT&year=2006&public=0 ] in the astronomical community. |
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Titania's Trenches
| Title |
Titania's Trenches |
| Explanation |
British astronomer Sir William Herschel [ http://star.arm.ac.uk/history/herschel.html ] discovered Titania and Oberon in January of 1787. He wasn't reading Shakespeare's [ http://the-tech.mit.edu/Shakespeare/works.html ]"A Midsummer Night's Dream" though, he was making the first telescopic observations of moons of the planet Uranus [ http://seds.lpl.arizona.edu/nineplanets/nineplanets/uranus.html ] (a planet which he himself discovered in 1781 [ http://www-groups.dcs.st-and.ac.uk/~history/ HistTopics/Neptune_and_Pluto.html ]). In January of 1986, nearly 200 years later, NASA's robot explorer Voyager 2 became the only spacecraft to visit the remote Uranian [ http://antwrp.gsfc.nasa.gov/apod/ap990531.html ] system [ http://antwrp.gsfc.nasa.gov/apod/ap971103.html ]. Above is Voyager's highest resolution picture of Titania [ http://photojournal.jpl.nasa.gov/cgi-bin/ PIAGenCatalogPage.pl?PIA00039 ], Uranus' largest moon. The picture is a composite of two images recorded from a distance of 229,000 miles. The icy, rocky world [ http://antwrp.gsfc.nasa.gov/apod/ap960304.html ] is seen to be covered with impact craters. A prominent system of fault valleys, some nearly 1,000 miles long, is visible as trench-like features near the terminator (shadow line). Deposits of highly reflective material which may represent frost can be seen along the sun-facing valley walls. The large impact crater near the top, known as Gertrude [ http://wwwflag.wr.usgs.gov/USGSFlag/Space/nomen/uranus/titacrat.html ], is about 180 miles across. At the bottom the 60 mile wide fault valley, Belmont Chasma [ http://wwwflag.wr.usgs.gov/USGSFlag/Space/nomen/uranus/titachas.html ], cuts into crater Ursula. Titania itself [ http://wwwflag.wr.usgs.gov/USGSFlag/Space/wall/titania.html ] is 1,000 miles in diameter. |
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Uranus: The Tilted Planet
| Title |
Uranus: The Tilted Planet |
| Explanation |
Uranus is the third largest planet in our Solar System [ http://www.nineplanets.org/overview.html ] after Jupiter [ http://antwrp.gsfc.nasa.gov/apod/jupiter.html ] and Saturn [ http://antwrp.gsfc.nasa.gov/apod/ap010826.html http://www.solarviews.com/eng/saturn.htm ]. Uranus [ http://www.nineplanets.org/uranus.html ] is composed mostly of rock and ices, but with a thick hydrogen [ http://chemlab.pc.maricopa.edu/periodic/h.html ] and helium [ http://antwrp.gsfc.nasa.gov/apod/ap010120.html ] atmosphere. The blue hue of Uranus' atmosphere arises from the small amount of methane [ http://scifun.chem.wisc.edu/chemweek/methane/methane.html ] which preferentially absorbs red light. This picture [ http://photojournal.jpl.nasa.gov/cgi-bin/PIAGenCatalogPage.pl?PIA01360 ] was snapped by the Voyager 2 spacecraft [ http://nssdc.gsfc.nasa.gov/planetary/voyager.html ] in 1986 - the only spacecraft ever to visit Uranus. Uranus [ http://antwrp.gsfc.nasa.gov/apod/uranus.html ] has many moons [ http://antwrp.gsfc.nasa.gov/apod/ap990531.html ] and a ring system [ http://antwrp.gsfc.nasa.gov/apod/ap971126.html ]. Uranus, like Venus [ http://antwrp.gsfc.nasa.gov/apod/venus.html ], has a rotation axis that is greatly tilted and sometimes points near the Sun. It remains an astronomical mystery why Uranus' axis [ http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1991LPIP...22...59S ] is so tilted. Uranus and Neptune [ http://antwrp.gsfc.nasa.gov/apod/neptune.html ] are quite similar: Uranus is slightly larger but less massive. |
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Neptune: Big Blue Giant
| Title |
Neptune: Big Blue Giant |
| Explanation |
This picture was taken by the Voyager 2 [ http://nssdc.gsfc.nasa.gov/planetary/voyager.html ] spacecraft in 1986 - the only spacecraft ever to visit Neptune. Neptune [ http://seds.lpl.arizona.edu/nineplanets/nineplanets/neptune.html ] will be the farthest planet from the Sun [ http://antwrp.gsfc.nasa.gov/apod/ap950813.html ] until 1999, when the elliptical orbit of Pluto will cause it to once again resume this status. Neptune [ http://www.c3.lanl.gov/~cjhamil/SolarSystem/neptune.html ], like Uranus [ http://antwrp.gsfc.nasa.gov/apod/ap950816.html ], is composed mostly of liquid water, methane and ammonia, is surrounded by a thick gas atmosphere of mostly hydrogen and helium, and has many moons and rings. Neptune [ http://nssdc.gsfc.nasa.gov/photo_gallery/photogallery-neptune.html ]'s moon Triton [ http://antwrp.gsfc.nasa.gov/apod/ap950805.html ] is unlike any other and has active volcanoes. The nature of Triton [ http://seds.lpl.arizona.edu/nineplanets/nineplanets/triton.html ]'s unusual orbit around Neptune is the focus of much discussion and speculation. Tomorrow's picture: Pluto: The Frozen Planet |
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Venus: Earth's Sister Planet
| Title |
Venus: Earth's Sister Planet |
| Explanation |
This picture in visible light was taken by the Galileo [ http://nssdc.gsfc.nasa.gov/planetary/galileo.html ] spacecraft. Venus [ http://seds.lpl.arizona.edu/nineplanets/nineplanets/venus.html ] is very similar to Earth [ http://antwrp.gsfc.nasa.gov/apod/ap950622.html ] in size and mass - and so is sometimes referred to as Earth's sister planet - but Venus has a quite different climate. Venus' [ http://www.c3.lanl.gov/~cjhamil/SolarSystem/venus.html ] thick clouds and closeness to the Sun [ http://antwrp.gsfc.nasa.gov/apod/ap950813.html ] (only Mercury [ http://antwrp.gsfc.nasa.gov/apod/ap950814.html ] is closer) make it the hottest planet - much hotter than the Earth. Humans could not survive there, and no life of any sort has ever been found. When Venus [ http://nssdc.gsfc.nasa.gov/photo_gallery/PhotoGallery-Venus.html ] is visible it is usually the brightest object in the sky after the Sun and the Moon. More than 20 spacecraft have visited Venus including Venera 9, which landed on the surface, and Magellan [ http://stardust.jpl.nasa.gov/mveg/guide.html ], which used radar to peer through the clouds and make a map of the surface. There are still many things about Venus's unusual atmosphere that astronomers don't understand. Tomorrow's picture: Uranus: The Tilted Planet |
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Uranus' Largest Moon: Titani
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| Title |
Uranus' Largest Moon: Titania |
| Explanation |
Titania's tortured terrain is a mix of valleys and craters. NASA's interplanetary robot spacecraft Voyager [ http://spacelink.msfc.nasa.gov/NASA.Projects/Planetary.Probes/Voyager/Mission.Summary ] 2 passed this moon of Uranus [ http://antwrp.gsfc.nasa.gov/apod/ap950816.html ] in 1986 and took the above photograph. The photograph was then transmitted back to earth by radio [ http://www.science.uwaterloo.ca/physics/p13news/number_2/maxnew-2.html ]. The valleys of Titania [ http://bang.lanl.gov/solarsys/titania.htm ] resemble those on Ariel [ http://antwrp.gsfc.nasa.gov/apod/ap960303.html ] indicate that Titania underwent some unknown tumultuous resurfacing event in its distant past. Although Titania [ http://seds.lpl.arizona.edu/nineplanets/nineplanets/titania.html ] is Uranus' largest moon, it is still much smaller than Triton [ http://antwrp.gsfc.nasa.gov/apod/ap950805.html ] - the largest moon of Uranus' sister planet Neptune [ http://antwrp.gsfc.nasa.gov/apod/ap950817.html ]. Titania is essentially a large dirty iceball that orbits Uranus - it is composed of about half water-ice and half rock. Titania was discovered by William Hershel [ http://www.dsi.unimi.it/Users/Students/amoroso/sun/fortunes/life-e.html ] in 1787. |
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Alpha Centauri: The Closest
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| Title |
Alpha Centauri: The Closest Star System |
| Explanation |
The closest star system to the Sun is the Alpha Centauri system [ http://monet.physik.unibas.ch/~schatzer/Alpha-Centauri.html ]. Of the three stars in the system, the dimmest -- called Proxima Centauri -- is actually the nearest star [ http://cassfos02.ucsd.edu/public/nearest.html ]. The bright stars Alpha Centauri A and B form a close binary as they are separated by only 23 times the Earth [ http://antwrp.gsfc.nasa.gov/apod/ap950622.html ]- Sun [ http://antwrp.gsfc.nasa.gov/apod/ap960521.html ] distance - slightly greater than the distance between Uranus [ http://antwrp.gsfc.nasa.gov/apod/ap950816.html ] and the Sun [ http://antwrp.gsfc.nasa.gov/apod/ap950813.html ]. In the above picture, the brightness of the stars overwhelm the photograph causing an illusion of great size, even though the stars are really just small points of light. The Alpha Centauri system is not visible in much of the northern hemisphere. Alpha Centauri A, also known as Rigil Kentaurus [ http://www.astro.wisc.edu/~dolan/constellations/hr/5459.html ], is the brightest star in the constellation of Centaurus [ http://galileo.gmu.edu/constellation/CEN.html ] and is the fourth brightest star [ http://www.entrenet.com/mizar/brt_star.html ] in the night sky. Sirius [ http://www.astro.wisc.edu/~dolan/constellations/hr/2491.html ] is the brightest even thought it is more than twice as far away. By an exciting coincidence, Alpha Centauri A is the same type of star as our Sun, causing many to speculate that it might contain planets that harbor life. |
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Alpha Centauri: The Closest
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| Title |
Alpha Centauri: The Closest Star System |
| Explanation |
The closest star system to the Sun is the Alpha Centauri system [ http://homepage.sunrise.ch/homepage/schatzer/Alpha-Centauri.html ]. Of the three stars in the system, the dimmest -- called Proxima Centauri [ http://antwrp.gsfc.nasa.gov/apod/ap020715.html ] -- is actually the nearest star [ http://antwrp.gsfc.nasa.gov/apod/ap010318.html ]. The bright stars Alpha Centauri [ http://www.astro.uiuc.edu/~kaler/sow/rigil-kent.html ] A and B form a close binary [ http://antwrp.gsfc.nasa.gov/apod/ap970219.html ] as they are separated by only 23 times the Earth- Sun distance [ http://neo.jpl.nasa.gov/glossary/au.html ] - slightly greater than the distance between Uranus [ http://www.nineplanets.org/uranus.html ] and the Sun [ http://antwrp.gsfc.nasa.gov/apod/sun.html ]. In the above picture [ http://www.eso.org/outreach/press-rel/pr-2003/pr-05-03.html ], the brightness of the stars overwhelm the photograph causing an illusion of great size, even though the stars are really just small points of light. The Alpha Centauri system [ http://www.eso.org/outreach/press-rel/pr-2003/pr-05-03.html ] is not visible in much of the northern hemisphere. Alpha Centauri A, also known as Rigil Kentaurus [ http://www.astro.wisc.edu/~dolan/constellations/hr/5459.html ], is the brightest star in the constellation of Centaurus [ http://www.astronomical.org/constellations/cen.html ] and is the fourth brightest star in the night sky. Sirius [ http://antwrp.gsfc.nasa.gov/apod/ap000611.html ] is the brightest even thought it is more than twice as far away. By an exciting coincidence, Alpha Centauri [ http://www.solstation.com/stars/alp-cent3.htm ] A is the same type of star as our Sun [ http://www.nineplanets.org/sol.html ], causing many to speculate [ http://homepage.sunrise.ch/homepage/schatzer/Alpha-Centauri.html ] that it might contain planets that harbor life. |
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Retrograde Mars
| Title |
Retrograde Mars |
| Explanation |
Why would Mars appear to move backwards? Most of the time, the apparent motion of Mars [ http://www.nineplanets.org/mars.html ] in Earth's sky [ http://antwrp.gsfc.nasa.gov/apod/ap010827.html ] is in one direction, slow but steady in front of the far distant stars. About every two years, however, the Earth passes Mars [ http://antwrp.gsfc.nasa.gov/apod/ap030902.html ] as they orbit around the Sun. During the most recent such pass in August, Mars loomed [ http://antwrp.gsfc.nasa.gov/apod/ap030828.html ] particularly large and bright [ http://antwrp.gsfc.nasa.gov/apod/ap030827.html ]. Also during this time, Mars appeared to move backwards in the sky, a phenomenon called retrograde motion [ http://alpha.lasalle.edu/~smithsc/Astronomy/retrograd.html ]. Pictured above is a series of images digitally stacked so that all of the stars images coincide. Here, Mars appears to trace out a loop [ http://mars.jpl.nasa.gov/allabout/nightsky/nightsky04.html ] in the sky. At the top of the loop, Earth passed Mars and the retrograde motion [ http://astron.berkeley.edu/~ay7/html/homework/hw1/node3.html ] was the highest. Retrograde motion [ http://antwrp.gsfc.nasa.gov/apod/ap011220.html ] can also be seen for other Solar System [ http://www.nineplanets.org/overview.html ] planets. In fact, by coincidence, the dotted line to the right of the image center is Uranus doing the same thing. |
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Solar System Portrait
| Title |
Solar System Portrait |
| Explanation |
On another Valentine's Day [ http://www.poets.org/poems/poems.cfm?prmID=1166 ] (February 14, 1990), cruising four billion miles from the Sun, the Voyager 1 [ http://antwrp.gsfc.nasa.gov/apod/ap031120.html ] spacecraft looked back to make this first ever family portrait [ http://photojournal.jpl.nasa.gov/cgi-bin/ PIAGenCatalogPage.pl?PIA00451 ] of our Solar System. The complete portrait is a 60 frame mosaic [ http://nssdc.gsfc.nasa.gov/photo_gallery/ photogallery-solarsystem.html ] made from a vantage point 32 degrees above the ecliptic plane [ http://antwrp.gsfc.nasa.gov/apod/ap001014.html ]. Voyager's wide angle camera frames sweep through the inner Solar System (far left) linking up with gas giant Neptune, at the time the Solar System's [ http://www.seds.org/nineplanets/nineplanets/overview.html ] outermost planet (scroll right). Positions [ http://nssdc.gsfc.nasa.gov/image/planetary/solar_system/ family_diagram.jpg ] for Venus, Earth [ http://www.seds.org/billa/psc/pbd.html ], Jupiter, Saturn, Uranus, and Neptune are indicated by the corresponding letters while the Sun is the bright spot near the center of the circle of frames. The inset frames [ http://antwrp.gsfc.nasa.gov/apod/ap980517.html ] for each of the planets are from Voyager's narrow field camera. Unseen in the portrait are Mercury [ http://www.psrd.hawaii.edu/Jan97/Mercury Unveiled.html ], too close to the Sun to be detected, and Mars, unfortunately hidden by sunlight scattered in the camera's optical system. Small, faint Pluto's [ http://antwrp.gsfc.nasa.gov/apod/ap011018.html ] position was not covered. |
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Irregular Moons Discovered A
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| Title |
Irregular Moons Discovered Around Uranus |
| Explanation |
Where did these two irregular moons of Uranus [ http://www.seds.org/nineplanets/nineplanets/uranus.html ] originate? Last week two previously undiscovered moons [ http://www.cita.utoronto.ca/~gladman/utpress.html ] of the distant gas planet were confirmed [ http://cfa-www.harvard.edu/iauc/06700/06765.html ], the first in irregular orbits. All fifteen previously known moons [ http://www.seds.org/nineplanets/nineplanets/uramoons.html ] of Uranus [ http://bang.lanl.gov/solarsys/uranus.htm ] are 'regular', circling near the planet's equator. Most of these were discovered by the passing Voyager 2 spacecraft [ http://vraptor.jpl.nasa.gov/voyager/voyager.html ] in 1986. These newly discovered moons [ http://www.news.cornell.edu/science/Oct97/new_moons.hrs.html ] are thought to be odd-shaped and about 100 km across. They are considered irregular, though, because they orbit in odd directions and far from Uranus [ http://antwrp.gsfc.nasa.gov/apod/ap950816.html ]. If Uranus' irregular moons have the same origin as those orbiting Jupiter [ http://www.seds.org/nineplanets/nineplanets/jupiter.html ], Saturn [ http://pds.jpl.nasa.gov/planets/welcome/saturn.htm ], and Neptune [ http://spacelink.msfc.nasa.gov/Instructional.Materials/Curriculum.Materials/Sciences/Astronomy/Our.Solar.System/Neptune/ ], then they were probably caught from orbits around the Sun [ http://antwrp.gsfc.nasa.gov/apod/ap960518.html ]. Moons like this are discovered by their motion. One of these moons is shown above [ http://www.cita.utoronto.ca/~gladman/ ] as the circled point of light moving from left to right. (To stop the movie from repeating, click "stop" on most browsers.) |
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Closer To Beta Pic
| Title |
Closer To Beta Pic |
| Explanation |
What did our Solar System [ http://antwrp.gsfc.nasa.gov/apod/ap961214.html ] look like as the planets were forming [ http://antwrp.gsfc.nasa.gov/apod/ap961207.html ]? Since the 1980s, astronomers have been pointing toward Beta Pictoris [ http://antwrp.gsfc.nasa.gov/apod/ap971128.html ], a young, sun-like star a mere 50 light-years distant, as a likely example. Beta Pic is surrounded by a disk of dust [ http://antwrp.gsfc.nasa.gov/apod/ap960119.html ] which we view nearly edge-on. The dust disk shines by reflected starlight and has been examined with ever increasing detail to search for signs [ http://hrssun.gsfc.nasa.gov/ghrsgh/02b.html ] of planetary formation. The trick is to follow the disk [ http://www.ifa.hawaii.edu/~jewitt/beta.html ] as close in to the star as possible, without being overwhelmed by the direct starlight. To make this Hubble Space Telescope image [ http://oposite.stsci.edu/pubinfo/pr/1998/03/a-js.html ], a coronagraph [ http://www.mpia-hd.mpg.de/MPIA/Projects/STARS/members/ kalas/corona.html ] was used to block the direct starlight and achieve the closest view yet. The false color picture shows [ http://oposite.stsci.edu/pubinfo/pr/1998/03/pr-gsfc.html ] the inner section of the dusty disk to within nearly 1.5 billion miles of the star itself, about the scale of the orbit of Uranus. The obvious warp is indirect evidence [ http://oposite.stsci.edu/pubinfo/pr/1998/03/ ] that a planet now orbits this young sun [ http://antwrp.gsfc.nasa.gov/apod/ap970611.html ], slightly inclined to the disk. The planet's gravitational pull would produce the visible distortion. |
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Infrared Uranus
| Title |
Infrared Uranus |
| Explanation |
The Sun [ http://www.seds.org/nineplanets/nineplanets/sol.html ]'s third largest planet usually looks quite dull. Uranus [ http://www.seds.org/nineplanets/nineplanets/uranus.html ] typically appears as a featureless small spot in a small telescope or a featureless large orb [ http://antwrp.gsfc.nasa.gov/apod/ap971115.html ] in a large telescope. Last August, however, the Hubble Space Telescope [ http://antwrp.gsfc.nasa.gov/apod/ap970306.html ] was able to photograph Uranus [ http://oposite.stsci.edu/pubinfo/pr/1998/35/index.html ] in infrared [ http://www.ipac.caltech.edu/Outreach/Edu/discovery.html ] light, where the distant planet better shows its unusual clouds, rings, and moons [ http://antwrp.gsfc.nasa.gov/apod/ap971126.html ]. Recent analysis indicates that clouds seen here in orange appear to circle Uranus [ http://www.hawastsoc.org/solar/eng/uranus.htm ] at speeds in excess of 500 kilometers per hour. Comparisons to earlier photographs [ http://antwrp.gsfc.nasa.gov/apod/ap971126.html ] show a slight precession shift in the brightest of Uranus' rings [ http://ringside.arc.nasa.gov/www/uranus/uranus.html ]. Several of Uranus' numerous small moons [ http://antwrp.gsfc.nasa.gov/apod/ap971103.html ] are visible. |
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A Solar System Portrait
| Title |
A Solar System Portrait |
| Explanation |
As the Voyager 1 spacecraft [ http://vraptor.jpl.nasa.gov/voyager/vgrfaqs.html ] headed out of our Solar System [ http://antwrp.gsfc.nasa.gov/apod/ap980517.html ], it looked back and took a parting family portrait [ http://nssdc.gsfc.nasa.gov/photo_gallery/photogallery-solarsystem.html ] of the Sun [ http://www.seds.org/nineplanets/nineplanets/sol.html ] and planets. From beyond Pluto [ http://antwrp.gsfc.nasa.gov/apod/ap990213.html ], our Solar System looks like a bright star surrounded by faint dots. In the above picture [ http://photojournal.jpl.nasa.gov/cgi-bin/PIAGenCatalogPage.pl?PIA00451 ], the Sun is so bright it is blocked out for contrast. The innermost dots visible, labeled E and V for Earth [ http://antwrp.gsfc.nasa.gov/apod/ap990131.html ] and Venus [ http://www.nasm.edu/ceps/RPIF/VENUS/rpifvenus.html ], are particularly hard to discern. Gas giants Jupiter [ http://www.jpl.nasa.gov/galileo/Jovian.html ] (J) and Saturn [ http://www.seds.org/nineplanets/nineplanets/saturn.html ] (S) are much more noticeable. The outermost planets visible are Uranus [ http://www.hawastsoc.org/solar/eng/uranus.htm ] (U) and Neptune [ http://antwrp.gsfc.nasa.gov/apod/ap980221.html ] (N). Each planet is shown labeled and digitally enhanced in an inset image. Voyager 1 is only one of four human-made objects to leave our Solar System, the other three being Voyager 2, and Pioneer 10 and Pioneer 11. |
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