11 Planets You Never Had to Memorize

??"My Very Early Mother Just Served Us Nine Pizzas.” Whatever mnemonic you used, you probably had to learn nine planets.  (Younger readers may be learning just eight!)  But our solar system actually has thousands more planets than that; those are just the biggies.  Here are eleven that you've probably never had to memorize.??

1. 1 Ceres

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1 Ceres, photographed by the Hubble Space Telescope and enhanced to show details of the surface; this is the best image currently available.?

Discovered in 1801 by Giuseppe Piazzi and named for the Roman goddess of agriculture, Ceres orbits the Sun between 2.5 and 3.0 Astronomical Units (AU), and with a diameter of 970 km at the equator, is the largest member of the main asteroid belt.  A year on Ceres is 4.6 of our years, and it rotates in about 9 hours.  Ceres seems to have a rocky core, and its icy mantle could contain more water than Earth does. It's been studied by ground instruments and the Hubble Space Telescope, but we'll find out more in 2015, when the Dawn spacecraft arrives in orbit and begins its survey.  Today, Ceres is classed as a dwarf planet.

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2. 136199 Eris?

136199 Eris and Dysnomia.

The most massive known dwarf planet, Eris was discovered on January 5, 2005, and immediately created a controversy: it's 27% more massive than Pluto and at 2400 km diameter, a hair wider. So is Eris the 10th planet or is Pluto not a major planet?

Mindful of the controversy, the discoverers named it after the Greek goddess of strife and discord, and indeed, the International Astronomical Union decided to demote Pluto instead of making Eris the tenth planet. Instead, the new category of "dwarf planet" was created, and promptly populated by Eris, Pluto, Ceres, and two other distant ice worlds, Haumea and Makemake. Orbiting from 38 to 98 AU, Eris is a scattered disk object that takes 557 of our years to go around the Sun, and rotates in probably a little more than an Earth day. Like other objects out there, it is mostly made of various ices and hydrocarbons.

On September 5, 2005, it was found to have a moon, eventually named Dysnomia, the daughter of Eris. Eris is near its aphelion, making it currently the most distant known object orbiting the Sun** (although it is known that many comets must lie beyond it, and Sedna's aphelion is vastly further).
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3. 4 Vesta?

4 Vesta's south polar region, taken by Dawn on July 24, 2011 at a distance of 5200 km; the peak at lower right is the central uplift of Rheasilva crater.

Dawn is currently orbiting Vesta, and capturing breathtaking images of it. Vesta was discovered in 1807, and was named for the Roman goddess of hearth and home. It orbits between 2.2 and 2.6 AU, and has a diameter of about 525 km.  It is the second largest main-belt asteroid, comprising about 9% of the mass of the main belt. Its year is 3.63 of our years, and it takes a little over 5 hours to rotate. A huge crater named Rheasilvia (after the mother of Romulus and Remus, and a priestess of Vesta) spans 505 km near the south pole; a whole family of asteroids (the Vesta family) probably came from that impact -- and it also produced a lot of meteors which rained down on Earth, making Vesta one of the few heavenly objects of which scientists have samples. Vesta has an iron-nickel core with an olivine mantle and a thin rocky crust, and is probably another protoplanet.

4. 433 Eros?

The northern hemisphere of 433 Eros, and NEAR's final image during descent; altitude is 120 meters, and the picture spans about 6 meters of the surface.?

The first asteroid ever to be orbited, Eros was discovered in 1898 and named for the Greek God of love. It is a stony near-earth asteroid with a Mars-crossing orbit, ranging from 1.1 to 1.8 AU, giving it a year of about 1.76 of our own years.  It's puny compared to Ceres and Vesta, and not at all round -- the peanut-shaped asteroid is 34.4 x 11.2 x 11.2 km.  It is dotted with craters and has a relatively thick dust layer.  The NEAR spacecraft went into orbit around it on Valentine's Day, 2000, and found orbiting to be a major challenge, because the lumpy moon rotates around its short axis once every 5 hours and 16 minutes.  The mission was a total success, and at the end, the NEAR spacecraft was gently set down on the surface of the asteroid, becoming the first man-made object to land on and transmit from the surface of an asteroid.

5. 243 Ida

243 Ida and little Dactyl, taken by Galileo at a distance of 10,500 km.

?Discovered on September 29, 1884, Ida is a stony main-belt asteroid, orbiting between 2.7 and 3.0 AU, and named for a mythical Greek nymph.  It's quite lumpy; the average diameter is about 31.4 km.  On its way to Jupiter, on August 28, 1993, the Galileo spacecraft flew by Ida, producing an interesting surprise: a tiny satellite, the first ever discovered around an asteroid.  The tiny moon, about 1.4 km across, was named Dactyl, for the mythical creatures who were said to inhabit Mount Ida.  There were not enough observations to determine Dactyl's orbit, but it did give enough information to work out the density of Ida.  It was found to be very low in metallic minerals.  Dactyl's spectrum was very similar to Ida's, so it is believed to be either a piece of Ida or a piece of a larger asteroid from which Ida was also cleaved.  Ida's year is about 4.8 of our years, and it completes a rotation in just 4.63 hours.

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6. 99942 Apophis?

99942 Apophis, from Osservatorio Astronomica Sormano in Italy, acquired December 30, 2004.

Discovered on June 19, 2004, Apophis made headlines for becoming the first object to reach Level 2 on the Torino Impact Hazard Scale, eventually setting the record at Level 4 ("current calculations give a 1% or greater chance of collision capable of regional devastation") before being downgraded to Level 0 based on additional observations.  Its year is about 324 of our days, ranging from 0.75 to 1.1 AU.  But orbital computations suggested there was a 2.7% chance it could impact the Earth in 2029.  This was later ruled out, but there was still the chance Apophis could pass through a gravitational "keyhole" on that date, deflecting its orbit enough to set up an impact on April 13, 2036.  By August 2006, the possibility of this was deemed extremely small.  Since Apophis is about 270 meters across, it would not be a planet-killer, but an impact with Earth would make for a very bad day.  The discoverers gave it the Greek name of Apep, the serpent enemy of Ra, and also one of the recurring major villains of the TV series "Stargate: SG-1".

7. 4179 Toutatis?

4179 Toutatis, imaged by the Deep Space Network's two largest radio telescopes, the 70-meter Arecibo Observatory in Puerto Rico, and the 34-meter antenna at the Goldstone Deep Space Communications Complex.
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Discovered on January 4, 1989, Toutatis is an asteroid in a 1:4 resonance with Earth and a 3:1 resonance with Jupiter, an arrangement that gives it a chaotic orbit that is difficult to predict. It had actually been sighted in 1934, but said chaotic orbit made it take an unusually long time to recover it, a prerequisite for claiming discovery.  Because of the possibility of becoming an Earth-crosser in the future, and many near passes, it is classed as a Potentially Hazardous Asteroid; there are no currently predicted risky passes, but its orbit cannot be accurately predicted more than 50 years out.  Radar observations suggest that it is more of a rubble pile than a simple object, and it has a chaotic rotation that makes day and night erratic.  It currently ranges from 0.9 to 4.1 AU, and is named for a Celtic god usually interpreted as a tribal protector. The modern spelling was popularized by the "Asterix" series of comic books, in which the plucky Gaulish protagonists would exclaim "By Toutatis!"

8. 5335 Damocles?

2011 Draconid meteor photographed by Gadget_Guru; some astronomers think the Draconids could be debris from 5335 Damocles.

There are worlds out beyond the stony planets of the inner solar system and the main asteroid belt.  One of them is Damocles, discovered on February 18, 1991.  It is named for a real person: a courtier of the 4th Century BC tyrant Dionysus I, who was famously reprimanded for saying how great it would be to be king. His punishment involved having a sword suspended precariously over his head by a thin strand.  The asteroid Damocles has a wildly eccentric orbit, ranging from 22.1 AU (beyond Uranus) to just 1.6 AU (inside the orbit of Mars).  It takes about 40.74 years to go around the Sun, and when it nears perihelion, the fastest point in any orbit, it is going incredibly fast.

Its size is unknown, but it is suspected to be a dead comet's nucleus, possibly related to Comet Halley; if it is, it is probably very dark, which combined with its known brightness would mean it is fairly large.  Such an object, if it impacted Earth, could cause major devastation.  Since then, more objects like Damocles have been discovered, the Damocloids.  Worryingly, some of them orbit retrograde, making them even harder to spot.

9. 3753 Cruithne?

Simulation of 3753 Cruithne's orbit with respect to Earth, and why it appears to go around the planet, even though it isn't orbiting it.

?Named for a medieval Irish ethnic group possibly related to the Picts and pronounced "KROOY-nyuh," this minor planet is a real peculiarity.  While it was iscovered on October 10, 1986, something odd was observed in 1997: Cruithne was making annual close approaches to the Earth, and from some perspectives, seemed to actually be going around it!  But it's an illusion.  Cruithne is co-orbital with Earth; it orbits the Sun, but with a 1:1 resonance to our own orbit.  Its distance from Earth is never less than 12 million km, so there is no danger of impact.  Its year is slightly shorter than ours, lasting 364 of our days, and ranges from 0.5 to 1.5 AU from the Sun.  Its rotational period is unknown, but it appears to be about 5 km across.  Since the discovery of Cruithne, a few more quasi-satellites have been discovered, along with the first known Earth trojan, a tiny rock with the provisional designation 2010 TK7.

10. 50000 Quaoar?

50000 Quaoar, taken by the Hubble Space Telescope in the summer of 2002; composite of 16 stacked images.

Orbiting far out in the Kuiper belt, a belt of small bodies orbiting past Neptune, from 30-50 AU, this object was discovered on June 4, 2002, and was soon realized to be quite large -- the second largest Kuiper Belt Object, after Pluto.  (It has since been beaten by more recent discoveries.)  Orbiting at 42 to 45 AU, its year is long: about 286 of our years.  Its rotational period is 17.7 hours, and it appears to be made of various ices, like other KBOs. Size estimates range from 844 to 1,170 km, and it is believed to be somewhat elongated.  It also may have cryovolcanoes, as observations suggest fresh frost deposits on its surface.  It was named for the creator god of the Tongva people, natives of the Los Angeles region.  When in 2007 a moon was discovered, the Tongva people were given the opportunity to name it; they selected the sky god Weywot, the son of Quaoar.  Weywot is probably about 74 km across, and orbits at 14,500 km from Quaoar.  Quaoar is probably a dwarf planet, but has not been officially recognized as one.

11. 90377 Sedna?

90377 Sedna through the Hubble Space Telescope; the image was taken as part of a search for a presumed satellite, which was not found.

This puzzling object has the most distant aphelion of any known object orbiting our Sun.  Discovered on November 14, 2003, Sedna's orbit lasts a staggering 11,400 years, and ranges from 76 to 937 AU, far beyond the Kuiper Belt.  It is sometimes classed as a scattered disk object, a group of objects ejected from the Kuiper Belt by Neptune, but it doesn't appear to have ever come near Neptune.  It may instead be the first known member of the Oort Cloud, a hypothesized cloud of objects at the most distant edges of the solar system, pushed in by an undiscovered large planet or by an encounter with a star sometime in the distant past.

It is this strange orbit that makes Sedna so interesting and so important: understanding it will help answer questions about the history of the solar system and the nature of its mysterious outer reaches.  Observations have fixed its rotational period at about 10 hours, and it is estimated to be somewhere between 995 and 1,600 km in diameter.  It is unusually red, almost as red as Mars, suggesting a covering of hydrocarbon sludge over its presumed icy interior.  Since it is so far away from the Sun, it was named for Sedna, the Inuit goddess of the sea, who was said to live beneath the frigid Arctic Ocean, the most remote and inaccessible sea of all.

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* 1 Astronomical Unit (AU) = approximately the average distance between the centers of Earth and Sun: about 150,000,000 km or 93,000,000 miles. ??** The twin Voyager spacecraft are further away, but are not orbiting the Sun.  They are on an escape trajectory.  Voyager 1 is 120 AUs from the Sun, and Voyager 2 is 99 AUs from the Sun.  As they are currently traversing the heliosheath, it's reasonable to believe that distant worlds such as Sedna are not always within the protection of the heliosphere.