I’m confused by all the news that’s been going back and forth in the world of astronomy: is my favorite planet, Pluto, still considered a planet and part of our solar system, or not?
Much to my surprise, the International Astronomical Union has indeed voted on a resolution that defines exactly what a planet is and Pluto no longer qualifies. This means that Pluto is indeed not a planet any more and that our solar system no longer has the nine planets I learned about as a kid, but only eight!
Don’t worry, though, Pluto is now to be known as a dwarf planet, so it’s not been completely ejected from our solar system, just relabeled. The eight planets in our solar system are now, in order from the Sun outwards, Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
As The Planetary Society explains, “a “planet” is now defined as a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit.”
“A dwarf planet, according to the new definition, is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighborhood around its orbit, and (d) is not a satellite.”
Got it?
There are now three dwarf planets in our solar system, so Pluto isn’t alone. It’s joined by the asteroids Ceres and Xena (aka UB313), though more than a dozen are on the IAU’s dwarf planet watch list, if you can believe it.
One key reason that Pluto has been given the boot is because its orbit is not in the same ecliptic plane as the rest of the planets and isn’t circular as are the planets, but is rather “eccentric”. Indeed, at certain points in its orbit, Pluto is actually closer to the sun than Neptune. You can see some of this shown here:
A wee bit of data about Pluto too: Discovered in 1930 by Clyde Tombaugh, Pluto has an orbital period of 90,613 days (what we on Earth would consider 248 years) and is only 18% the size of Earth with a surface area of far less than 1% of our own planet. Gravity is far less than our planet’s gravity too, at 0.58 m/s2 (about 0.06 Earth Gee). Sound like somewhere worth visiting? Maybe not. The average surface temperature is a chilly 44K which converts to -380F. Pluto also has three moons, Charon, about 35% the size of Earth’s moon, and two much smaller moons provisionally called Nix and Hydra.
Additional reading:
- The International Astronomical Union
- NASA’s Pluto fact sheet
- Windows to the Universe (NCAR)
- Orbital simulator of Pluto
- Kids guide to Pluto
- Wikipedia Entry on Pluto
Not being funny but Pluto has always been one of the 9 planets of our solar system. That’s what I was taught and that’s what I’m sticking to. The rest is semantics and drivel 🙂
thank you for answering my question….
Indy, apparently Pluto isn’t called 2003 UB313: that’s actually a Kuiper Belt Object “Eris” that’s been called the potential tenth planet in our solar system. It’s actually bigger than Pluto, as it happens.
The way these names work is that the first part is the year of initial discovery, then the next letter defines when during the year the discovery was made (A = first half of January, B = second half of January, etc). So “U” = the second half of October. The B313 is just a numeric sequence as far as I can tell, and it means that during the last two weeks of October, 2003, this was the 313th object to be identified and registered.
Pluto itself? It’s considered minor planet 134340. Why that big a number? I have no clue. 🙂
hello my school is researching this now in class why was it called 2003 ub313 i know why it had 2003 but why ub313?
thank you very much for clearing things for me
Wow. I can’t believe that They’ve revoked Pluto’s planetary status. I was taught that there were nine! My Very Educated Mother Just Served Us Nine Pizzas. Now what’ll it be? Did she serve us noodles? Young children rarely like noodles unless they’re covered in spaghetti sauce and have meatballs with them.
I just wonder if these people actually thought their decision through completely before passing it.
If Pluto is called a dwarf planet, can Jupiter be called an “obese planet”?
For me Pluto is a Plutoplanet!
So by the new standards, would not Neptune also fail the criteria seeing as it has also not cleared its own orbit, passing outside the orbit of Pluto? I’m sure they probably thought of some great excuse to get Neptune off the hook!
Ugh…Pluto is a planet – These yo-yo’s can’t find the bathroom without a map.
And now that I’ve actually READ their idiot-definition, I am even MORE convinced that I’m right.
How about this:
Cessna’s Law of Planetary Description
A ‘planet’ is defined as any celestial body that meets ALL of the following:
a) It generates its own gravity
b) is at least 1,000 miles in diameter
c) It occupies an independent solar orbit conforming to Bode’s Law
d) it is not a satellite of another planet
Anything not conforming to the above is a ‘rogue’ planet, a ‘planetoid’, an asteroid, a meteor, or a comet.
Thus:
*Pluto is a planet, but Ceres is not.
*The Moon is still a ‘moon’, as are Titan, Ganymede, Europa, etc.
LOL! A swarf! Well, it’s always been a little small! Good show! Now let’s work on reclassifying earth.
then it is still a planet is’nt it??
Arvin, it’s now called a “dwarf planet”. My opinion? It’s kind of silly…
then if pluto is not still a planet waht is it now????
There is also more than 1 dwarf planet. I think one is called Ceres, and one is called UB131 2007? Or something like that – unfortunately I forgot the exact name of the last one, but it’s nickname is Eris. I think =/ Thanks for the info though! It’s pretty handy!
According to this link, pluto will retain as a planet…
http://science.slashdot.org/article.pl?sid=06/08/10/1645259&from=rss
Thanks for the info. I can’t believe this. How the heck are the kids going to learn all of the planets now that they can’t sing the song I learned them to that goes “My (Mars) very (Venus) educated (Earth) mother (Mars) just (Jupiter) served (Saturn) us (Uranus) nine (Neptune) pizza pies (Pluto).”
Is pluto still a planet????
don’t write so much i get so confused
Hey!! thanks for that. it helped me understand some stuff and prove some people wrong 🙂
Shivania
i’d like start by saying thanks. ur short,but still clear and interesting, summery of pluto helped me finish my project and now i know wat to say on display day. it s also a huge part of my grade. second i’de like to say that i m mad that they would do that to pluto. its been a planet for……ever! watever, i’ll still say mercury,v,e,m,j,s,u,neptune,PLUTO!
what’s new about the planet why did they move the pluto out of the nine planet
Thanks sooooooooooooooooooooooo much man!!!
Thanks so much, this is really helpful.
Thank you for clearing this, I am using it for a essay in school that is a huge chunk of my grade
DAVE TAYLOR,
THANK YOU.
…for clearing that up so in such a timely manner for us, now i know what to tell my younger sister.
and can spend my time painting:)