Essentially, a planet orbits a star. All other descriptions are mere refinement.
In 2006 the International Astronomical Union (IAU) issued an updated definition of what a planet is. This came along because more planets had been discovered on beyond Pluto. Once they realised they'd have dozens, potentially hundreds of new planets, they thought they'd come up with some definitions to help sort or categorise the planets. This definition is based off the gravitational interaction of a planet with its environment:
"A planet is a celestial body that:
- is in orbit around the Sun,
- has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and
- has cleared the neighbourhood around its orbit."
"But what about Pluto?" you cry! "They demoted it! It's no longer a planet!"
Uh, yes but no. Pluto is still a planet. Go ahead and call it one if you want. It's new classification may be "dwarf planet", but that is still a planet.
|Shot this pic of Venus a month ago|
with my smartphone thru my telescope.
So, when the 2006 definition came along, Ceres got re-promoted and classified as a planet once more, albeit in the "dwarf planet" category. I didn't mind (Okay, I was ecstatic!). But many people who weren't around for the 1850 demotion of Ceres, didn't realise the historical precedence for this sort of card-shuffling of our Solar System.
Just like in the 1850's, the 2006's brought a new definition of a classification, because lots more of whatever it was they were classifying were found. Kuiper Belt Objects (KBOs) were being discovered all the time, and some of them were large enough they could be classified as planets. Also, planets were being discovered around other stars. Everyone went, "That's so cool!" Astronomers went, "Yeah, but we need to sort our rocks."
So the definition was born.
A dwarf planet's not just about size. The only change between the classification of a dwarf planet and a terrestrial planet is that a dwarf planet hasn't cleared the neighbourhood around its orbit. So, Ceres wanders along in the Asteroid Belt and Pluto and Eris and Sedna and company wander about in the Kuiper Belt. (Trojans don't count, because they're gravitationally shepherded.)
|Our Moon, as shot by Your Truly.|
I notice you didn't ask about the Moon (any of them). Couldn't the 2006 planet definition qualify them as well?
Enter Refinement #2: If an object is in orbit around another object, it's a satellite (aka a moon).
Just thought I'd mention this, as we have moons (like Ganymede) that are bigger than Mercury and Pluto. But, as Ganymede and her sisters are in orbit around Jupiter, our Luna (Moon) is in orbit around Earth, and Titan and his mates are in orbit around Saturn, we're calling them satellites.
So, what about rocks that aren't round and aren't orbiting anything else? We call them Small Solar System Bodies (SSSBs). These include asteroids, comets, pebbles and anything else with a gravitational connection to the Sun.
So go outside tonight and look up. Can you spot any planets?
Want to know if that bright light up in the sky is a planet or a star? Easy; stars twinkle. Planets don't. (If the light blinks regularly and is moving, it's a plane. If it grows bright then fades and is moving, it's a satellite. If it flares up really bright, enough that you could see it during the day, and lasts several weeks before fading, it's a supernova. If it's fuzzy and grows a tail, it's a naked-eye comet. If it's sudden, really really bright, and takes up your whole field of vision, it's the flashlight of a police officer who's wondering why you're laying on the ground staring up at the sky.)
So, want to know what planets are currently up? Why, all of them, if you know where and when to look.
The early evening sky favours us with a glimpse of Mercury in Pisces. Uranus is also close by, but twilight might be too bright at the moment to spot it well. (Give it a go in about six months' time. Under the right conditions, Uranus can be spotted with the naked eye and some skill in very dark skies. But if you've got binoculars or a telescope, I recommend this for better luck.) Mars isn't too far behind in Aries.
|My wee shot of Saturn.|
You'll have to stay up late, or get up early to see Saturn between Scorpio and Sagittarius. This year and next year is excellent to view Saturn through a telescope, as the rings are pretty much as full on tilted our way as they get.
Neptune is a morning star in Aquarius and Venus is close by, just before dawn.
Her Grace loves to stare up at the sky, and does so nearly every single night.