If the barycentric coordinates (the point objects orbit around where both masses are balanced) of an object are in (or on) the sun, and it meets other requirements like clearing the neighborhood and having hydrostatic equilibrium, it is a planet. If the coordinates lie in (or on) a planet, it is a satellite. Pluto's coordinates lie above the planets surface near it's moon Charon, as they orbit each other in a binary system. This is the best way to explain the planetary definition IMO.
I hadn’t even really considered orbital points as coordinates, but that’s a wonderful way to think about it - I’m on board.
If one of the qualifications for Pluto to be a planet is "(d) is not a satellite", how are any of the planets considered to be planets? They are all satellites of the sun.
A fine question! At some level, these things come down to semantics. Sure, all of the planets are, indeed, satellites around the sun in the sense that we take ‘satellite’ to mean “something separated from or on the periphery of something else but is nevertheless dependent on or controlled by it,” of which orbiting is a great example.
The generally accepted astronomical definition for satellite is “a celestial body orbiting the earth or another planet.” Just like how our typical, everyday usage of words like “theory” can get confused with the scientific meaning, “satellite” is another example of that.
Since there are so many different things out there in space, it’s hard to come up with a definition for each one to fit into neatly (this graphic is a good example). Thanks for asking!
If every cell in our bodies is replaced in seven years, how do tattoos stay on?
RIGHT!? Isn’t that FREAKING COOL! Your skin cells only last for a few weeks, and yet your tattoo lasts YOUR WHOLE LIFE!
WELL! The outer layer of our skin is made of collagen, a flexible but durable protein. This is constantly sloughing off and being replaced from below as cells die leaving only their collagen-filled shells behind.
But when you get a tattoo, the needle punches past the outer layers of skin, doing quite a bit of damage. The result is that scar collagen forms around the dye that’s been placed in the dermis. Scar collagen, unlike skin collagen, doesn’t replace (which is why scars last forever.) The particles of ink are too large for white blood cells to surround them and carry them off to lymph nodes so they just sit there, surrounded by small amounts of scar collagen outside of your skin cells…pretty much forever.
This is the key to tattoo removal, by the way. Lasers are used to break the ink into smaller bits, so your white blood cells can take care of them.