Third Rock was a TV sitcom popular back in the '90's. I don't watch the tube so I never saw the show, but I've heard it was fun. Something about a family of clueless aliens trying to live on Earth in human form. The title Third Rock was just a clever way of saying that their new home orbited in the solar system's number three position going out from the Sun.
If you're willing to get up before dawn on these cold February mornings you can currently see all four of the rocky planets at the same time. Mercury, Venus and Mars are visible in the eastern sky before sunrise and you'll be standing on the third rock ... Earth. Dazzling Venus is by far the 'star' of the show. Mars is dim by comparison and Mercury can be tough to spot down lower and closer to the brightening horizon as day approaches.
From Sky and Telescope website
These four relatively small terrestrial bodies are contrasted with the four giant gas/ice planets much further out from the Sun. Jupiter, Saturn, Uranus and Neptune may (or may not) have solid cores but mostly they are huge balls of hydrogen and helium (Jupiter and Saturn) or water, methane and ammonia (Uranus and Neptune). While Jupiter and Saturn are made of the same stuff as the Sun they lack sufficient mass to 'ignite'. That's why they are sometimes called failed stars.
Why are there a group of small, solid inner planets, then an asteroid belt of even smaller rocks followed by four entirely different objects? Astronomers posit that the solar system formed from a rotating disk of gas and dust 4.6 billion years ago. Gravity caused the material to collapse and concentrate into a core that became the Sun. When this young natal star had accreted sufficient mass, pressures and temperatures rose to the point where nuclear fusion could begin. Once a star begins to produce and radiate energy (light, heat and the solar wind) the process of gravitational collapse ends and is reversed with leftover material being pushed outward.
Gases, being lighter, were pushed further away before eventually coalescing into those four distant behemoths while heavier solids stayed closer to the Sun and formed the 'rocks' that you can see in the dawn sky (plus the one you live on). At least that's one theory. Of course, it's more complicated than that and as we learn more about exo-planets around distant stars hopefully we'll understand our own local family better. In the meantime, bundle up and see if you can spot some of our nearest neighbors on the next clear morning.
NASA's famous Blue Marble photo
* Here's a link to Neil Young's After the Goldrush. Is there a 'new home in the Sun' or is it 'all in a dream'.
