I think it's :
The Solar System's planets in reverse order.
My first child was a rebel. He is estranged from us.
Pluto, is no longer a planet. -So many things made Pluto differ from all 8 other planets - and so many new solar system bodies kept being discovered to which Pluto shared more similarities. It eventually had the definition of a Planet revised and the Dwarf Planet category introduced (to which Pluto now belongs)
My second used to be much closer to me, but we are losing touch.
My third is similar in temperament to my second, but smarter, although sometimes he lets my second persuade him into doing things for him.
Neptune & Uranus - Almost twins, nearly the same size. Uranus is bigger, Neptune has greater mass. Both have rings (although Neptune's are fragmented). They are similar in composition (hydrogen & helium).
(Per @Going hamateur) Neptune was discovered as it perturbed Uranus' orbit so it was theorized to exist and pinned down relatively easily. Speculating that at an earlier point it was closer in orbit to the sun but got knocked away (sorta).
My fourth is quite a snappy dresser, and is very popular.
Saturn and its fancy rings has always been popular. It also has a TON of moons, almost as many as Jupiter (again you could say it's popular). One of those moons is Titan - which is again a very popular subject of interest in science-fiction (and space exploration).
My fifth is the strongest, and the most help around the home.
Biggest Planet in the Solar System, Depending on it's position relative to the sun, Jupiter is the only planet that has a barycenter with the Sun that lies outside the volume of the Sun, Along with the Sun, the gravitational influence of Jupiter has helped shape the Solar System. The orbits of most of the system's planets lie closer to Jupiter's orbital plane than the Sun's equatorial plane (Mercury is the only planet that is closer to the Sun's equator in orbital tilt), the Kirkwood gaps in the asteroid belt are mostly caused by Jupiter, and the planet may have been responsible for the Late Heavy Bombardment of the inner Solar System's history
My sixth and seventh are quite similar, but my seventh is friendlier. My sixth and fifth don't get on well.
6th is Mars, very similar to Earth(7th) - they are similar size, gravity, composition etc... - but I think everyone will agree Earth is much nicer (having an atmosphere that supports human life). Mars and Jupiter(5th) are separated by the asteroid belt which also separates the Inner Solar System from the Outer Solar System.
My eighth likes to keep an air of mystery about herself.
Venus, which atmospheres' makes it almost impossible(difficult) to learn anything of it's surface.
I keep my baby closest to me. He can be very temperamental.
Mercury. Closest to the sun. Mercury's lack of any substantial atmosphere means the surface exposed to the sun is very hot where as the surface in the shadows will see a crazy temperature drop. It's temperature can range from -173'C to 426'C.
for those that are thinking of Mercurial, that's putting the cart before the horse. Guess where that adjective comes from. (1350-1400; Middle English < Latin mercuriālis of, pertaining to the god or planet Mercury. See mercury)
What is my name?