Do planets need a nearby star and moon to be a planet?

[dreamer45]

Do planets need to have a moon and/or a sun to be known as a planet?

 

[MeteorWayne]

Strictly speaking, there is only a definition for a planet within our own solar system. 2 do not have moons.

There is no formal definition for exoplanets, so all we can say is moons are not required. Is a star? The at remains to be seen as we learn more about our galactic environment.

MW

 

[SpaceTas]

Expanding on MW's post. There is a working definition but not official definition of an exoplanet. It is an object with a mass less than about 20-30 Jupiter masses. Above this are brown dwarfs. The dividing line is uncertain. Not even a working definition has been set at the low end; because we can't detect them. Thus it is quite possible to have free floating planets; ie not bound to any star. There are even searches underway for such objects ...

 

[ramparts]

Expanding on MW's post. There is a working definition but not official definition of an exoplanet. It is an object with a mass less than about 20-30 Jupiter masses. Above this are brown dwarfs. The dividing line is uncertain. Not even a working definition has been set at the low end; because we can't detect them. Thus it is quite possible to have free floating planets; ie not bound to any star. There are even searches underway for such objects ...

The common dividing line is actually 13 Jupiter masses, which is the limit for deuterium burning. Brown dwarfs can't burn hydrogen (mass cutoff of about 65 Jupiter masses) but can still make some of their own energy through, among other things, deuterium burning. Below that there's almost no way to make their own energy (besides leftover gravitational energy from its formation) so that's a convenient cutoff line between brown dwarfs and planets. If you ask most brown dwarf researchers, though, what the ideal distinction is, it's formation - stars and planets form in very different ways, and brown dwarfs tend to form the way stars do. Unfortunately, formation is difficult if not impossible to figure out observationally in many cases (unless you see it in orbit around a star) so the deuterium burning ends up being a convenient cutoff because in principle it can be determined observationally.

Anyway - that's the high mass cutoff for planets, as opposed to the low mass cutoff where we start distinguishing between planets and non-planets like asteroids and Kuiper Belt objects. The important thing to remember is that there's no right answer; "planet" is a manmade term, and man can define a planet to be whatever he pleases. There's only (highly imperfect) current definitions which are incomplete and not well-agreed upon. The International Astronomical Union now has an official definition of planet but that's kind of for show; astronomers are under no obligation to follow any organization's definitions, so how we define things comes down more to what the hive mind decides over time than what some organization's vote declares.

 

[neilsox]

As the others posted, a moon is not necessary, but is desirable. Likely most planets orbit a star, or small group of stars early in the planets life. Most astronomers would still call it a planet if it left orbit. Possibly a very few never orbited except for two or less turns, but we likely would not know that. Few or no free planets are known, but logic suggest that they may be as numerous as orbiting planets. In recent years we have confirmed several new to us extra solar orbiting planets, each year. Neil