Is there any theory that prohibits planets that are not in solar systems?

Is there any theory that prohibits planets that are not in solar systems?

I see we keep looking for planets in other galaxies.

Couldn’t there be some between us and the next star, just floating free?

Maybe formed on their own without enough mass to become even dead “stars”.
Or maybe ejected from an exploding star.

Whatever the case, they should be possible. And they could have enough internal heat for volcanic action and thus sustain life without a sun.

But I’ve never heard anyone looking for something like that.

Isn’t a “free-floating” planet the same thing as an asteroid?

There wouldn’t really be any way to detect a planet just floating free in interstellar space. The only way to detect a planet (AFAIK) is to observe it’s effect on the star it orbits.

Are you sure about searching for planets outside our galaxy? I’m not sure we can even discern individual stars in most galaxies, much less planets.


Asteroids are orbiting the sun, just like planets. Sometimes they cross the orbit of another planet and change their own orbit, or impact a planet, but most have been orbiting the Sun since the Beginning.

I really don’t know that much about astronomy, but I thought I’d look around, so I found This list of known planets outside our solar system, all of which orbit other stars.

However, on the same site, I found this:

Which seems to indicate that planets may depart solar systems and go off to roam on their own. Like Arjuna, I have no idea how one would go about detecting such a planet.

[nitpick] And in the quote I posted above… aren’t orbits supposed to be ellipses, not circles? [/nitpick]

The OP is basically looking for something called “brown dwarfs.” Those are bodies with enough mass to have drawn everything together in the first place, but not enough mass to ever get the fusion reactions started that make a star a star. According to some theories, they’re quite numerous, but pretty darned difficult to detect. Just do a google search on “brown dwarf” and you’ll probably get plenty of hits.

Orbits are elliptical. What they meant to say is that the orbits of Jupiter and Earth are relatively circular, not very eccentric. There are models of solar system formation where some Earth-like planets form in some systems and then eventually get thrown out of orbit by close encounters with Jupiter-like planets that are in chaotic, eccentric orbits. If these models are right, there may be a lot of Earth-like planets (Earth-like in the sense of size, density, etc., but not climate) floating around free in space. Detecting them would be difficult, since the only planets we’ve detected in other systems have been much larger than Earth and we detected them through their influence on the stars they orbit.

You’re not the first person to have thought of this.

Not quite. Brown dwarfs would have to be larger than any planet in our solar system. They are not quite large enough to start the fusion reactions, but nearly so. Jupiter is our largest planet and it fails to be a brown dwarf.

Um, no. Galaxies are the huge collections of stars millions of light-years away. We’d be lucky to discern individual stars in other galaxies.

We are looking for planets in other solar systems, i.e., around other stars, in our own galaxy, the Milky Way.

Hey, I said “basically,” barbitu8! And in the sense of “large accretions of matter that aren’t stars and do not orbit another body,” I think that’s basically correct. Now if we’re talking about solid, Earth-like objects in interstellar space, we’re pretty much left with planets that have been chucked out there by passing Jupiters, stars, science fiction plot devices, etc.

minty green
3 hours away from a minor in astronomy instead of poli sci

Here ya go!
It’s a recent discovery.

BTW, as others said, astronomers look for other planets within this galaxy only…modern telescopes are not powerful enough to see planets in other galaxies. (Remember not to confuse the terms “galaxy” and “solar system”.)