That is to say, could a planet be big enough to have other planets orbiting around, while not orbiting around any star? Cha 110913 seems to be close to what I'm looking for, but its classification is still a bit vague.

And if yes, would life be possible, on any planet of that system? I know life is possible in lightless environments, but I wonder if the various variables here would change this.

And pushing the stretch even further, could human life be possible there? Not necessarily originating from there, but maybe as a colony.

  • $\begingroup$ I know this is more semantics than anything else, but isn't a object which orbits a planet, by definition, a moon? Unless there are all objects of decently equal size that they are all basically orbiting each other (irregular, non-circular orbits). In which case, they are planets orbiting each other? $\endgroup$ Dec 4, 2014 at 18:49
  • $\begingroup$ @out-null Good question, although I assume you mean a celestial body rather than simply any object (e.g. artificial satellites). However I am not entirely sure of the scientific definition of a natural satellite, so I don't know if we could indeed say that those orbiting planets are moons of the center planet. If a planet can be a moon, then I guess than Yes, we could call them moons too. $\endgroup$
    – Linkyu
    Dec 5, 2014 at 1:24

5 Answers 5


I'll first address Cha 110913-773444 specifically.

Checking the Wikipedia page yields some interesting information:

  • Mass: 5-15 Jupiter masses. This means that it is likely a planet - extremely low-mass brown dwarfs are typically in the mid-20s, in terms of Jupiter masses
  • Spectral type: L-dwarf. This is consistent with it being a brown dwarf, as many brown dwarfs are of spectral class L or "lower" (i.e. colder). The gist of this is that it doesn't give off a lot of light.
  • Luminosity: 0.000096 solar luminosities. This is really, really dim, dimmer than many (possibly most) brown dwarfs. That sounds like a planet.
  • Temperature: 1,350 K. This is really low for a brown dwarf, although it may still be one. I'm not sure if that's surface temperature or estimated core temperature. In the latter case, it could be a sign of a massive gas giant.

My completely-non-expert assessment is that it's a planet. But you can disregard that, if you want. Some experts are saying it's a brown dwarf, so you may want to just agree with them. However, some are on the fence:

There are two camps when it comes to defining planets versus brown dwarfs," said Dr. Giovanni Fazio, co-author of the new paper from the Harvard-Smithsonian Center for Astrophysics. "Some go by size and others go by how the object formed. For instance, this new object would be called a planet based on its size, but a brown dwarf based on how it formed. The question then becomes what do we call any little bodies that might be born from this disk - planets or moons?"

Either way, it doesn't seem too conducive for life.

Pre-planet-forming materials have been found around other brown dwarfs, though. As the first article said,

Astronomers have become more confident in recent years that brown dwarfs share another trait in common with stars - planets. The evidence is in the planet-forming disks. Such disks are well-documented around stars, but only recently have they been located in increasing numbers around brown dwarfs. So far, Spitzer has found dozens of disk-sporting brown dwarfs, five of which show the initial stages of the planet-building process. The dust in these five disks is beginning to stick together into what may be the "seeds" of planets.

The second article states,

Apai and his team used Spitzer to collect detailed information on the minerals that make up the dust disks of six young brown dwarfs located 520 light-years away, in the Chamaeleon constellation. The six objects range in mass from about 40 to 70 times that of Jupiter, and they are roughly 1 to 3 million years old.

The astronomers discovered that five of the six disks contain dust particles that have crystallized and are sticking together in what may be the early phases of planet assembling. They found relatively large grains and many small crystals of a mineral called olivine.

The issue is, this thing is small, and its disk may not be massive enough to form planets. If Cha 110913-773444 is a planet, it may not have captured a lot of matter in its disk, because it may have been chucked out of its home system. This means that large bodies may not have formed. If it's a brown dwarf, then it may have accreted more matter. Still, the disk may not be massive enough to form large bodies.

The other issue is the luminosity of the object. Life generally needs solar energy (although tidal forces can produce geothermal heat on a body's satellite) to grow and spread. If this is a planet, it will have a low luminosity, and thus life won't have it easy. If it's a brown dwarf, it will give off more energy, but still not a lot.

In summary, I think it's unlikely life will form. The disk may not be massive enough to form planet-sized (or moon-sized) bodies, and there may not be enough energy for life.

Optimistic Note

I recently asked this question, and it made me realize that it's very possible for this object to accrete matter from a passing star with a debris disk. If someone answers that question, I'll be able to tell you just how much, but I can assume that, given a close enough passing distance and a high enough density, the brown dwarf can accrete a decent amount of matter. It's doubtful as to whether it could capture enough to form a protoplanetary disk (because it's so low-mass), but it's possible

  • $\begingroup$ Considering the number of moons Jupiter has why wouldn't this planet (considerably larger than Jupiter after all) have moons? $\endgroup$
    – Tim B
    Dec 3, 2014 at 9:36
  • $\begingroup$ @TimB Jupiter had plenty of material to pick from in the protoplanetary disk; there was a lot of material there. If this thing was ejected from the system, it might not have as much material to choose from. $\endgroup$
    – HDE 226868
    Dec 3, 2014 at 16:10
  • $\begingroup$ Presumably the rogue planet could capture moons even if it was unable to make them from its own disk during its formation. $\endgroup$
    – Oldcat
    Dec 3, 2014 at 20:36
  • 2
    $\begingroup$ @Oldcat From where? It would have to capture them before it was chucked out of the system, and since a gravitational encounter would have brought it away, any moons may very well have been lost. After that, it wouldn't have a great chance of capturing anything. . . $\endgroup$
    – HDE 226868
    Dec 3, 2014 at 21:34
  • 1
    $\begingroup$ @Oldcat If it's ejected out of the system, it's moving pretty fast. Even if it did catch some bits, it would be hard to get them together into a moon. But I shouldn't have been so harsh before - it's definitely possible for any of the things you mentioned to happen. $\endgroup$
    – HDE 226868
    Dec 3, 2014 at 23:14

Habitability of a planet orbiting a brown dwarf or other very low luminosity body would presumably be dependent on geothermal energy. Plenty of ink has been spilled on the possibility of Europa (a moon of Jupiter) hosting life. It's well out of the habitable zone for Sol, and doesn't get much energy from Jupiter either, so these theories depend on geothermal energy.

Colonization of such a geothermally-driven planet might be possible, but it would either have to draw resources from a more earthlike world, depend on future-magic (replicators), or (more interestingly) have a radically weird (possibly designed/terraformed) ecosystem that derives energy from geothermal vents rather than solar radiation. You could probably even tell a just-so story that leads to humanlike life evolving on such a world, but it would stretch the limits of suspension of disbelief if they were TOO humanlike (for example, having eyes that detect 430-770 THz.)

One thing to keep in mind is that widespread natural availability of geothermal energy would more or less be dependent on very high levels of volcanic activity. It would be a chaotic world for sure, and intelligent life would likely be forced to remain nomadic even well beyond our level of technical advancement. (Stray too far from volcanic activity/geothermal vents, there's nothing to eat and you'll freeze, stay too close and you'll get crispy fried.)

  • 1
    $\begingroup$ Welcome to the site Sudo, good answer. $\endgroup$
    – James
    Oct 9, 2015 at 18:21

Adding to what HDE said. Anything is possible, but originating life on any of the moons/planets around such a planet is unlikely. However, that doesn't preclude the ability for things to survive on the 'moons' surrounding it. I would guess the moons would likely be bodies it picked up along the way, so an earth sized planet would be unlikely, though Ganymede is slightly larger than Mercury so you never know. I just think it would be less likely for it to pick up a large body as it is floating through the cosmos.

If there is enough of an energy source, say a solar array that can capture large amount of the planets radiation or if there is enough gravitational forces or a molten core to tap for energy or even large amounts of fissionable material, then yes humans could live there. I would consider it mostly to be either an outpost as a way point for travelers/traders, a science station or refugees that want to be left alone to pick such a lonely and difficult place to live.


There are proposals that true rogue planets (i.e. an Earth sized(?) planet ejected into deep space) could retain life, either under ice (a Steppenwolf planet: http://arxiv.org/abs/1102.1108) or under a high-pressure hydrogen atmosphere (http://www.nature.com/nature/journal/v400/n6739/abs/400032a0.html). Certainly a planet like one of these could be orbiting a sub-brown dwarf (under 13 times the mass of Jupiter) which would meet your criteria. It is also possible that a large moon could be heated by tidal forces if it orbits close to the rogue planet.


Let me list these other two very interesting objects along with Cha 110913:

WISE 0855−0714

  • This is the most interesting one because the estimated temperature (derived from luminosity, distance - only 8 ly and mass) is in the 225-260 K (-40 to -13 C) degrees, that is, extremely cold for a sub-brown-dwarf
  • It masses 3-10 Mj, a lot less than the currently accepted 13Mj needed to be able to fuse deuterium.
  • It does not orbit any other star, thus it can be considered a rogue planet

PSO J318.5-22

This is very interesting as well, because even if it hotter and bigger than WISE 0855−0714 (> 1000 K and ~ 6.5 Mj), it is thought to be just 12 million years old.


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