Is there possibly such a thing as miniature gas planets?

[banquo's_bumble_puppy]

Is there possibly such a thing as miniature gas planets? I am thinking of something Earth sized or maybe smaller. Could such a world have moons? Would it technically be a planet or a new class of objects?

[antoniseb]

It seems likely to me that there can be Earth-mass gas giants, but they would have to be quite large geometrically (i.e. not very dense), and would have to be far away from any star, as they would be very loosely held together. I suspect that even far from a star, they would be cosmologically short-lived (less than a billion years). The way they would form is similar to how brown dwarfs form, except that the accreting envelope around them gets blown away much earlier in the process, leaving this puffball of an object. I suspect the balancing act required to make it would occur quite rarely.

[chornedsnorkack]

In the outer Solar System, Pluto has escape speed of 1,22 km/s - and holds on to nitrogen atmosphere. What would the escape speed need to be to hold on to hydrogen? Helium?

In outer Solar System, there happens not to be anybody with escape speed between 2,74 km/s (Ganymedes) and 21,3 km/s (Uranus). Big gap! If a body did exist in that range, how small could it be and still consist of hydrogen and helium?

Eris is officially a dwarf planet. If an Oort cloud object bigger than Eris were found, how can it be checked whether it has "cleared orbit" and is a planet or whether it has not, and is a dwarf planet?

[antoniseb]

... If an Oort cloud object bigger than Eris were found, how can it be checked whether it has "cleared orbit" and is a planet or whether it has not, and is a dwarf planet?

Ahhh. Good point. He *did* say "planet" in the OP, and my eye skipped that. My notion was about a rogue object, at least a large fraction of a light year from any star, but by the definition of planet, such a fragile object could only exist briefly during an accretion near a prestellar core, and could never clear its orbit before either getting bigger or being destroyed.

[chornedsnorkack]

Ahhh. Good point. He *did* say "planet" in the OP, and my eye skipped that. My notion was about a rogue object, at least a large fraction of a light year from any star, but by the definition of planet, such a fragile object could only exist briefly during an accretion near a prestellar core, and could never clear its orbit before either getting bigger or being destroyed.

Sedna has been detected because it is near its perihelion of 76 AU briefly - just happened there now in its 11 000 year orbit. Aphelion of 937 AU. How many objects bigger than Sedna exist outside the orbit of Sedna? Epsilon Eridany has companions at 1800 AU which we happen to know about because they are massive enough to shine as brown dwarfs.

Say you have an Earth sized object at a low eccentricity orbit near Sedna´s aphelion. We would not have seen it.

Would existence of Sedna prove that a yet unseen body on Oort cloud could not have "cleared its orbit" and qualify as planet? Well, Pluto crosses Neptune - does this disqualify Neptune from being a planet?

[Ara Pacis]

If Titan was in orbit around the sun instead of Saturn...

[Doodler]

If Titan was in orbit around the sun instead of Saturn...

Here lies the most blatant flaw in the planet/dwarf planet definition. If Titan were to be in Mercury's orbit or closer, it'd be a planet, hands down. There's nothing much smaller than it that would stand much of a chance against the Sun. Beyond the Belt, in Saturn's place, for instance, it's a dwarf. That far out, scattered material has too many places to orbit that could easily avoid its gravitational influence. Bad, bad, bad, bad definition.