48

It’s because they are much smaller than Io. Tidal forces are differential forces, that is, they result from the difference in gravitational pull on one side of a body compared to the other. When an object is small, the difference in distance to the two sides of it is necessarily small as well. According to Wikipedia, Amalthea, the largest of those four ...


38

Liquid water can't exist in a vacuum. If there is no pressure, then the boiling point will drop to the freezing point and so there will either be ice or water vapour. And if the world is "small" then its gravity won't hold on to any water vapour, and it will be lost to space. The Earth can have liquid water because its gravity is strong enough to ...


17

There are four moons that are closer to Jupiter than Io with higher eccentricities, yet they don't seem to have any volcanism at their surface. Only one of those innermost moons (Thebe) has an eccentricity higher than that of Io. The other three have lower eccentricities. The reason they don't exhibit volcanism is because they are too small. The largest of ...


15

Do celestial objects need to be big to have liquid water on their surfaces? Yes. In a nutshell: liquid surface water needs an atmosphere. To sustain an atmosphere, a planet must be sufficiently massive, therefore sufficiently large. The warmer a planet, the more mass it needs to sustain an atmosphere. A planet warm enough for liquid water must thus also ...


13

This question has been asked before on the Space Exploration page. In summary, the term used is system, e.g. the 'Jupiter system'.


13

Uranus rotates once every 0.718 days. 11 of its satellites have a shorter orbital period. These are inner satellites of Uranus which are roughly in the equatorial plane of Uranus. I don't quite understand what direction they orbit in compared to the direction in which Uranus spins. Neptune rotates once every 0.671 days. 5 of its satellites have a shorter ...


11

Certainly 4 (perhaps 5): The longest certainly-known chain is "3": Sun-Earth-Moon. Rhea (a moon of Saturn) has been observed to have a ring of orbiting material: That gives Sun-Saturn-Rhea-Ring for four levels (including unresolved dust) Looking further afield: DH Tauri and DI Tauri are a binary pair of similarly sized T-Tauri stars (both small ...


10

A moon will describe a path like this if its orbital speed relative to its parent planet is greater than the parent planet's orbital speed about the Sun. (Assuming the moon's orbit about the planet and the planet's orbit about the sun are roughly coplanar; I'll ignore Uranus for the remainder of this discussion.) Going through the planets: Neptune: All ...


9

The Moon's orbit about the Earth is only approximately elliptical. The Moon's orbit precesses both axially and nodally, and the eccentricity of the Moon's orbit varies. That the Moon's orbit precesses axially means that perigee sometimes occurs when the Moon is close to new, sometimes when it is close to half-full, and sometimes when it is close to full. ...


9

gerrit's answer has done an excellent job of showing that (1) there are a narrow set of temperatures and pressures where liquid water exists and (2) a planet has to be pretty big to have enough gravity to keep water in the atmosphere. However, I wanted to mention this: However, the conditions required for liquid water can be extended by mixing it with other ...


7

According to Thomas et al. (2007), these craters are not unusually deep: It is unlikely that unusual crater depths significantly enhance the sponge-like appearance. Crater depth-to-diameter ratios for the 13 examples that can be measured reliably using shadow lengths average 0.21 ± 0.05 (s.d.). These ratios are similar to values for fresh lunar craters, and ...


7

Planetoids can have moons and the minimum size is "pretty small". For example 2003 SS84is a small Near-Earth asteroid, with a diameter of 120m and a moon of about 60m in diameter, which orbits at a distance of 270m ever 24 hours. It probably didn't form by "attracting the moon" but the moon probably formed as a result of impact splitting ...


7

Jupiter has a sidereal rotation period or day of 9.925 hours. https://en.wikipedia.org/wiki/Jupiter The two innermost moons of Jupiter, Metis and Adrastea, have orbital periods of 7 hours 10 minutes 16 seconds and 7 hours 15 minutes 21 seconds respectively. https://en.wikipedia.org/wiki/Moons_of_Jupiter#List Saturn has a sidereal rotation period or day of 10 ...


7

This is directly related to another question: Why are asteroids with zero orbital inclination rare? If captured, irregular moons are randomly oriented in space then there is very little chance of them having either inclination angles near zero or near $180^\circ$. This is because, if they are uniformly distributed in space, the fraction of orbits within a ...


7

Could an orphan planet have a moon? Yes. s there some reason why an orphan or rogue planet wouldn't have a moon? Yes, this answer lists some possibilities. What you didn't ask was: Is there some reason why no orphan nor rogue planet could ever have a moon? and based on the previous two, No. Any non-contact gravitational acceleration that could pull a ...


6

tl;dr The inner moons are much to small (and too stiff) to experience significant tidal work. They also cool down much faster though I don't think this is relevant in this case. There're two things to look at, here: The total amount of tidal work that is done on a moon The amount of energy that is radiated away over time When looking at the list of Jupiter'...


5

The moon's orbit is elliptical. This is what the moon's orbit looks like from above (A is the Earth, and C is moon, c is the orbit of the moon around the Earth) As you see with an eccentricity of 0.056 it doesn't get very close to Earth, but if you look carefully enough, there is some variation. The moon in the diagram is at apoapsis, as far from the Earth ...


4

They might escape from the solar system, if the angles are right. If not, they'll probably wind up in elliptical orbits around the Sun. We'll use a simplifying assumption that the orbits are circular to make the calculations easier; All objects mentioned have orbital eccentricity of less than 0.05. The mean orbital velocity of Jupiter is 13.1 km/s, which ...


4

As in real estate, so in astronomy: Location, Location, Location Where an object forms in a system will significantly impact its makeup. For example, during the stage of planet(esimal) formation, due to the radiation of the (proto-)star, various chemicals and atoms will be expelled from the innermost areas, but may stay around long enough to be part of ...


4

Let’s start with an easy example and say the orbit of the planet with radius $r_p$ around the sun with radius $r_s$ is circular, the orbit of the moon around the planet is circular, the inclination of the moon's orbit is 90 degrees from the planet's orbital plane, and the orbital period of the moon is 1/360th the orbital period of the planet. Let's call the ...


4

Certainly an orphan planet could retain its moon. One doesn't need to apply impulse to a planet in order to make it go rogue. One could just as easily apply impulse to the star it orbits instead (though the impulse would have to be orders of magnitude greater). As soon as the difference in velocities between a planet and star exceed escape velocity, the ...


4

Short Answer: It might be possible to have a chain of as many as twelve objects, but that depends on how many of the objects in that chain actually have stable orbits around other objects in the chain. Long Answer: My first guess would be something like: Virgo Supercluster of galaxies > the gravitational center of the Local Group of Galaxies > The ...


3

To find the angular diameter of a satellite you need to find $$\arctan\left(\frac{\text{diameter of satellite}}{\text{distance to satellite}}\right)$$ As you have been using the diameter of the planet, your formulae are wrong. You should also make use of the small-angle approximations $\arctan(x)\approx x$ for small x in radians. So to get the value in ...


3

A moon with a sixth month period would be about 1.3 million km from Earth. That puts it close to the edge of the Earth's Hill sphere, and probably isn't stable in the long term. So the biggest thing you might notice is "goodbye moon" (and having the moon in an Earth crossing orbit wouldn't be pleasant in the longer term, a collision would melt the ...


3

The brightness of light received from a light source (or an object that reflects light) is inversely proportional to the square of the distance. So if an astronomical object A which reflects light from the Sun back to Earth orbits at a distance of two AU from the Sun and an idental astronomic object B orbits at twice that distance, or at 4 AU, how much light ...


3

How can I do this by not knowing the gravitational constant? Sarcasm: You could do what Henry Cavendish did, which was to measure how the horizontal deflection rather heavy balls separated horizontally by a small distance and held in place by torsion rods. The concept of the Newtonian gravitational constant did not exist in Cavendish's time, and would not ...


3

As Rob Jeffries says, no moons are in retrograde equatorial orbit in our solar system. One reason why prograde equatorial orbits are more likely than retrograde equatorial orbits has to do with tidal locking. Our Moon, for example, is tidal locked with the Earth. The Earth is actually also spinning down to tidal lock with the Moon. However, most ...


Only top voted, non community-wiki answers of a minimum length are eligible