# Tag Info

## Hot answers tagged planets

42

Let's assume mass of the person plus spacesuit to be $m_1$=100kg Asteroid density: $\rho=$2g/cm$^3$ (source) that is 2 000kg/m$^3$ 15km/hour is a good common run. That's roughly v=4m/s The orbital height is negligible comparing to the radius, assume 0 over surface. Linear to angular velocity (1): $$\omega = {v \over r }$$ Centripetal force (2):  F = ...

40

One thing to keep in mind is that objects that are bound gravitationally actually revolve around each other around a point called a barycenter. The fact that the earth looks like its revolving around the sun is because the sun is much more massive and its radius is large enough that it encompasses the barycenter. This is a similar situation with the Earth ...

33

Anything the mass of a star is going to get hot like a star and fuse hydrogen like a star. In other words it will be a star not a planet! While it's technically possible to have a rocky planet the mass of a star, in practice when stellar systems form there aren't enough metals available to build such a large object. Large objects are invariably built from ...

33

The answer kind of depends on how old you are. At a very introductory level, say, maybe middle school or younger, it's "okay" to refer to Jupiter as a failed star to get the idea across that a gas giant planet is sort of similar to a star in composition. But around middle school and above (where "middle school" refers to around 6-8 grade, or age ~12-14), I ...

25

And you jumped in. No, I'd refuse, You should do that Yourself. :=( Do You want to "solve" this with or without friction by air? Without friction, You would fall and reach maximum speed in the center of earth, going on until You reach the antipods, where You would stand still for a fraction of a second, then You would go down again. Very boring ...

24

Pluto is now classified as a dwarf planet. The main difference between a planet and a dwarf planet has to do with the requirement that a planet clear out the material in and near its orbit. Planets do this, dwarf planets do not. The reclassification was triggered by the discovery of many additional object (the Edgeworth-Kuiper Belt) out beyond the orbit ...

24

The other answers provide a first-order approximation, assuming uniform density (though Adam Zalcman's does allude to deviations from linearity). (Summary: All the mass farther away from the center cancels out, and gravity decreases linearly with depth from 1 g at the surface to zero at the center.) But in fact, the Earth's core is substantially more dense ...

22

Correct. If you split the earth up into spherical shells, then the gravity from the shells "above" you cancels out, and you only feel the shells "below" you. When you are in the middle there is nothing "below" you. Refrence from Wikipedia Gauss & Shell Theorem. {I am using some simplistic terms, but I don't want to break out surface integrals and ...

19

This is a gravitational phenomenon known as tidal lock. It is closely related to the phenomenon of tides on Earth, hence the name. Tidal locking is an effect caused by the gravitational gradient from the near side to the far side of the moon. (That is, the continuous variation of the gravitational field strength across the Moon.) The end result is that the ...

19

This is a really rough calculation that doesn't take into account the realistic direction of the bow shock, or calculation of the drag force. I just take the net momentum flow in the solar wind and direct it so as to produce the maximum decceleration and see what happens. Apparently the solar wind pressure is of the order of a nanoPascal. As I write this ...

18

Officially, no -- but there is a weak case to be made that the Moon orbits the Sun rather than the Earth. If you trace the Moon's path in a Sun-centric frame of reference, that path is completely convex. Quoting this Wikipedia article: Unlike most other moons in the Solar System, the trajectory of the Moon is very similar to that of its planet. The ...

18

I think you are confused as to what the 'surface' of Jupiter or Saturn are. They have a large liquid hydrogen centre, but this is surrounded by an incredibly thick layer of atmosphere, which has clouds, gases, liquids etc. So you would first pass through the outer layers of atmosphere, falling through denser and denser gas until you float at a height which ...

18

No, not by jumping. Jumping gives you an acceleration only from the location on the surface. As soon as you leave the surface, you have no way of adjusting your orbit. Either you reach escape velocity, or you will return to your initial location after exactly one orbit. The only way to prevent this would be to have an additional acceleration once you ...

17

A "Trojan" object is any smaller object that shares the same orbit as a larger body but leads or trails it by about 60 degrees in the orbit. These positions are the L4 and L5 Lagrange points (respectively) in the larger body's orbit about its parent object. The L4 and L5 Lagrange points are locations of stable gravitational equallibrium between the larger ...

17

There are a few things that keep Saturn's rings roughly the way they are. First, Saturn's D ring actually is "raining" down on Saturn currently. But, the phenomenon of shepherd moons prevents the vast majority of material from leaving the other rings: "The gravity of shepherd moons serves to maintain a sharply defined edge to the ring; material that ...

17

Well, motion is relative so you can choose a frame of reference where one is stationary. If you do though, it makes the equations of motion quite complicated. Even in our solar system, the Sun isn't stationary. It orbits the center of mass of the whole solar system (barycenter), just as each planet orbits the center of mass. The center of mass of our ...

17

In a planetary system you would expect the orbits of the large bodies to be almost but not quite circular. This is because there are two opposing effects: tidal forces tend to make orbits circular but perturbations between planets tends to make orbits elliptical. In a two body system, just the star and one planet, tidal deformation of the planet and the ...

16

Non-physical/philosophical answer (see comment of @annav): We orbit Sun because we called the star we're orbiting "Sun". (I think that's not actually as "stupid" as it may sound initially. See also Anthropic principle ) Physical answer: The Earth formed from matter near to the Sun, so it ended up near the Sun, and is orbiting it, because gravity depends ...

16

The simplest way to think about it is that there is mass all around you in the center of the Earth so you get an equal gravitational "pull" from all directions. The pulls cancel out so you get no acceleration. If one assumes constant density for the Earth (which isn't strictly speaking true but it is close enough for this illustration) the gravitational ...

16

Begin by imagining that the moon isn't quite a perfect sphere. One side is just a little bigger than the other. As the moon rotates, the heavier face will swing around towards the earth a little faster, and it will swing away from the earth a little slower, since it feels a stronger gravitational attraction via its larger mass. Since gravity is a ...

16

There are a lot of factors that go into whether or not a planet has an atmosphere. First, the mass and size of the planet. Really what it comes down to is the escape velocity. The higher the escape velocity (ve), the easier it is for a planet (or moon) to retain any atmosphere it gets as the gases that make up the atmosphere have to be moving faster to ...

15

This correlation is known as Titius-Bode's law, which is often stated as $$d=0.4 + 0.3 \cdot 2^n$$ where d represents planet's mean distance from the Sun in Astronomical Units and n = -∞, 0, 1, 2... for Mercury, Venus, Earth, Mars, asteroid belt, Jupiter and so on. The rule is not satisfied exactly with Neptune's orbit (n=7) ...

15

The leading theory is that at a distant point in its past, Uranus was struck by a very large object, which knocked it to its side, and current tilt. Imagine if you took a top, and smacked it with a rock. The top might be turning perfectly alright at first, but after it had been hit, the top would most likely be wobbling significantly. Similarly, after an ...

15

To some extent the universe exhibits something called self-organized criticality where a dynamic, non-linear system with many degrees of freedom (the gas after the Big Bang but before the emergence of structure) eventually forms a system with a notable degree of scale invariance (moons orbiting planets, planets orbiting stars, stars orbiting galactic ...

15

I doubt if anyone has come up with a complete explanation, but some laboratory simulations have created similar patterns. They happen if the central and surrounding areas in a flat, circular disk of fluid have different velocities. Emily Lakdawalla at The Planetary Society covers it at this site. She also explains how other patterns (triangles & ...

14

I like answers that appeal to symmetry, so I answer this one with a question: If you were at the center, which way would you fall? That tells us you could stay floating there.

14

The angular size of the object can be calculated by basic trigonometry: $\theta=2\cdot \arctan(r/d)$, where $r$ is the radius of the object you're viewing, and $d$ is the distance between you and the object ($\theta$ is the angle). The average (volumetric) radius of Saturn is 58,232 km. The distance between Titan and Saturn is 1,221,830 km. Plugging the ...

14

A circle is a very difficult shape to maintain. Even the slightest deviation, and a circle is bypassed. Orbits are elliptical when any of the following things happen: Another object strikes the planet in such a way to change its orbit. It would have to be massive compared to the primary object, at least a sizable fraction. Gravitational interaction with ...

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