It isn’t difficult to find rankings for the most massive objects in the solar system, rankings of objects in terms of increasing distance from the sun, or rankings of objects by radius. However, ranking objects by surface gravitational field strength is another matter, and is more complicated, for it is affected by both the mass and radius of the object in question, but in different ways. If two objects have different masses, but the same radius, the gravitational field strength will be greater for the more massive object. However, increasing the radius of an object decreases its surface gravitational field strength, in an inverse-square relationship.
Gravitational field strength is measured in N/kg, which are equivalent to m/s², the units for acceleration. The terms “gravitational field strength” and “acceleration due to gravity,” both of which are symbolized “g,” are actually synonymous. I prefer “gravitational field strength” because referring to acceleration, when discussing the weight of a stationary object on the surface of a planet, can cause confusion.
Use of the numbers given below is easy: given the mass of a thing (an imaginary astronaut, for example), in kilograms, simply multiply this figure by the given gravitational field strength, and you’ll have the weight of the thing, in newtons, on the surface of that planet (or other solar system object). If, for some odd reason, you want the weight in the popular non-metric unit known as the “pound,” simply divide the weight (in newtons) by 4.45, and then change the units to pounds.
How is surface gravitational field strength determined? To explain that, a diagram is helpful.
The large green circle represents a planet, or some other solar system object, and the blue thing on its surface, which I’ll call object x, can be pretty much anything on the solar system object’s surface. There are two formulas for Fg, the force of gravity pulling the planet and the thing on its surface toward each other. One is simply Fg= mxg, a form of Newton’s Second Law of Motion, where “g” is the gravitational field strength, and mx is the mass of the object at the surface. The other formula is more complicated: Fg= (Gmxmp)/r². This is Newton’s Law of Universal Gravitation, where “G” (not to be confused with “g”) is the universal gravitational constant, 6.67259 x 10-11 Nm²/kg², and mp and r are the mass and radius of the planet (or other solar system object). Because they each equal Fg, the expressions mxg and (Gmxmp)/r² can be set equal to each other, yielding the equation mxg = (Gmxmp)/r², which becomes g = (Gmp)/r² after mx is cancelled. The mass of the object on the surface is not needed — “g” is simply a function of mp and r.
There is a problem, however, with the idea of “surface” gravitational field strength — and that is the fact that the five largest objects in the solar system, the sun and the gas giants, all lack visible solid surfaces. One cannot stand on Jupiter — if you tried, you’d simply fall inside the planet. Therefore, for Jupiter, picture a solid platform floating at the top of the visible clouds there, and place the test object on this solid platform. Under those conditions, multiplying the test object’s mass by the Jovian value of “g” will, indeed, yield the weight of the object there, as it could be measured by placing it on a bathroom scale, at rest on the floating platform. For the other gas giants, as well as the sun, the idea is the same.
The objects included in the list below are the sun, all eight major planets, all dwarf planets (and dwarf planet candidates) with known values of “g,” all major satellites, some minor satellites, and a few of the largest asteroids. Many more objects exist, of course, but most have values for “g” which are not yet known.
Here are the top five:
Sun/Sol, 274.0 N/kg
Jupiter, 24.79 N/kg
Neptune, 11.15 N/kg
Saturn, 10.44 N/kg
Earth/Terra, 9.806 65 N/kg
The top five, alone, make me glad I undertook this project, for I did not realize, before doing this, that our planet has the highest surface gravitational field strength of any object in the solar system with a visible solid surface.
The next five include the rest of the major planets, plus one Jovian moon.
Venus, 8.87 N/kg
Uranus, 8.69 N/kg
Mars, 3.711 N/kg
Mercury, 3.7 N/kg
Io, 1.796 N/kg
The third set of five are all planetary moons, starting with earth’s own moon. The others are Jovian moons, except for Titan, which orbits Saturn.
Moon/Luna, 1.622 N/kg
Ganymede, 1.428 N/kg
Titan, 1.352 N/kg
Europa, 1.314 N/kg
Callisto, 1.235 N/kg
The fourth set of five begins with the largest dwarf planet, Eris, and includes two other dwarf planets as well.
Eris, 0.827 N/kg (dwarf planet)
Triton, 0.779 N/kg (Neptune’s largest moon)
Pluto, 0.658 N/kg (dwarf planet)
Haumea, 0.63 N/kg (dwarf planet)
Titania, 0.38 N/kg (largest moon of Uranus)
The fifth set of five includes the remaining dwarf planets with known values of “g.”
Oberon, 0.348 N/kg (moon of Uranus)
1 Ceres, 0.28 N/kg (dually classfied: dwarf planet and largest asteroid)
Charon, 0.278 N/kg (largest moon of Pluto)
Ariel, 0.27 N/kg (moon of Uranus)
90482 Orcus, 0.27 N/kg (probable dwarf planet)
The sixth set of five are dominated by Saturnian moons.
Rhea, 0.265 N/kg (Saturnian moon)
4 Vesta, 0.25 N/kg (2nd largest asteroid)
Dione, 0.233 N/kg (Saturnian moon)
Iapetus, 0.224 N/kg (Saturnian moon)
Umbriel, 0.2 N/kg (moon of Uranus)
The seventh set of five are mostly asteroids.
704 Interamnia, 0.186 N/kg (5th most massive asteroid)
2 Pallas, 0.18 N/kg (3rd most massive asteroid)
Tethys, 0.147 N/kg (Saturnian moon)
52 Europa, 0.14 N/kg (7th most massive asteroid)
3 Juno, 0.12 N/kg (large asteroid, w/~1% of mass of the asteroid belt)
Starting with the eighth group of five, I have much less certainty that something may have been omitted, although I did try to be thorough. My guess is that most future revisions of this list will be necessitated by the discovery of additional dwarf planets. Dwarf planets are hard to find, and there may be hundreds of them awaiting discovery.
Enceladus, 0.114 N/kg (Saturnian moon)
Vanth, 0.11 N/kg (moon of probable dwarf planet 90482 Orcus)
10 Hygiea, 0.091 N/kg (4th most massive asteroid)
15 Eunomia, 0.08 N/kg (large asteroid, with ~1% of mass of asteroid belt)
Miranda, 0.079 N/kg (moon of Uranus)
Here is the ninth group of five:
Nereid, 0.072 N/kg (Neptunian moon; irregular in shape)
Proteus, 0.07 N/kg (Neptunian moon; irregular in shape)
Mimas, 0.064 N/kg (Saturnian moon / smallest gravitationally-rounded object in the solar system)
Puck, 0.028 N/kg (6th largest moon of Uranus)
Amalthea, 0.020 N/kg (5th largest Jovian moon)
Finally, here are “g” values for the two tiny moons of Mars, included because they are nearby, and are the only moons Mars has to offer. A more exhaustive search would reveal many asteroids and minor satellites with “g” values greater than either Martian moon, but smaller than Amalthea, the last solar system object shown in the last set of five.
Phobos, 0.0057 N/kg
Deimos, 0.003 N/kg
It’s interesting that Saturn is less dense (on average) than water at STP, yet still has such a high “surface” gravity.
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Your comment makes it sound as though you believe the Earth is comprised mostly of water. This is true only for the surface: most of Earth’s mass derives from its molten core and the thousands of kilometers of dense rock in between the core and the surface.
What’s *really* interesting, thus, is that Saturn manages to have more surface gravity than Earth despite being made of gas. One would think gas is much lighter than rock, so it seems counterintuitive. However, gas is also much, much more highly compressible than rock. I would assume the gas density near Saturn’s core is absurd. Saturn is able to pack much more matter in a much tighter space than Earth. Thus how it has more than enough mass to compensate it’s higher radius, and have a greater surface gravity than Earth despite being made of lighter stuff.
That’s how I reason it, at least.
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The earth, on average, is over 5 times as dense as water at STP, of course due to its rocky core. The other gas giants, i.e. except Saturn, are more dense than water at STP despite being made of gas. I find all these things quite interesting indeed. That Saturn is a gas giant less dense than water at STP I find even more interesting. The gas giants have layers and slightly different compositions (perhaps even with a very tiny, percentage-wise, rocky or iron core). The sun, primarily consisting of hydrogen and helium (but also layered), shows things at an extreme level. For example, on earth, hydrogen and helium gases are less dense than air. But in the sun, on average, hydrogren and helium gas (really, plasma, as they’re ionizied) are more dense than water at STP (by 40%). Of course, the gas giants and sun derive their huge gravitational fields from their enormous masses, made possible by their extreme volumes, and benefit from the fact that gravity goes as 1/R^2 (inverse square) whereas volume is proportional to R^3 (R cubed). Unlike the sun, however, the gas giants lie outside the frost line that divides the warm, rocky, small inner planets from the cold, gaseous outer planets (and both sets of planets are surrounded by planetesmals, rocky asteroids in one case and an icy Kuiper Belt in the other). Though it’s easy to understand and reason, I still find it all quite fascinating. 🙂 (By the way, my background is particle physics. We are probably both drawn to Robert’s fascinating geometry blog for similar reasons.)
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Very deep stuff and also very interesting. The thing that surprised me the most was that Uranus was not in the same group with the other gas giants.
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