# How could a smaller planet than Earth have a higher gravity?

I want to build a planet (or satellite) that:

• is smaller than Earth,
• has a thicker atmosphere than Earth but breathable,
• has neither intense volcanism, nor any extreme condition of that sort that would increase atmosphere density,
• revolves around a binary star similar to BY Draconis

A higher gravity makes for a higher atmosphere density. If feels like this is the best option in order to keep the planet a peaceful place, hence my question.

Question:
How is that possible? Is it coherent in such a system with at least two (small young) stars?

Bonus questions: What would the stellar system be like? Would the other telluric planets in the system necessarily look the same? Would there still be gas giants?

• Is the planet orbiting components A and B, or component C? – HDE 226868 Dec 7 '14 at 13:43
• Well, both? It is said on the wiki that they are close to one another... – dyarob Dec 7 '14 at 16:53
• No. A and B are close together, while C is 260 AU away (give or take). – HDE 226868 Dec 7 '14 at 16:54
• Have a look at neutron stars. They're not planets but they're dense and they'd give you some serious gravity. If you can reapply the principles you're sorted. – ArtOfCode Dec 7 '14 at 23:04
• Earth is already more dense than usual for a planet of its size. It is the densest planet in our solar system. – Philipp Dec 8 '14 at 15:27

I think the others have done a good job of answering the main question of what it would be composed of (I'd have said iron, too, or something similar), so I'll address some of the other stuff.

The BY Draconis system is young. Really, really, young. Components A and B certainly aren't well developed, because they haven't yet exited the protostar phase of their lives. As far as I know, there is no protoplanetary disk in the system. A good rule of thumb is this: No protoplanetary disk $\implies$ No planets. This would seem to rule out this entire scenario - at least at this point in time. I'm also doubtful that the system could capture any rogue planets. They just aren't old enough for the probability of that to happen.

Any planet orbiting component A or B would not be conducive to life. These two stars constitute a BY Draconis variable. This means that there can be drastic changes in luminosity due to surface activity, such as star spots (the extrasolar equivalent of sunspots). Variable stars in general aren't great for life, because of their variability. Some are periodic, though, which does make them regular, but BY Draconis variables are not periodic.

This doesn't rule out component C, though. It appears to be a red dwarf, far out from the two others. (The whole system reminds me of a younger version of the Alpha Centauri/Proxima Centauri system) The danger here is that if the red dwarf is a flare star, it, too, may not be friendly to life. Also, I'm unsure of how easy it would be for a planet to get out here, given that much of the system's mass is at the center of the binary pair, meaning that they would be more likely to scoop up any possible planet-forming material.

So I highly doubt that any planets could form in the system, and if they did, they most likely would (certainly at the moment) not be habitable. I think this also rules out the "other planets" part of your question, although Wikipedia does say this:

There may be a fourth component to the system, orbiting with a 114 day period, but this has not been visually confirmed.

So that gives us some hope.

• So iron is most common heavy element, and by definition is created in old stars by nova and supernova explosion. So you are right, answer is "substantially denser planets are very improbable, especially around young stars". Physics. It just works. – Peter M. Dec 7 '14 at 19:00
• @PeterMasiar IIRC, it's elements heavier than iron that are produced by supernova. Iron is the last and heaviest element created in the normal fusion processes, before one of the end-of-life processes (brown dwarf, supernova, neutron star, black hole, etc.) – RBarryYoung Dec 8 '14 at 2:33
• @RBarryYoung Iron is produced in some supernovae, but you're right that in general, it's the heavier elements that are produced. – HDE 226868 Dec 8 '14 at 2:38
• @Peter Why would it be uncommon around young stars if the materials are ejected by dying stars? I mean, these stars are dead, so what prevents new stars to form near them and attract some of this heavy material? Is it just that we are too far away from the galactic center? – dyarob Dec 8 '14 at 12:32
• @dyarob Young stars form in giant molecular clouds (see the Wiki article), which aren't necessarily near supernova remnants or dead stars. – HDE 226868 Dec 8 '14 at 15:59

It is unlikely for a natural planet to have a density significantly higher than iron. Barring some outside force elements lighter than iron will be more common than elements heavier than iron. A collision of two planets might work. The collision could separate a part of a core from the rest of the mass, probably into a moon. The core fragment could, I suppose, have density higher than iron. You'd then need a second "just right" collision to strip the moon of lighter materials ejected at the same time. I don't think it is flat out impossible, but it is very unlikely.

Incidentally, gravity and atmospheric density are not directly related, but atmospheric composition and gravity are. So you'd actually want only near 1G gravity, not something significantly higher, which should help. Also "breathable" puts a limit on the atmosphere. It might be better not to go exotic here,and just have a smaller, denser Earth.

• The only scenario of two planets colliding I know about is the impact theory for the formation of the moon: en.wikipedia.org/wiki/Giant_impact_hypothesis It doesn't end up with the creation of a very heavy satellite. How do you see this happen to form a heavy moon? – dyarob Dec 7 '14 at 11:54
• @dyarob You need two collisions. Both "just right" in an unlikely way. – Ville Niemi Dec 7 '14 at 12:04
• You think that if all the light material was removed from the moon, its gravity would be higher than that of Earth? (I'm not sure I follow you here) – dyarob Dec 7 '14 at 12:14
• Maybe a collision with the naked core of a dead planet would make the whole planet a lot heavier in just one impact? (gizmodo.com/…) – dyarob Dec 7 '14 at 12:16
• @dyarob True. Unfortunately elements denser than iron are relative rare. I am basically betting that 2+ planets of starting materials would have enough raw materials to make something significantly denser than earth possible in planet-size. – Ville Niemi Dec 7 '14 at 13:34

Surface gravity is a function of both the mass of the planet and its radius. By Newton's law of gravitation, the force experienced by a mass at the surface of a planet with mass $M$ and radius $r$ is proportional to $M/r^2\!$. If the planet's density is $\rho$, then its mass is proportional to $\rho r^3\!$, so the gravity at the surface is proportional to $\rho r$. Therefore, if you want a smaller planet than Earth to have higher surface gravity, you're going to need it to have higher density.

One way to achieve that might be to have a proportionally larger core than the Earth's, or denser crust. But I've no idea how feasible that would be.

How could a smaller planet than Earth have a higher gravity?

The easiest way is to make the core mostly Gold. Now is this likely? No, Is it possible? Yes. It would certainly be fun to have a 'gold' planet in a sci-fi world.

• For planets made of solid gold, contact Magrathea. – Clockwork-Muse Dec 7 '14 at 14:49
• Maybe King Midas touched this planet. – Jarred Allen Dec 16 '16 at 0:54

It is possible. You would need either:

1. A larger core relative to the mantle.
2. A denser core, perhaps gold, lead or uranium.
3. An accretion around some ancient alien tech, perhaps a gravity drive.
4. Some other source of mass. Perhaps the planet is an extrusion into our universe of a 4 dimensional hypersphere. Perhaps it is locked with a larger object shifted sideways out of our dimension.
• Uranium core could have certain peculiar side effects – PTwr Dec 8 '14 at 8:50

I'm not an expert in this, but it seems to me that if the planet has a heavier core than Earth's, it would also have higher gravity.

Earth has an iron-nickel alloy core. A look at a periodic table shows several elements heavier than iron, some of which might be suitable as a planet's core.

• That's an interesting point, but is it even possible to have a telluric planet core with a significant amount of heavier elements than iron? I doubt so, but I found out that a high initial metallicity of the accretion cloud results in more metal in the stars and planets born out of it (www2.cnrs.fr/en/703.htm) – dyarob Dec 7 '14 at 12:11
• @dyarob: Consider conjectured "iron planets": en.wikipedia.org/wiki/Iron_planet It needn't be a question of having a high proportion of heavier-than-iron elements, rather having much less silicate. For example Mercury's density is about the same as Earth's, but an Earth-mass body with Mercury's composition would be denser than Mercury (because higher pressure in the middle). By, ummm.... 20% or so? (en.wikipedia.org/w/…). So it would be smaller than Earth and hence have higher surface gravity. – Steve Jessop Dec 8 '14 at 1:41

To have a gravity equal to or greater than earth, the planet would have to have a mass equal to or greater than earth. Given the gravitational effects at play with that kind of mass and binary stars, you will have volcanic activity.

• Hi, Geo., welcome to Worldbuilding! (FYI, your answer's fine; I'm not criticizing it - don't get the wrong idea here) How would the volcanic activity follow from the greater mass, or the binary stars? – HDE 226868 Dec 7 '14 at 23:52
• Unless the planet has a solid core, which would be purely fiction with no science involved, the gravitational forces of the planet interacting with a binary star would cause more volcanic action that we have on our planet just interacting with the sun and the moon. In theory, by eliminating the moon and making the planet's day equal to a full year (i.e., the same side of the planet always faces the star it orbits), you could minimize volcanic activity to below the noise level. But, given that you have a binary star, that is not possible. – mcgeo52 Dec 9 '14 at 6:06
• To have volcanic activity related to the Star's attraction, you would have to be real close to the star (AFAIK, in the Solar System it has only been observed in a few of Jupiter's moons). Even if BY Draconis appears to be slightly colder than Sun, I doubt you could get an inhabitable planet so close to it. And the two main stars appears to be so close between them that planets around them probably just get the same pull as they would get from a single object at their barycenter. – SJuan76 Dec 9 '14 at 14:37

# Make it a Failed Alien Experiment

As other answers have put it, it's pretty unlikely an object similar to your description will naturally occur. So my suggestion to you is to make it into a failed alien experiment. This will add mystery to your story, and as a bonus makes great grounds for prequels and sequels.

Some long extinct alien race have accumulated a large amount of very dense material in an orbit around the system. About a million years have passed now, in which time comets and the remnants of the proto-stars have created a very thin (~50-100 meter) crust on the object. Atmosphere was also created by the aliens, who seem to have breathed Oxygen, just like humans.

Densest material known to humans is the quark-gluon plasma, but that might be too exotic, and needs special containers, which are unlikely to survive million years. The densest known element is Osmium, and Iridium follows closely. If we use an Osmium-Iridium alloy, the density will be slightly more than 2.2 kgm-3. If we account for the thin crust and settle for a mean density of 2.2 kgm-3, for a planetoid with 0.25 Earth radii you will get a gravitational acceleration of very nearly 1 g (9.78 ms-2). For a Moon-sized planetoid, gravity will be 10.68 ms-2, and will give you the slightly dense atmosphere you wanted.