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?
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1 answer
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.
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