Dec 11, 2025
How long does it take to grow plants
If you’re Earth, about three billion years.
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How long does it take to grow plants
If you’re Earth, about three billion years.
Worldbuilding: Something I Learned About Planetary Surface Gravity
Last year (hehe; it's always fun to say that at the start of a new year; actual time about 4 months ago), I made a strange discovery!
I won't slow-roll you. Here's the bottom line: In reality, i.e. based on observational data, there appears to be a plateau of surface gravity for planets up to about 500 times more massive than Earth. Most planets between 1 Earth mass and ~500 Earth masses have a surface gravity that is close to our own, i.e. 1 g. So if you're worldbuilding, and need a planet with a surface gravity that is more than a few percentage points higher than Earth's, be careful.
All right, on to the story!
I had need of such a planet for Galaxy Federal. About 125% percent or so, i.e. 1.25 g. And it was in the back of my mind that Neptunian planets often have a "surface" gravity similar to that of Earth's. "Okay," I figured, "that's probably because their giant atmospheres reduce the average planetary density. On a rocky world that wouldn't be an issue. But I'd better check to be safe."
Oh, it is always a good idea to check!
I found a paper which was reporting the plateau that I mentioned in the TL;DR. Here's what it looks like:
Gravity vs mass for Solar System bodies and exoplanets. Ballesteros, Fernando & Luque, Bartolo. (2016). Walking on Exoplanets: Is Star Wars Right?. Astrobiology. 16. 10.1089/ast.2016.1475.
What they found is that Super Earths, i.e. rocky planets with a mass greater than Earth's, did not exhibit the increase in surface gravity seen in smaller celestial bodies. There is this "plateau" where increases in planetary mass and radius are such that the surface gravity essentially levels out for a while.
This isn't something I think you could predict from amateur astronomical principles. What you would predict (certainly, what I predicted) would be what we see on the left side of the above figure, where more mass equals more surface gravity. All of the small rocky bodies in our own Solar System fall onto that left side of the graph. The plateau only really begins in the Earth range of planetary mass. You can see how Venus is included in the plateau almost perfectly, but the much less massive Mars is not.
This is a truly remarkable finding about planetary formation, and has serious implications both for fictional worldbuilding and someday for planetary colonization. That is, if these observations are truly representative of the Universe. This is a pretty big "if" that I don't have the knowledge to be able to check, except to say that there may be some bias unaccounted for because of how we detect exoplanets, such that perhaps the ones we detect are not representative of the ones we do not—so take it all with a grain of salt. This paper is 8 years old but I couldn't find anything newer following up on it.
Fully understanding how it would work is still a question mark for me. I mean, it's a question mark for the professional planetary scientists, too, so there's that, but, in addition to the unknowns that curtail their comprehension, there's a lot I don't know about astronomy that further curtails mine. My understanding is that this isn't really about the intrinsic density of the materials (i.e. substances that are more massive or less massive per mole); it would mainly have to be a consequence of how planets form, how the geological processes change in different planetary mass categories; that sort of thing. But I don't really know! I'm out of my depth.
Anyway, you can download a copy of the full paper on its entry page on ResearchGate; look for the button that says "Download full-text." And the website Centauri Dreams has a nice write-up which is where I originally discovered this.
"A Rocky Planet Forms
An artist's rendition of how a rocky planet forms.
As a rocky planet forms, the planet-forming material gathers in a process known as 'accretion.' It grows larger in size, and increases in temperature, along with the pressure at its core. The energy from this initial planet forming process causes the planet's elements to heat up and melt. Upon melting, layers form and separate. The heavier elements sink to the bottom, the lighter ones float to the top. This material then separates into layers as it cools, which is known as 'differentiation.' A fully formed planet slowly emerges, with an upper layer known as the crust, the mantle in the middle, and a solid iron core.
InSight is short for Interior Exploration using Seismic Investigations, Geodesy and Heat Transport. The InSight mission will help answer key questions about how the rocky planets of the solar system, as well as how rocky exoplanets, formed. So while InSight is a Mars mission, it's also more than a Mars mission.
The lander seeks the fingerprints of the processes that formed the rocky planets of the solar system, more than 4 billion years ago. It measures the planet's 'vital signs:' its 'pulse' (seismology), 'temperature' (heat flow) and 'reflexes' (precision tracking)."
Date: January 25, 2018
NASA ID: PIA22233
The Making of BEASTies
Astronomers studying the formation of exoplanets had noticed that there is an odd abundance of planets orbiting B type stars, giant blue stars only really outdone by their O type siblings. Both O and B type stars live very short lives and pump out so much radiation, that planetary formation theories had suggested that massive Jupiter style planets simply shouldn't form in such systems.
BEAST (B-star Exoplanet Abundance STudy) finds that despite the theory, there's a huge amount of far flung Jupiter (and larger) planets orbiting.
When our star formed 4.6 billion years ago (give or take a few days) it didn't do so in isolation, rather, it was born in a nebula that would have created a large amount of varying sizes of stars, a cluster, and such clusters contain a large amount of B type stars.
Being born in a cluster means, that for the first few hundred million years at least, our Sun was gravitationally bound to its birthing cluster, a large amount of stars all within 10-20 light years of one another, and in such a condensed space, material is going to be interchanged between star systems, and that doesn't just include dust and gas, it includes planets.
What has been termed the Planetary Heist, B type stars with huge amounts of gravity may not be great places to form planets, but they sure can steal them ! and this the astronomers suspect is exactly what was going on in the early days of our own system.
What's interesting about this is, it also means that planets like Jupiter and Saturn may not have even formed in our own solar system, they may have been captured planets during those early years, as too any of our planets.
Our star certainly had the ability to create it's own planets, but it may be some of those ended up out there orbiting other stars, it's also possible that some ended up wondering around the galaxy without a star at all, just waiting for a near enough flyby that could capture it.
🪐🔭✨ Giant planets spin faster than brown dwarfs — and the reason goes back to how they were born. A Northwestern University study just cracked open one of planetary science's oldest identification puzzles. Read the full story at SKYCR.ORG.
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Scientists have captured the clearest images of the interstellar comet "3I/ATLAS," revealing its bright core and tail. The findings offer new insights into planetary formation in distant star systems. 🌌☄️ #Astronomy #3IATLAS #InterstellarComet #SpaceDiscovery
Water on Rocks in Space
You will be aware of the famous line, “Water water everywhere, and not a drop to drink.” In fact it appears that water IS, if not everywhere, it is fairly close to it. The latest discovery, according to Physics World, is that water has been found on two stony S-type asteroids, which have been considered to have been formed dry. “Hydrated minerals” have been detected on hundreds of asteroids, but…
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Scientists have discovered a rare sight in a nearby star system: Six planets orbiting their central star in a rhythmic beat. The planets move in an orbital waltz that repeats itself so precisely it can be readily set to music.
This discovery is going to become a benchmark system to study how sub-Neptunes, the most common type of planets outside of the solar system, form, evolve, what are they made of, and if they possess the right conditions to support the existence of liquid water in their surfaces.
A rare resonance
The six planets orbit a star known as HD110067, which lies around 100 light-years away in the northern constellation of Coma Berenices.
Scientists have discovered a rare sight in a nearby star system: Six planets orbiting their central star in a rhythmic beat. The planets mov