The planet is a barren rocky dwarf planet which orbits at 0.05 AU from its star of 0.9 solar masses, at 6 billion years old. Because of this, it’s surface is melting and evaporating, forming a thin atmosphere, which is continually being blown off by solar wind, recondensing into dust which is spread throughout the star system.
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1$\begingroup$ how long do you want your planet to exist? As depicted, it is melting away at a rather fast rate... $\endgroup$– TrishCommented Nov 9, 2022 at 14:16
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1$\begingroup$ What do you mean by "feasible?" Do you mean, "can exist in the Real World?" Or do you mean "exceeds suspension-of-disbelief?" Humanity knows no example of a world like this other than Mercury and no example of outgassing like this other than comets in our own system. If you're looking for "can exist..." then you need to provide complete characteristics for the star and should provide the basic chemical components of the planet. Note that an evaporating rock surface won't create a breathable atmosphere. However, if the dust is the end goal of this idea... it's a cool idea. $\endgroup$– JBHCommented Nov 9, 2022 at 16:25
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$\begingroup$ +1, interesting question. I've atempted to put an acceptable story building answer for your world building problem. $\endgroup$– GoodiesCommented Nov 12, 2022 at 11:20
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5 Answers
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Yes hot hotness. Rock comets are close to what you describe. From previous answer:
The plausibility of Rock comets and Molten Asteroids
https://astronomy.com/news/2021/09/sodium-may-make-asteroid-phaethon-fizzle
Aptly named after the son of the Sun god in Greek mythology, Phaethon has a 524-day orbit that brings it within just 0.14 astronomical units — where 1 AU is the average distance between the Earth and Sun — of our star, well within Mercury’s orbit. At that distance, the Sun heats the asteroid’s surface to about 1,390 degrees Fahrenheit (750 degrees Celsius). While any water, carbon dioxide, or carbon monoxide ices just under the surface would have evaporated long ago, sodium — an element abundant in asteroids — could be fizzling just under its surface.
Phaeton and the similar "rock comet" Icarus are stony bodies that get very close to the sun. Each of these bodies is thought to have given rise to a cloud of little fragments that rain down on the Earth as meteor showers - The Geminids from Phaeton and the Areitids from Icarus. They get so close to the sun that comety stuff like ice is long gone. There is a thought that boiling sodium inside them might blast off fragments that turn into the meteors which is what you are thinking about - stony stuff melting because of the heat.
But Phaeton is twice the distance from our sun as your planet is from its star. The Parker solar probe is going to get as close as you want: 0.04 AU
https://www.nasa.gov/content/goddard/parker-solar-probe-humanity-s-first-visit-to-a-star
The spacecraft will fly through the Sun’s atmosphere as close as 3.8 million miles to our star’s surface, well within the orbit of Mercury and more than seven times closer than any spacecraft has come before. (Earth’s average distance to the Sun is 93 million miles.)...
At closest approach to the Sun, the front of Parker Solar Probe's solar shield faces temperatures approaching 2,500 F (1,377 C). The spacecraft's payload will be near room temperature.
To perform these unprecedented investigations, the spacecraft and instruments are protected from the Sun’s heat by a 4.5-inch-thick (11.43 cm) carbon-composite shield, which needs to withstand temperatures outside the spacecraft that reach nearly 2,500 F (1,377 C).
Sodium and sulfur will be long gone. What remains would be nickel and iron and silica. Oxides of aluminum and magnesium will not melt at 1377 but the metals will. The thin atmosphere you describe will be metal gases.
Note that so close to the star your planet will be tidally locked. It will have a cooler dark side. The evaporated metals may rain down on the dark side. Like this giant exoplanet which is hot and close to its star like your planet.
https://earthsky.org/space/wasp-76b-exoplanet-iron-rain-espresso/
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$\begingroup$ This is a great explanation! Although I didn’t include this information, the planet is tidally locked, I predicted the atmosphere would be at least partially metallic, and I had suspected what you described by evaporated metals depositing on the dark side. I also had not previously heard of the rock comets you described, and I was thinking of making it cause seasonal meteor showers, and metal seeding on other planets. I wonder what would happen to a probe if it were to land on the dark side, maybe it would slowly become encased in metal. $\endgroup$ Commented Nov 11, 2022 at 13:48
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$\begingroup$ / if it were to land on the dark side, maybe it would slowly become encased in metal./ Probably not that slowly - I suspect that it would find the surface to be liquid or a soft solid. It will sink in. As the iron rain begins to fall... $\endgroup$– WillkCommented Nov 11, 2022 at 22:16
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It's a napkin stretch, so not much information
What is the surface temperature of the planet? If it is high enough, then the rocks can get hot enough to evaporate and outgas from the planet.
If your planet has a sufficiently high escape velocity, then it can retain rock vapor on its surface. That means you need to specify the mass of the planet, so that the escape velocity can be calculated.
What is the loss rate of the planet's mass? This is useful in determining the lifespan of the planet. Too high and the planet evaporates away in just a few million years. Too low, and the planet is going to last for billions of years.
0.05 AU sounds too small for a orbit. Although there are a few planets that orbit much closer (WASP-12B at 0.02 AU), their orbits tend to be highly unstable and they quickly fall into the parent star. You need to make the orbit a bit bigger, and a bit stabler. Still hot as lava, but less chances of falling into parent star.
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$\begingroup$ Yeah, even knowing physical characteristics the rate of attrition would be very hard to determine. At 0.05 AU (~7.5M km), it orbits well outside the bulk of the star's atmosphere. The planet would need to intercept a large proportion of its own mass in the medium to deorbit, so I wouldn't worry too much about it. (I'd ballpark at least a billion years.) $\endgroup$– BMFCommented Nov 9, 2022 at 17:13
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Don't worry about the distance
Don't both trying to calculate the distance to the Sun. The exact number of astronomical units has no qualitative effect on the world or story.
What I can tell you is that a suitable distance certainly exists. And I'll tell you how I know too:
Put your planet too far away and it will not evaporate at all. Put it too close and it evaporates in ten minutes. Somewhere in between is a distance where it takes 10 billion years to evaporate fully.
Put your planet there.
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1$\begingroup$ This is good worldbuilding advice. Don't stress the details. (I'm extremely guilty of it lol.) A planet so close that it actively disintegrates from raw solar output is a completely reasonable setting imo. $\endgroup$– BMFCommented Nov 9, 2022 at 17:19
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$\begingroup$ @BMF This is one of those "can this happen?" questions where the answer is an obvious "yes". It only becomes interesting once you ask "and then what?". For example "Can we have the outer surface burn off while keeping the inside solid?" $\endgroup$– DaronCommented Nov 9, 2022 at 19:18
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Silicate rock starts melting at around 1,200 °C. Mercury has a surface temperature as high as 430 °C. Your planet is around 9.5x closer to its solar analogue than Mercury is to the Sun. If mean solar output is roughly the same, your planet recieves something like 90x the insolation that Mercury does.1
Without going into much math, I think it's safe to say there will be a lot outgassing coming from your world.
1 that's likely an overestimate because at these distances the disk of the star takes up an appreciable fraction of the sky. But, it'll still be a heck of a lot.
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$\begingroup$ Not to mention the lack of atmospheric pressure, though I can’t find any straightforward phase diagrams for silicate rock. $\endgroup$ Commented Nov 11, 2022 at 13:36
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$\begingroup$ @SeanHolm Yeah, I thought of that, but I don't think atmospheric pressure will have much influence on the phase of molten rock (maybe if you're dealing with >1 bar, which in these conditions I doubt). Molten rock should be hotter when exposed to near-vacuum as it can only lose heat via radiative cooling, whereas in atmosphere, the air steals some of the heat in addition to radiative cooling. $\endgroup$– BMFCommented Nov 11, 2022 at 17:19
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$\begingroup$ There'll be some kind of atmosphere, severe outgassing would contribute to it, but it's just hard to say how thick it could be. $\endgroup$– BMFCommented Nov 11, 2022 at 17:20
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Story in the present, something happened in the distant past
Think of your story time window
How much "past" would you need ? You can have a planet that age, but you don't need this "lava planet" to exist there for 6 billion years. You just need a planet that is 6 billion years old. When your story has a time window of say, thousands of years, the situation can have developed like you describe. Just put an explanation for it, e.g.
Unfortunate stellar dynamics in the distant past caused this
Something happened a few million years ago. Due to a near-collision with a neighboring planet, your poor dwarf was moved into its current apocalyptic orbit. At first, when the near collision happened, it need not be a mass extinction event: apart from the view, the unusual irregular temperatures and seasons, no one on your planet noticed what had happened, until it got very warm. Nowadays, everything turned into lava and the dwarf planet is approaching its definite end: but your planet could exist for another, say 100 thousand years, before it has evaporated, or spiraled into the sun. Your story will have a lot of time to develop, your lava planet age (and its past) can be maintained.
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$\begingroup$ 100k years is too fast imo. The planet is too far outside the stellar atmosphere to be abraded or deorbited by it. It's main assailant is 90x the sunlight Mercury gets, which would def create some outgassing but not enough to blow a planet to dust on that timescale. $\endgroup$– BMFCommented Nov 12, 2022 at 13:58
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$\begingroup$ If the planet is Earth-sized, 100k years represents about 64 meters of surface abrasion per year. $\endgroup$– BMFCommented Nov 12, 2022 at 13:59
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$\begingroup$ @BMF my answer is about the time frame.. it could be longer as you say, or much shorter. My proposal is to let this planet exist, but it has not always been at the location it is now. Story writer must fillin the details.. $\endgroup$– GoodiesCommented Nov 13, 2022 at 12:09