Is it possible for a rocky planet to have the same diameter as a star?

  1. I love the link to the figure but i don't think it's quite so simple. I'll see if I can dig up a reference later, but I think you might be able to get there with a rocky planet that managed to hold onto its primordial atmosphere, since it could have a very inflated radius with enough instellation. If the star were a very late and very old M-dwarf, so like 1 Jupiter radius (or is that just for brown dwarfs?), then maybe you could approach approximately equal radii.

  2. The limit on a planet's diameter comes from its mass, composition (ie: what elements), and especially for gaseous planets temperature. I will constantly suggest

  3. No. The accretion of a planet like this would generate so much heat that even if it could gather that much rocky stuff it would never cool. It would, at the very least, be a gigantic molten planet so hot and bright that it would look a lot like a star. It might even be a star if it starts fusing elements in its core. It might even get hot enough that it boils all that iron and other heavy elements. Whatever it would be, it wouldn't be a "planet" as we think of them.

  4. I read somewhere that the upper limit on a rocky planet forming naturally is probably 2-4 times Earth radii.

  5. I like this question. I would have never thought to ask this. I'm very interested in these comments.

  6. I just googled how much mass is needed to make a black hole. Google says 2-3 times the mass of our sun.

  7. There are plenty of celestial objects bigger than that though. Our Sun is not very big at all, really. Did it say how dense that material had to be?

  8. Let's take the average density of a rocky planet (e.g. Earth) at 5.5g/cm^3 or 5.5 * 10^3 kg/m^3 and look at the mass this would give us for something like the sun:

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