12
$\begingroup$

Suppose I am an alien, living on a distant planet, a very BIG distant planet.

One day, I thought of making a planet on my own. I went to a nearby land and using my ultra heavy duty land moving machine, drilled out some $10^{26}$ kilos of mud. Then I dumped it into my space vehicle, and went off in search for a gravity free space.

Using my supersensitive gravimeter, I found a place, and opened my vehicle and set all the mud to float freely in space.

I knew that the particles would eventually coagulate, according to the $\frac{Gm_1m_2}{R^2}$ rule .

Given that my life time is very long,

Will I ever get to see a planet out of my mud ?

Now, if I wait long enough,

Will I make a Star out of my planet?

Thinking a little deeper,

Will My Planet Be Watery..?

Since water evaporates AND suddenly freezes in space, according to Water in vacuum (or space) and temperature in space,

The water in the mud instantly vaporizes and spreads out. WHICH MEANS, I won't get a watery planet after-all;

BUT the same link says the water recrystallizes into ice , once thrown out in space.

DOES THAT mean, that the ice would return to the planet due to its gravity, and due to pressure, form water, making my planet watery again...?

$\endgroup$
  • 11
    $\begingroup$ Would Worldbuilding.SE be a better place for this question? $\endgroup$ – Ruslan Sep 24 '17 at 13:52
  • 5
    $\begingroup$ You're not going to end up with a mud planet--the pressure will drive the water out of the interior. You're going to end up with a planet with a deep ocean on top. $\endgroup$ – Loren Pechtel Sep 24 '17 at 22:19
14
$\begingroup$

$10^{26}$ kg is about 5% of the mass of Jupiter and much larger than the mass of the Earth. Gravitational contraction would ensure that your mass of "mud" would indeed form a (roughly) spherical planet. The timescale upon which it does so is entirely dependent on what sort of radius you began with. The "dynamical" freefall timescale is given approximately by $(G\rho)^{-1/2}$, where $\rho$ is the average density.

Hence if the initial mass of $10^{26}$ kg was spread over an astronomical unit in diameter, then it would take about 15 years. However, this timescale is a lower limit to the time it takes to settle into an equilibrium configuration, since it assumes the material can fall freely. As it becomes more compact, the material will heat up considerably - according to the virial theorem about half the released gravitational potential will be radiated away (largely in the infrared) and half will heat the material up. The interior will become a gas/fluid and exert a pressure that will slow the contraction.

Whether the contracting mass can then initiate nuclear fusion and become a star depends on a competition between whether the temperature can become hot enough to allow nuclei to have enough kinetic energy to approach each other closely enough to interact via the strong nuclear force (although quantum mechanical tunneling is of vital importance here) and whether the density becomes so high that the material is supported by electron degeneracy, whereby the Pauli exclusion principle demands that electrons in the gas occupy different quantum states and results in an almost temperature-independent pressure that prevents further contraction. In the latter case, the "planet" could simply cool-off at roughly constant size and nuclear fusion would never occur.

By convention, "a star" is defined as a body which can initiate the fusion of hydrogen (protons). Assuming your mud contains water and hence plenty of hydrogen, then in a pure hydrogen gas, the threshold mass at which gravity wins this particular battle and the gas heats up sufficiently to start hydrogen fusion is around 75 Jupiter masses. But "mud" also contains a mixture of carbon, silicon and oxygen. In this case the threshold will be much higher because the Coulomb barrier between a pair of carbon nuclei is much higher than between a pair of hydrogen nuclei. Roughly speaking, a mass of a bit more than the mass of the Sun is capable of being supported by electron degeneracy alone before carbon fusion can occur.

Your mud is somewhere between these two extremes - it has hydrogen to fuse, but most of its mass is carbon, oxygen and silicon. In any case $10^{26}$ kg is too low by orders of magnitude to become a star. There is a lower threshold at about 13 times the mass of Jupiter where the deuterium (a trace isotope of hydrogen) would fuse at lower temperatures than hydrogen. However, this is not classed as "a star" since the entire deuterium content is burned in (astrophysically-speaking) a short time period of a few tens of millions of years.

$\endgroup$
9
$\begingroup$

The entire life history (  path on the HR-diagram https://en.wikipedia.org/wiki/Hertzsprung%E2%80%93Russell_diagram  ) of a star is determined by its initial mass, chemical composition, and angular momentum. Too much angular momentum and all your initial mass goes spinning off into space. But "float freely" sounds like no angular momentum. So, depending on your definition of "planet", you'll certainly see any such initial mass contract, and $10^{26}\mbox{kg}$ sounds like enough to make it pretty much spherical.

But even if your "mud" were pure hydrogen, that's not enough mass such that gravitational contraction would heat it up enough to "ignite". So no star. But some off-main-sequence stars can eventually burn carbon (  https://en.wikipedia.org/wiki/Carbon-burning_process  ), so if you bumped up your mass enough, and if "mud" is carbon plus some trace elements, it might just start off with that carbon process. But I don't offhand know what initial mass (of carbon) would be required for that. (For an initial mass of hydrogen, and if memory serves, you need about ten Jovian masses for ignition, which is $\sim2\times10^{28}\mbox{kg}$.)

$\endgroup$
  • 3
    $\begingroup$ Paragraph 2 is inaccurate, in that deuterium fusion is not the requirement for "starhood". The requirements for hydrogen burning are much greater and those for carbon burning even an order of magnitude more. Also, you have not explained why gravitational contraction does not heat the core to the required temperature. $\endgroup$ – Rob Jeffries Sep 24 '17 at 9:04
  • 2
    $\begingroup$ @RobJeffries Okay, so please post a more complete and correct answer. But, if memory serves again, even 2nd (and higher) generation stars, with initial compositions containing non-negligible carbon, start off burning hydrogen on the main sequence. And nothing but hydrogen until they leave the main sequence. Wouldn't they ignite carbon first if the pressure/temperature required were lower? Especially since the heavier carbon would displace hydrogen at the core. $\endgroup$ – John Forkosh Sep 24 '17 at 9:14
  • $\begingroup$ Please read my comment more carefully, that is not what I said. $\endgroup$ – Rob Jeffries Sep 24 '17 at 13:02
2
$\begingroup$

If you take all your mud in one trip, then the planet will start to form inside your vehicle and by the time you find the place, it already close to being a planet.

But if you do it in multiple/numerous small trips, then, even if you find a place to float freely, you have to know how the first dump is moving wrt your big planet so that you can locate it when you bring over the second load.

Suppose you are able to dump all the trips pretty close to one another.

Even in this case, it seems, by the time you are done dumping, it should already be very close to be a planet because the formation will not wait for you to finish dumping.

As others have said, it will be a watery planet. You better make it orbit in habitable zone of some mid age sun like star.

You will not ever see a star from this. No calculations are needed here. Because, you are digging a single "big planet" to make a new planet. That says it all.

$\endgroup$
  • $\begingroup$ I doubt the existence of a watery planet. Water doesn't exist as a liquid in space. Hence,it would evaporate away as soon as the mud enters space, AND, recrystallize to ice particles. But wait... can the ice particles get attracted by the mud planet due to its building gravity, and RETURN to form the watery planet again...? $\endgroup$ – Krishnanand J Oct 1 '17 at 15:06
  • $\begingroup$ @KRISHNANANDJ: I think you have a point. Thinking about it more, It can be more complex though. If the water freezes (due to temperature) then it would not evaporate. If it is hotter, then the outer layer would dry first thereby trapping the wet mud inside. Liquid water will distill up due to gravity only after the planet has significant mass. At that point there likely will be enough gravity to hold the water on surface. This would require more complex calculations. That is why, you have a good point but can not say for sure. $\endgroup$ – kpv Oct 1 '17 at 17:10
  • 1
    $\begingroup$ @KRISHNANANDJ: It will depend on temperature and the dumping mechanism etc. One special example - if you dump in few trips, say 4, there will always be enough gravity that water will stay, provided temperature is suitable. Re-reading your question, it seems you are doing it in one trip, so it will be a watery planet if temperature allows so. $\endgroup$ – kpv Oct 1 '17 at 17:13
0
$\begingroup$

Your question depends on what you define as a "planet". If a "planet" is just a huge sphere high in kilograms, then the mud would definitely form a planet.

In your situation, with "no gravity", the only gravitational forces would be between the mud bits. This would inevitably cause them to all come together, and slowly form a more and more perfect sphere.

If you defined a planet as something made up of more than "mud molecules", then you wouldn't be able to make a planet.

As for making a star, it's tough to say. Stars need tremendous amounts of heat in order to create fusion. When the mud bits collapse in on each other due to their gravity, this could cause the core to heat up. Eventually, the core could become a star. However, based off my research this seems to be mainly the case with dust and gas collapsing in on each other.

This NASA article explained it pretty well to me:

https://science.nasa.gov/astrophysics/focus-areas/how-do-stars-form-and-evolve

$\endgroup$
  • $\begingroup$ The mud ball would have a high pressure inside. It would also melt inside (heat from gravitational energy and from decay of radioactive elements in the mud). $\endgroup$ – Pieter Sep 24 '17 at 10:07
  • $\begingroup$ @Pieter, Yes, I suppose the pressure and heat from the gravitational energy could cause the core to melt. This would satisfy our gas requirement. It seems as if creating a star from mud may be more plausible than first assumed. $\endgroup$ – Inertial Ignorance Sep 24 '17 at 10:10
  • 2
    $\begingroup$ The mass mentioned by the OP is less than Saturn's mass, so it could not become a star (with nuclear fusion) by several orders of magnitude. $\endgroup$ – Pieter Sep 24 '17 at 12:51

protected by Qmechanic Sep 24 '17 at 14:23

Thank you for your interest in this question. Because it has attracted low-quality or spam answers that had to be removed, posting an answer now requires 10 reputation on this site (the association bonus does not count).

Would you like to answer one of these unanswered questions instead?

Not the answer you're looking for? Browse other questions tagged or ask your own question.