<p>Escape velocity can be descibed by</p>
<p><span class="math-container">$$v=\sqrt{\frac{2GM}{r}}\tag{1}$$</span></p>
<p>Where <span class="math-container">$v$</span> is the escape velocity, <span class="math-container">$r$</span> is the distance from the mass (in case of the planet, the minimum distance is the radius of the planet), <span class="math-container">$M$</span> is the mass and <span class="math-container">$G$</span> is Newton's Gravitational Constant.</p>
<p>If the planet should have the excape velocity of the speed of light, it has to be so dense that you have to consider relativistic effects. In General Relativity, the Schwarzschild Radius of a black hole (this radius is the distance at which the escape velocity equals <span class="math-container">$c$</span>, which is exactly what you want) is described by:</p>
<p><span class="math-container">$$r=\frac{2GM}{c^2}\tag{2}$$</span></p>
<p>Whch can be solved for <span class="math-container">$m$</span>:</p>
<p><span class="math-container">$$M=\frac{rc^2}{2G}\tag{3}$$</span></p>
<p>So if you have either a fixed radius of mass, you can easily calculate the missing parameter using these equations.</p>
<p>The problem, as mentioned before, is that for a planet (or any other body) to have an escape velocity of the speed of light, it must be extremely dense. In fact, if it is dense enough to have <span class="math-container">$v_{esc} = c$</span>, the body is a black hole (Think about it - the event horizon of a blackhole if the distance where the escape velocity equals the speed of light, so anything beyond this horizon cannot escape as it would require a speed greater than <span class="math-container">$c$</span>).</p>

CC BY-SA 4.0
2020-12-06T17:38:22.913Z
It is kind of a coincidence that the Newtonian escape velocity calculation results in the the same value as the Schwartzchild radius in GR. #
James K
CC BY-SA 4.0
2020-12-06T18:14:21.117Z
@JamesK Thanks for your comment. I always thought that one of these formulae is derived from the other, but it actually doesn't make sense because you have to consider relativistic effects. I will edit my answer
jng224
CC BY-SA 4.0
2020-12-07T05:04:59.897Z
Thank you for your great explanation
Nikhil Alapati
CC BY-SA 4.0
2020-12-06T09:22:44.797Z
2020-12-06T18:17:22.600Z
2020-12-06T18:17:22.600Z
jng224
<p>As @KeithMcClary says in his comment, escape velocity depends on both mass and radius. The smaller the radius for a given mass the higher the escape velocity. So if you could somehow compress the Earth until it was just a few centimeters across, its escape velocity would approach the speed of light.</p>
<p>At the other extreme, if you filled a space twice the diameter of Earth's orbit with copies of the Earth, without doing any compressing at all, it would already be a black hole.</p>
<p>If you just pile matter onto a planet and let its own gravity compress it, you need about 2 solar masses before it is close to being a black hole (at which point it is 10-20 km across).</p>

CC BY-SA 4.0
2020-12-07T05:04:56.337Z
Thank you for your great explanation
Nikhil Alapati
CC BY-SA 4.0
2020-12-06T08:15:49.830Z
2020-12-06T08:15:49.830Z
Steve Linton
<p>What should the mass of a planet be in-order for its escape velocity to be the speed of light?  Is it even possible? What will it look like from an outside viewer? Will it even be visible in the human eye spectrum?</p>

CC BY-SA 4.0
2020-12-06T04:00:40.093Z
Escape velocity depends on mass and radius. If you keep adding rocks and atmosphere to a planet, it will collapse into a neutron star, and no longer be a planet. If you keep adding mass it will collapse to a black hole at about 2 solar masses.
Keith McClary
CC BY-SA 4.0
2020-12-06T05:14:47.310Z
*Welcome to Astronomy Stack Exchange!* This is a great question, don't worry if this is closed as duplicate of a previously posted question with an answer that also answers this one.
uhoh
CC BY-SA 4.0
2020-12-06T07:19:26.590Z
How does the linked question (can you escape a black hole) relate to this (what is the mass of body with a escape vel near speed of light)  This question is explictly not about black holes.
James K
CC BY-SA 4.0
2020-12-06T08:07:54.740Z
@JamesK Any object whose escape velocity actually reaches the speed of light is a black hole by definition.
Steve Linton
CC BY-SA 4.0
2020-12-06T14:43:14.690Z
Escape velocity doesn't really relate to why light is said not to escape from inside a black hole.  When you start talking about and "near the speed of light" and gravity you need to be using general relativity.
StephenG - Help Ukraine
CC BY-SA 4.0
2020-12-06T15:45:47.823Z
Kind of related: https://astronomy.stackexchange.com/q/39921/16685 which is about building a giant rocky planet.
PM 2Ring
CC BY-SA 4.0
2020-12-06T15:56:46.220Z
I agree with JamesK that the linked question is not a suitable dupe target for this question. And as StephenG suggests, the impossibility of escaping from a black hole is because there are no paths leading away from the BH, your velocity isn't really relevant.
PM 2Ring
<planet><mass><escape-velocity>
What should the mass of a planet be in-order for its escape velocity to be near the speed of light?
Nikhil Alapati
# What should the mass of a planet be in-order for its escape velocity to be near the speed of light?

(Asked by: username_0 on 2020-12-06)

What should the mass of a planet be in-order for its escape velocity to be the speed of light? Is it even possible? What will it look like from an outside viewer? Will it even be visible in the human eye spectrum?

**Tags:** planet, mass, escape-velocity

> **Comments:**
> **username_1**: Escape velocity depends on mass and radius. If you keep adding rocks and atmosphere to a planet, it will collapse into a neutron star, and no longer be a planet. If you keep adding mass it will collapse to a black hole at about 2 solar masses.  
> **username_2**: *Welcome to Astronomy Stack Exchange!* This is a great question, don't worry if this is closed as duplicate of a previously posted question with an answer that also answers this one.  
> **username_3**: How does the linked question (can you escape a black hole) relate to this (what is the mass of body with a escape vel near speed of light)  This question is explictly not about black holes.  
> **username_4**: @username_3 Any object whose escape velocity actually reaches the speed of light is a black hole by definition.  
> **username_5**: Escape velocity doesn't really relate to why light is said not to escape from inside a black hole.  When you start talking about and "near the speed of light" and gravity you need to be using general relativity.  
> **username_6**: Kind of related: https://astronomy.stackexchange.com/q/39921/16685 which is about building a giant rocky planet.  
> **username_6**: I agree with username_3 that the linked question is not a suitable dupe target for this question. And as username_5 suggests, the impossibility of escaping from a black hole is because there are no paths leading away from the BH, your velocity isn't really relevant.  

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## 2 answers

### Answer by username_4 on 2020-12-06. Score: 5 | Accepted answer

As @username_1 says in his comment, escape velocity depends on both mass and radius. The smaller the radius for a given mass the higher the escape velocity. So if you could somehow compress the Earth until it was just a few centimeters across, its escape velocity would approach the speed of light.

At the other extreme, if you filled a space twice the diameter of Earth's orbit with copies of the Earth, without doing any compressing at all, it would already be a black hole.

If you just pile matter onto a planet and let its own gravity compress it, you need about 2 solar masses before it is close to being a black hole (at which point it is 10-20 km across).

> **Comments:**
> **username_0**: Thank you for your great explanation  

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### Answer by username_7 on 2020-12-06. Score: 4

Escape velocity can be descibed by

$$v=\sqrt{\frac{2GM}{r}}\tag{1}$$

Where $v$ is the escape velocity, $r$ is the distance from the mass (in case of the planet, the minimum distance is the radius of the planet), $M$ is the mass and $G$ is Newton's Gravitational Constant.

If the planet should have the excape velocity of the speed of light, it has to be so dense that you have to consider relativistic effects. In General Relativity, the Schwarzschild Radius of a black hole (this radius is the distance at which the escape velocity equals $c$, which is exactly what you want) is described by:

$$r=\frac{2GM}{c^2}\tag{2}$$

Whch can be solved for $m$:

$$M=\frac{rc^2}{2G}\tag{3}$$

So if you have either a fixed radius of mass, you can easily calculate the missing parameter using these equations.

The problem, as mentioned before, is that for a planet (or any other body) to have an escape velocity of the speed of light, it must be extremely dense. In fact, if it is dense enough to have $v\_{esc} = c$, the body is a black hole (Think about it - the event horizon of a blackhole if the distance where the escape velocity equals the speed of light, so anything beyond this horizon cannot escape as it would require a speed greater than $c$).

> **Comments:**
> **username_3**: It is kind of a coincidence that the Newtonian escape velocity calculation results in the the same value as the Schwartzchild radius in GR. #  
> **username_7**: @username_3 Thanks for your comment. I always thought that one of these formulae is derived from the other, but it actually doesn't make sense because you have to consider relativistic effects. I will edit my answer  
> **username_0**: Thank you for your great explanation  

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