<p>Yes, Vulcanoids larger than 5.7 km diameter are considered to be ruled out:
<a href="http://arxiv.org/abs/1301.3804" rel="noreferrer">A Search for Vulcanoids with the STEREO Heliospheric Imager</a>.</p>

<p>An object sufficiently large to be called "planet Vulcan" would change Mercury's orbit in a detectable way by gravity.</p>

<p>So, there are two lines of evidence, that such a planet doesn't exist.</p>

<p>More to read in Wikipedia:
<a href="https://en.wikipedia.org/wiki/Vulcan_(hypothetical_planet)" rel="noreferrer">https://en.wikipedia.org/wiki/Vulcan_(hypothetical_planet)</a></p>

CC BY-SA 3.0
2015-12-03T17:52:21.193Z
2015-12-03T17:52:21.193Z
Gerald
<p>Currently, most explorations about new planets are pointing to area outside Neptune's orbit, but how about inside Mercury orbit? Can we say for certain there are no planets inside Mercury's orbit?</p>

CC BY-SA 3.0
2015-11-30T06:16:26.457Z
Using the IAU definition of a planet, I'm pretty sure we can. Observations of Mercury's orbit place significant upper bounds on the mass of any additional bodies potentially existing between Mercury and Sun, while part of the IAU definitional criteria for being a planet stipulates that it must be massive enough to reach hydrostatic equilibrium.
David H
CC BY-SA 3.0
2015-11-30T20:16:13.503Z
See https://en.wikipedia.org/wiki/Vulcan_%28hypothetical_planet%29 and https://en.wikipedia.org/wiki/Vulcanoid.
Martin Schröder
<planet><mercury>
Are we sure there are no planets inside Mercury's orbit?
Gstestso
# Are we sure there are no planets inside Mercury's orbit?

(Asked by: username_0 on 2015-11-30)

Currently, most explorations about new planets are pointing to area outside Neptune's orbit, but how about inside Mercury orbit? Can we say for certain there are no planets inside Mercury's orbit?

**Tags:** planet, mercury

> **Comments:**
> **username_1**: Using the IAU definition of a planet, I'm pretty sure we can. Observations of Mercury's orbit place significant upper bounds on the mass of any additional bodies potentially existing between Mercury and Sun, while part of the IAU definitional criteria for being a planet stipulates that it must be massive enough to reach hydrostatic equilibrium.  
> **username_2**: See https://en.wikipedia.org/wiki/Vulcan_%28hypothetical_planet%29 and https://en.wikipedia.org/wiki/Vulcanoid.  

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## 1 answers

### Answer by username_3 on 2015-12-03. Score: 5 | Accepted answer

Yes, Vulcanoids larger than 5.7 km diameter are considered to be ruled out:
[A Search for Vulcanoids with the STEREO Heliospheric Imager](http://arxiv.org/abs/1301.3804).

An object sufficiently large to be called "planet Vulcan" would change Mercury's orbit in a detectable way by gravity.

So, there are two lines of evidence, that such a planet doesn't exist.

More to read in Wikipedia:
<https://en.wikipedia.org/wiki/Vulcan_(hypothetical_planet)>


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