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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?

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    $\begingroup$ 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. $\endgroup$ – David H Nov 30 '15 at 6:16
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Yes, Vulcanoids larger than 5.7 km diameter are considered to be ruled out: A Search for Vulcanoids with the STEREO Heliospheric Imager.

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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