Hello you cant take a space ship very far away because it take about a month to get to the moon and how long did it take to get to Mars? Exactly my point. So by the time im dead, people will be trying to land on Neptune and titan and Not ONE of those idiots are gonna make it.
Yes, in fact, for awhile now, several researchers have theorized the existence of at least two planets beyond Pluto (http://www.Businessinsider.Com/afp-two-more-planets-in-our-solar-system-say-astronomers-2015-1).
However, there was one other unique theory in which I have heard: Say a planet were in orbit around two different stars, each of which are in two separate solar systems. Although the eccentricity of this planet's orbiting period would be quite significant, this still technically would qualify as a planet 'within' our solar system. Simply, its orbiting period would be so vast in time, that it would be nearly impossible to adequately recognize the planet without a local star to shadow. After all, Keplar's design is for pinpointing these shadows, requiring a prominent light source.
Yes, there are likely undiscovered planets in our solar system. This is possible because of very recent changes in technology and also in growing understanding of the definition of a planet. The definition of a planet must take into account the size of the planet, the behavior of the planet in gravitational terms, and the relation of that planet to other planets and to the sun.
Absolutely, there could be undiscovered planets in our solar system. Every day we are creating new technology to further space travel and exploration. As well we are also finding "new" species on the planet that we live on so there would be no reason as to why there couldn't be an undiscovered planet.
We are still learning about our universe and solar system. Pluto was recently removed as a planet and now we have planet 9. As we get better technology and become more innovative in our electronic robotic designs that we can send out for exploration, we are bound to find new planets billions of miles away.
Based upon the amount of research we have done and the amount of space probes we have sent out it into the solar system it would be a huge surprise to the scientific community if a planet was discovered. The only possibility seems to be on the outer fringes of the solar system where Pluto size dwarf planets have recently been discovered.