It seems that Caltech may have discovered a TENTH planet in the solar system (Pluto is a planet and always will be*). Gravitational disturbances within the Kuiper Belt have led to a number of researchers hypothesising the existence of a massive planet very very far from the Sun. This planet, if it exists, is estimated to be 10 times as massive as the Earth and is at a distance of 700 AU, and it takes between 10,000 and 20,000 years to complete one orbit (90 times longer than Neptune). It may well be the long hypothesised fifth gas giant.
The planet has not yet been directly observed, only its gravitational effects on other bodies. This is exactly how Neptune was discovered.
notes that the putative ninth planet—at 5,000 times the mass of
Pluto—is sufficiently large that there should be no debate about whether
it is a true planet. Unlike the class of smaller objects now known as
dwarf planets, Planet Nine gravitationally dominates its neighborhood of
the solar system. In fact, it dominates a region larger than any of the
other known planets—a fact that Brown says makes it "the most planet-y
of the planets in the whole solar system."
One key prediction of the planet hypothesis is that a group of Kuiper objects should be detected inclined perpendicular to the ecliptic, and five such objects were discovered. Unlike global warming, the computer models for the tenth planet actually match the observed data. This is how to do real science, not political science.
Making visual observations of this new planet, if it exists, will be difficult. It should be quite dim, and no one knows exactly how far away it is. It is not known where to look, but at least there is good evidence that there is something out there to look for.
* The International Astronomical Union, a good ol' boy's club for elite astronomers, decided to cancel Pluto's membership in the planet club. They took a vote. But a vote isn’t exactly a rigorous scientific argument, so to give its decision the flavor of science, the IAU came up with a definition of "planet" so convoluted it seemed entirely arbitrary.
To qualify as a planet, a body must orbit the sun and be large enough to be at least roughly spherical, two rules that make sense. But it must also have gravitationally "cleared its neighborhood" of other bodies, meaning it has its orbital traffic lane all to itself, which Pluto doesn’t, at least during the most remote portion of its journey around the sun. The rule seemed carefully crafted so that "dwarf planets" like Pluto, Eris and the asteroid Ceres didn’t make the cut.
But that's not the half of it. If the Earth were placed where Pluto is it would not have enough mass to "clear its neighborhood", and so the Earth would not be a planet! That's right. An earth-sized object situated between 30 and 40 AU from the Sun would not have enough mass to qualify as a planet. The farther from the Sun the more area must be dominated by the object's mass, so it gets harder and harder the farther away you get for a large spherical object to qualify as a planet.
But the Earth IS a planet where it is in space, meaning the third criterion doesn't signify anything intrinsic about the body itself! The definition of what is and is not a planet should not rest on some factor independent of the planet itself.
Until someone can come up with a compelling, intrinsic reason (a reason that would not disqualify the Earth!) why Pluto should not be a planet, then I am considering it a planet.