"outermost planet" is a correct and usable phrase in written English.
It refers to the furthest planet from the sun in a particular system. Example: Neptune is the outermost planet in our solar system, with an average distance from the sun of about 30 AU.
But the outermost planet soaks in just enough energy for surface water to stay liquid.
Neptune is now the system's outermost planet, 2.8 billion miles from Earth.
(By comparison, Neptune, the outermost planet, is 30 AU from the Sun).
Pluto, for decades classified as the outermost planet, doesn't fit well into any of these catagories.
Of course, a trendy, Pikettyish reason might also be deduced for the promised renaissance of the outermost planet.
Neptune, third most massive planet of the solar system and the eighth and outermost planet from the Sun.
Comets made more news in 1995 when scientists led by Anita L. Cochran of the University of Texas, using the HST, discovered 30 objects lying in a region beyond the orbits of the outermost planets Pluto and Neptune.
From their modeling, the scientists estimate that the innermost planets lost more than 20 times Earth's current oceanic water stores over their 8-billion-year journey toward their star, while the outermost planets lost much less, equivalent to around three times the ocean stores on Earth.
Our Sun's influence continues far beyond the outermost planets, and it's tricky saying where it ends.
But it turned out that the minerals in the comet dust that Stardust managed to collect weren't that pristine: They had been heated, melted, and totally transformed somewhere near the nascent sun and then carried outward to be incorporated into comets in the ultradeep freeze far beyond the outermost planets.
The two largest, the gas giants Jupiter and Saturn, are composed mainly of hydrogen and helium; the two outermost planets, the ice giants Uranus and Neptune, are composed largely of substances with relatively high melting points compared with hydrogen and helium, called ices, such as water, ammonia and methane.
