Is Pluto a Planet?

In the past, I know I've been adamant to say that Pluto is not a planet. You can read them again here, here, and here. However, I've thought about things and may be coming around to categorizing Pluto as a planet.


Before, I had always said that there are two types of planets: terrestrial planets, like Earth, and Jovian planets, like Jupiter. But what if we redefined a planet to have a third type: dwarf planets. These are bodies in the solar system that are too large to be asteroids or comets but are not necessarily big enough to have swept all the smaller bodies from its orbit (meaning it could have co-orbital bodies), have densities somewhere between Jovian planets and terrestrial planets, and can be found anywhere in the solar system.


I did discuss dwarf planets in more detail in a previous post, when discussing Pluto (see first link above), so I may have unwittingly decided already that Pluto is a planet. In fact, if we make Pluto a planet again (which probably will happen sooner rather than later), we will also have add a few more planets to our solar system.

Trans-Neptunian Objects (objects orbiting the Sun outside of Neptune's orbit)

Image Credit:

NASA/Hubble

Eris and Makemake will definitely be added. Haumea, Sedna, and 2007 OR10 may as well. It all depends on where the lower limit of a dwarf planet's size will be if the IAU determines that.

This is what I love about science. It's a fluid subject that can easily change based on new information. I've changed my mind about Pluto (for now), and there may be other things I might change my mind about.

Neptune, The Other Blue Planet

Neptune with a Great Dark Spot prominent in the center

Image Credit:

NASA

Earth is not the only blue planet in our Solar System. The planet farthest from the Sun (and Pluto is still not a planet, nor should it be), is also blue. However, Neptune's color does not come from water but from the atmosphere composed mainly of methane. Neptune is also the fourth largest planet in diameter (smaller than Uranus and larger than Earth), but the third largest in mass (between Saturn and Uranus). It orbits at a semi-major axis distance of 30 AU which gives it a orbital period of 165 Earth years. Based on its eccentricity of 0.0087, it is by far the planet with the most circular orbit.

Neptune is also known as the first object to be predicted to exist before it was discovered. In the 1840s, two different astronomers predicted another planet orbited outside the orbit of Uranus and those predictions were confirmed in 1846. Since its discovery, Neptune has only completed one full orbit, way back in 2011.

Neptune also is home to one of the larger moons in the Solar System, Triton. It also has 13 confirmed moons, and may have more awaiting discovery. And just like the other gas planets, Neptune does have a ring system.

As shown in the above image, Neptune has a series of prominent storms on its surface that have persisted since their discovery in 1989 by Voyager 2. Much like Jupiter's Great Red Spot, they are a fascinating set of storms that we are still learning about.

Uranus

Image Credit:

NASA/Voyager 2

 

No planet or celestial object causes as much snickering as the seventh planet from the Sun. Depending on whom you ask, it can be pronounced one of two ways. The first way is the one that causes the snickering, especially among high school and college students (I know, I taught college astronomy). The other way, the way that I prefer to use, does not make it sound so humorous. Of course, I am talking about the planet, Uranus.

I pronounce Uranus as if it sounds like "You're A Nus" or "You're a Nis". Trust me, it helps people from giggling when you say its name. The name Uranus itself comes from Greek mythology as Uranus was the father of the first Titans (including Cronus) and the grandfather of the Olympians, the Greek gods. It is the first planet (other than Earth) not named for a Roman god. If the tradition had held using Roman names, Uranus should have been called Caelus, the father of Saturn and in turn, the grandfather of Jupiter.

Uranus is also the first planet discovered with a telescope. Up to its discovery in 1781 by William Herschel, only the six planets were known (Mercury, Venus, Earth, Mars, Jupiter, and Saturn). Of course, before the heliocentric model of the Solar System, Earth was not considered a planet. There is evidence that Galileo saw Uranus, but mistook it for a star. Two other astronomers also observed Uranus, but did not identify it as a planet.

Like Jupiter and Saturn, Uranus is a Jovian planet, i.e. a gas giant planet. It does not have a solid surface, but contains hydrogen, helium, methane, ammonia, and water in the outer layers. Its "mantle" is a mixture of ice and rock and has a heavy element core. The density of Uranus is 1.29 g/cm³, making it the seventh densest planet, only ahead of Saturn.

Uranus is the third largest planet in terms of diameter, but the fourth largest in mass (Neptune is larger in mass, but has a smaller diameter). It orbits 19.18 AU from the Sun, taking just over 84 years to orbit the Sun. Since its discovery in 1781, it has only completed two orbits, its third orbit won't be complete until 2033.

Since it is a Jovian planet, it also exhibits two properties that Jupiter and Saturn display: a ring system and multiple moons. The rings are more similar to Jupiter's rings than Saturn's rings as they are very faint and were not confirmed until Voyager 2 imaged them directly. Uranus has 27 confirmed moons which are all icy bodies. Five are considered to have planetary mass which means that they are spherical. Titania and Oberon were actually first discovered by William Herschel in 1787, after his discovery of Uranus.

Image Credit:

W. M. Keck Observatory

Lastly, the most amazing thing about Uranus is its day. Recall that Venus has an inclination of almost 180°. Uranus' inclination is not as severe, but may considered stranger. Its inclination is 98° which means that its axis of rotation is almost parallel to Uranus' orbital plane.