Tomorrow, July 12, will mark the one-year anniversary of the discovery of Neptune. Of course I don’t mean one Earth year — one Neptune year, which is about 165 Earth years long. A year as we typically mean is measured by the time it takes Earth to complete one circuit around the Sun (relative to the position distant stars, if you want to be technical): it’s defined relative to our planet, and other worlds don’t have orbits directly corresponding to our own.
Neptune is the farthest planet from the Sun (yes yes, Pluto blah blah): its orbit is about 30 times larger than Earth’s, so it’s not that easy to observe. It’s nearly four times the diameter of Earth — you could fit nearly 60 Earths inside its volume — but being made of hydrogen, water, ammonia, and other hydrogen compounds, it’s a lot less dense than our rocky planet. The best images and data we have about it still are from Voyager 2, which visited in 1989, before most of my current astronomy students were born!
Neptune still holds a lot of mysteries, not least of which is how it formed: according to the most accepted theory of Solar System formation, an object as far out from the Sun as Neptune would have trouble gathering enough material to grow large. Perhaps it formed closer to the Sun and migrated outward, or (in a far less likely scenario) formed elsewhere in the galaxy and was captured by the Sun’s gravity. No matter what, it’s an interesting object with a lot of characteristics astronomers are still trying to work out.
One characteristic that was recently determined is the length of the Neptunian day: how long it takes Neptune to rotate on its axis. That might seem like a trivial question, but it’s not. Think about Earth, which has a solid core of iron and nickel, and a solid crust made of rocky materials — every point (excepting the poles) on the Earth’s surface takes the same amount of time to rotate. (The amount of sunlight is a different story, of course — we’re just thinking about the total 24-hour day.) Neptune lacks a solid surface, and may not even have a solid core (though I think most planetary scientists suspect it does). What we see isn’t a sea with an ocean floor below — that beautiful blue we see is a dense mixture of water and other molecules that extends deep into the planet’s interior. In other words, there isn’t really a “surface” in the same sense Earth has, or Mars, or the Moon.
In fact, up until recently, Jupiter was the only one of the outer planets whose rotation was determined to any degree of precision. Saturn, Jupiter’s smaller sibling, exhibits different rotation rates in its northern and southern hemispheres — probably due to the large tilt of its axis, which creates strong seasonal variations. As you can see in the video above, Neptune has a lot of turbulence in its atmosphere — not as much as Jupiter or Saturn, to be sure, but enough that determining features to use in clocking rotation is a challenge. This challenge was met by University of Arizona planetary scientist Erich Karkoschka; I encourage you to read the EarthSky article, which goes into some detail about how he made the measurement.
For his classic orchestral suite The Planets, Gustav Holst chose the title “Neptune the Mystic” for the seventh and final movement. Although we need not assign astrological significance to Neptune, the name of the piece is relevant: Neptune is still largely mysterious to us, even one long Neptunian year after its discovery.
Update: I intended to point this out in the first paragraph, but in the China Miéville novel Embassytown, the narrator mentions her age as 11 years. She is a human living on an alien planet — 11 years on that planet puts her fully into adulthood by human terms. There’s my necessary pop-culture reference, and a plug for a truly awesome novel.