Triton is the largest moon of Neptune. It is distinctive because it is the largest moon in the solar system with a retrograde orbit, meaning its orbit is opposite to Neptune’s rotational direction. This means Triton didn’t form along with Neptune, but rather was captured once Neptune formed. Just how Triton was captured isn’t clear, but one possibility is that Triton was once a binary object, and a close encounter with Neptune led to the capture of Triton and the expulsion of its partner.

Triton’s orbit is almost perfectly circular, which is a bit surprising. It likely had a much more elliptical orbit when it was captured, and over time its orbit was circularized by tidal forces as well as gas and dust particles moving in a prograde orbit. The tidal forces also worked to sync its rotation with its orbit, so that now one side of Triton always faces Neptune, just as one side of our Moon faces Earth. When it was initially captured, tidal forces would have been much stronger, which heated the moon’s interior. To this day Triton is still geologically active, though it is now driven by radioactive heating in its core. It’s possible that Triton has liquid water in its interior similar to Europa.

Triton has a chemical composition similar to Pluto, so there has been some speculation that the two bodies share a common origin. What we do know is that Triton is a Kuiper belt object like Pluto. When New Horizons makes its flyby of Pluto later this year, one of the questions we’ll try to address is just how closely related Pluto and Triton actually are.

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At the moment there are 8 known planets in the solar system (alas, poor Pluto) and (officially) 5 dwarf planets. With the exception of Ceres, the dwarf planets are all trans-Neptunian objects. We know of several other such objects of similar size, but these haven’t yet been designated as dwarf planets. But what about larger bodies on the edge of our solar system?

At the moment, we know of nothing larger than Eris beyond the orbit of Neptune, but a recent paper in MNRAS hints at the possibility of larger trans-Neptunian planets. The paper looks at the orbits of 13 trans-Neptunian objects and notices an interesting pattern. According to current models, these small distant bodies should fairly close to the invariable plane of the solar system (where most planetary orbits are) and they should have an average orbital distance (semi-major axis) of about 150 AU (astronomical units). What the team found was that the bodies have a semi-major axis of 150 – 520 AU, and their orbits can tilt 20 degrees or more from the invariable plane.

So the team looked at gravitational simulations of these orbits, to see how they might be affected by larger gravitational bodies. In particular, they looked at in interaction known as the Kozai mechanism, where the gravity of a larger body can pull the orbit of a smaller one toward the orbital plane of the larger one at the cost of making orbits less circular. They found that the orbits of these trans-Neptunian objects could be explained by the presence of one or two larger bodies in the region. Their results suggest super-Earth massed bodies in the region of 250 AU.

This wouldn’t be the first time a new planet has been discovered by its gravitational influence, though it should be stressed that the evidence here is not particularly strong. Thirteen bodies is a small statistical sample, so don’t bet on a large planet beyond Neptune just yet. But the hint is definitely there, and we are bound to find more objects waiting in the dark as our detection methods continue to improve.

Paper: C. de la Fuente Marcos, R. de la Fuente Marcos. Extreme trans-Neptunian objects and the Kozai mechanism: signalling the presence of trans-Plutonian planets? Monthly Notices of the Royal Astronomical Society Letters 443(1): L59-L63  (2014)

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In August of 1989, Voyager 2 made its close approach of Neptune.  It had been traveling for nearly 12 years, and had now reach the last planet on its journey.  As the probe streamed images and data from the flyby PBS ran live coverage known as Neptune All Night.  I was in college at the time, and happened to be working on a final for a summer class that evening, so I spent the whole night oscillating between and the latest data from Neptune.  To this day I can’t think of Neptune without being reminded of Neptune All Night.

In some ways, associating Neptune with the night is rather fitting.  Neptune is 30 times farther from the Sun than Earth, which means the Sun appears about 1/30th the width of Earth’s sun, and about 1/1000th as bright.  That is 400 times brighter than the Moon, but it is hardly what we might consider daylight bright.  It is a cold planet with a rock and ice core, which is why it is sometimes called an ice giant.

High altitude clouds on Neptune.
Credit: NASA / Jet Propulsion Lab

Unlike Uranus, Neptune has an active weather system.  It has weather storms, such as the great dark spot, which was observed by Voyager 2.  That particular storm appears to have faded, but other “dark spot” storms have appeared.  This storm activity is likely driven by the planet’s internal heat. Neptune radiates about 2.5 times the heat it receives from the Sun.

Neptune is the third most massive planet in the solar system (after Jupiter and Saturn), which is unusual given its great distance from the Sun.  We’re still not entirely sure how Neptune formed, but the most widely accepted model is known as the Nice model.  In this model, Neptune formed about 10 AU from the Sun, which is roughly where Saturn is now.  It was actually closer to the Sun than Uranus at the time.  Through a gravitational dance, the gas planets migrated outward and Neptune was flung to the outer region where it is now.

Most of this gravitational dance was driven by Jupiter. Since Jupiter has similarities to Thor, it is probably fitting that Thor drove the ice giants to the outer edge of our solar system.

Last but not least: Pluto

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