Astronomers have discovered a new moon for Uranus and two for Neptune! The new moons are called S/2023 U1, S/2021 N1 and S/2002 N5, and bring the total for the two planets to 28 and 16, respectively [link to journal paper]. 

The moons were found by taking very long exposures of the space around the two planets, then shifting and adding the images together to find the very dim objects (I describe this technique in more detail back on The Old Blog™ when it was used to find faint moons of Saturn). The moons are incredibly faint; the faintest star you can see by eye is about 10 million times brighter than these distant satellites!

The moon for Uranus is about 7 km in diameter (it’s too small to see its size directly, but is instead inferred from its brightness and likely reflectivity; darker objects are bigger then shinier ones at a given brightness). It orbits the planet on a highly tilted orbit, and moves backwards relative to Uranus’s spin. The orbit is very similar to two other moons (Caliban and Stephano; moons of Uranus are named after Shakespeare characters), implying all three came from the breakup of a larger object, probably shattered when it was impacted by an asteroid or comet).

The two moons of Neptune are about 14 and 25 km wide, with 2021 N1 orbiting backwards and 2002 N5 forwards (technically, retrograde and prograde). Both share orbital similarities with other moons, again implying something got whacked and made smaller moons.

If you’re wondering about the names, the S stands for planetary satellite, then the year of the observations when it was discovered, then the planet’s initial, and the final number is the number in order for the satellite found in that year (so the first one is, say, N1, then N2, and so on).

All these satellites were just discovered, so why does S/2002 N5 have that earlier year in its name? The observations were made in 2021 – 2023, and when they found that moon they were able to compute an orbit for it. They then ran that orbit backwards to see if any earlier observations of it were made, and it turns out it was observed in 2002, but not confirmed at the time. As a nod to those observations, it was given the earlier year’s designation.

The solar system is pretty big — Earth is considered an inner planet even though it’s 150 million km from the Sun! — and these planets are incredibly far away; Uranus is 20 times farther out from the Sun than Earth, and Neptune 30. Finding tiny moons around them is amazing work, taking deep images on some of the biggest telescopes in the world! And it goes to show you that even though we live here, there’s still plenty of stuff to discover in our neighborhood.

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Oh great. Now I’m just waiting for some Flat-Earther to claim all of orbital physics is wrong.

An announcement from the journal Research Notes of the American Astronomical Society:

Explaining a joke always ruins it, but because this is a tad esoteric…

This is an announcement that a paper was retracted due to an error. What mistake was made? Well.

The astronomer Johannes Kepler, after analyzing painstaking observations of planets made by Tycho Brahe (before the telescope was invented, mind you!), found three laws that planets seemed to obey as they orbited the Sun. The first was that planets orbit the Sun in ellipses, not circles (in general all orbits are conic sections, of which the ellipse is one). The second is that the Sun isn’t at the center of the ellipse, but at one focus.

The third is that the semimajor axis of a planet (half of the long axis of the ellipse; it’s not entirely unfair to think of it as an average distance) from the Sun is related to its period, the time it takes to complete one orbit — specifically, the cube of the semimajor axis is proportional to the period squared (if you use the Earth’s semimajor axis as your unit of length and Earth’s year as the unit of time, the proportionality becomes an equality). As an example, a planet four times as far from the Sun as Earth will have a period of 8 Earth years — 4 cubed is 64, and 64 is 8 squared.

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