Scientists are finding so many moons of the solar system’s giant planets that they can hardly keep up with them. By the end of last year, Saturn was in the lead, with 146 confirmed moons. Jupiter — the biggest planet — came in second, with 95. But some of its moons are the most amazing of all — worlds with giant volcanoes, or hidden oceans that could host microscopic life.

Most of the moons of both planets are a lot less interesting. They’re basically big chunks of rock and ice. Some of them could be captured asteroids, but most probably are debris from shattered moons.

The population of known moons has exploded in the past decade or two. In part, that’s because modern technology makes it possible to find these tiny, faint objects. Also, scientists are putting a lot more effort into the search. That’s because, while the recent discoveries are all fairly boring on their own, they reveal a lot about the histories of both worlds. In particular, they tell us how the systems of moons and rings may have evolved — producing the busy planetary systems we see today.

Jupiter stands quite near our own moon tonight. It’s to the lower left of the crescent Moon at nightfall. Jupiter looks like a brilliant star — only the Moon and the planet Venus outshine it. It’s a world with plenty of known moons — and probably many more awaiting discovery.

We’ll talk about the Moon and some stellar companions tomorrow.

Script by Damond Benningfield

Moon and Jupiter

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Most of the “star pictures” in the night sky look nothing like their namesakes. But one beautiful exception lunges across the southwestern sky on May evenings: Leo, the lion. It’s high in the sky at nightfall.



Leo consists of two patterns of stars that the brain puts together to make a lion. A backward question mark represents the head and mane. And a triangle of stars to the lower left forms the lion’s hindquarters and tail.



Leo is best known for its bright stars — especially Regulus, its brilliant heart. But the constellation also contains quite a few bright galaxies. Leading the list are three galaxies that form the Leo Triplet: M65, M66, and NGC 3628.



NGC 3628 is the most interesting of the three. Like our own Milky Way, it’s a spiral — a pinwheel that spans at least a hundred thousand light-years. We see it edge-on, so it looks like a streak of light with lines of dark dust running down the middle.



Close encounters with the other galaxies have pumped up NGC 3628. They’ve triggered the birth of millions of new stars near the galaxy’s core. And they’ve pulled out a tail of gas that spans a quarter of a million light-years — enough gas to make half a billion stars as massive as the Sun. In fact, the tail has given birth to millions of stars already.



The Leo Triplet is just one of the wonders in one of the night sky’s most easily recognizable constellations: the lion.



Script by Damond Benningfield

Leo Triplet

Over the centuries, no planet has been as frustrating to study as Mercury. The Sun’s closest planet never moves far from the Sun in our sky. So when astronomers pointed telescopes at Mercury, it was almost always screened by twilight and a thick layer of Earth’s atmosphere. So most of what we know about Mercury has come from spacecraft that visited the planet.



You can see the difficulty yourself the next few days. Mercury is quite low in the east not long before sunrise. It’s almost at its farthest point from the Sun, and it looks like a fairly bright star. But at that low altitude, its light has to pass through a thick layer of air, which blurs the view. The glow of twilight makes the view even murkier.



Without the ability to see surface features, it was hard to nail down the length of Mercury’s day. The planet always shows the same face when it’s closest to Earth. So it looked like Mercury was locked so that the same hemisphere always faced the Sun, just as the same side of the Moon always faces Earth.



But that’s not right. Radio telescopes have showed that Mercury makes three turns on its axis for every two orbits around the Sun. That means a “day” on Mercury — the time from one noon to the next — lasts 176 Earth days — long days and nights for the Sun’s closest planet.



Mercury appears near the Moon the next couple of mornings. It’s to the lower left of the Moon tomorrow, and closer to the right on Monday.



Script by Damond Benningfield

Moon and Mercury

The atmosphere of Mars is quite different from Earth’s. It’s colder, thinner, and it’s made mainly of carbon dioxide. In one way, though, the skies of the two planets are similar: both of them have clouds. And for the most part, the clouds themselves are alike as well.



Because Mars is so cold, the clouds are made entirely of ice — frozen water or carbon dioxide. There’s not much water in the Martian atmosphere, so the clouds are thin. And they’re usually much higher than even the highest clouds of Earth.



But Mars also has some clouds that are a bit different from those on Earth. That includes “dots” and some long, skinny ribbons.



An orbiting spacecraft has seen the dots in a specific region of Mars, south of the equator. The clouds are almost perfectly round. They’re up to 60 miles in diameter, a few miles thick, and they top out at altitudes of 30 to 50 miles. Some are seen at dawn, while others are seen in clumps in the dawn or evening twilight. The clouds could form as the atmosphere interacts with some odd spots in the Martian magnetic field.



The elongated clouds are seen in volcanic regions. The longest span about 1500 miles, and are up to a hundred miles wide. They may be sculpted by winds flowing over volcanic peaks — some odd clouds for the Red Planet.



Mars stands close to the lower left of the Moon at dawn tomorrow. The planet Saturn is about the same distance to the upper right of the Moon.



Script by Damond Benningfield

Moon and Planets

The solar system is pretty settled. The planets appear to be following orbits that have remained stable for billions of years. But in the early days, things might have been a lot more chaotic. According to one model, in fact, the giant planets Jupiter and Saturn might have moved much closer to the Sun before they moved back out again.



The planets probably took shape from a disk of gas and dust around the Sun. Small bits of material stuck together to make bigger bits, all the way up to planets. But much of the material remained free. There was enough of it to exert both a drag and a pull on the giant planets.



In this model, Jupiter — the Sun’s heaviest planet — was born at about two-thirds of its present distance from the Sun. It quickly moved inward, though, all the way to the orbit of Mars. Saturn — the second-most-massive planet — was dragged inward as well. Jupiter and Saturn thinned out the supply of gas and dust and the leftover planetary building blocks — either by scooping them up or kicking them away from the Sun. That changed the gravitational balance of the solar system. Jupiter and Saturn moved outward — settling into their current stable orbits around the Sun.



Saturn is in the dawn sky now, and looks like a bright golden star. Unlike a star, though, it doesn’t twinkle — its light holds steady. Tomorrow, it will stand close to the left of the Moon. The Moon will pass between Saturn and Mars the next morning; more about that tomorrow.



Script by Damond Benningfield

Moon and Saturn

One of the larger moons of the planet Neptune has been through a lot. It might have started as an asteroid, and was captured by Neptune’s gravity. Or it might have started as a moon, but was hurled into a wild orbit when Neptune grabbed its largest moon. And since then, it’s been battered by impacts with other space rocks.



Nereid was discovered 75 years ago today, by Gerard Kuiper. It was only the second moon seen around the giant planet, and it’s the third-largest of Neptune’s 16 known moons.



Kuiper was observing Neptune with the 82-inch telescope at McDonald Observatory. In a pair of 40-minute exposures, the moon showed up as a tiny star near the planet. Kuiper suggested the name Nereid because, in mythology, the Nereids were daughters of Neptune.



We don’t know a lot more about the moon today than when it was discovered. It’s more than 200 miles in diameter, its gray surface probably is coated with ice and rock, and the surface is rough — perhaps the result of billions of years of impacts.



Nereid follows the most lopsided orbit of any good-sized moon in the solar system. It ranges from less than a million miles from Neptune to about six million miles. That suggests that Nereid could be the last of Neptune’s original moons. When Neptune captured its biggest moon, Triton, Triton’s gravity could have kicked out all the others, leaving only Nereid — in a wild orbit around a giant planet.



Script by Damond Benningfield

Moon and Jupiter

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