StarDate

Our planet is a water world — the wet stuff covers more than two-thirds of Earth’s surface. But that may be the proverbial drop in the bucket compared to some other worlds. Vast oceans may hide beneath the surfaces of several bodies in the solar system. And even bigger oceans may cover planets in other star systems.

Missions to the other planets in the solar system have found evidence of water on several moons, as well as two “dwarf” planets.

At least three of those water worlds are moons of Jupiter. The most intriguing is Europa. It appears to have a sub-surface ocean that could be miles deep. The ocean might hold several times as much water as all of Earth’s oceans combined. Hot, mineral-rich fountains might squirt up from far below the ocean floor — perhaps providing conditions that are comfortable for life.

Water is squirting out of a subsurface ocean on Enceladus, a moon of Saturn. And a huge ocean could lurk far below the surface of Saturn’s biggest moon, Titan.

There’s also evidence of water on Pluto, the little world that’s just beyond the realm of the major planets, and on Ceres, the largest member of the asteroid belt; more about that tomorrow.

And a recent study suggests that oceans could almost completely cover some planets in other star systems. Dry land might make up just 10 percent of the surface of such a planet. So if anyone wants a home planet with almost unlimited ocean views, there could be plenty to choose from.

Script by Damond Benningfield

If you look south two or three hours after sunset tonight, you’ll be gazing into one of the largest of all the 88 constellations — Eridanus, the river. It’s long and winding, and not easy to see. It hosts one of the brightest stars in the sky, but the star is so far south that it’s out of sight for most Americans.

Eridanus also is home to a cluster of galaxies. The galaxies spread over a range of distances, but the center of the cluster is probably about 75 million light-years from Earth.

The cluster is a grander gathering than the Local Group, the collection of galaxies that includes our home galaxy, the Milky Way. For one thing, the Eridanus cluster has more galaxies. For another, its largest galaxies have more diversity: some are elliptical galaxies and some are spirals. In contrast, the only two giants in the Local Group are both spirals.

One of the most impressive members of the Eridanus cluster is NGC 1300. It’s a breath-taking barred spiral that looks like a backward letter S. It has two enormous spiral arms that are spawning new stars. The spiral arms emerge from opposite ends of a long bar, which houses mainly older stars. The Milky Way is also a barred spiral, but its bar is much less prominent.

Next door to the Eridanus cluster is the better-known Fornax cluster, which is slightly closer to us. Both clusters are part of a long filament of galaxies, which stretches across the southern sky tonight.

Script by Ken Croswell

The Moon and two bright planets form a beautiful triangle at dawn tomorrow. Brilliant Jupiter stands to the right of the Moon, with fainter orange Mars close below them.

The surfaces of the Moon and Mars have been sculpted in part by collisions with asteroids — big space rocks that orbit the Sun. The impacts have gouged craters. And on the Moon, they punched holes in the crust that allowed molten rock to bubble up from below, creating dark volcanic plains.

Jupiter gets hit by asteroids, too. In fact, because it’s a bigger target and has a stronger gravitational pull, it gets hit a lot. Over the last quarter of a century, in fact, astronomers have seen several asteroids and comets slam into the planet.

Jupiter doesn’t have a solid surface, though, so the impacts don’t leave permanent scars. Instead, they leave temporary markings — dark splotches in the planet’s upper atmosphere. They can persist for weeks or even months, and stretch across thousands of miles. Eventually, though, the atmosphere heals itself and the scars disappear.

Jupiter and Mars have received many asteroid visitors because they flank the asteroid belt — a wide band of debris left over from the birth of the planets. In fact, the gravity of nearby Jupiter kept the pieces from clumping together to make a planet.

Collisions between asteroids may send debris zipping out of the belt and toward the planets — creating potential hazards for these and other worlds.

Script by Damond Benningfield

The Surveyor program of the 1960s was designed to prove that it was safe for Apollo astronauts to land on the Moon. So the first few missions touched down on the smooth volcanic plains where the first astronauts would land. By the time the final Surveyor took flight, though, Apollo planners had all the intel they needed to land safely. So Surveyor 7 was dedicated strictly to science.

The three-legged craft landed on the Moon 50 years ago tonight, on the flank of Tycho, a big, bright crater in the southern hemisphere. The crater was gouged by an impact more than 100 million years ago. On the airless Moon, that’s fairly recent — the crater looks fresh, and it’s surrounded by bright “rays” of material blasted out by the collision. Surveyor 7 landed in that debris field, which included boulders gouged from deep below the surface.

Over the following few weeks, Surveyor snapped more than 21,000 pictures of its surrounding landscape — the roughest seen by any Surveyor. Just as important, it carried instruments for digging into the powdery lunar dirt and analyzing the dirt and rocks.

Those instruments found that the chemistry around Tycho was similar to that of the volcanic plains, with a little less iron and related elements.

Surveyor 7 also saw a glow above the horizon at sunset. That could have been caused by grains of electrically charged moondust levitating above the surface — a bonus discovery from America’s final robotic lander on the Moon.

Script by Damond Benningfield

It’s usually best to stay clear of lions. But the Moon will ignore that bit of wisdom tonight. It will pass quite close to Regulus, the star that marks the heart of Leo, the celestial lion. And for skywatchers from Alaska to western Europe, the Moon will pass in front of the star for a while.

The passage is known as an occultation. It’s a type of eclipse, like last year’s eclipse of the Sun. The Moon will cover the star, blocking it from view.

Over the years, astronomers have used occultations to hunt for close companions around stars. If a star is single, its light will wink off in an instant. If the star has a companion, though, the light from the system will fade in steps, as first one star moves behind the Moon, then the other.

The technique doesn’t always work, though, and Regulus is an example. The bright star we see as Regulus has a small companion. It’s so faint, and so close to the bright star, that there’s no two-step drop when the system passes behind the Moon — the light blinks off all at once. Instead, astronomers discovered the companion through its gravitational tug — it causes a slight shift in the light produced by brilliant Regulus.

And Regulus will stay close to the Moon after they climb into view around 9 or 10 p.m. For most of the country, they’ll pass within a couple of degrees of each other — the width of a finger at arm’s length. The occultation will be visible from Alaska, as the Moon gets close to the lion.

Script by Damond Benningfield

For sheer spectacle, it’s hard to beat the great light shows on a New Year’s Eve — everything from the ball dropping in Times Square to fireworks above the Queen Mary.

And the starry night offers up a pretty spectacular light show of its own. The almost-full Moon is encircled by some of the brightest stars in the night sky. The whole array is in great view by a couple of hours after sunset, and remains in view all night. And the stars are all so bright that they’re visible even from light-polluted cities.

The faintest members of the stellar wreath stand to the lower left of the Moon: Pollux and Castor, the twins of Gemini. Pollux is the brighter of the two. And the brightest member of the wreath stands well above them, to the upper left of the Moon: Capella, the leading light of Auriga the charioteer. It’s the sixth-brightest star in the night sky, so it’s hard to miss.

To the upper right of the Moon, there’s Aldebaran, the eye of the bull. And down below Aldebaran, look for the bright stars of Orion the hunter. Betelgeuse stands closest to the Moon, with Orion’s three-star belt to the right, and Rigel to the right of the belt.

Several of these impressive stars show a bit of color — from orange to blue. The tints are subtle, though. In the glare of the brilliant Moon they can be hard to see. Even so, the stars will stand out — bright natural lights in the busy New Year’s Eve sky.

Script by Damond Benningfield

The Moon has a bright companion tonight: Aldebaran, the eye of Taurus, the bull. Skywatchers across much of the United States won’t see the star during part of the evening, though, because it’ll be hidden behind the Moon. The best view of that disappearing act comes from the northeast.

Even when it isn’t covered up, though, Aldebaran might be tough to spot through the Moon’s glare. The Moon is so bright that it can be hard to look at, especially from a dark skywatching location.

The Moon looks so bright because it’s a big, close target. In reality, though, it’s quite dark — it reflects only about 12 percent of the sunlight that strikes it.

Just how bright the Moon looks is important for folks who operate Earth-watching spacecraft. They sometimes use the Moon to calibrate their instruments. But no one has cataloged how bright the Moon looks at different phases and viewing angles. So scientists have to look at the Moon from space and the ground at the same time to get a good match.

But scientists at the National Institute for Standards and Technology have launched a project to measure the Moon’s appearance at just about every set of circumstances, in multiple wavelengths.

They’re using a small telescope in Maryland to look at both the Moon and at special artificial light sources. And next year, they’ll move the project to Hawaii. The observations should provide a good catalog of the Moon’s brightness, no matter how you look at it.

Tomorrow: Bright lights in the holiday sky.

Script by Damond Benningfield

The planets Mars and Jupiter execute a pincer maneuver as the old year ends and the new one begins: They bracket one of the ancient claws of Scorpius. Jupiter looks like a brilliant star in the southeast at dawn, with fainter Mars to its upper right. And just below Mars is Zubenelgenubi, the second-brightest star of Libra.

Libra represents a set of balance scales. But the name “Zubenelgenubi” comes from an ancient Arabic phrase that means “the southern claw.” That tells us that when the scorpion was first drawn, thousands of years ago, the star represented one of its claws.

Centuries later, though, the claws were separated from Scorpius, and the stars were assigned to the new constellation Libra. That may have been because the Sun appeared against those stars at the time of the fall equinox, a time of equal daylight and darkness — a “balance” in the sky.

Zubenelgenubi consists of two widely separated stars. Both of them are visible through binoculars. But each of the stars is in fact a pair of stars on its own, making four stars in all.

The stars in each pair are quite close together. But the two pairs are separated by more than 5,000 times the distance from Earth to the Sun. At that range, it takes more than 200,000 years for the two pairs to orbit each other.

So if you’re up and about at dawn tomorrow, look for the scorpion’s ancient claw close to two bright planets in the dawn sky — a beautiful decoration for any year.

Script by Damond Benningfield

Two stars in the constellation Cancer look like they’re puckering up for a kiss. And in a way, they are — the stars are about to make contact.

One star in the system is bigger, heavier, and brighter than the Sun. The other is smaller and fainter than the Sun. The stars orbit each other once every 11 hours.

A team of astronomers has been watching the system for 15 years. The observations show that each star is bulging toward the other. That makes the system look like a pair of teardrops, with the tapered ends aiming toward each other — like two cartoon characters getting ready for a smooch.

Before long, the stars may make contact. Both stars are quite bloated. The bigger star has puffed up as much as it can. But the other has a little room left to reach its maximum size. So right now, the tips of the teardrops are separated by a few thousand miles. Once the second star is fully inflated, though, the two stars will likely make contact.

The stars then will begin to share the gas in their outer layers. Eventually, they’ll merge to form a single star. That may produce an outburst known as a red nova. After that, the merged system should settle down. The single star will be bright and hot, and it’ll be spinning rapidly — the result of a stellar coalescence that begins with a kiss.

The system is too faint to see with the eye alone. But its constellation, Cancer, rises in the east by around 9 p.m., and climbs high across the sky later on.

Script by Damond Benningfield

Things are heating up for a planet that orbits the brightest star in Aries, the ram. The star is expanding to become a giant, so it’s pumping more energy into space. That will make temperatures extremely uncomfortable on the planet.

The star is Hamal. It’s high in the south in early evening, almost directly above the Moon.

Hamal is at the end of its life. It’s converted the original hydrogen fuel in its core to helium. Now, it’s getting ready to fuse the helium to make other elements. That’s making the core hotter, which, in turn, has caused the star’s outer layers to puff up — to more than a dozen times the diameter of the Sun. And that’s made Hamal about 90 times brighter than the Sun.

Hamal has only one known planet — a giant that’s heavier than Jupiter, the biggest planet in our own solar system. On average, it’s only about as far from Hamal as Earth is from the Sun — much closer in than Jupiter is. So any given area of the planet’s surface receives hundreds of times more energy than Jupiter does.

If the planet is a ball of gas like Jupiter, then the extra heat is causing its atmosphere to puff up — and causing a lot of it to stream away into space.

And over the next few million years, the planet will get even hotter, because Hamal will get even bigger. The extra energy may eventually erode the planet’s entire atmosphere. Or the planet may spiral into the star. Either way, things are going to get a lot hotter for Hamal’s only known planet.

Script by Damond Benningfield