StarDate

Researchers recently added a new term to the astronomy vocabulary: ploonet. It describes an object that started out as a moon, but became a planet.

The researchers were studying a class of exoplanets known as hot Jupiters. They’re giants like Jupiter, the biggest planet in our own solar system. But they’re extremely close to their parent stars. As a result, their upper atmospheres are heated to hundreds or thousands of degrees.

Astronomers have expected to find giant moons orbiting some of these planets. But so far, not a single “exomoon” has been seen.

A team of astronomers from Australia and South America came up with a possible explanation: the moons might escape.

Most models say that hot Jupiters are born farther from their host stars and migrate closer in. As they move closer, the combined gravity of the planet and star could give any orbiting moons a boost. The moons would enter higher and higher orbits, and eventually drift off on their own.

Escaping might not be such a good thing for the moons, though. The astronomers found that about half of the runaway moons would slam into their planet or star within a million years. Such an impact might leave a giant ring of debris around the parent star — perhaps explaining the rings seen in several star systems.

If it survives that initial trial period, though, a former moon could enter a stable orbit around the star. So it would take up a new life as a ploonet — a planet that started as a moon.

Script by Damond Benningfield

Few Americans have to hunt wild game to survive the long, cold winter these days. But that hasn’t always been the case. Native tribes and European settlers alike had to stock up as the weather grew cooler. And they relied on an autumn full Moon for help: the Hunter’s Moon.

And that Moon is coming up tonight. It’s the full Moon after the Harvest Moon, which appeared in September.

Like the Harvest Moon, the Hunter’s Moon helped people prepare for winter. Its light made it easier to see deer and other animals. And at high northern latitudes, the Moon rises only about a half-hour later each night around full Moon — a much smaller gap than average. So the big, bright Moon was in view by the time evening twilight faded away — allowing hunters to stay in the fields long into the night.

Despite what many people think, though, the Harvest and Hunter’s Moons aren’t any bigger and brighter than any other full Moons. The Moon’s orbit and its cycle of phases aren’t synchronized. So some years the Hunter’s Moon is closer than average. In other years, though, it’s farther than average — including this year. The Moon was farthest from Earth for its current orbit on Thursday. So we’re still thousands of miles farther than the average distance.

That means this year’s Hunter’s Moon is actually a little below average for a full Moon. Even so, it’s a beautiful sight, lighting the autumn fields for hunters — and everyone else, too.

Tomorrow: moons become planets.

Script by Damond Benningfield

We’ve seen details on the surfaces of only a few stars other than the Sun. Stars are just so far away that most of them look like nothing more than pinpoints of light, even to the biggest telescopes. And most of the ones where we have seen details are either giants or supergiants.

One exception is Altair, one of the members of the bright Summer Triangle. It’s high in the south as darkness falls, and drops down the western sky during the night.

Altair is in the prime of life. It’s a couple of times wider and heavier than the Sun, though, so it’s hotter and brighter than the Sun.

Altair is only 17 light-years away. By astronomical standards, that’s just down the block. That’s allowed astronomers to measure its size and shape, and to get a rough view of its surface.

They observed Altair with a set of small telescopes. They were linked through a technique known as interferometry. The combined telescopes see more detail than a single giant telescope could reveal.

The interferometer confirmed estimates of the star’s size made with other techniques. And they showed that the star is squished — it’s about 20 percent thicker through the equator than through the poles. That’s because the star rotates much faster than the Sun does, which pushes gas at the equator outward.

Finally, the technique produced rough images of Altair, showing big areas of light and dark on its surface — details that have been seen on few other stars.

Tomorrow: Stalking the full Moon.

Script by Damond Benningfield

Much of what makes science work is following up. After the discoveries are made, more research is needed to fill in the details. And the launch window for a spacecraft that’s all about the follow-up opens next week.

CHEOPS is a European mission that will study exoplanets — planets that orbit stars other than the Sun. It won’t hunt for new planets. Instead, it’ll make detailed observations of star systems with known planets.

CHEOPS will look for the planets to pass in front of their parent stars. Such a passage is called a transit. It causes a tiny “dip” in a star’s light. How much the star dims, and how long the transit lasts, reveals the size of the planet. Scientists especially want to study worlds that are between the size of Earth and the size of Neptune, the Sun’s fourth-largest planet.

The size will be combined with measurements of the planet’s mass made from the ground. The combination will tell astronomers how dense a planet is. And that will give them an idea of the planet’s composition. Less-dense planets are made mainly of gas, while heavy planets are made mainly of rock.

CHEOPS will keep an eye on target stars between transits as well. That’ll allow it to see how much starlight a planet reflects. From that, astronomers can deduce whether the planet has an atmosphere, and whether the atmosphere is topped by clouds.

It’s possible that CHEOPS will find more planets orbiting its target stars — giving it a chance to follow up on its own discoveries.

Script by Damond Benningfield

The Sun is a giant nuclear reactor. Every second, it “fuses” together about 650 million tons of hydrogen atoms to make helium. Some of the hydrogen is converted to energy — less than one percent. That’s enough to keep the Sun shining brightly.

Scientists have been trying to replicate that process here on Earth since the 1950s. They use two forms of heavy hydrogen. One is common in seawater, while the other has to be manufactured.

Success could provide abundant power with no climate effects and almost no radioactive residue. But controlled nuclear fusion has proved elusive. There’s a long-standing joke that it’s about 20 years away — and always will be.

The big problem is temperature. For fusion to work in reactors on Earth, the hydrogen must be heated to about 270 million degrees Fahrenheit — 10 times hotter than the center of the Sun. It’s hard to achieve those temperatures. And it’s even harder to contain the hot gases, known as plasma. You need strong magnetic fields, which require superconducting magnets and other high-tech equipment.

Scientists may be making progress, though. A giant project in Europe should be ready to start experiments in a few years, and achieve stable fusion a decade later. And several companies are entering the field as well. Some will use new techniques to hold the fusion reactions. And others may try different processes.

Most of the projects report that success could lead to commercial reactors — in about 20 years.

Script by Damond Benningfield

A mine in California holds the residue of collisions between dead stars. So do even bigger mines in China. The stellar “ash” consists of a set of chemical elements that are used to forge key parts of modern technology — from cell phones to batteries for electric cars.

They’re known as rare-earth elements. Some are born inside stars similar to the Sun that have reached the ends of their lives. A slow “drip” of neutrons builds up heavier elements, which are expelled into space when the stars die.

But most rare-earth elements are born in collisions between neutron stars — the tiny, ultra-dense corpses of once-massive stars. The impacts produce the extremely high temperatures needed to forge heavy elements — not just rare earths, but gold, silver, and similar elements as well. As they scatter through space, these elements may be incorporated into planets — like the newly forming Earth.

Despite their name, rare-earth elements are fairly common in Earth’s crust. But they don’t clump together — they’re sprinkled through the crust like bits of coconut on top of a cake. So it’s rare to find high concentrations of them.

Yet rare earths are critical ingredients in modern technology. They’re used in lighting fixtures, fighter jets, medical equipment, televisions, wind turbines, and much more. So the next time you use your cell phone or turn on an LED lamp, keep in mind that some of its raw materials were forged in the collisions of dead stars.

Script by Damond Benningfield

It’s summertime! Not here on Earth, of course. Instead, summer arrives today in the northern hemisphere of Mars. It’s the Red Planet’s longest season. It lasts about 178 Mars days, which is 183 Earth days.

The seasons on Mars have the same cause as those on Earth: the planet’s tilt on its axis. Right now, Mars’s north pole tilts toward the Sun. That brings the long days of summer north of the equator.

South of the equator, though, it’s the start of winter. And it’s a frigid season. Not only is it long, but Mars is farthest from the Sun during southern winter. So southern winter brings the coldest temperatures of the year. Lows in the southern hemisphere drop below minus 200 degrees Fahrenheit. And the highs typically stay well below minus 100.

The seasons bring big changes to the Martian ice caps. In the north, much of the ice cap is vaporizing, adding water and carbon dioxide to the atmosphere. And the opposite is happening in the south. Water and CO2 are freezing on the ice cap, making it bigger.

As a cap vaporizes, it stirs up dust around it, which can cause dust storms. Most northern storms are fairly small. Storms that start in the south, on the other hand, can blanket the entire planet — massive storms created by the changing of the season.

And Mars is just peeking into the dawn sky. It’s quite low in the east during twilight, so it’s hard to spot. It’s climbing higher each day, though, so it’ll be easier to see in a couple of weeks.

Script by Damond Benningfield

It’s not easy to take a picture of a planet orbiting another star. A planet is much smaller than a star, and it’s so faint that it’s drowned out by the star’s brilliant light. A decade ago, though, Hubble Space Telescope managed to snap the first picture of an exoplanet. It orbits Fomalhaut, the autumn star.

Fomalhaut is about 25 light-years away. It’s twice as big and heavy as the Sun, and about 16 times brighter. And it’s only one-tenth of the Sun’s age — about 450 million years.

It’s surrounded by a wide disk of dust. The disk is divided into several bands, with gaps between them. The inner band comes within a few million miles of Fomalhaut. The outer band extends almost 20 billion miles away from it.

Hubble found the planet in the outer regions of the disk. That distance was one reason Hubble could see the planet — it’s well away from Fomalhaut’s glare. Even so, it appears as no more than a tiny dot of light.

A few years ago, the planet was given a formal name: Dagon, for a Semitic god. Dagon probably is a giant, similar to Jupiter, the biggest planet of our own solar system. And there are hints that at least one more planet lurks inside Fomalhaut’s disk — perhaps waiting to have its own picture taken.

Fomalhaut is low in the southeast as night falls. It stays low in the sky as it arcs across the south during the night. It’s the only bright star in that region of the sky, though, so it’s easy to pick out.

Script by Damond Benningfield

Two well-known star patterns highlight the northern sky tonight. The Big Dipper is low in the north-northwest at nightfall, and in the northeast at first light tomorrow. And W-shaped Cassiopeia is just the opposite — in the northeast at nightfall, and north-northwest at first light.

As that sequence suggests, both patterns make a big circle around the sky during the night. They circle the North Star, Polaris — the hub of the northern sky. All the stars in the northern hemisphere appear to move around Polaris — the result of Earth turning on its axis.

For much of the United States, the stars of the Big Dipper and Cassiopeia never set at all. They appear close enough to Polaris that they never drop below the horizon. So they’re in the sky every day and night of the year, endlessly circling the North Star.

Such stars are called “circumpolar.” The number of such stars from any given location depends on latitude. From 30 degrees north — the latitude of Austin or New Orleans — anything within 30 degrees of Polaris always remains above the horizon. From 50 degrees north — roughly Paris or Vancouver — it’s anything within 50 degrees of Polaris. So as you go farther north, more stars are circumpolar.

And if you go all the way north — to the north pole — all the stars are circumpolar — nothing ever rises or sets. Instead, each star follows the same path across the sky night after night, circling the hub of the northern sky: Polaris.

Script by Damond Benningfield

The Moon slides down the southwestern sky this evening. It’s accompanied by Saturn, the second-biggest planet in the solar system. Saturn looks like a bright star to the right of the Moon.

Sunlight illuminates about half of the lunar hemisphere that faces our way. In other words, the Moon is at first quarter.

It probably sounds odd that it’s both a half Moon and a quarter Moon. But the two fractions describe different things. At first quarter, the Moon is one-fourth of the way through its cycle of phases. Daylight spreads across half of the visible lunar hemisphere, making it look like a half Moon.

If you look at that “half,” you’ll see several dark splotches. Those are lunar “seas” — another term that can be a bit bewildering. They’re filled not with water, but with volcanic rock. But long-ago skywatchers thought they looked like water. So they called these features lakes, seas, or oceans.

Among those visible tonight is the Sea of Crises — a small circular spot away from the other dark areas. Three larger ones are joined together: the Sea of Serenity, Sea of Tranquility, and the Sea of Fertility.

And this is a special night for watching the Moon: International Observe the Moon Night. Many groups around the country will host Moon-watching events.

If you miss the Moon tonight, though, don’t worry about it. It’ll be in good view tomorrow night, too, and most other nights of the year — a steady and beautiful presence in the night sky.

Script by Damond Benningfield