Scientists have been searching for dark matter for decades. They haven’t found it – every experiment they’ve devised has come up empty. But they haven’t given up. Among other ideas, they’re thinking about ways to use moons, planets, and stars as detectors. Dark matter appears to make up about 85 percent of all the matter in the universe. We know it’s there because its gravity pulls on the visible stars and galaxies around it. Dark matter may consist of a type of particle that almost never interacts with normal matter. But it should interact just enough to reveal its nature. Experiments here on Earth haven’t seen any such interactions. So some scientists recommend using astronomical objects instead of lab experiments. Blobs of dark matter might enfold a binary star system. The dark matter’s gravity could pull the two stars away from each other. And dark matter might clump together to make a special kind of star. Both of those might be detectable with current telescopes. Smaller blobs might slam into an icy moon, creating a special kind of crater. Such craters could be visible on Ganymede, the largest moon of Jupiter. Two missions on their way to Jupiter might be able to see them. And dark matter might fall into the center of a planet and hang around. If enough builds up, it could heat the planet’s interior. So by studying many planets in other star systems, we might see some that are unusually warm – heated up by encounters with dark matter. Script by Damond Benningfield

Things are heating up for a planet that orbits the brightest star of Aries. 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. Hamal is at the end of its life. It’s converted the hydrogen in its core to helium. Now, it’s getting ready to fuse the helium to make other elements. That’s made the core hotter. And that’s caused the star’s outer layers to puff up – to more than a dozen times the diameter of the Sun. So Hamal is about 75 times brighter than the Sun. Hamal has one known possible planet. It’s heavier than Jupiter, the giant of our own solar system. On average, the planet is about as far from Hamal as Earth is from the Sun – much closer in than Jupiter is. So every square foot of the planet’s surface receives dozens of times more energy than the same area on 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. Over the next few million years, the planet will get even hotter, because Hamal will get even bigger. The extra energy may erode the planet’s atmosphere completely. On the other hand, the planet may spiral into the star. Either way, things are going to get much hotter for Hamal’s only known planet. Look for Hamal in the east at nightfall, well to the left of the Moon. Script by Damond Benningfield

As most parents can tell you, coming up with names isn’t easy. It sometimes takes a while to settle on something that sounds just right. It wasn’t easy for the people who named the constellations, either. Some of the names sound like they just gave up. They picked a region of the sky with few stars, gave it the name of a nearby bright constellation, then added the word “minor.” All three of these minor constellations are in good view at dawn: Ursa Minor, Canis Minor, and Leo Minor. The most famous of the bunch is Ursa Minor – the little bear. Seven of its stars form the Little Dipper, which is in the north – directly below the Big Dipper, which is part of Ursa Major. The constellation is especially well known because its brightest star is Polaris, the Pole Star. It’s at the tip of the little bear’s tail. Canis Minor is the little dog. It’s about half way up the sky in the west-southwest. It has only a couple of bright stars. The brightest is Procyon – a name that means “before the dog.” That’s because the little dog leads the big dog across the sky. In ancient Greece, in fact, the constellation was known as Procyon. Finally, Leo Minor is high overhead. It’s the little lion, standing on the shoulder of Leo. That region of the sky wasn’t depicted as a separate constellation until 1687. Today, though, it’s one of the 88 official constellations – even if it is a “minor” one. Script by Damond Benningfield

The shortest season on the planet Mars begins today – autumn in the northern hemisphere, and spring in the southern hemisphere. It will last for 142 Mars days – almost eight weeks less than the longest season. Mars has seasons for the same reason that Earth does – it’s tilted on its axis. And the tilt is at almost the same angle as Earth’s. But the seasons on Mars are more exaggerated because the planet’s orbit is more lopsided. A planet moves fastest when it’s closest to the Sun, and slowest when it’s farthest from the Sun. That stretches out some seasons, and compresses others. It also changes the intensity of the seasons. Mars is farthest from the Sun when it’s summer in the northern hemisphere. So northern summers are fairly mild, while southern winters are bitterly cold. On the flip side of that, northern winters are less severe, while southern summers are the warmest time on the whole planet. The start of northern autumn also marks the beginning of dust-storm season. Rising currents of air can carry along grains of dust. Enough dust can be carried aloft to form storms that cover thousands of square miles. And every few Martian years, a storm gets big enough to cover the entire planet. The storms usually peak around the start of southern summer. Mars is about to pass behind the Sun, so it’s hidden in the Sun’s glare. It’ll return to view, in the dawn sky, in early spring – on Earth. Script by Damond Benningfield

The Moon slides by Saturn the next couple of nights. The planet looks like a bright star. It’s to the left of the Moon as night falls this evening, and to the lower right of the Moon tomorrow night. Saturn is best known for its rings. They’re almost wide enough to span the distance from Earth to the Moon. Right now, we’re viewing them almost edge-on, so they look like a thin line across the planet’s disk. Saturn isn’t the only world with rings. The solar system’s three other giant outer planets also have them. But they’re dark and thin, so they’re hard to see. Several asteroids and dwarf planets have rings, too. But the biggest set of rings yet seen may encircle a “rogue” planet about 450 light-years away. The possible rings were discovered years ago. Over a period of eight weeks, the light of a star in Centaurus flickered – sometimes dropping to just five percent of its normal level. The most likely cause was the passage of a set of rings in front of the star. And it’s quite a set. The rings are more than a hundred million miles across – greater than the distance from Earth to the Sun. The ringed planet appears to be traveling through the galaxy alone, and it just happened to pass in front of the star. It could be up to six times the mass of Jupiter, the giant of our own solar system. And moons could be orbiting inside the rings – the most impressive rings we’ve seen anywhere in the galaxy. Script by Damond Benningfield

Planets are tough little buggers. They can form and survive in some extreme environments. In fact, the first confirmed planets outside our own solar system orbit the remnant of a dead star – a pulsar. A pulsar is tiny – the size of a small city. But it’s more massive than the Sun. A teaspoon of its matter would weigh as much as a mountain. Yet a pulsar spins rapidly – up to several hundred times per second. It has an extreme magnetic field. The field shoots “jets” of particles out into space. As the pulsar spins, the jets can sweep across Earth like a lighthouse beacon, producing short pulses of energy. The timing of those pulses is extremely precise. That makes pulsars some of the best clocks in the universe. But the timing can be changed by a companion – another star, or even a planet. And that’s how pulsar planets are discovered – through tiny changes in the timing of the pulses. Eight pulsar planets have been confirmed. But they present quite a challenge. A pulsar is the remnant of a titanic explosion – a supernova. It’s hard to see how any planets could survive such a blast. So it’s likely that the planets formed after the blast – perhaps from debris from the explosion’s aftermath. Regardless of how they formed, the planets aren’t friendly places. They’re blasted with charged particles, X-rays, and gamma rays from the pulsar. That may slowly erode the planets – no matter how tough they are. Script by Damond Benningfield

[pulsar audio] This is the rhythm of the stars – the beat of dead stars. It’s the “pulses” of radio waves produced by rapidly spinning stellar corpses. They produce beams of energy that sweep around like the beacon of a lighthouse. Radio telescopes detect the beams when they sweep across Earth. The stars are known as pulsars. They’re some of the most extreme objects in the universe. They’re neutron stars – the dead cores of some of the most massive stars. When a heavy star can no longer produce nuclear reactions in its core, the core collapses. Gravity squeezes the core down to the size of a small city. But that tiny ball is heavier than the Sun. The star is rotating as it dies. As the core collapses, it keeps on spinning. But the smaller it gets, the faster it spins. So newborn neutron stars can spin a few dozen to a few hundred times per second. Particles trapped in the neutron star’s magnetic field produce energy that’s beamed into space – the source of the pulses. The neutron star spins down over time, slowing the pulses. But if it has a close companion, it can be revved up even faster. The neutron star can pull gas from the surface of the companion. As it hits the neutron star, the gas acts like an accelerator – creating some of the fastest pulsars in the universe. These extreme stars can still host planets; more about that tomorrow. Script by Damond Benningfield

Getting too close to a black hole is bad news. The black hole’s gravity can pull apart anything that’s falling into it atom by atom. A magnetar can do the same thing. And it’s not just its gravity you have to worry about. Its magnetic field can do the job as well – from hundreds of miles away. A magnetar is a neutron star -the crushed corpse of a once mighty star. It’s heavier than the Sun, but only a little bigger than Washington, D.C. It’s born when a massive star can no longer produce nuclear reactions in its core. The core collapses, while the star’s outer layers explode. The original star generated a strong magnetic field. As the core collapsed, the field was mashed inward as well, making it extremely powerful. It’s boosted by the turbulent sloshing inside the newly formed neutron star. So a typical magnetar’s magnetic field is a million billion times the strength of Earth’s field. The neutron star sticks around, but its magnetic field weakens in a hurry. So there aren’t many magnetars around – only about 30 have been discovered. The magnetic field can help produce titanic explosions. Interactions with the field can cause the crust of a neutron star to crack in a “starquake.” Energy from the quake is beamed out by the magnetic field, producing an outburst of gamma rays. The most powerful quake yet seen generated more energy in a tenth of a second than the Sun will emit in 150,000 years – the enormous power of a magnetar. More about neutron stars tomorrow. Script by Damond Benningfield

When the most massive stars die, they can leave behind two types of corpse. The heaviest ones probably form black holes. But the fate of the others is no less exotic. They form neutron stars – ultra-dense balls that are more massive than the Sun, but no bigger than a small city. A massive star “dies” when its core can no longer produce nuclear reactions. For a star of about eight to 20 or more times the mass of the Sun, the core collapses, while the star’s outer layers explode as a supernova. The gravity of the collapsing core squishes together protons and electrons to make neutrons – particles with no electric charge. The neutrons can be squished together only so much before they halt the collapse. By then, the core is trillions of times as dense as Earth. So a chunk of a neutron star the size of a sugar cube would weigh as much as a mountain. A neutron star probably has a solid crust made of iron or other elements, with no features more than a couple of millimeters tall. The gravity at the center of a neutron star is so strong that we don’t really know what the conditions are like – there’s just nothing to compare it to. There could be as many as a billion neutron stars in the galaxy. But they’re hard to find. Some of them make it a little easier, though. They produce the most powerful magnetic fields in the universe – and some of the most powerful outbursts. More about that tomorrow. Script by Damond Benningfield

You can always count on the constellations. Over the course of a human lifetime, their configuration doesn’t change – they don’t appear to move at all. That’s an illusion, though. The stars are all so far away that we don’t see any motion. But they’re all moving in a hurry. And one of the fastest is in view on autumn evenings. Gamma Piscium is the second-brightest member of Pisces, the fishes. The constellation stretches across the east and southeast at nightfall. Gamma Piscium is near its top right corner – part of a pentagon of faint stars. Gamma Piscium is a giant. It’s nearing the end of its life, so it’s getting bigger and brighter. Right now, it’s about 10 times the diameter of the Sun, and more than 60 times the Sun’s brightness. That makes it faintly visible to the eye alone, even though it’s 135 light-years away. Perhaps the most interesting fact about Gamma Piscium is its speed: It’s moving through the galaxy at about 340,000 miles per hour – faster than all but a few other visible stars. At that rate, it’ll move the equivalent of the Moon’s diameter in less than 3,000 years. The star’s composition hints that it came from outside the disk of the Milky Way – the part of the galaxy that includes the Sun. The star has very few heavy elements. That suggests it formed outside the disk, and just happens to be passing by – zipping through the galaxy like a speeding rocket. Script by Damond Benningfield