The planet Mercury is shrinking. It’s contracted by several miles since its birth. And it’s continuing to get smaller even now. Mercury is the closest planet to the Sun. It’s also the smallest major planet in the solar system – a little more than 3,000 miles in diameter – about the width of the 48 states. It has a core of iron and nickel, surrounded by dense layers of rock. And it’s topped by a thin crust. The surface of the planet is marked by lots of impressive cliffs. The biggest is more than 600 miles long and about two miles high. They formed as Mercury lost heat from its interior. As the planet cooled, it shrank. Estimates of how much it’s contracted have ranged from about a mile to about nine miles. A recent study narrowed the range a little bit. It measured the most dramatic features, then scaled that to the surface of the entire planet. The result suggests that Mercury has shrunk by about three to five miles as a result of its cooling. And when you add in some other causes, the total contraction is about four to seven miles. And Mercury is still getting smaller today. This incredible shrinking planet is quite low in the southeast in the dawn twilight for the next few days. It looks like a bright star, but you need a clear horizon to spot it. And because of the viewing angle, it’s easier to spot from more southern latitudes. Tomorrow, the Moon stands to its right or upper right. Script by Damond Benningfield

Here’s what we know for sure about the planet K2-18b. It’s about 125 light-years away. It’s bigger and heavier than Earth. It orbits a cool, faint star once every 33 days. It receives about the same amount of energy from its star as Earth gets from the Sun. And it has an atmosphere. After that, things get muddled. Astronomers aren’t sure about the structure of the planet or the make-up of its atmosphere. And ideas about whether it might be habitable are all over the place. The confusion highlights the challenges of studying planets in other star systems. K2-18b passes in front of its star on every orbit. And as it does so, the chemical “fingerprints” of its atmosphere are added to the starlight. Substracting the starlight provides a profile of the atmosphere. But the profile is hard to read. Many of the fingerprints are subtle, and can be produced by different compounds. Earlier this year, a team announced the discovery of compounds in the atmosphere that could be produced by microscopic life. Follow-up studies by other groups contradicted that finding. But the original study team has stuck by its conclusions. So it’ll take a lot more work to know for sure what’s going on at K2-18b. The K2-18 system is in Leo, which climbs into good view after midnight. K2-18 is to the right of Denebola, the star that marks the lion’s tail. But it’s too faint to see without a telescope. Script by Damond Benningfield

For stars that are similar to the Sun, the end comes in stages. And each stage is triggered by changes in the star’s core. One star that’s going through those changes is Diphda, the brightest star of Cetus, the sea monster. The star is several hundred million years old – billions of years younger than the Sun. For most of its life, it “fused” hydrogen atoms in its core to make helium. When the hydrogen was gone, it began fusing hydrogen in a shell around the core. That made the star puff up, so it was classified as a red giant. Now, it’s finished off the shell, so it’s fusing the helium in the core to make carbon and oxygen. This phase is generally lumped into the red-giant category. Technically, though, it has its own name: the red clump. In a hundred million years or so, Diphda will have used up all the helium. The star isn’t massive enough to fuse the carbon and oxygen to make heavier elements. Without that energy, the core will collapse to about the size of Earth. It’ll be extremely hot, though, so it’ll blow away Diphda’s outer layers. For a while, the star will enter one more phase: a planetary nebula – a colorful cloud of gas and dust. When the cloud disperses, only the dead core will remain: a white dwarf – the hot but tiny remnant of a star. Cetus spreads across the southeastern quadrant of the sky at nightfall. Diphda is near its lower right corner, roughly a third of the way up the sky. Script by Damond Benningfield

People collect all kinds of things, from baseball cards to Persian rugs. Over the past 40 years, some NASA aircraft have collected dust – grains of dust from beyond Earth. Many of the collection efforts have taken place during meteor showers. That’s included the Geminid shower, which is at its peak tonight. A meteor shower takes place when Earth flies through a trail of particles that were shed by a comet or asteroid. Many of the particles burn up in the upper atmosphere, creating the streaks of light known as meteors. But many more grains are too small to burn up. They float down through the atmosphere. Some of them stop at a height of about 10 miles. And that’s where the research aircraft head. Once there, they open up small boxes that catch whatever is drifting along – pollen grains, parts of bugs, bits of volcanic ash, and even exhaust from rocket engines. Analysis reveals whether the captured particles are from Earth or from outside. The cosmic particles can then be tied to the meteor shower that was under way. And that can tell scientists about the shower’s parent body – a sample-return mission that never leaves Earth. The Geminids are in good view tonight. The meteors are visible from mid-evening on. At its best, the shower might produce a hundred or so meteors per hour. And you don’t need to look in a particular direction to see them – just look up and wait for the fireworks. Script by Damond Benningfield

A couple of thousand years ago, a large asteroid or comet might have been blasted apart. And we’re still seeing the fireworks from its destruction – as the Geminid meteor shower, which will reach its peak tomorrow night. Most meteor showers flare to life when Earth passes through the orbital path of a comet. The comet sheds bits of rock and dirt, which spread out along its orbit. As Earth flies through this trail of debris, the solid grains ram into the atmosphere, forming the glowing streaks known as meteors. But the Geminids are a bit odd. For one thing, their parent body – 3200 Phaethon – appears to be an asteroid or a “dead” comet, not an active comet. For another, the meteor stream contains way more material than we’d expect to see from a body the size of Phaethon. A couple of years ago, scientists came up with a possible explanation. They used observations by a Sun-orbiting spacecraft that passed through the meteor stream. They then used computer models to calculate a possible cause for the stream. They concluded that a larger body could have been destroyed. That produced Phaethon and a couple of other large remnants. But it also produced a giant cloud of dust and pebbles. So while some of the material that makes up the Geminids comes from Phaethon, a lot of it also comes from that cloud – shrapnel that makes fireworks in Earth’s night sky. More about the Geminids tomorrow. Script by Damond Benningfield

The Sun isn’t easy to influence. It’s more than a thousand times the mass of Jupiter, the solar system’s largest planet, and more than 330,000 times the mass of Earth. Even so, a recent study says the planets might influence our star’s magnetic cycle – perhaps making conditions more comfortable for life. The Sun goes through many cycles of magnetic activity. The best known lasts an average of 11 years. At the cycle’s peak, the Sun is much more active than average. It pelts Earth and the other planets with higher levels of radiation and charged particles. That can wreak havoc with everything from satellites to blood pressure. Another cycle lasts an average of less than two years. It produces “mini” peaks and valleys in the 11-year cycle. And it lines up well with the longer cycle. In the recent study, researchers from Germany compared these cycles to the orbits of the planets. They found that the peaks and valleys of the shorter cycle correspond to some planetary alignments. One was a lineup of Earth, Jupiter, and Venus. The other was an alignment of Jupiter and Saturn. The researchers said the planets may help control the solar cycles. The planets might even tamp down the Sun’s activity, which is weaker than that of many Sun-like stars. Less activity means that Earth gets bombarded by less of the nasty stuff – making our planet a much more comfortable home for life. Tomorrow: cosmic shrapnel. Script by Damond Benningfield

Storms on the Sun can cause all kinds of problems. They can knock out satellites and black out power grids. They can interfere with GPS and disrupt some radio broadcasts. They can even have an impact on human health. Solar storms happen when the Sun’s magnetic field gets tangled up. Lines of magnetic force can snap, then reconnect. That produces outbursts of radiation and charged particles. When the particles hit Earth, they’re funneled toward the surface by our planet’s own magnetic field. And that’s what causes the problems. Among the health concerns, particles and radiation can penetrate deeper into the atmosphere around the magnetic poles. That zaps anyone who’s flying at high altitudes in those regions. It’s not a fatal dose, but it’s enough to cause concerns. So airlines divert flights to avoid exposing passengers and crew. There’s also evidence that these bouts of “space weather” can boost people’s blood pressure. In one study, researchers in China looked at half a million blood pressure readings taken over six years. And they found a definite jump around the time of solar storms – especially among women and those with hypertension. An American team found similar results among older men. There’s no consensus about how space weather might cause blood pressure to spike. For now, all we know is that stormy skies on the Sun can cause lots of problems for the people on Earth. Script by Damond Benningfield

The Moon and the heart of the lion just miss each other tonight – at least as seen from the United States. As they climb into good view, after midnight, the Moon and the star Regulus will be separated by just a skosh. The farther north and east your location, the closer together they’ll appear. From some spots, they’ll be almost touching. And from much of Canada across to northern Norway they will touch – the Moon will occult the star. It’ll pass directly in front of Regulus, blocking it from view. The Moon can occult Regulus because the star lies almost atop the ecliptic – the Sun’s path across the sky. The Moon stays close to the ecliptic as well, but it does move a few degrees to either side. As a result, occultations of Regulus come in groups. This one is part of a cycle of that began earlier this year and will continue through the end of next year. Each occultation is visible from a different part of Earth. In part, that’s because each one lasts only a few minutes to a few hours, so the Moon and Regulus are below the horizon as seen from much of the world. Also, the Moon is so close to us that there’s a big difference in the viewing angle across the globe – up to two degrees – four times the width of the Moon itself. From any specific location, sometimes the angle is just right, but more often it’s a little off – providing a beautiful close encounter between the Moon and the heart of the lion. Script by Damond Benningfield

A couple of years ago, a space telescope discovered something odd about NGC 6505. The galaxy is encircled by a ring. It isn’t part of the galaxy itself. Instead, it’s an image of a background galaxy – one that’s billions of light-years farther. Einstein Rings are named for Albert Einstein because they were predicted by his theory of gravity. The gravity of a foreground object acts as a lens – it bends and magnifies the light of a background object. On small scales, gravitational lenses have revealed everything from black holes to rogue planets. Galaxies are much bigger and heavier, so they produce more dramatic lenses. Many of them create bright arcs. But when the alignment is just right, they can create a full circle. NGC 6505 is a good example. The galaxy is about twice the diameter of the Milky Way, and several times its mass. It’s about 600 million light-years away. The background galaxy is four billion light-years farther. The lensing effect has allowed astronomers to measure the amount of dark matter in the center of NGC 6505, as well as details about its stars – discoveries made possible by its beautiful ring. NGC 6505 is enwrapped in the coils of Draco, the dragon. The galaxy is more than a third of the way up the northwestern sky at nightfall. It’s visible through a small telescope. But you need a big telescope and a long exposure to make out its ring. Script by Damond Benningfield

The Moon is a “dead” world. It trembles with a few small moonquakes, and there may be occasional “burps” of gas. But for the most part, not much happens inside it. That’s definitely not the case for one of the moons of the giant planet Jupiter. Io is the most volcanically active world in the solar system. It’s covered by hundreds of volcanoes and pools of hot lava. Some of the volcanoes are larger than anything on Earth, and the lava is much hotter. The volcanoes can send gas and ash hundreds of miles high. Some of this material escapes Io completely – about one ton every second. It forms a wide “doughnut” around Jupiter. The activity is powered by a gravitational tug-of-war between Jupiter and some of its other big moons. They pull on Io in different directions. That heats Io’s interior, melting some of its rocks. A couple of recent studies found that Io has been at least this active since it was born. That suggests that Io and the other big moons have been locked into their current configuration since shortly after the birth of Jupiter itself. If that’s the case, then Io has been caught in a terrific tug-of-war for four and a half billion years. Jupiter rises above our moon this evening. The planet looks like a brilliant star – only the Moon and Venus outshine it. But you need binoculars to pick out Io and the planet’s other big moons. Tomorrow: gravitational “rings” around a galaxy. Script by Damond Benningfield