A third of a century ago, we knew of only two solar-system bodies beyond the orbit of Neptune: Pluto and its largest moon. Today, the known population of such bodies is in the thousands. And quite a few of them are in the same class as Pluto itself: dwarf planets. One of the newest members of that class is 2017 OF201. It was discovered in 2017. A recent study found that it may be about a third the size of Pluto. If so, then it most likely would qualify as a dwarf planet. The object follows a highly elongated orbit around the Sun. It ranges from about 45 times to 1600 times the distance from Earth to the Sun. And it takes almost 25,000 years to complete a single orbit. Today, it’s about 90 times the Earth-Sun distance, and moving outward. Before long, it’ll be so remote that not even the biggest telescopes can see it. Researchers say the object could be bad news for a possible Planet Nine. Studies of other objects in the outer solar system suggest that some of them may have been pushed around by the gravity of a much larger body. That body could be a planet roughly five to 10 times the mass of Earth, orbiting far from the Sun. But the orbit of 2017 OF201 shows no influence of such a planet. There’s a lot to be done to understand the orbits of the bodies in the outer solar system – and use them to pinpoint a possible planet far from the Sun. Script by Damond Benningfield

The roster of “dwarf planets” keeps growing. But it’s not official – there’s no league office to tell us who’s on the roster and who’s not. Various groups keep their own lists, but they don’t agree on which objects belong. The dwarf-planet category was formalized a couple of decades ago. Astronomers had discovered some new Pluto-like objects beyond the orbit of Neptune. They had to decide whether to add those objects to the roster of planets, or to put them in a new category. So in 2006, the International Astronomical Union voted to create the “dwarf planet” designation. A dwarf planet was defined as a body that’s large enough for its gravity to pull it into a rounded shape, but not large enough to clear its orbit of other bodies. The initial list included Pluto and three other distant objects, plus Ceres, the largest asteroid. Since then, astronomers have discovered thousands more objects in the realm of Pluto and beyond. Most of them are fairly small. But some are larger. Because they’re so far away, though, it can be tough to figure out an exact size and mass. So that makes it harder to decide whether some of these bodies are dwarf planets, or just big comets or asteroids. Today, most planetary scientists agree on a core list of about 10 dwarf planets. Another dozen or so are considered good candidates. And many more are possibilities – including a recently discovered one that we’ll talk about tomorrow. Script by Damond Benningfield

A bright star and planet team up with the Moon early tomorrow to form a tight, beautiful triangle. Pollux will stand close to the lower left of the Moon, with much brighter Jupiter about the same distance to the lower right of the Moon. Pollux is the brightest star of Gemini, while Jupiter is a planet. Jupiter is by far the giant of the solar system. It’s more than twice as massive as all the other planets combined. And it’s about 11 times the diameter of Earth. That makes it big enough to hold 1300 Earths. But a recent study says that Jupiter might have been much bigger during its infancy – about two or two-and-a-half times its current diameter. That would have made it big enough to hold thousands of Earths. Scientists came to that conclusion by studying the orbits of two of Jupiter’s small, close-in moons. The orbits are slightly tilted. Simulations showed that the moons were pushed into those orbits by the larger moon Io as it moved away from Jupiter. Those calculations revealed Jupiter’s original size and other details. Jupiter probably formed in just a few million years – much quicker than most of the other planets. By then, the supply of planet-making materials had dried up. So Jupiter’s gravity began squeezing it and making it spin faster. Eventually, the planet reached a point where it couldn’t shrink any farther – leaving the smaller but still-giant world we see today. Script by Damond Benningfield

A star that may be in a death spiral wants the universe to know about it. Every four and a half days it creates a burst of X-rays. The cause of those outbursts may be leading to the star’s demise. The possibly dying star is in a galaxy that’s about 300 million light-years away. During evening twilight now, that spot is quite low in the west, below the bright star Arcturus. According to a recent study, the story probably involves the star; a black hole, nicknamed Ansky, that’s a million times the mass of the Sun; and a wide disk of hot gas around the black hole. The star is following a tilted orbit around the black hole. Every few days, the star plunges through the disk. That heats the gas around the star, so gas blows away from the disk in bubbles that may be as massive as the planet Jupiter. Each passage robs the star of a bit of its orbital energy, so it spirals closer to the black hole. If the star is the mass of the Sun, it could last another five or six years before it dives into the black hole or is ripped apart by the black hole’s gravity. If the star is heavier, it could survive a little longer. Astronomers discovered the system in observations by two X-ray telescopes in space. They’ll use those same telescopes to watch the system in the years ahead. If the outbursts get more frequent, it’ll confirm they’re on the right track, and the star is on the wrong one – headed toward its destruction. Script by Damond Benningfield

There’s a season for everything, from football to Broadway to allergies. There are seasons in the heavens as well. And the next act in one of those seasons plays out early tomorrow: an occultation by the Moon of the star Elnath – the tip of one of the horns of Taurus. An occultation takes place when one object covers up another. The Moon occults a few fairly bright stars every month. And the occultations occur in seasons. That’s because the Moon’s orbit is tilted with respect to the ecliptic – the Sun’s path across the sky. The Moon moves back and forth across the ecliptic, allowing it to occult any star within a few degrees of that path. But its position relative to any particular star changes from year to year. As a result, occultations occur in bunches – in seasons. Now, the Moon is in the middle of its occultation season with Elnath. The season began in 2023, and continues into 2027. Because of the different angles to the Moon and star, and the short length of each event, only a few of the occultations are visible from a particular location. This occultation will be visible from the far-southwestern United States. Elnath and the Moon rise into good view after midnight, with the star to the lower left of the Moon. The Moon will slip toward Elnath as they climb higher. From most of the country, the Moon and star will just miss each other – a “seasonal” encounter in the dawn sky. Script by Damond Benningfield

The Milky Way is packed with star clusters – thousands of them. They contain anywhere from a few dozen stars to more than a million. And the most impressive of them all is right in the middle – it surrounds the supermassive black hole at the heart of the galaxy. The Nuclear Star Cluster contains up to 10 million stars. They extend a couple of dozen light-years from the black hole in every direction. But most of them are packed in close. If our part of the galaxy were that densely settled, we’d have a million stars closer to us than our current closest neighbor, Alpha Centauri. So any planets in the cluster would never see a dark night. Most of the stars in the cluster formed about 10 billion years ago, when the galaxy was young. But there was another wave of starbirth about three billion years ago, and a smaller one just a hundred million years ago. Each wave might have been triggered when the Milky Way swallowed a smaller galaxy. As the galaxies merged, clouds of gas and dust settled in the middle, around the Milky Way’s black hole. That gave birth to new stars – populating the galaxy’s most impressive cluster. The cluster is in Sagittarius, which is due south at nightfall. The constellation looks like a teapot. The center of the galaxy is in the “steam” rising from the spout. But giant clouds of dust absorb the light from the galaxy’s heart, so it takes special instruments to see the cluster. Script by Damond Benningfield

Guillaume Le Gentil spent more than 11 years away from his native France just to witness two brief astronomical events. Along the way, he had to survive war, a hurricane, disease, and grumpy officials. When he got home, he’d lost his job and been declared dead. But the real hardship? He missed both events. Le Gentil was born 300 years ago this week. He studied theology, but decided on astronomy as a career. He became a member of the Royal Academy of Science at age 28. Le Gentil and other astronomers hoped to measure a 1761 transit of Venus across the Sun from many locations on Earth. The details would reveal the Sun’s distance – the basic “yardstick” for the entire solar system. Le Gentil planned to watch from India. He headed out in March of 1760. War with England complicated the trip, and his ship was blown off course. On the day of the transit he was still at sea, where it was impossible to make observations. The next transit was just eight years away, so Le Gentil decided to hang around. He planned to watch from the Philippines. But he got a chilly reception, so he returned to India. He set up an observatory and waited. But the day of the transit was cloudy – until shortly after it was over. Heartbroken, Le Gentil headed home. It took two hard years to get there – only to encounter even more problems. But he worked things out, and published two volumes about his travels in the name of science. Script by Damond Benningfield

If you’d like to travel into the future – even the far-distant future – you don’t need a time machine. Instead, a starship will do just fine. Fire up the engines, head into space, and keep your foot on the gas. The laws of physics seem to make it impossible – or nearly so – to travel through time in anything like the modern concept of a time machine – something that allows you to move through the centuries at will. Yet those same laws make it possible to zoom into the future. The concept is known as time dilation. As you travel faster, your clock ticks more slowly compared to the clocks of those you left behind. It’s been proven by putting atomic clocks in airplanes and aboard GPS satellites. In fact, if GPS clocks weren’t adjusted to account for it, the entire system would fail. At the speed of a satellite, the difference is tiny – a few millionths of a second per day. As speed increases, though, the effect becomes more significant. If you could travel at 90 percent of the speed of light for one year as measured by the clock on your ship, more than two-and-a-quarter years would pass back on Earth. At 99 percent of lightspeed, it’s more than seven years per ship year. And at 99.99 percent, the ratio is 70 Earth years per ship year. Of course, there is the problem of finding a fast starship to carry you. But so far, that’s the only known way to beat Time – and travel into the future. Script by Damond Benningfield

Based on the number of books, movies, and TV shows about it, you might assume that traveling through time is almost as easy as ambling through the park on a sunny day: Just build a TARDIS or soup up your Delorean, and off you go. Alas, the arrow of time moves in only one direction. It allows you to travel into the future, but roadblocks seem to prevent any method that scientists can envision for traveling in the other direction. Wormholes, for example, are theoretical “tunnels” through space and time. They seem to allow travel to other times – past or future. But there’s a problem: The wormhole may collapse as soon as anything enters it – a person, a spaceship, or even a radio beam. Another possibility for traveling into the past is moving really fast. Albert Einstein’s theories of relativity suggest that anything moving faster than light might move backward in time. But any physical object moving at lightspeed would become infinitely massive. That means you’d need an infinite amount of energy just to reach lightspeed – and even more to go faster. A few decades ago, Stephen Hawking suggested that the universe doesn’t like time travel. He wrote that the laws of physics may stop anyone from ever building a time machine – keeping the past safe from its own future. Even so, physics provides some tricks that allow travel to the future, and we’ll have more about that tomorrow. Script by Damond Benningfield

If a cosmic giant sat on a big, gassy planet, it would look a lot like Saturn, the second-largest planet in the solar system. It’s 10 percent wider through its equator than through the poles. But Saturn flattened itself – a result of its low density and fast rotation. Saturn consists of a series of layers. Its core is a dense ball of metal and rock. Around that is a layer of hydrogen that’s squeezed so tightly that it forms a metal. Around that is a layer of liquid hydrogen – the lightest and simplest chemical element. And the planet is topped by an atmosphere that contains methane, ammonia, water, and other compounds. Despite its great size, Saturn spins once every 10.7 hours. That pushes material outward, making the planet fatter through the equator. The combination of its composition and rotation makes Saturn especially light – it’s less dense than water. Saturn doesn’t have a solid surface. But scientists have defined a “surface” as the depth in its atmosphere where the pressure equals the surface pressure on Earth. At that level, Saturn’s gravity is only a bit stronger than Earth’s gravity. So if you were floating at that altitude, you’d feel like you’d added a few pounds. And because of Saturn’s flattened shape, you’d feel heavier at the poles than the equator. Look for Saturn near the Moon tonight. It looks like a bright star to the right of the Moon in early evening, and farther below the Moon at dawn. Script by Damond Benningfield