Welcome back, and thanks for spending another evening here. Tonight we have sixteen stops, and a surprising number of them are about things that aren't there.
We'll meet a star barely bigger than Saturn, watch a comet break apart while Hubble happened to be looking, find a cloud of dark matter that never managed to become a galaxy, follow slushy hailstones down into Jupiter, and go looking for a planet nobody has seen. At the end, we'll ask the simplest question in astronomy: why is the night sky dark?
Get comfortable. 🌙
1. ⭐ EBLM J0555-57Ab is a star barely bigger than Saturn
Stars are supposed to be big. EBLM J0555-57Ab, about 600 light-years away, is just a sliver larger than Saturn.
It was found by WASP, a project built to hunt planets. Astronomers saw a Sun-like star dim slightly every 7.8 days as something small passed in front of it. That's what a planet looks like. But when they measured how hard the small object tugged on its partner, it turned out to be far too heavy: about 85 times the mass of Jupiter.
Squeeze that much mass into a Saturn-sized ball and you get gravity at the surface about 300 times stronger than Earth's.
It's also about as small as a star can be. It has just enough mass to fuse hydrogen in its core. "Had this star formed with only a slightly lower mass," said lead author Alexander von Boetticher, the fusion "could not be sustained," and it would have become a brown dwarf instead.
It's smaller, and probably colder, than many of the giant planets we've found around other stars. Sometimes it's harder to find a star than a planet.
2. ☄️ Comet K1 broke apart while Hubble was watching
Comets do sometimes fall apart, usually after a close pass by the Sun. Astronomers had tried for years to schedule Hubble to catch one in the act, and never managed it. By the time anyone noticed a breakup, it was weeks old.
Then luck stepped in. A team's original comet became impossible to observe, so they switched to a backup: Comet C/2025 K1 (ATLAS). When John Noonan opened the images, taken from November 8 to 10, 2025, "there were four comets in those images when we only proposed to look at one." K1 had split into at least four pieces, each with its own fuzzy cloud of gas and dust, and one smaller piece broke up again while Hubble watched.
K1 was about 8 kilometers across. A month earlier it had passed the Sun inside Mercury's orbit, which is when comets feel the most heat and stress. The team traced the pieces back and found it began breaking up just eight days before Hubble looked.
That raised a puzzle. Freshly exposed ice should make a comet brighten almost immediately, but the outbursts seen from the ground came later. One idea is that a layer of dust has to form on the new ice before gas can blow it off.
"The irony is now we're just studying a regular comet and it crumbles in front of our eyes," said Dennis Bodewits. Sometimes the best science comes from the backup plan.
3. 🌌 Cloud-9 is a galaxy that never switched on
Galaxies grow inside clumps of dark matter, the invisible stuff that makes up most of the universe's mass. Theory predicted that some small clumps would gather a little gas but never enough to make stars: failed galaxies, left over from the early universe. Nobody had found one.
Then China's FAST radio telescope spotted a small, round cloud of hydrogen near the spiral galaxy M94. It was the ninth cloud found there, so it was named Cloud-9. Radio telescopes in the US confirmed it. But was it really starless, or just a faint galaxy too dim for telescopes on the ground?
Hubble settled it. "With Hubble's Advanced Camera for Surveys, we're able to nail down that there's nothing there," said Gagandeep Anand. Not a single star.
The cloud's core is about 4,900 light-years across, holding about a million Suns' worth of hydrogen. For that gas to hold together, something much heavier must be pulling on it: roughly 5 billion Suns' worth of dark matter.
Much heavier and it would have collapsed into an ordinary galaxy. Much lighter and its gas would have been blown away. "This is a tale of a failed galaxy," said Alejandro Benitez-Llambay. Here, the stars that aren't there are the proof.
4. 🪐 Barnard's Star finally has planets, after decades of false alarms
Barnard's Star is a small red dwarf just six light-years away. It's the closest single star to the Sun, and only the Alpha Centauri system is nearer. It's also notorious among astronomers for planets that were announced and then turned out not to be real.
The problem is how small the signals are. Planets make their star sway back and forth very slightly, and that shows up as a tiny shift in its light. The four planets now confirmed make Barnard's Star move at just 0.2 to 0.5 meters per second, slower than a person strolls. The star's own surface jitters at about 2 meters per second, almost ten times more.
In March 2025, a team led by Ritvik Basant of the University of Chicago used the MAROON-X instrument on the Gemini North telescope in Hawaii, along with earlier data from ESO's Very Large Telescope. By carefully modeling the star's own noise and subtracting it, they confirmed one planet found earlier and revealed three more.
Each has between a fifth and a third of Earth's mass. Their years last from about two days to just under seven, so they're far too hot for life.
After all those false starts, our nearest single neighbor finally has a family.
5. ⚡ Jupiter's storms make hail out of slush
When Voyager first saw lightning on Jupiter in 1979, it seemed familiar. Scientists assumed it worked like ours, in clouds of water deep below the visible cloud tops.
NASA's Juno spacecraft, flying low over Jupiter's night side, saw something else: small, shallow flashes far higher up, where it's colder than −88 °C. Pure liquid water can't exist there.
Heidi Becker's team thinks Jupiter's powerful storms fling ice crystals more than 25 kilometers above the water clouds. Up there, ammonia gas melts the ice, acting like antifreeze, and forms droplets of ammonia-water. Those droplets collide with rising ice and charge the clouds. "Ammonia-water clouds do not exist on Earth," Becker said.
The same mix seeds hailstones nicknamed mushballs: layers of ammonia-water slush inside a crust of ice, growing as they bounce up and down through the storm. When they get too heavy, they fall deep into the planet and evaporate.
That solved another puzzle. Juno had found ammonia missing from much of Jupiter's atmosphere. "The ammonia isn't actually missing," said Scott Bolton. "It is just transported down while in disguise." On Jupiter, the weather hides its own ingredients.
6. 💥 SN 2025pht's star was hiding in its own dust
On June 29, 2025, a survey of the sky caught a new supernova, SN 2025pht, in the galaxy NGC 1637. Its light had been traveling for 40 million years. While others studied the explosion, Charlie Kilpatrick's team went back to old pictures to find out which star had died.
Webb had photographed that galaxy in 2024. Right where the supernova now shines, it showed a single red supergiant, a huge, cool star near the end of its life. Hubble had pictures of the same spot, and saw nothing at all.
The star was wrapped in so much dust that its visible light was blocked. Only Webb's infrared eyes could see it. "It's the reddest, most dusty red supergiant that we've seen explode as a supernova," said Aswin Suresh. It's the first time Webb has been used to identify the star behind a supernova.
That may solve a long-running puzzle. The most massive red supergiants should be the brightest stars in pre-explosion images, but astronomers kept failing to find them. If the biggest ones are also the dustiest, they've been hiding.
There was a second surprise: the dust seems to be rich in carbon, not the silicate dust astronomers expected. The team suspects the star dredged carbon up from its interior shortly before it exploded. The missing giants may have been there all along, hidden behind their own smoke.
7. 🪐 Planet Nine is a planet nobody has seen
In 2016, Caltech's Konstantin Batygin and Mike Brown announced evidence for a ninth planet, far beyond Neptune.
Their clue was a handful of icy objects at the edge of the Solar System. The six most distant had long, stretched orbits that all pointed the same way, and were all tilted the same way too. "It's almost like having six hands on a clock all moving at different rates, and when you happen to look up, they're all in exactly the same place," Brown said. The chance of that happening randomly, they estimated, was about 0.007%.
Their explanation was a hidden planet about 10 times Earth's mass, on average 20 times farther from the Sun than Neptune, taking 10,000 to 20,000 years to orbit once. Its gravity would herd the small bodies into line.
Not everyone agrees there's anything to explain. Surveys only find faint objects where and when they happen to look. In 2021, one team accounted for exactly where and how deeply three surveys had searched, and found the 14 objects they'd discovered were consistent with orbits scattered at random.
So far, no telescope has spotted the planet itself. Either a world bigger than Earth is hiding in the dark, or a pattern fooled us.
8. ⭐ KOI-54 rings like a bell every 42 days
NASA's Kepler telescope was built to find planets by watching stars for tiny dips. In one star, KOI-54, it saw the opposite: a sharp brightening every 41.8 days, with a gentle ripple of light running in between.
It turned out to be two nearly identical stars, each hotter and heavier than the Sun, on a very stretched orbit. Most of the time they're far apart. Then, once each orbit, they swing in close. Their gravity pulls each star out of shape, and they heat each other's facing sides, so the system briefly brightens.
The ripple is the stranger part. The close pass sets the stars vibrating, the way striking a bell makes it ring. The two strongest vibrations beat exactly 90 and 91 times per orbit, locked in step with the stars' approach.
A year later, astronomers found 17 more in Kepler's data, and KOI-54 is the prototype of what they call heartbeat stars. The name fits: a steady pulse, and then a ripple.
We see the pair almost face-on, so they never pass in front of each other. Everything we know comes from how their combined light changes. Two stars keep time like a heart and ring like a bell.
9. 🌌 Terzan 5 is a building block of the Milky Way that never got used
Terzan 5 was discovered in 1968 and filed away as a globular cluster, one of the tight, ancient balls of stars that orbit our galaxy. It sits in the Milky Way's bulge, the crowded, dusty region around the center.
But globular clusters usually hold one generation of stars, all born together. In 2009, astronomers found Terzan 5 had two. Now Giorgia Zullo, a PhD student at the University of Bologna, and colleagues have combined Webb's infrared view with 12 years of Hubble images. Hubble's long baseline let them see which stars move with the cluster and which are just passing in front.
The result: four generations of stars, born about 12.5, 4.7, 3.8 and 2.5 billion years ago. Each generation formed from gas enriched by supernovae from the one before.
That makes Terzan 5 something rarer than a cluster. Astronomers think the bulge was built when many clumps of gas and stars merged and mixed together billions of years ago. Terzan 5 seems to be one of those clumps that never mixed in. It's like a lump in an otherwise smooth cake batter.
The team calls it a bulge fossil fragment. Only one other is known so far. Part of our galaxy's original building material is still sitting there, intact.
10. 🔴 Mars's methane comes out at night
On Earth, much of the methane in the air is made by living things. So when NASA's Curiosity rover kept detecting methane in Gale Crater on Mars, people paid attention. The amounts are tiny, less than half a part per billion on average, like a pinch of salt in an Olympic swimming pool, with occasional spikes up to 20 parts per billion.
Then ESA's Trace Gas Orbiter, designed to be the best methane detector ever sent to Mars, found none at all. "I was definitely shocked," said Chris Webster, who leads Curiosity's methane instrument.
John Moores asked, "What if Curiosity and the Trace Gas Orbiter are both right?" Curiosity measures mostly at night, when Martian air is still and gas seeping from the ground can pool near the surface. The orbiter needs sunlight and looks a few kilometers up, after the day's churning has diluted the methane. In 2021, Curiosity took daytime measurements, and the methane dropped to essentially zero, just as Moores predicted.
That still leaves a puzzle. Methane should last about 300 years on Mars, so it should be building up in the air. Something seems to be destroying it faster. It could come from geology or from life, and nobody knows which. Mars breathes out a little methane at night, and by morning it's gone.
11. 🪐 29 Cygni b is a giant that formed like a planet
What separates the biggest planets from the smallest stars? One answer is how they're born. Planets grow from the bottom up, as dust clumps into pebbles and rocks and then gathers gas. Stars form from the top down, as a cloud of gas breaks apart and each piece collapses.
Objects about 15 times Jupiter's mass sit right on the line, where either process could work. 29 Cygni b is one of them, orbiting its star at about the distance of Uranus from the Sun.
Webb blocked its star's light and photographed the planet directly, measuring carbon dioxide and carbon monoxide in its air. That revealed that it's rich in heavier elements compared with its star, which is similar to the Sun. In total, it holds about 150 Earths' worth of heavy elements. A collapsing gas cloud would have the same mix as its star. A planet built by sweeping up solid material would be enriched, and 29 Cygni b is.
Another telescope array, CHARA, showed that its orbit lines up with its star's spin, as expected if it formed in a flat disk around the star.
Both clues point the same way. At 15 times Jupiter's mass, it still grew up like a planet.
12. ☀️ On Mercury, a day lasts two years
Before 1965, astronomers thought Mercury always kept one face toward the Sun, the way the Moon does toward Earth. Observers watching its faint markings had settled on a rotation of 88 days, the same as its year.
In April 1965, G. H. Pettengill and R. B. Dyce bounced radar signals off Mercury with the giant Arecibo dish in Puerto Rico and measured how fast it spins. The answer was about 59 days, in complete disagreement with the old value. It had been wrong all along.
Mercury spins three times for every two trips around the Sun. Put that together with its fast orbit, and from one sunrise to the next takes 176 Earth days, two full Mercury years.
Because its orbit is so stretched, things get stranger still. Near its closest approach to the Sun, Mercury races along its orbit so fast that it briefly outpaces its own spin. From some places on the surface, the Sun rises, sets again, and then rises a second time. Elsewhere it does the same at sunset.
Those long days and nights swing the surface from about 430 °C down to −180 °C. Stand on Mercury, and you could wait a whole year for the sun to set.
13. 🌙 Kepler-1625b's giant moon may not be there
Our Solar System has more than 200 moons. Finding even one around a planet orbiting another star has proved incredibly hard.
The most famous candidate orbits Kepler-1625b, a giant planet first found by NASA's Kepler telescope. In 2018, Alex Teachey and David Kipping watched it cross its star with Hubble and found evidence for a moon: an extra small dip in the star's light, and the planet's timing slightly off, as if something were tugging on it. The suggested moon was about the size and mass of Neptune, orbiting a planet several times heavier than Jupiter.
They were careful to say that it rested mainly on one Hubble observation and needed checking.
In 2023, René Heller and Michael Hippke reanalyzed the data. They found the moon's evidence came almost entirely from that single Hubble crossing, and argued it was an artifact of modeling how the star's disk looks darker near its edge. They concluded that neither Kepler-1625b nor a second candidate, Kepler-1708b, is likely to have a large moon. The original team has posted a reply defending its analysis, and the debate goes on.
The first moon beyond the Solar System is still waiting to be confirmed.
14. ⭐ Gliese 710 is heading for the Solar System
Somewhere in the constellation Serpens, about 60 light-years away, an orange dwarf star called Gliese 710 is coming almost straight toward us.
Using data from ESA's Gaia satellite, Berski and Dybczyński calculated in 2016 that it will pass about 13,000 times Earth's distance from the Sun, roughly a fifth of a light-year, in about 1.35 million years. Using Gaia's next data release in 2018, a second team confirmed the flyby, and found it may come a little closer, a little sooner.
That's well inside the Oort Cloud, the vast shell of icy bodies that surrounds the Solar System. A separate search of more than 7 million stars found that Gliese 710 is the closest encounter known, and the one that will disturb the Oort Cloud most.
The planets should barely notice. But the star's gravity may nudge many icy bodies inward, possibly setting off a major shower of comets into the inner Solar System.
A visitor is already on its way, and it's been traveling toward us since long before anyone could look up.
15. 🌠 The Murchison meteorite holds stardust older than the Sun
In 1969, a meteorite fell near the town of Murchison in Australia. The people of Murchison made it available to science, and the Field Museum in Chicago now holds the largest portion. Hidden inside are presolar grains, specks of mineral that formed around other stars before the Sun existed.
They're tiny: a hundred of the biggest would fit on a period. To get them out, scientists crush the rock into a paste that, as Jennika Greer put it, "smells like rotten peanut butter," then dissolve away everything else with acid. "It's like burning down the haystack to find the needle," said Philipp Heck.
To date each grain, Heck's team measured how long it had been hit by cosmic rays drifting through the galaxy, which slowly create new elements inside solid matter. Heck compares it to a bucket left out in steady rain. The more water inside, the longer it's been out.
Most grains were 4.6 to 4.9 billion years old, and some were older than 5.5 billion. The Sun is 4.6 billion years old. Some of this stardust formed 5 to 7 billion years ago, making it the oldest solid material ever found on Earth.
The unexpected crowd of slightly younger grains suggests a burst of star formation before the Sun was born. You could hold, in a museum, dust from stars that died before our own was lit.
16. 🌌 The night sky is dark because the universe had a beginning
Here's a question a child might ask: why is the night sky dark?
It sounds silly until you try to answer it. Imagine a universe that goes on forever, filled with stars. Every direction you look, your line of sight would eventually end on a star, the way every line of sight in a deep forest ends on a tree. Distant stars look fainter, but there are more of them, and the two effects cancel exactly. The whole sky would blaze as bright as the surface of the Sun.
This puzzle is called Olbers' paradox, after the German astronomer Heinrich Olbers. Dust doesn't solve it. In a sky that bright, the dust would heat up and glow too.
The answer is time. The universe isn't infinitely old, and light travels at a finite speed. Light from the most distant stars and galaxies simply hasn't had time to reach us. The dark between the stars is a horizon in time.
Curiously, the first published version of this answer is credited not to an astronomer but to the poet Edgar Allan Poe, in his essay Eureka.
So the darkness is a clue. Every night, the black between the stars tells you that the universe had a beginning.
🌙 One last thought before you sleep
So much of tonight was found in what's missing.
Cloud-9 was proved by the stars it doesn't have. A dying supergiant was found where Hubble saw nothing. Jupiter's ammonia turned out to be hiding, not missing, and Mars's methane vanishes every morning. Planet Nine is a gap in the sky shaped like a planet, if it's there at all.
Astronomers learn to read silences. An empty patch of sky, a dip that doesn't happen, a signal that isn't where it should be: each one is information.
And the biggest absence of all is the one you'll see if you look out the window tonight. The darkness between the stars isn't nothing. It's the edge of time, the place where light hasn't arrived yet.
Even the dark is telling us a story.
That's all for tonight. Thanks for wondering along with me. Good night, and clear skies. 🌌
— Kasi